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■ セミナー (2019年度)

December/12/2019 (Thu.) 15:30-16:45 @ Room 431
Prof. Takamasa Momose
Dept. of Chemistry, The University of British Columbia
''Deceleration and trapping of cold free radicals''
Since the realization of Bose-Einstein condensation (BEC) of ultracold atoms, experimental observation of correlated motion of matter waves of particles in an ensemble has drawn considerable attention in various fields, revealing new quantum aspects of matter. Especially, the research of cold and ultracold molecules has expanded rapidly over the past decade because of their importance in various fields from fundamental physics to interstellar chemistry.
Recently we have demonstrated that a supersonic beam of methyl radical (CH3), one of the most important hydrocarbon free radicals, has been slowed down to standstill with a magnetic molecular decelerator, and successfully captured spatially in an anti-Helmholtz magnetic trap for over 1 s [1, 2]. The present method is, in general, applicable to trap any molecule (or atom) that has one or more unpaired electrons, i.e. free radicals. The demonstrated trapping capability of polyatomic radicals opens up various possibilities for realizing ultracold ensembles of molecules towards Bose-Einstein condensation of polyatomic molecules, investigations of reactions governed by quantum statistics, and applications to quantum information processes.
Furthermore, high-resolution spectroscopy of trapped cold radicals may reveal parity violation in molecules, which might be related to the homochirality of biological molecules. We will discuss the details of our experiments and various future applications of trapped free radicals.
[1] Y. Liu et al., Phys. Rev. Lett. 118, 093201 (2017)
[2] A. G. Smart, Physics Today, 70, 4, 18 (2017)

December/06/2019 (Fri.) 17:00-18:30 @ Room 1320 [物理学教室談話会]
Prof. Pertti Hakonen
Low Temperature Laboratory, Department of Applied Physics, Aalto University, Finland
''Suspended Nanocarbon Devices in Creation and Sensing of Condensed Matter States''
Graphene and carbon nanotube (CNT) mechanical resonators make ultra-sensitive sensors due to their small mass and nearly ideal structure. At low temperatures, high quality factors can be obtained, which facilitates detection of tiny frequency shifts as well as small variations in the resonance amplitude. The frequency shifts are caused by a modification in the resonator mass or in the spring constant, while the amplitude changes are due to altered dissipation.
I will review a few of our recent sensing experiments. For example, we have employed mechanical resonance frequency in a combined graphene-gold resonator mode to identify ordered states of two-dimensional electron gas in graphene: de Haas–van Alphen effect in the quantum Hall regime and magnetization of composite fermions have been investigated. The other examples to be discussed deal with charge detection sensitivity, interplay of superconducting phase and mechanical motion, detection of ultrasonic shear modes, and sub-monolayer 3He films on carbon nanotubes.

December/04/2019 (Wed.) 10:00-12:00 @ Room 431
Jun Usami
''Non superfluid - superfluid transition in 2D''
Helium on graphite above the third layers shows the superfluidity, which is consistent with thin helium films on non-uniform substrate. The second layers also show the frequency shift in the torsional oscillator studies. It was only observed in the limited density region with no universal jump. Contrary, the universal jump caused by the vortex pairs unbinding appears above the critical density in other non-uniform substrate cases. The several studies found the quantum phase transition from the gapped localized phase to the gapless superfluid phase at the quantum critical point in thin layers of helium. I will discuss the experimental results, the theoretical study of Mott insulator-superfluid transition using the idea of percolation and other studies.

November/27/2019 (Wed.) 10:00-12:00 @ Room 431
Dr. Tomohiro Matsui
''ULT-MF-SPM and Studies of the Zigzag Edge States in Graphene''
In this colloquium, I want to overview our experimental set-up and recent STM/S studies about the zigzag edge states on graphite surfaces.
As the first topic, I want to remind about our multi-functional scanning probe microscope (MF-SPM), which can work not only as STM but also as atomic force microscope (AFM). In addition, since there are six electrodes on the sample stage, electronic transport property can also be obtained for exactly the same sample in exactly the same environment as STM and AFM studies. If the AFM tip is metallic, this MF-SPM can be operated as, for example, scanning gate microscope (SGM) and scanning nuclear-resonance microscope (SNRM). Some studies using SGM and SNRM are planning to be shown in this colloquium.
As the second part, I will show our STM/S results about the zigzag edges fabricated on graphite surfaces by using H-plasma etching technique. Here, I want to show two topics; one is the edge states in magnetic fields, discussing about the relation between the zigzag edge state and the 0th Landau level, and the other is the spin polarized states observed in the zigzag graphene nanoribbons.

October/30/2019 (Wed.) 10:00-12:00 @ Room 431
Shohei Takimoto
''Quantized Vortex Tangle in Superfluid Helium''
In recent years, study on superfluid turbulence has been more important. Superfluid turbulence consists of quantized vortices which has quantized (i.e. discretized) circulation. That means superfluid turbulence is thought to be a simple prototype of classical turbulence. Therefore, an understanding of superfluid turbulence leads to an understanding of classical turbulence.
Particle Tracking Velocimetry (PTV) is a method to make quantitative measurements of quantized vortices. This method provides us new knowledge of quantized vortices.
In this colloquium, I will explain theoretical background of quantum hydrodynamics and latest results of superfluid turbulence in steady-state He-II counterflow by means of PTV.

October/23/2019 (Wed.) 10:00-12:00 @ Room 431
Keisuke Kinoshita
''Studies of Quantum Phase in monolayer Helium3 on Bilayer HD''
Helium-3 layer adsorbed on grafoil is an ideal 2D Fermion system with spin S = 1/2. Among the particles, exchange interaction exists arising from quantum mechanic tunnering. In 2D solid Helium film, this exchange interaction plays important role because whether the number of exchange particles is odd or even is strongly related to magnetism. In this study, the data of heat capacity and magnetism give hints to understand the structure of Helium-3 in each phase.

July/17/2019 (Wed.) 10:00-12:00 @ Room 431
Andre E. B. Amend
''Moiré pattern and electronic properties of epitaxial graphene on SiC(000-1)''
Epitaxial graphene grown on the C-face SiC interacts only weakly with the substrate and thus grows with different orientations. Multilayer epitaxial graphene can therefore posses orientational misalignments that create Moiré superlattices and restore the symmetry between the two graphene sublattices. This latter property causes the Fermi velocity of epitaxial graphene grown on SiC(000-1) to be higher than on graphene systems with broken sub lattice symmetry, such as graphite or epitaxial graphene on SiC(0001). Therefore C-face epitaxial graphene posses a lot of potential for study and applications. In this colloquium I want to present the properties of Moiré patterns of graphene, and the electronic features and tunneling spectra of epitaxial graphene on SiC(000-1).

July/03/2019 (Wed.) 10:00-12:00 @ Room 431
吉田 研介
''超低温温度計としてのQTFとその理論''
超流動ヘリウム3-B相における音叉型水晶振動子(QTF)のふるまいについて紹介します． QTFを超流動中を運動する剛体としてモデル化することで，共鳴曲線から得られる半値幅の情報をもとに温度測定に関する定量的な議論を行います．

June/20/2019 (Thu.) 13:30-15:30 @ Room 431
Jun Usami
''Consideration of monolayers of 4He from the view point of the temperature or the density dependence''
Torsional oscillator studies showed the superfluidity of monolayers of 4He on Grafoil, which has the unusual density and temperature dependence. Those unusual properties can be because of the coexisting of the superfluidity and spatial order.
KT theory can explain the superfluidity in two dimensions experimentally found in the torsional oscillator, the third sound and the thermal resistivity studies and also superconductivity. I will discuss the temperature or density dependence of the superfluidity in such studies and compare them with monolayers of 4He on Grafoil.

May/29/2019 (Wed.) 10:00-12:00 @ Seminar room on 3rd floor, CRC
Shohei Takimoto
''Visualization of Quantum Vortices''
The method which can visualize quantum vortices by using small particles of solid hydrogen was established [1]. Small particles of solid hydrogen are trapped by quantum vortex cores due to pressure gradient. Thus, we can visualize the quantum vortices by tracking the small particles of solid hydrogen.
I would like to explain the details of the visualization method. And I will also explain the property of motion of solid hydrogen particles trapped by quantum vortices.
[1] G.P.Bewley et al., Nature 441, 588 (2006)

May/22/2019 (Wed.) 10:00-12:00 @ Room 431
Dr. Tomohiro Matsui
''STM/S Studies of the Spin-polarized zigzag edge states''
Recent STM/S studies of the zigzag graphene nanoribbon (z-GNR) prepared by H-plasma etching on graphite, measured at temperatures down to 60 mK and in magnetic fields up to 13 T, will be shown in this colloquium. Using the actual STM/S results, I want to show the way to analyze them and discuss how one can understand them.

March/27/2019 (Wed.) 10:00-12:00 @ Room 431
Dr. Tomohiro Matsui
''Toward experimental observations of the spin polarized zigzag edge state''
Recently, we believe we have observed the spin polarized local density of states (LDOS) for graphene nanoribbons (GNRs) terminated by two zigzag edges (ZGNRs), where observed dI/dV are qualitatively consistent to the theoretical considerations. However, this consistence is not perfect, and there are still some features which cannot be understood theoretically. Aiming to consider and understand the experimentally observed LDOS on our ZGNR on graphite, I want to discuss some topics which can modify the band structure and LDOS of free-standing ZGNRs, in this colloquium.

February/6/2019 (Wed.) 10:00-12:00 @ Room 431
Andre E. B. Amend
''Quasi-free-standing Graphene on SiC(0001) and Graphene on SiC(000-1) ''
Epitaxial graphene grown on SiC offers a lot of potential to study various graphene systems. On the Si-face of hexagonal SiC strong substrate coupling determines the alignment of grown graphene, thus it forms commensurate stacks. On this face, graphene can be decoupled from the substrate by hydrogen intercalation, yielding “quasi-free-standing” graphene. On the C-face, on the other hand, due to the weak substrate coupling, newly grown graphene layers can have many different alignments, thus epitaxial graphene stacks grown on this face posses rotational disorder between layers. The relative rotation of layers removes the inter layer symmetry breaking (that is present in AB stacked graphene) and gives rise to additional van Hove singularities. Both QFS graphene on the Si-face and graphene grown on the C-face will be the topic of this colloquium.

November/14/2018 (Wed.) 10:00-12:00 @ Room 431
Takamasa Suzuki
''Magnetic Property of monolayer Helium-3''
Helium-3 absorbed on graphite is ideal fermion 2D-system. Monolayer of Helium-3 shows various kinds of phases depending on areal density. I will introduce several spin susceptibility and NMR experiments on each phase of 3He/HD/HD/graphite and 3He/4He/graphite.

November/07/2018 (Wed.) 10:00-12:00 @ Room 431
Jun Usami
''The density problems of monolayers of He4 ~layer promotion, KT transition~ ''
Monolayer of helium has attracted much interest since it has not only a rich phase diagram but also the superfluidity. In the second layers of He4 on graphite substrate, the quantum liquid crystal phase existing between the liquid and solid phase was predicted by the heat capacity measurement. The superfluidity was also observed from the torsional oscillator experiment around that phase. However the density dependence of the superfluid response were in consistent among those previous observation by several groups. This ambiguity of density scale is because the inhomogeneities of graphite substrate.
I summarize those inconsistencies in the critical densities in this colloquium. The way of comparing the superfluid response given in higher densities with the torsional oscillator measurement with KT theory will be explained, where the relation among the signal size, the critical temperature and the density is focused. Finally, I talk about the unusual third sound observed in the monolayers of He4 on hydrogen.

November/6/2018 (Tue.) 15:30-17:00 @ Seminar room on 3rd floor, CRC
Dr. Christpher Bauerle
''Manipulating single electrons on the fly using a sound wave?''
Surface acoustic waves (SAW) provide a promising platform to realize quantum optics experiments with electrons at the single particle level. Pioneering single-shot experiments have shown SAW-assisted electron transport between spatially separated quantum dots over a distance of 4 μm with an efficiency of about 92% [1,2]. More recently we have also been able to transfer spin information [3], demonstrating the potential of this technique for coherent long distant transfer between two static qubits.
Here we go an important step further. We couple two quantum channels by a tunnel barrier along a region of 2 μm. At the end of each channel a quantum dot is placed serving as single-electron source and detector, respectively. We demonstrate single-electron transport over a distance of 22 μm with efficiencies above 99%. Changing the energy detuning in the coupling region we can partition the electron on-demand into two paths. By gradually changing the barrier height we additionally observe tunnel oscillations of the probability that the electron ends up at the upper or the lower detector quantum dot. This finding demonstrates coherent manipulation of the electron quantum state on the fly [4].
Our results pave the way for the implementation of a solid-state flying qubit having high relevance in fundamental research and quantum information technology.
[1] S. Hermelin et al., Nature 477, 435 (2011).
[2] R. P. G. McNeil et al., Nature 477, 439 (2011).
[3] B. Bertrand et al. Nature Nanotechnology 11, 672 (2016).
[4] S. Takada, H. Edlbauer et al, to be published.

October/24/2018 (Wed.) 10:00-12:00 @ Seminar room on 3rd floor, CRC
Shota Yamazaki
''Blackbody radiator in 3He-B''
The dilute gas of excitations from 3He-B in the temperature range below about 0.2Tc can be regarded as the ideal medium for a quasiparticle experiment because of their very long mean free paths. A blackbody radiator is composed of two vibrating wire resonators(VWRs) and a box with a small hole, and it can be used as a quasiparticle detector and source.
In my colloquium, I explain the experiments about (i)calibration of blackbody radiator, (ii)the blackbody as source, (iii) the beam spread experiment with the blackbody as detector.

October/10/2018 (Wed.) 10:00-12:00 @ Seminar room on 3rd floor, CRC
Shohei Takimoto
''quantized vortex tangle and knot theory''
Vortex is important phenomena both in classical and quantum fluid. And vortex lines are tangled in turbulence. Knot theory is one of the tools to analyze the vortex tangle because tangled vortex lines form knots and links. I will explain knot theory briefly and how to apply knot theory to the vortex tangle.

July/11/2018 (Wed.) 10:00-12:00 @ Room 431
Andre E. B. Amend
''Spin-Orbit Coupling in Graphene''
Single layer pristine graphene is conventionally thought of as a zero-gap semiconductor, where, without doping, low-energy excitations posses a linear dispersion relation. However, even in this idealized case a band-gap exists theoretically that is induced by the atomic spin-orbit interaction, but the gap size is usually so small that it is neglected. Nevertheless, spin-orbit interaction in graphene lifts the degeneracy between spin up and down states at the two Dirac points and enables the control of the spin by external electric fields via the Rashba effect. It also introduces additional spin scattering mechanisms, the commonly low spin-orbit coupling in graphene thus makes possible longer spin coherence lengths that are desireable in spintronic devices. Aside from external fields, the spin-orbit coupling can also be strongly influenced by adsorbates, defects and curvature. In this presentation some theoretical background on the spin-orbit coupling in graphene as well as experiments that measure and modify it will be presented.

July/04/2018 (Wed.) 10:00-12:00 @ Seminar room on 3rd floor, CRC
Shota Yamazaki
''The theory and the experiments of two-dimensional topological insulator HgTe/CdTe''
In the quantum Hall system, the quantized hall conductivity is explained by TKNN number, and the system can be regarded as a kind of topological insulators in which the time-reversal symmetry is broken. The quantum spin Hall system has time-reversal symmetry because of the strong spin-orbit interaction, and the Z2 invariant is important for classifying the topological property. In my colloquium , first I will introduce the time-reversal operator, Kramers pair, and Z2 invariant. Then I explain the two-dimensional topological insulator HgTe/CdTe through the tight binding calculation and the experimental results.

June/13/2018 (Wed.) 10:00-12:00 @ Room 431
Dr. Tomohiro Matsui
''Quasi-Free-standing Graphene''
We are recently studying the H-plasma etching of graphene epitaxially grown on SiC(0001). For this sample preparation, one can expect not only the formation of the zigzag edges as in the case of graphite surfaces, but also the realization of so-called quasi-free-standing graphene (QFG), which is a graphene layer decoupled from the SiC substrate. In this colloquium, I want to show the properties of such QFG and its related amterials.

May/30/2018 (Wed.) 10:00-12:00 @ Room 431
Jun Usami
''Two dimensional melting -property of hexatic phase-''
The two dimensional melting is explained by KTHNY theory. KTHNY theory predicts the continuous phase transition solid to liquid via hexatic phase, which has quasi-long-range bond orientational order. There are several real systems for two dimensional system. To my knowledge, there is not the real two dimensional system completely consistent with KTHNY theory or other theories.
The two dimensional hexatic phase is first observed in liquid crystal systems. I show the properties of such systems and the relation to the theory.

May/09/2018 (Wed.) 10:00-12:00 @ Room 431
Takamasa Suzuki
''Frustrated magnetism of YbMgGaO4''
I will introduce one of QSL material, YbMgGaO4.
YbMgGaO4 is triangular-lattice material and have very high frustration.(even higher than some kagome-lattice material) and this leads to peculiar nature.(example: specific heat)
I will introduce several experiments and theories of YbMgGaO4 tomorrow.

April/25/2018 (Wed.) 10:00-12:00 @ Seminar room on 3rd floor, CRC
Kensuke Yoshida
''量子力学における幾何学的位相と量子ホール効果''
近年注目を集めているトポロジカル物質の理論的基礎について発表します． 量子系の断熱的変化を考えることによって出てくる幾何学的位相の導入から，量子ホール効果の説明まで行う予定です． 基礎となる量子力学をできるだけ厳密に紹介したいので数式多めになると思います．

April/11/2018 (Wed.) 10:00-12:00 @ Seminar room on 3rd floor, CRC
Shohei Takimoto
''Development of a compact and continuous nuclear demagnetization refrigerator''
A copper nuclear demagnetization refrigerator(NDR)is a method to reach sub-mK temperature. However, it requires a large superconducting magnet. And it is also a one-shot refrigerator. To solve these problems, we are developing a compact and continuous NDR with two PrNi5 nuclear stages.
I designed a superconducting magnet with a FeCoV shield and simulated the distribution of heat and magnetic field. In this colloquium, the details of simulation and its results are discussed.

February/07/2018 (Wed.) 10:00-12:00 @ Room 431
Andre E. B. Amend
''Experimental Indications of Magnetic Polarization and Electron Confinement on Graphene Nano-ribbons''
I will introduce one of QSL material, YbMgGaO4.
Zigzag-type graphene edges host electronic edge-localized states. The high density of states at the edges opens the possibility of magnetic polarization of the spins at the edges. On zigzag edge ribbons with nano-scale width, the ground state is expected to be spin-polarized, due to the interaction of the edge-states along opposite edges. Anti-ferromagnetic edge interaction is expected to be most stable, and cause a spin-gap in the LDOS with one or two peaks on either side. In addition, to the edge-states, the nano-ribbons can feature carrier confinement, where the borders of the valence and conduction bands may also give rise to density of state peaks. In this colloquium the theoretical calculations and possible experimental results on different substrates of spin-split states and electron confinement on graphene nano-ribbons will be reviewed.

February/05/2018 (Wed.) 10:00-12:00 @ Seminar room on 3rd floor, CRC
Shota Yamazaki
''Mobility of ions trapped below a surface of 3He-B''
A copper nuclear demagnetization refrigerator(NDR)is a method to reach sub-mK temperature. However, it requires a large superconducting magnet. And it is also a one-shot refrigerator. To solve these problems, we are developing a compact and continuous NDR with two PrNi5 nuclear stages.
I designed a superconducting magnet with a FeCoV shield and simulated the distribution of heat and magnetic field. In this colloquium, the details of simulation and its results are discussed.The surface Andreev bound states (SABSs) are formed at the surfaces of 3He-B. On my colloquium, I explain the experiment to detect SABSs from the mobility of the impurity trapped below a free surface of 3He-B. First, I show the mobility data of positive and negative ions.The data don't depend on the depth from the surface, which indicate that the contribution from SABSs can be ignored. Second, I introduce the scattering theory of the Majorana fermions in the SABSs. The theory suggests that the transport cross section increases as the depth deeper. The theory quantitatively reproduces the mobility, and we conclude that the mobility experiment succeeded to detect the SABSs.

December/25/2017 (Wed.) 10:00-12:00 @ Room 431
Takamasa Suzuki
''Triangular-lattice quantum spin liquid''
Due to strong magnetic frustration, several materials which have triangular lattice don't show magnetic ordering until very low temperature. Tommorrow,I introduce basic theory for spin liquid at first.(maybe mean field approach to spin liquid,and so on) Next,I review theoretical papers which discuss the possibility of spin liquid on triangular-lattice. Finally, I review several experimental results of triangular-lattice.

December/06/2017 (Wed.) 10:00-12:00 @ Room 431
Dr. Tomohiro Matsui
''Lifting of the four-fold degeneracy of graphene''
Graphene is fourfold degenerated reflecting its peculiar honeycomb structure. This fourfold degeneracy can be lifted by breaking the honeycomb symmetry; for example by introduce substrate potential or by applying high magnetic field to enhance the electron-electron interaction. In this colloquium, I want to show some experimental observations of the lifting of the four-fold degeneracy of graphene. Direct observation of Landau quantization by use of the scanning tunneling spectroscopy can clearly show the lifting. In addition, the observation of the Landau level on the surface layer can be used to characterize graphene itself, such as number of layers.

November/20/2017 (Wed.) 10:00-12:00 @ Seminar room on 3rd floor, CRC
Kensuke Yoshida
''Andreev Reflection and Surface Andreev bound state''
I will introduce about Andreev reflection and Andreev bound state with this colloquium. Andreev reflection was theoretically clarified by Andreev in 1964. This theory explains the increase in thermal resistance at the NS junction. The same phenomenon is generally observed in the region where the density of state changes spatially. In connection with this, I will also introduce the state that appears in the surface state of p-wave superconductors. This state is called the Andreeff bound state and also appears on the surface of the superfluid helium three B phase. At the end of the presentation, I will also describe the prospect of further experiments on this surface state.

November/08/2017 (Wed.) 10:00-12:00 @ Room 431
Masahiro Kamada
''Heat Capacity of 3He monolayer on HD plated graphite''
Helium three (3He) monoatomic layer adsorbed on graphite surface is an ideal strongly correlated fermion system with S = 1/2. Especially the second layer 3He shows interesting properties such as the quantum spin liquid state. We measured heat capacity of 3He monolayer on graphite preplated with a bilayer of HD, and found quantum phase diagram of this system is qualitatively different from that of the second layer 3He on graphite.
Recently I reanalyzed our heat capacity data based on new calibration for thermometers and reconsidered amorphous contribution. This analysis requires modification about the phase diagram in the vicinity of solidification. In this colloquium, the detail of the reanalysis and the resulting heat capacity data are discussed.

November/06/2017 (Wed.) 10:00-12:00 @ Room 431
Kazuma Kita
''Fabrication of Zigzag edges on Graphite/Exfoliated Graphene and its Electrical transport properties''
Graphene nanostructure with zigzag edges is one of the most interesting materials for diverse electronic and spintronic devices. To obtain zigzag edges hydrogen plasma etching is commonly used because it allows precise, top-down fabrication and tailoring of graphene nanostructures. Recently, a lot of evidence for patterning of graphene with zigzag edges by hydrogen-plasma treatment has been reported. In this colloquium, first, the detail of the mechanism of hydrogen plasma etching is discussed and compared with the experimental results. Second, there is a possibility to observe zigzag edges studying the Raman spectrum and electrical transport properties. Therefore, how to distinguish zigzag edges based on the Raman spectrum and scattering behavior of electrons in graphene is discussed.

October/30/2017 (Wed.) 10:00-12:00 @ Room 431
Katsuyoshi Ogawa
''NMR measurements of 3He adsorbed on graphite''
In solid 3He, the higher order cyclic exchanges such as three- or four-particle exchange are favored over the simple interchange of two atoms, because of a strong hard core repulsive potential between atoms. Exchange of an even number of particles is AFM, while that of odd number is FM. The existence of two competing interactions makes the system intrinsically frustrated, leading to various peculiar magnetic properties both in bulk and in adsorbed 3He. Recent studies of solid 3He films adsorbed on graphite have proved to provide a ruly two-dimensional frustrated S=1/2 quantum spin system. In this Colloquium, I will introduce some kinds of NMR measurement of the frustrated two-dimensional system and theoretical interpretation. In latter part, I discuss what is expected in NMR measurement of 3He on HD bilayer system from earlier experiments.

October/18/2017 (Wed.) 10:00-12:00 @ Room 431
Jun Usami
''vortex in superflow -phase slip and Josephson effect-''
Superfluid is named because of zero viscosity of helium. However superfluid looks to has viscosity due to quantized vortex. In toroidal experiment, vortex line can be detected one by one. In thin aperture experiments, phase slip can be seen as the flow increase to reach the critical velocity. When the size of aperture is comparable to the healing length, Josephson effect appears as same as superconductor in both 4He and 3He system. In this colloquium, I introduce those experiments and theoretical explanation.

July/11/2017 (Wed.) 10:00-12:00 @ Room 431
Katsuyoshi Ogawa
''Anisotropic superconductivity state in Sr2RuO4''
Following the discovery of high-Tc superconductivity, cuprates, a large number of related compounds have been found that are superconducting at relatively high temperatures. The superconductivity of Sr2RuO4 was discovered in 1994 and has attracted much attention. Not only because it has the same structure as La2-xSrxCuO4 of high-Tc cuprates but also because the experimental data are strongly suggestive of a triplet order parameter being different from conventional s-wave superconductivity and high-Tc cuprates. In this colloquium, I will introduce some unconventional properties of Sr2RuO4 superconductivity and discuss the possible p-wave order-parameter symmetry of Sr2RuO4 by introducing the concept of the d-vector.

July/6/2017 (Thu.) 13:30-14:30 @ Room 445
Dr. Aron Beekman
''Dislocation-mediated melting to quantum liquid crystals''
While liquid crystals can be viewed as rotational symmetry breaking of a liquid, such order can also be obtained by translational symmetry restoration in a solid. This Kosterlitz-Thouless dislocation-mediated melting process was shown to give rise to the so-called hexatic liquid crystal phase by Nelson, Halperin and Young in 1978-1979. Here we consider the analogous zero-temperature quantum phase transition from a two-dimensional bosonic solid to a quantum liquid crystal by condensation of dislocations. We employ a duality mapping where the phonons of the solid are represented by gauge bosons ("stress photons") that mediate the elastic interactions between dislocations. In the liquid crystal, transverse shear phonons undergo a dual "stress Meissner effect" and can no longer propagate long-range shear forces.
In this talk I shall focus on the phenomenology of such quantum hexatic phases, mostly in the form of the spectrum of low-lying excitations. Here we find two gapless modes: the longitudinal phonon (sound) but with a reduced velocity, and a Nambu-Goldstone mode due to rotational symmetry breaking. There are also two gapped modes coming from the gapped shear phonons and from the dislocation condensate itself. I will also discuss the limitations of this idealized model and the possible modifications needed when applying it to real systems.
Reference: Physics Reports 683, 1-110 (2017), arXiv:1603.04254

June/28/2017 (Wed.) 10:35-12:00 @ Room 431
Kazuma Kita
''Transport Properties in Graphene and Multilayer Graphene''
Graphene is a model system for the study of electrons confined to a strictly two-dimensional layer and a large number of electronic phenomena have been demonstrated in graphene, from the fractional quantum Hall effect to superconductivity. In this colloquium I will present (1)general transport properties such like temperature dependence and layer number dependence, (2)scattering properties due to defects, (3) some characteristic oscillation in magnetic filed which is related to our experiments.

May/31/2017 (Wed.) 10:00-12:00 @ Room 431
Dr. Tomohiro Matsui
''How Robust Graphene Edge Magnetism is ?''
It is widely believed for graphene nanoribbon (GNR) with zigzag edges that the spins are polarized ferromagnetically along the edges and anti-ferroomagnetically interacting between the edges in its ground state. However, in principle, long-range order is prohibited in one dimensions. One dimensional spin ordering is observed only with a help of large anisotropy (which is not expected for graphene because of its small spin-orbit interaction) and the substrate. In addition, such a picture is for ideal system and the realistic situation can be dependent on the environment. Here, in this colloquium, I will introduce some theoretical considerations of graphene edge magnetism with fluctuations, charge doping and the substrate. Please note that there are plenty of theoretical studies in this field and what I will show are just a tiny part of them. I just hope they can be good starting points for further considerations.

May/22/2017 (Mon.) 10:00-12:00 @ Room 431
Andre Amend
''STS study of Graphene Edges Fabricated by Hydrogen Plasma Etching''
Imbalance between the sublattice sites in a bipartite system such, as graphene near Dirac point energy, induces zero-energy modes. In the presence of sublattice interaction these modes can persist and thus electrical localized states can be measured on disordered real graphene. The local density of state (LDOS) depends on the disorder type. The extended Zigzag edge state is of particular interest since it may support magnetic polarization. The electrical states produced by single vacancies and Zigzag edges, as well as methods to produce Zigzag edges, are reviewed. An analysis of our STM/S measurements of Zigzag edges and Zigzag graphene nano-ribbons (GNR) on graphene, fabricated by Hydrogen-plasma etching, is presented. The Zigzag edge spectra show the characteristic low-energy peak in the LDOS and a pseudo-gap surrounding the peak, which may stem from the edge state degeneracy. The GNRs also hosted the edge state, but no evidence of a LDOS double-peak was observed, which is expected for spin-polarization.

Feb/20/2017 (Mon) 14:00-15:00 @ Room431
松元和幸 氏
室蘭工業大学　応用理化学系応用物理
『2次元吸着固体³Heにおける核磁性のスケーリング理論』
　グラファイトなどの基板上に吸着させた³Heは2次元”固体“を形成し核磁性を 示す。その研究は1980年代に始まったが、その様子が分かってきたのはつい最近のこ とである。理論的には2次元三角格子反強磁性ハイゼンベルク模型が提案されてきた が、最近の超低温領域実験によって、こうした簡単な模型では説明できないデータが 蓄積されつつある。具体的には、磁化率はT^-1/3、比熱はT^2/3の温度依存性を 示すことが明らかになりつつある。これらは、2次元三角格子反強磁性ハイゼンベル ク模型では説明できない。そこで、この講演では2次元三角格子反強磁性ランダムハ イゼンベルク模型を提案したい。実際、この模型を解析することによって、磁化率、 比熱の温度依存性が説明できそうなのである。解析方法としては、バットとリーに よって拡張された数値的スケーリングを用いてみた。実際に計算を実行してみると磁 化率および比熱の温度依存性は、計算条件の詳細にはよらないことが分かった。そし て、ランダムネスが十分につよい時にはさきほどの指数、すなわち-1/3と2/3と矛盾 のない数値が得られた。このことから2次元三角格子反強磁性ランダムハイゼンベル ク模型がこの系に適用できる可能性は十分に高い。さらに、これらの結果をサポート するある強相関系におけるスケーリング理論も紹介したい。ただし、問題にしている 2次元”固体“のような整合相で、はたして相互作用がランダムであるかどうかは確 かに疑問の残るところである。この点については今後さらなる考察が必要となるであ ろう。

Feb/8/2017 (Wed) 10:00-12:00 @ 低温センター3階セミナー室
小川 勝之
''Multiple spin exchange in three- and two-dimensional solid 3He''
Three-dimensional solid helium 3 forms bcc structure under 3 < P < 10 MPa. The nuclear spin system in this solid has an unusual magnetic phase diagram which cannot be explained by the Hamiltonian including only nearest neighbor interactions. However, the phase diagram can be explained by the Hamiltonian including multiple spin exchange (MSE) proposed by Thouless in 1965. In MSE, exchanges among the even number of atoms favor anti-ferromagnetic interaction. On the other hand, those among the odd number of atoms favor ferromagnetic one. The competition among them make the system magnetically frustrated. When the dimensionality is reduced from three to two, the frustration is enhanced. In two-dimensional triangular lattice an intriguing ground state, quantum spin liquid (QSL), is expected with MSE. The existence of QSL is also indicated from experimental studies on helium 3 monoatomic layer adsorbed on graphite expected to form triangular lattice.

Dec/15/2016 (Thu) 16:00-17:00 @ Room445
Prof. Keith E. Gubbins
Dept. of Chemical & Biomolecular Engn., North Carolina State University, USA
''Confinement-Induced High Pressure Phases in Nanopoers: Can the Pressure be in the Megabar Range?''
There is an abundance of anecdotal evidence that nanophases adsorbed within nanoporous materials can exhibit high pressures as a result of the confinement1,2. For example, phase changes and chemical reactions that only occur at high pressures in the bulk phase occur in the confined phase at bulk phase pressures that are orders of magnitude lower. The pressure in the pore is a second order tensor, and for simple pore geometries has both a normal pressure component (normal to the walls) and one or more tangential components (parallel to the walls).
For simple fluids in pores that are up to a few nanometers in width, molecular simulations show that both the normal and tangential pressures can be locally very high (thousands or tens of thousands of bars) in the pore, even though the bulk phase in equilibrium with the pore is at a pressure of one bar or less. The cause of these high in-pore pressures will be discussed, and where possible comparison with experimental results will be made3.
When the molecules in the confined nanophase react with each other chemically it may be possible to achieve even higher tangential pressures, in the megabar range. Evidence for this is provided by recent experiments on sulfur (an insulator at ambient conditions) in narrow single-walled carbon nanotubes, carried out by Kaneko and coworkers4. They find that the sulfur atoms within the pore covalently bond to form a one-dimensional phase that is metallic. In the bulk phase sulfur forms a metallic phase only at pressures above 95 GPa. In our recent molecular dynamics simulations of this system we find that the sulfur atoms are covalently bonded in the pore and that they experience tangential pressures in excess of 100 GPa as a result of the strong confinement5.
1. Y. Long, J.C. Palmer, B. Coasne, M. Sliwinska -Bartkowiak and K.E. Gubbins, "Pressure enhancement in carbon nanopores": A major confinement effectモ, Physical Chemistry Chemical Physics, 13, 17163 (2011).
2. Y. Long, J.C. Palmer, B. Coasne, M. Sliwinska- Bartkowiak, G. Jackson, E.A. Muller and K.E. Gubbins, "On the Molecular Origin of High Pressure Effects in Nanoconfinement: Effects of Surface Chemistry and Roughness", Journal of Chemical Physics, 139, 144701 (2013).
3. M. Sliwinska-Bartkowiak, H. Drozdowski, M. Kempinski, M. Jazdzewska, Y. Long, J.C. Palmer and K.E. Gubbins, "Structural Analysis of the Behavior of Water Adsorbed in Activated Carbon Fibers", Physical Chemistry Chemical Physics, 14, 7145 (2012).
4. Y. Fujimori, A. Morelos-Gomez, Z. Zhu, et al., "Conducting Linear Chains of Sulphur Inside Carbon Nanotubes", Nature Comm., 4, 2162 (2013).
5. K.E. Gubbins, C.A Addington and J.M. Mansell, to be published.

Dec/14/2016 (Wed) 10:00-12:00 @ Room431
松井 朋裕
''Magnetic Property in Graphite''
Since magnetic order is observed for pi-electrons in non-metallic organic polymer-based materials in 1991, possible magnetism in carbon-based structure are also attracting great interest. In fact, ferromagnetic and superconducting magnetization hysteresis loops were observed in HOPG in a broad temperature range. However, the details of the hysteresis depend on the sample, sample heat treatment and on the direction of the applied field. The origin of magnetic order in such a light element is only poorly understood. In this colloquium, I want to overview the ferromagnetic properties observed in graphite.

Nov/29/2016 (Tue) 10:00-12:00 @ Room431
中村 祥子
''Mass transport measurement of 4He''
To consider alternatives to torsional-oscillator experiments for detection of superfluidity in monolayer 4He, recent mass transport measurements on solid 4He will be reviewed. Previous studies on 4He films related to forthcoming mass transport measurements will be introduced as well.

Oct/12/2016 (Wed) 10:00-12:00 @ Room431
Andre Amend
''Graphene Zig-Zag direction Edge States''
Due to the theoretical bipartite Hamiltonian of graphene it is expected that sublattice vacancy imbalances give rise to localized zero-energy states. Such localized states increase the electrical conductivity near the defects and may produce a magnetic moment in some cases. The graphene Zig-Zag edge has attracted much interest since it features a large sublattice imbalance and thus supports localized edge-states. Our recent STS studies of edges in Zig-Zag direction have probed the edge-state localization and have revealed features beyond the single or double (polarized edge) peak spectrum which is expected for Zig-Zag edges. The observed edge-states may be the result of Zig-Zag direction edges other than the pure Zig-Zag edge.

Sep/21/2016 (Wed) 10:00-12:00 @ 低温センター3階セミナー室
中村 龍司
''Kitaev Spin Liquids''
The Kitaev model is one of the few exactly solvable models in physics. In recent years, this model has attracted both experimental and theoretical physicists’ attentions.A. Kitaev proved that the ground state of the model has both gapless and gapped quantum spin liquid phases.
In this colloquium, I will show you the novel spin liquid behaviors of the Kitaev model. This model is thought have a relevance for undiscovered Majorana fermions, so I will discuss the relationship with our experimental results.

Sep/12/2016 (Mon) 10:00-11:30 @ Room431
喜田 和馬
''Electronic transport properties in graphene and graphene nanoribbon''
Graphene samples can have a very high carrier mobility, allowing to observe a ballistic transport, the fractional quantum Hall effect or quantum interference in bulk graphene. Another interesting properties can be seen in graphene-based antidot lattice with lattice periods down to 50 nm. They have pronounced commensurability features stemming from ballistic orbits. Finally, I will show that the effects of edge and bulk disorder on the conductance of graphene nanoribbons comparing zigzag edge with armchair edge.

Jun/30/2016 (Thu) 13:30-14:30 @ Room431
高野 安正 教授
Dept. of Physics, Univ. of Florida
''Spin-1/2 Heisenberg antiferromagnet in one dimension''
The one-dimensional spin-1/2 Heisenberg model is one of the few exactly solvable non-trivial models in physics. When the interaction is antiferromagnetic, the ground state of this model is a Tomonaga-Luttinger liquid, in which dynamic and static properties are inextricably linked. Low-energy excitations are spinons instead of bosonic magnons, with a unique gapless dispersion. These and other properties of the model have been extensively studied since the pioneering work by Bethe, published in 1931. This seminar describes our recent experiment that puts some of the theoretical predictions to tests.

Jun/13/2016 (Mon) 10:00-11:30 @ 低温センター3階セミナー室
中村 龍司
''The possibility of two dimensional superfluid 3He''
The superfluidity is a one of the states of matter, which behaves like a fluid without viscosity. For superfluid 3He, the symmetry of Cooper pairs is an odd parity (p-wave). We could describe the mechanism of the superfluid 3He by using the Legget equations. On the other hand, for 2D superfluid 3He, the symmetry of Cooper is considered to have a density dependence by some theoretical physicists. In this colloquium, I show two theories for 2D and 3D super fluidity and compare them. I will talk about a new NMR experiment to seek 2D 3He superfluidity.

Jun/6/2016 (Mon) 10:00-11:30 @ Room431
松井 朋裕
''Graphene Antidot Lattices''
Introducing a periodic array of holes, i.e., an antidot lattice, in a graphene has been suggested as a route towards the band gap opening in otherwise zero-gap semiconductor. However, so far, most studies in this field are restricted to experiments for circular antidots and theoretical discussions without spin polarization. Recently we have succeeded to control the shape of antidot between circular and hexagonal, in other words, between irregular edges and zigzag edges, by changing the temperature during hydrogen plasma etching of graphene/graphite. This technique is potentially able to extend the non-magnetic properties of graphene antidot lattices (GALs) further to spin dependent properties. In this colloquium, I want to overview the electronic properties of GALs and to provide a base to consider the electronic and magnetic properties of graphene with either circular or hexagonal nanopits.

Apr/25/2016 (Mon) 10:00-11:30 @ Room431
Andre Amend
''Zero-energy states in disordered graphene''
Any real honeycomb graphene lattice is subject to different types disorder, which can change the system's electrical properties. Defects and disorder can often be modeled as vacancy configurations in the hexagonal lattice, such as single vacancies or edges. For low energy excitations the system can be described by a bipartite Hamiltonian. It is shown that for for all types of vacancy configurations, the number of induced zero-energy states has a lower bound which is equal to the local site imbalance between the graphene sublattices. These induced states were found to be (quasi-) localized, depending on the disorder configuration. The localization of such states for some vacancy configurations is reviewed.

Apr/18/2016 (Mon) 10:00-11:30 @ Room431
喜田 和馬
''Quantum transport in GaN/AlGaN two-dimensional electron system under periodic potentials induced by GaN nanocolums''
We are trying to detect the influence of electronic state in GaN nanocolumns array through the measurement of the magneto-resistance in GaN/AlGaN 2D-electron gas underneath the nanocolumns.The magneto-resistance shows characteristics oscillations which are the Shubnikov-de Haas oscillations and the Aharonov-Bohm oscillations.

小川 勝之
''Relation between symmetric property of the gap energy and thermodynamic quantities'' In BCS theory, the expectation value of Cooper pairs is adopted as order parameter. It is assumed in the theory that Cooper pairs are isotropic. In this work, an anisotropic Cooper pair like line node (order parameter is 0 on equator) is considered. Thermodynamic quantities are derived from the gap equation and the Hamiltonian numerically. As a result, the temperature dependence of thermodynamic quantities assuming anisotropic Cooper pairs are different from that assuming isotropic Cooper pairs. For example, specific heat became proportional to the third power of temperature at very low temperatures in the line node case.

Apr/11/2016 (Mon) 10:00-11:30 @ 低温センター3階セミナー室
鎌田 雅博
''Domain wall in adsorbed monoatomic layer''

■ セミナー (2015年度)

Mar/7/2016 (Mon) 10:00-11:30 @ Room431
中村 祥子
''Localization of Quantum Molecules''

Feb/1/2016 (Mon) 10:00-11:30 @ Room431
中山 和貴
''Tomonaga Luttinger Liquid in CNT network''

Dec/7/2015 (Mon) 10:00-11:30 @ Room431
松井 朋裕
''From Graphene to Graphite: Band Structure and Electronic States in Magnetic Fields.''
Graphene is known as a zero-gap semi-conductor with a linear Dirac type energy dispersion. On the other hand, graphite which is a multi-layer graphene coupled with van der Waals interaction shows a semi-metallic behavior. It is interesting to investigate, for both fundamental and industrial reasons, how the electronic structure changes as a function of the number of graphene layers and how many layers are necessary to obtain the graphite behavior. Indeed, it has been studied that the electronic properties of multi-layer graphene are sensitive to the stacking geometry and the next-nearest-layer couplings, etc. In this colloquium, I am planning to overview the band structure and Landau level differences depending on the number of graphene layers and stacking geometry. If possible, I will also discuss the effect of the edges to them.

Nov/16/2015 (Mon) 10:00-11:30 @ Room431
鎌田 雅博
''helium-three adsorbed on heterogeneous surfaces''
Adsorbed helium-three films are ideal systems to study strongly correlated Fermions. Especially, helium-three adsorbed on atomically flat surfaces of the graphite substrate show interesting nuclear magnetism. However, there are some effects from helium-three solid adsorbed on inhomogeneities of the substrate. In this colloquium, previous studies on helium-three solid adsorbed on rough surfaces is reviewed, and then helium-three on inhomogeneities of the graphite substrate is discussed.

Nov/2/2015 (Mon) 10:00-11:30 @ Room431
佐藤 秀樹
''Hydrogen-Plasma etching of graphite surfaces and their STM/S studies''
There are two types of edges for graphene: zigzag edge and armchair edge. It is known that localized states exist at zigzag edge. In addition, it is expected that spin-polarized states exist at zigzag edge and it is attracting attention both from the viewpoint of fundamental physics and electronic device applications. But, it is not easy to obtain zigzag edges controlled in an atomic scale selectively. In this colloquium, I will introduce hydrogen plasma etching as one way of obtaining zigzag edge. In particular, I will introduce the details of the hexagonal pit formation process.

Oct/19/2015 (Mon) 10:00-11:30 @ Room431
Andre Amend
''Introduction of the Hong Kong University of Science and Technology and Prisoner's Dilemma Games with adapting players.''
Introduction of myself and my undergraduate university in Hong Kong. Introduction of the Prisoner's Dilemma game. The evolution of players with certain initial characteristics that repeatedly interact with other players through the Prisoner's Dilemma game while adapting their strategy was studied. While interacting the players can change their strategy according to a partial imitation rule depending on their success, which is measured by the payoff players gain through the Prisoner’s Dilemma game. The tendencies that the players show in their strategy adaption under different conditions was investigated. Some interesting features of the results could be explained heuristically: in the repeated interaction between an adaptive cheater against a patient nice player in a noisy environment, a minimum amount of cooperation of the cheater towards the nice player in the Prisoner's Dilemma was observed at a specific noise level.

Oct/5/2015 (Mon) 10:00-11:30 @ Room431
中村 龍司
''The Ground States of Quantum Spin System''
The quantum spin system is a localized spin system which has strong quantum effects. In low dimensional system, we can observe curious physical phenomena, but it is difficult to solve many-body problem of the spins exactly.
In this colloquium, I will focus on the ground states and the elementary excitations of quantum spin systems in low dimensions (d= 1,2). The various ground states and elementary excitations will teach us important clues to understand frustrated magnetism.

Jul/12/2015 (Mon) 10:00-11:30 @ 低温センター3階セミナー室
中村 祥子
''4He Third Sound Studies using Graphite Substrate''
グラファイト基板を用いたヘリウム４薄膜の第三音波測定に ついて概観し、現在、我々が、ヘリウム４薄膜の超流動測定における 新しいグラファイト基板として着目している熱分解黒鉛シート（PGS）の 評価に第三音波測定が有意義であるかどうかを考察する。

Jul/6/2015 (Mon) 10:00-12:00 @ 低温センター3階セミナー室
戸田 亮
''Operation and Optimization of the Continuous Demagnetization Refrigerator''
10 mK以下の温度を得るための方法として核スピンの断熱消磁冷却(NDR)がある。 断熱消磁サイクルは原理的にシングルショットであるが、複数の独立に制御可能 な断熱消磁ユニットを組み合わせれば、連続的に冷却能力を得ることが可能であ る。これは電子スピンを用いた断熱消磁冷凍機（最低温度50 mK）おいては実現 されている。本コロキウムでは、既存の連続断熱消磁冷凍機の運転方法、最適化 についてレビューし、これを核断熱消磁冷凍機に適用することについて議論する。

Jun/29/2015 (Mon) 10:00-11:30 @ 低温センター3階セミナー室
藤井 武則
''Evidence of Anisotropic Impurity Scattering in Iron Pnictide :Measurement of Anisotropy in the Thermopower of Ba(Fe1-xCox)2As2''
強相関電子系においては、通常の金属や半導体では無視し得る電子間の相互作用が重要となり、複雑な電子相が現れる。 そこでは、スピン、電荷、軌道の自由度が絡み合い、様々な秩序が競合、共存し、その結果、高温超伝導や、 巨大磁気抵抗のような特異な物性が表れると考えられる。 我々は、近年鉄系超伝導体において報告されている、自発的に回転対称性を破るネマティック相という電子状態を 熱輸送特性の測定から観測し、超伝導との競合・共存関係を調べている。 鉄系超伝導体Ba(Fe1-xCox)2As2では、反強磁性転移及び構造転移温度より遥かに高温において、正方晶にもかかわらず 抵抗率に面内異方性が観測されており、ネマティック秩序との関係が議論されている。 抵抗率の異方性の起源としては、①電子状態の異方性（スピンもしくは軌道秩序によって再構築されたフェルミ面の 異方性）、②散乱時間の異方性（Coによる異方的な不純物散乱）、の2つが考えられるが、未だ結論は出ていない。 今回我々はこの異常な異方性の起源を調べるために、熱起電力の面内異方性を測定した。 その結果、反強磁性転移温度以上においては、熱起電力に異方性は見られず、転移温度以下で明瞭に異方性が見られた。 熱起電力は基本的に散乱時間に依存しない物理量であるため、抵抗率に見られる転移温度以上の異方性は、 異方的散乱時間によるものと考えられる。 コロキウムでは、以上の結果を踏まえて、反強磁性転移温度以上の電子・スピン状態と超伝導の関係について議論する。

Jun/22/2015 (Mon) 10:00-11:30 @ Room431
松井 朋裕
''Edge states in condensed matter physics''
The edge states in 2D systems include a challenging problem in solid state physics. For instance, edge states are predicted to give rise to a novel type of magnetic ordering and may lead to the realization of novel spintronic devices. They are also known to play an important role in quantum hall systems and topological insulators. In this colloquium, I will focus on the edge states in condensed matter physics and introduce how such edge states are observed by scanning tunneling microscopy and spectroscopy technique.

Jun/8/2015 (Mon) 10:00-11:30 @ Room431
佐藤 秀樹
''Modulated structure appearing around the edges''
金属中に不純物や欠陥が存在すると、ポテンシャルからの影響を受けた自由電子によって、その周辺には変調構造が生じる。 この変調構造の例として、電子定在波やフリーデル振動などがよく知られている。
グラフェンのエッジやアイランドを中心に、これらの変調構造やエッジに特有な状態などを紹介する。

May/25/2015 (Mon) 10:00-12:00 @ 低温センター3階セミナー室
鎌田 雅博
''Doped Mott Insulator and 2D 3He''
Mott insulators are what should be metal under band theory, but are insulators due to electron-electron interaction. Career doping into Mott insulators show various phenomenon such as high-Tc superconductivity. In this colloquium, basics of doped Mott insulator are reviewed, and liquid-solid phase transition of 2D 3He on grahpite is discussed in view of hole-doped Mott localized state.

May/11/2015 (Mon) 10:00-12:00 @ Room431
中山 和貴
''Basic properties of carbon nanotubes and how to measure them''
Many believes that the discovery of carbon nanotubes(CNT) is attributed to Sumio Iijima in 1991. Nowadays, CNT is common material even among those who are not familiar with science.
In this colloquium, basics of CNT and structures made of CNT are explained. Then, the techniques for measuring properties of CNT are reviewed, mainly focusing on elecrical transport measurements.

Apr/6/2015 (Mon) 10:00-11:30 @ Room431
中村 龍司
''Exact two-magnon states in a multiple-spin exchange model on a zigzag ladder''
Solid helium-3 is known as a quantum solid, which exhibits exotic behavior at ultra-low temperatures. The magnetic properties of solid helium-3 are well described by a multiple-spin exchange model, which includes both Heisenberg and ring-exchange interactions among nuclear spins. As a simple model of helium-3 absorbed on graphite, we consider a quantum spin-1/2 model on a zigzag ladder with competing Heisenberg (J) and 4-spin cyclic exchange (K) interactions. We show that one-magnon states are strictly localized when K=-J/2. Then we find in this particular case that the exact two-magnon dispersion can be obtained analytically. We also present numerical results for the ground state in a magnetic field.

■ セミナー (2014年度)

Feb/23/2015 (Mon) 10:00-11:30 @ Room431
松井 朋裕
''Graphite and Graphene Surfaces reacted with Hydrogen''
The step edges of graphite / graphene terminated by unknown materials, are expected to be refreshed and re-terminated by hydrogen by exposing graphite / graphene surfaces to atomic hydrogen at elevated temperatures. However, such edge termination cannot be an only reaction. Etched pits and step edges are found to be created additionally by atomic hydrogen. Moreover, it is reported that the sp2 bonding of graphite can be broken to form tetrahedral amorphous structures at the edges. In this colloquium, such reactions of graphite / graphene surfaces against hydrogen and resultant electronic properties will be discussed.

Feb/19/2015 (Thu) 10:00-11:30 @ 低温センター3階セミナー室
鎌田 雅博
''Heat Capacity of 3He of Bilayer HD Preplated Graphite''
グラファイト基板上の吸着第2層目の2次元3He整合固体相（C2相）をはじめとし た2次元三角格子をもつ量子スピン液体候補物質が盛んに研究されており、これ らの物質の多くが低温で温度に 比例する比熱を示している。
しかし、最近我々が行った2層HDをプレ コートしたグラファイト上^3 Heの比熱 測定では従来の報告とは異なり温度の2/3乗に比例する比熱が観測された。この 特異な温度依存性 はスピノン或いはマヨラナフェルミオンといった素励起の理 論で理解できる可能性があり、先行研究の帯磁率測定の結果と合わせて議論する。

Feb/10/2015 (Tue) 13:30-15:00 @ 低温センター3階セミナー室
戸田 亮
『超小型サブミリケルビン連続冷凍システムの開発』
既存の一般的な希釈冷凍機に搭載して、1 mK以下までの 温度を連続的に発生させられる小型の核断熱消磁冷凍システムの 開発を進めている。まず、一般に使われている核断熱消磁冷凍に ついて紹介した後、本システムの特徴である小型化、 連続冷凍の方法およびその設計について議論したい。

Dec/22/2014 (Mon) 10:00-11:30 @ Room431
中村 祥子
''XRD Studies of Graphite Sheets''
熱分解黒鉛シートの構造を、X線回折の手法で調べたいと考えている。
試料形状が薄板状なので、薄膜試料に特化した装置や測定手法を用いることが好ましい。
本コロキウムでは、まず粉末X線回折の復習を行い、次に、現在計画している、薄膜試料に対するX線測定の手法を紹介する。
最後に、グラファイトのX線回折の先行研究を紹介したい。

Dec/8/2014 (Mon) 10:00-11:30 @ Room431
中山 和貴
''Properties and utilization of suspended graphene''
Suspended graphene is commonly used technique to look into the intrinsic properties of graphene. Not merely to show high quality transport properties, spatially-isolated graphene enables variety of experiments.
In this colloquium, previous experiments utilizing suspended graphene are reviewed, focusing mainly on its unique features and differences with non-suspended graphene.

Oct/27/2014 (Mon) 10:00-11:30 @ Room431
佐藤 秀樹
''An anisotropic etching of graphene/graphite by hydrogen-plasma''
水素プラズマによって、グラフェン・グラファイトに異方性エッチングを施した 研究を紹介する。 このようにエッチングされたエッジは、主にジグザグエッジになると報告されて いる。 また、この方法によって得られたエッジやグラフェンナノリボンについて、ラマ ン分光法による測定や電気的特性を測定した研究も紹介する。

Jul/14/2014 (Mon) 10:00-11:30 @ Room431
鎌田 雅博
''Isotope Effect on 2D Hydrogen on Graphite''

Jul/7/2014 (Mon) 10:00-11:30 @ Room431
中村 祥子
''Preferential Adsorption of Helium Isotopes''
Because of its smaller zero-point energy, it is to be expected that helium-4 (4He) will be preferentially adsorbed over helium-3 (3He) next to the substrate. Previous measurements directly measured isotope ratio in films adsorbed in equilibrium with a bulk liquid mixture and with a vapor. In film-liquid equilibrium such adsorption is successfully confirmed. However, the isotope ratio of whole films is independent of the film thickness in film-vapor equilibrium, possibly because of too thick films or too high temperatures to obtain high enough vapor pressures. Direct measurements might be too difficult for film-vapor equilibrium. Our recent heat-capacity measurements on bilayer mixture films adsorbed on ZYX graphite show an extra heat capacity which is not seen in isotopically pure bilayers. Energetically the films should comprise a 3He layer (2nd layer) on a layer of 4He (1st layer). I suggest a simple partition function of inter-layer swap of helium isotopes and calculated the heat capacity due to the swap between the 1st layer 3He and 2nd layer 4He. The model includes only one fitting parameter, swap energy, but it describes the extra heat capacities quite well.

Jun/2/2014 (Mon) 10:00-11:30 @ Room431
松井 朋裕
''Experimental Studies of Graphene Edge State''
Peculiar edge states on Graphene and the magnetism expected for the zigzag structure were discussed in the last colloquium of mine. However, in these discussions, the edges show an "ideal" structure keeping honeycomb symmetry of Graphene. In reality, variety of elements, such as N, O, H, Fe and Si, are know to attach to the edge, and/or the edge may also be reconstructed. In this colloquium, I want to clarify the edge structure and introduce some studies under, more ore less, controlled conditions.

Apr/28/2014 (Mon) 10:00-11:30 @ Room431
中山 和貴
''Characterizing graphene with Raman spectroscopy''
Raman spectroscopy is a tool that enables understanding the electronic and vibrational properties of a system nondestructively by means of the inelastic scattering of light. It is widely used to characterize or evaluate nanocarbons including graphene. In this colloquium, scattering mechanisms responsible for the characteristic Raman peaks seen on graphene are reviewed. Then, Raman spectrum of the damaged graphene samples we obtained (accidentally) are considered.

Apr/14/2014 (Mon) 10:00-11:00 @ Room431
佐藤 秀樹
『自己紹介と特別実験の報告』

■ セミナー (2013年度)

Mar/19/2014 (Wed) 14:00-15:00 @ Room431
松本 洋介 氏
東京大学 物性研究所
''Novel quantum criticality in the valence fluctuating heavy-fermions''
   “Novel quantum phases formed in the vicinity of a quantum critical point (QCP)” is one of the most important topics in condensed matter physics on strongly correlated electrons. Archetypical examples are found in the heavy-fermion (HF) intermetallics, for which, owing to its relatively small energy scale, we can fine tune the ground state of the systems by controlling magnetic field, pressure and chemical doping etc. Indeed, unconventional superconductivity and non-Fermi liquid behavior have been found in the vicinity of the magnetic QCP of the several compounds such as CeIn₃, CeRhIn₅, CeCu₂Si₂ and have been studied extensively over the past few decades [1].
   So far, all the quantum critical (QC) materials in the HF intermetallics are known to have an almost integral valence which stabilizes the local moments considered essential for the criticality. In contrast, departures from integral valence associated with valence fluctuations are thought to promote screening of local moments, suppressing critical phenomena. On the other hand, the first Yb-based HF superconductor β-YbAlB₄ provides a unique example of a QC in the mixed valent compounds [2-4]. In addition, the QC cannot be explained by spin fluctuation and emerges without tuning any control parameter, suggesting realization of a novel metallic phase [4].
   Here I review the novel quantum criticality in β-YbAlB₄. I will further discuss the future scope of the studies on QC materials in HF systems introducing a few more examples. Possible extensions of these studies into the ultra-low temperatures will be also discussed.
[1] H. Löhneysen, A. Rosch, M. Vojta, P. Wölfle, Rev. Mod. Phys. 79, 1015 (2007), and references therein.
[2] S. Nakatsuji, K.Kuga, Y. Machida, T. Tayama, T. Sakakibara , Y. Karaki, H. Ishimoto, S. Yonezawa, Y. Maeno, E. Pearson, L.Balicas, H. Lee, Z. Fisk, Nature Phys. 4, 603-607 (2008).
[3] K. Kuga, Y. Karaki, Y. Matsumoto, Y. Machida, and S. Nakatsuji, Phys. Rev. Lett. 101, 137004 (2008).

Mar/10/2014 (Mon) 13:30-15:30 @ Room431
仲嶋 孝友起
『三次元混合系における理論/実験および二次元混合系における実験の紹介』

低温センター談話会
Mar/6/2014 (Thu) 14:30-15:30 @ Room445
村川　智 氏
慶応義塾大学 理工学部
『トポロジカル超流動の表面状態　-超流動³He-B相の表面アンドレーエフ束縛状態とマヨラナ状態-』
　近年、トポロジカル絶縁体から着想を得たトポロジカル超伝導・超流動が大きな注目を集めている。このトポロジカル超伝導・超流動はバルク‐エッジ対応により、表面にギャップレスな状態が現れることが知られているため、表面状態の研究が活発に行われている。
　多くの超流動体・超伝導体の中から我々は超流動ヘリウム3(³He)に着目している。超流動³Heはp波スピン3重項超流動体であるため、複数の相が存在することが知られ、そのバルクの性質は詳細に研究されてきた。その中でもバルクでは等方的なギャップ2Δを持つB相と呼ばれる相は、トポロジカル超流動体であることが理論的に示されている[1]。また超流動³Heは不純物が極端に少ないため、表面状態の研究を行う試料として理想的である。バルク‐エッジ対応から予想される表面状態は、超流動³He-B相では表面アンドレーエフ束縛状態(Surface Andreev bound states; SABS )として現れるが、それがマヨラナフェルミ粒子であるとの指摘がされた。SABSは壁での準粒子散乱の境界条件である鏡面度Sに大きく依存し、特に鏡面散乱極限(S = 1)では、SABSバンドの幅Δ^*が広がりギャップレスになり、エネルギーに比例する表面状態密度が現れ、表面にマヨラナコーンが存在することが示されている[2]。
　我々はその超流動³He-B相のSABSを、液体³Heに浸したずれ振動するAC-cut水晶振動子の複素音響インピーダンス(Z)測定により調べてきた。Sは壁を⁴He薄膜でコートすることで制御することができる。
　この測定から、ギャップ内に構造を持つ低エネルギー励起状態のSABSがあること[3]、およびSABSのバンド幅Δ^*がSの増大ともに大きくなることが示され、それは理論計算と定性的に一致した[4]。
　また、鏡面度が拡散的散乱条件ではない(S > 0)とき、新たなピークがZの温度依存性に現れ、そのピークはSの増加にともない成長した。この低温ピークはSABSのゼロエネルギー状態が減少したことから生じると理論計算から示され、その成長はマヨラナコーンへと漸近する振る舞いと見なせる。これは鏡面散乱極限(S = 1)の極限で現れると予測されているマヨラナコーンの存在を強く支持する実験結果である[5,6]。
[1] A. P. Schnyder et al., Phys. Rev. B 78, 195125 (2008).
[2] Y. Nagato et al., J. Low Temp. Phys. 149, 294 (2007).
[3] Y. Aoki et al., Phys. Rev. Lett. 95, 075301 (2005); M. Saitoh et al., Phys. Rev. B 74, 220505(R) (2006).
[4] Y. Wada et al., Phys. Rev. B 78, 214516 (2008).
[5] S. Murakawa et al., Phys. Rev. Lett. 103, 155301 (2009)
[6] S. Murakawa et al., J. Phys. Soc. Jpn. 80, 013602 (2011).

Jan/27/2014 (Mon) 13:30-14:30 @ Room445
Prof. Yoonseok Lee
Dept. of Physics, University of Florida, USA
''MEMS-based Probe for the Study of Quantum Fluids''
   One of the most distinct properties of unconventional superfluids/superconductors is the extreme fragility of Cooper pairs against any types of impurity or disorder. The surface scattering in these systems readily breaks Cooper pairs and consequently induces quasi- particle bound states near the surface within the coherence length, often called Andreev surface bound states.
   This generic nature of the unconventional order parameter near the boundaries becomes prominent in a film. This property combined with the exotic symmetries in the superfluid phases of 3He with p-wave spin triplet pairing is at the heart of many fascinating phenomena in confined geometry. However, it has been an experimental challenge to form thin films and to make sensitive measurements on them.
   We have developed a Micro-Electro-Mechanical System (MEMS) based probe for this purpose. Using a commercial micro-machining process, MEMS oscillators were designed, manufactured, and characterized. Each device consists of a pair of parallel plates with a well-defined gap in which a film forms when immersed in liquid. The mobile plate is suspended above the fixed plate (substrate) by four springs. This geometry allows to study the properties of the surrounding liquid through the resonant behavior of the mobile plate. We will discuss the design and the operation of the device, and will present results obtained using these devices in air, liquid 3He [1] and also in liquid 4He [2] in a wide range of temperature down to submillikelvin range. Our work demonstrates great potential of the device in a wide range of experiments in quantum fluids.
[1] M. Gonzalez et al., Rev. Sci. Instr. 84, 025003 (2013).
[2] M. Gonzalez et al., J. Low Temp. Phys. 171, 200 (2013).

Jan/14/2014 (Tue) 11:00-12:00 @ Room431
Prof. Markus Morgenstern
II. Institute of Physics and JARA-FIT, RWTH Aachen, Germany
''Probing Graphene on the Nanoscale by Scanning Tunneling Microcopy''
We use ultra-high-vacuum scanning tunneling microscopy at low temperature (6 K) to probe basic features of graphene relevant to mechanical and quantum mechanical applications.
Firstly, I present wave function mapping in graphene quantum dots deposited on Ir(111) [1]. The quantum dots are confined exclusively by zig-zag edges. However, edge states are absent due to an exchange interaction of the π-bands of graphene with the dz2 surface states of the Ir(111) [2]. The exchange interaction gets continuously smaller away from the edges, which leads to weak confinement of the quantum dot states being decisive for a rather regular wave function shape observed experimentally. The wave functions are additionally influenced by the penetration of an sp-like surface state of Ir(111) into graphene [1]. Concerning the edge state, DFT calculations reveal that H-terminated graphene nanoribbons on Au(111) should exhibit magnetic edge states [3].
Secondly, the mechanical properties of graphene flakes deposited by the scotch tape method are probed by STM. The graphene is partly not conformal to the substrate, but provides areas which are freely suspended [4]. The valleys of this corrugation can be further lifted by the van-der Waals forces and the electrostatic forces of the tip of the STM [5]. During continuous lifting, we often observe a breaking of atomic symmetry, i.e. the atomic structure switches reproducibly between a hexagonal and a triangular appearance.
[1] D. Subramaniam et al., Phys. Rev. Lett. 108 (2012) 046801.
[2] Y. Li et al., Adv. Materials, 25, 1967 (2013)
[3] Y. Li et al., arXiv:1210.2876.
[4] V. Geringer et al., Phys. Rev. Lett. 102 (2009) 076102.
[5] T. Mashoff et al., Nano. Lett. 10 (2010) 461.

Dec/5/2013 (Thu) 11:00-12:00 @ Room445
Prof. Eunseong Kim
KAIST, Korea
''How is the torsional oscillator response connected with shear modulus of solid helium?''
Supersolidity, the appearance of viscousless flow in solids, was first indicated in 4He torsional oscillator (TO) experiments [1]. Despite many efforts to uncover the mechanism of supersolidity since the first observation of non-classical rotational inertia (NCRI), the microscopic origin and physical interpretation of the observed phenomena has been under considerable debate. Recently, the TO response was considerably suppressed when the effect of shear modulus on TO was removed, suggesting alternative non-superfluid explanations responsible for the TO response [2]. On the other hand, quantum statistics dependence of the TO response [3], heat capacity signatures [4], and rotation effects on the TO response [5] are conflicting with non-supersolid explanations. In this talk, I will present simultaneous measurement of the TO response and shear modulus to understand the fundamental connection between two phenomena.
[1] E. Kim and M. H. W. Chan, Nature 427, 225 (2004).
[2] D. Y. Kim and M. H. W. Chan, Phys. Rev. Lett. 109, 155301 (2012).
[3] J. T. West et. al, Nat. Phys. 5, 598 (2009).
[4] X. Lin, A. C. Clark, and M. H. W. Chan, Nature, 449, 1025 (2007).
[5] H. Choi et. al, Science 330, 1512 (2010).

Dec/2/2013 (Mon) 10:00-12:00 @ Room431
中山 和貴
''Understanding the oxidative etching in graphite/graphene''
Pit formation by oxidative etching in graphite has long been studied since 1960s. Nowadays, the etching technique is utilized as a tool for the fabrication of graphite/graphene nanostructures.
In this colloquium, the experiments concerning environmental dependence of oxidative etching and their mechanism are reviewed. Based on that, the way to optimize the etching parameters to obtain desired etch pits is discussed.

Nov/18/2013 (Mon) 10:00-12:00 @ Room431
久保田 雄也
''Development of a Torsional Oscillator for Studies of Superfluid Responses in Monolayers of Helium Four''
Recent heat capacity measurements for the second layer of helium four adsorbed on ZYX graphite clearly show the existence of a commensurate phase (C2) at a density C2 = 19.7 nm^－2 in between a low density liquid phase and a high density incommensurate phase. In addition, previous torsional oscillator measurements of the second layer of helium four by three different groups using Grafoil show frequency shifts below 300 mK at densities near C2 suggesting unusual superfluidity. However, the observed reentrant density variations of superfluid response are rather different each other, and the shifts are too small to convince the supersolidity.
In this colloquium, we report details of experimental setup and preliminary results of our new torsional oscillator measurement for studies of superfluid responses in monolayers of helium four.

Nov/11/2013 (Mon) 10:00-12:00 @ Room431
鎌田 雅博
『2次元³He整合固体相の磁気基底状態』

Oct/28/2013 (Mon) 10:00-12:00 @ Room431
松井 朋裕
''Magnetic Properties of Zigzag edge in Graphene''
The understanding of the edge state is one of the central issues in graphene research, since the edge contribution becomes larger in nano-devices as nano-ribbons and nano-islands. It is especially the case when the edge is constructed in zigzag structure because spin polarized localized state is expected around Fermi energy in zigzag edges as had been discussed in last colloquium of mine. In this colloquium, I want to focus on some structures with zigzag edges such as zigzag nano-ribbons, zigzag islands and zigzag antidots, and review their electronic properties obtained in transport measurements and Raman spectroscopies.

Sep/9/2013 (Mon) 10:00-12:00 @ Room431
仲嶋 孝友起
『比熱測定法と熱スイッチ』
系の微視的状態を知るために比熱は極めて重要な物理量である。
そのような比熱を測定するには主に３つの方法がある（断熱ヒートパルス法、緩和法、交流法）
測定原理、特徴とともに実際どのように使われているのか、実験データとともに紹介する 予定である。熱スイッチも主に３つあり（超伝導熱スイッチ、ガスーギャップ式、機械的熱スイッチ） 特徴、実験論文も紹介し、今回自身の研究で用いるインジウムの超伝導熱スイッチについても考察する予定である。

Jul/30/2013 (Tue) 14:00-15:00 @ Room431
Prof. Dr. Christian Enss
Universitat Heidelberg
Kirchhoff-Institut fur Physik
Germany
''New Frontiers at Ultralow Temperatures: Micro Calorimetry and Noise Thermometry''
In recent years important advances have been made in astro/particle physics that have deepened our understanding of the nature of the universe and the fundamental properties of matter. Much of this experimental progress has been made possible by technological developments in others fields like optics, computers, semiconductors, etc. In this vein, the emerging technology of cryogenic particle detection is now poised to make major contributions to a variety of experiments in astrophysics, atomic physics and material science. We will discuss the status of development of metallic magnetic calorimeters and their applications. In addition, we present a new concept for precision thermometry at ultralow temperatures based on magnetic flux noise.

Jul/22/2013 (Mon) 10:00-12:00 @ Room431
中山 和貴
『グラフェンの伝導特性の物理』

Jul/8/2013 (Mon) 10:00-12:00 @ Room431
松井 朋裕
''Peculiar Electronic States at Graphene Edges''
It is a central question how graphene behaves when it is patterned at nanometer scale with different edge geometries. The graphene nanoribbon (GNR), a narrow strip of graphene, is a good playground to study the electronic states at graphene edges and their effect to the ribbon. And there are many predictions for GNRs to exhibit a wide range of behavior, including tunable energy gaps and the presence of one-dimensional (1D) edge states with unusual magnetic structure. In this colloquium, I want to overview the peculiar electronic states of graphene edges, based on the STM/S point of view.

Jun/10/2013 (Mon) 12:30-14:30 @ Room431
中村 祥子
''Phase diagram of the second layer of 4He on graphite''
I will show the coverage-temperature phase diagram of the second helium-4 layer adsorbed on ZYX graphite, based on our thermodynamical studies.
Our coverage scale might include absolute errors, as our experiment is not a diffraction study, however, we obtained the coverage scale with a small relative error using the 1/3 commensurate solid phase of the first layer of helium or nitrogen, and we believe, the same batch of ZYX graphite behaves similarly in determination of the coverage scale.
Therefore, for our on-going measurement of the superfluid responce using ZYX graphite, to be discussed in the next talk, the phase diagram of 4He on the same substrate is very important.
I will also review the heat capacity and the vapor pressure of the second layer of 4He adsorbed on ZYX graphite and related isotope systems, namely, bilayer 3He, and 3He on a layer of 4He. Substitutions of a few isotope particles by other groups will be dealt with briefly.

久保田 雄也
''Search for Possible Supersolidity in 2D Solid ⁴He by Torsional Oscillator Measurement''
Recent heat capacity measurements for the second layer of 4He adsorbed on graphite clearly show the existence of a commensurate phase　(C2) at a density C2 = 19.7 nm−2. The C2 phase is the lowest density quantum solid ever found with substantially fast　exchanges of atoms and vacancies. Therefore, it is a hopeful candidate for the novel supersolid phase. Previous torsional oscillator measurements of the second layer of 4He by three different groups using Grafoil show frequency shifts below 300 mK at densities near C2 suggesting unusual superfluidity. However, the observed reentrant density variations of superfluid response are rather different each other, and the shifts are too small to convince the supersolidity. This is presumably because of large uncertainties in their density scales and poor connectivity of platelet boundaries in Grafoil. Here, we report details of experimental setup and preliminary results of our new torsional oscillator measurement down to 10 mK. The oscillator is made from coin silver containing ZYX substrate which is an exfoliated graphite with much larger platelet size than Grafoil.

May/27/2013 (Mon) 10:00-12:00 @ Room431
鎌田 雅博
『フラストレートしたスピン系の磁性』

Apr/15/2013 (Mon) 10:00-12:00 @ Room431
中村 祥子
''Possible Substrates for our Future Adsorption Study''
グラファイト類似物質である六方晶窒化ホウ素を中心に、壁かい方法の異なる膨張黒鉛や、近年見出されたグラフェン、グラファン、フルオログラフェンといった新しい炭素系物質について、特徴や先行研究、合成方法や理論計算を紹介し、今後、それらを吸着基板として使っていく可能性について考察する。

■ セミナー (2012年度)

Feb/28/2013 (Thu) 10:00-12:00 @ Room445
松井 朋裕
『STM/S Studies of Rare Gas Atoms on Graphite』
Rare gas atoms on various surfaces are regarded as two-dimensional (2D) model systems in surface science due to their closed-shell electronic structure and weak interaction with substrates and among themselves. These systems provided ideal testing grounds for 2D adsorbate phases and phase transitions as well as for growth mechanism of physisorbed films. Among several techniques, STM/S is a powerful tool to study such systems since it is possible to obtain real-space images of the rare gas adlayer structure from submonolayer to monolayer coverage. However, it had not been well cleared yet why rare-gas atoms are visible in the STM. In principle, an adsorbate is visible when it modifies the LDOS near /E/_F, but it is unlikely the case for the rare gas atoms. In this colloquium, I want to review the experimental examples of the rare gas observations and discuss about their mechanisms.

Feb/21/2013 (Thu) 10:00-12:00 @ Room445
松井 幸太
『低密度2次元He系の気液相転移』

Feb/14/2013 (Thu) 10:00-12:00 @ Room445
佐藤 大輔
『グラファイト上単原子層³Heのこれまでの研究』

Dec/20/2012 (Thu) 10:00-12:00 @ Room445
鎌田 雅博
『グラファイト吸着基盤のエッジ状態と磁気比熱』
グラファイトはバルクでは反磁性と示すが、エッジ状態では強磁性になる可能性が指摘されている。
今回のコロキウムでは我々が2次元3He整合相の吸着基盤として用いているグラファイト基盤に対して、エッジ状態の強磁性を仮定して求めた磁気比熱と実際に測定されたアデンダ比熱から基盤の内部磁場 を見積もる。

Dec/10/2012 (Mon) 13:30-15:30 @ Room431
久保田 雄也
『zero-point vacancyとその超固体への寄与』
AndreevとLifshitzによってzero-point vacancy（ZPV)の存在およびそれがBECを起こす可能性があると提唱されて以来、超固体の研究においてZPVが注目されてきた。我々が測定する２次元ヘリウムにおいてもその寄与が期待される。
今回はそのZPVの概観とグラファイト上吸着ヘリウム4/7整合相近傍での寄与について見ていく。

Nov/15/2012 (Thu) 10:00-12:00 @ Room445
中山 和貴
『グラフェン上の酸素吸着』
グラフェンはガスセンサーとしての応用が目されていることもあり、種々の気体中に暴露した際の反応について研究が行われてきている。
本コロキウムでは、グラフェン上の酸素吸着について概観し、電気伝導特性の変化について先行研究とこれまで取得したデータとの比較を試みる。

Nov/8/2012 (Thu) 10:00-12:00 @ Room445
中村 祥子
『ZYX上に吸着した第１層目ヘリウム４と第２層目ヘリウム３』
これまで、第２層目のヘリウム４の実験結果のみ紹介してきたので、本コロキ ウムでは、最近取得した第１層目ヘリウム４の熱容量データと、第１層目が ヘリウム４で第２層目がヘリウム３のときのデータについて紹介したい。
第１層目のヘリウム４は、全てやり尽されているというのが通説であったが、 整合固体相の融解比熱の密度依存性について、顕著な非対称性が今回新たに 観測された。
第２層目のヘリウム３においては、第１層目がヘリウム４であることによる 効果だけでなく、おそらく基板の質に依存したと期待される変化が観測されて いる。
これらを中心に、その他のデータも交えつつ概観したい。

Sep/5/2012 (Wed) 10:00-11:30 @ Room445
鎌田 雅博
''Studies on Quantum Spin Liquid''
My research theme is studying the ground state of 2 dimensional 3He in the 4/7 phase, which is expected to be the gapless spin liquid state.
In this colloquium, I will talk about the idea of Resonating Valence Bond (RVB), some systems show the possibility to be spin liquid state, and our study on 4/7 phase of 3He.

Jul/5/2012 (Thu) 16:30-18:00 @ Room431
Kouta Matsui
''LEED Calculations for Complex Structures''
LEEDの動力学計算は計算量が単位格子に含まれる散乱体の数の2-3乗で増えてしまうため、構造が複雑になると計算量が現実的でなくなってしまうという問題があった。
グラファイト吸着2次元ヘリウム系は密度に応じて複雑な構造をとるため、単純な格子モデルの範疇で計算できるのはせいぜい1層目整合層のみである。
そこで今コロキウムではこのような複雑な系を計算するために必要な近似手法をいくつか紹介する。

Jun/28/2012 (Thu) 16:30-18:00 @ Room431
松井 朋裕
''SiC graphene -STM/S studies ant its modification by adsorbates-''
Various strategies exist for preparing graphene samples, for example popular micromechanical exfoliation of graphite, high temperature decomposition of polar surface of SiC crystal, and chemical vapor deposition on metallic substrate. In each case, however, there are unavoidable influences from the substrate to graphene, which modifies the genuine electronic properties of graphene. Here I want to focus on the electronic properties of the graphene grown on SiC(0001) and its appearance in STM/S measurement. In addition, I want to show some examples of the effect of adsorbates on SiC-graphene and our experimental results of Kr deposited on SiC-graphene.

Jun/21/2012 (Thu) 16:30-18:00 @ Room431
中村 祥子
『ヘリウム４吸着第二層目4/7相について』
ヘリウム４吸着第二層目4/7相について、最近得られた熱容量と蒸気圧のデータをどう解釈するか、他の系を参照しつつ考える。
熱容量の特異性付近に対して、ポッツモデルを実現している吸着系と同様の解析を試みたが、うまくいかなかった。回転自由度を持つ系とも振る舞いは異なった。そのため、過去の吸着系の実験から少し離れて、他の可能性を考えてみる。

Jun/7/2012 (Thu) 14:30-15:30 @ Room414
池上　弘樹　博士　（理研）
''Probing chirality of superfluid 3He-A''
In this talk, we report the first detection of chirality of superfluid 3He-A using a novel effect, intrinsic Magnus effect, experienced by a traveling impurity [1,2]. When an impurity is traveling in a plane perpendicular to the l vector, the impurity experiences the intrinsic Magnus force in the direction perpendicular to both its velocity and the l vector, as a result of the skew scattering of quasiparticles by the impurity. We investigated the intrinsic Magnus effect for charged impurities, electron bubbles, trapped below the free surface of superfluid 3He at a depth ~30 nm by transport of electron bubbles along the surface. The free surface aligns the l vector uniformly normal to the surface. We observed the current associated with the intrinsic Magnus effect in the direction transverse to the electric field. Two behaviors in the transverse current are observed, which are equal in magnitude but opposite in sign, and these two correspond to the l vector pointing upward (+l) and downward (-l). After repeating many cooling runs from normal statewe find that the ratio between +l and -l emerging just below Tc is asymmetric about the direction of magnetic field.
[1] R. H. Salmelin et al., Phys. Rev. Lett. 63, 868 (1989).
[2] R. H. Salmelin and M. M. Salomaa, Phys. Rev. B 41, 4142 (1990).

Jun/4/2012 (Mon) 10:00-10:45 @ Room445
石川　修六　教授　（大阪市立大学　大学院理学研究科　数物専攻）
''Experimental Study on Intrinsic Angular Momentum in Superfluid 3He-A Phase''
NMR study has been done to investigate intrinsic angular momentum of Cooper pairs as a macroscopic quantity. Recent experiments using a rotating nuclear demagnetization cryostat at ISSP shows a novel result related to this problem. A sample cell is composed of one long cylinder with 0.1mm in diameter, in which there exists a special texture, so-called Mermin-Ho texture. Texture is a spacial pattern of order parameter describing superfluid 3He. A small change of this texture by rotation is detectable by NMR and gives us an information about macroscopic quantity of intrinsic angular momentum.

May/31/2012 (Thu) 17:00-18:00 @ Room431
久保田 雄也
『ヘリウム超固体観測に向けてのねじれ振り子研究について』
現在ねじれ振り子を用いた実験にむけて準備中である。そこで今回は、まずこれまで行われてきた超固体に関する研究について簡単に紹介し、そのひとつであるねじれ振り子の原理について述べる。さらに、ねじれ振り子を用いた先行研究を発表し、最後に今回設計した装置について報告する。

Apr/26/2012 (Thu) 16:30-18:00 @ Room431
中山 和貴
『希ガス／グラフェン系のギャップ生成検証実験の現状での結論』
まず、現在行っている、電気伝導測定用グラフェンサンプルの作成方法について説明する。特にインジウムを用いた端子付けの手法について詳説する。次に、今まで行ってきた、グラフェン上に希ガスを物理吸着させギャップが生成するか確認する検証実験について、現在の実験セットアップにおける一応の結論を得たため、これを報告する。

■ セミナー (2011年度)

Feb/21/2012 (Tue) 16:30-18:00 @ Room431
Tomohiro MATSUI
''STM/S studies of Indium nanoparticles on graphite and SiC-graphene exposed to Kr atoms''
I will overview our ULT-STM activities in 2011 in this colloquium. As a first part, I will talk about Indium nanoparticles on a surface of graphite. In this measurement, unexpectedly large and deep superconducting gap with small coherence peaks were observed suggesting the localization and/or confinement of Cooper pairs in superconducting particles. STM/S studies of epitaxial graphene on SiC substrate exposed to Kr atoms would be presented as a second part. The analysis is still on-going but I hope I can show some hints for future experiments.

Dec/13/2011 (Tue) 15:00-16:15 @ Room431
Yu. M. Bunkov (Institut Neel, Grenoble, France)
''Magnon Bose-Einstein condensation''
Superfluid ³He can be considered as a quantum vacuum carrying various types of quasiparticles and topological defects. The structure of this system shows many similarities to that of our Universe. It can act as a model system for the study of many types of general physics experiments, which are difficult or even impossible in Cosmology, Atomic or Nuclear physics. There is a complete analogy between the Bose-Einstein condensation of atomic gases and the Bose-Einstein condensation of magnons in superfluid ³He. Five different states of magnon condensation have been found; the homogeneously precessing domain (HPD) in ³He-B; the persistent signal, which is formed by a Q-ball in ³He-B at very low temperatures; coherent precession with fractional magnetization in ³He-B and coherent precession of magnetization in ³He-A and ³He-B in a squeezed aerogel [1]. All these cases are examples of the Bose-Einstein condensation of magnons with the interaction potential provided by specific spin-orbit coupling. The BEC phenomenon in the gas of magnons is readily accessible owing to the possibility of modifying the spin-orbit coupling. In some cases the BEC of magnons corresponds to almost 100% condensation.
In Fig.1 the frequency and phase distribution of BEC radiation is demonstrated. The accuracy of phase and frequency measurements is mixed due to the analogy with the uncertainty. Fig 2 shows the BEC radiation broadening, which is about 0.2 Hz, the 4000 time smaller then it should be due to the inhomogeneity of magnetic field. The broadening of magnetic field on the sample corresponds to an 800 Hz!
Finally the magnon BEC state was found on a 55Mn nuclear in antiferromagnetic CsMnF₃ [3].
[1] Yuriy M. Bunkov and Grigoriy Volovik, J. Phys.: Condens. Matter 22 164210 (2010)
[2] Ю. М. Буньков, УФН, 180, 884; Physics–Uspekhi, 53:8, 848–853(2010)
[3] Yu.M. Bunkov, E.M. Alakshin, R.R. Gazizulin, A.V. Klochkov, V.V. Kuzmin, T.R. Safin, M.S. Tagirov. « Discovery of the Classical Bose–Einstein Condensation of Magnons in Solid Antiferromagnets » JETP Letters, 94, 68–72 (2011)
[*] 図はこちらのPDFファイルを参照

Dec/8/2011 (Thu) 17:00-19:00 @ Room431
中山 和貴
『原子吸着によるグラフェンのギャップ生成メカニズム』
　グラフェンはその特異な性質から研究が盛んに行われているが、デバイスへの応用などの観点から重要となるのがグラフェンのギャップ生成である。本コロキウムでは、グラフェン上に原子を物理吸着させることによりグラフェンの対称性が破れギャップが開くという理論予測について、そのメカニズムを解説する。また、現在我々が行っている検証実験の現状についても紹介する予定である。

Nov/1/2011 (Tue) 16:30-18:30 @ Room431
成瀬 公暁
『ヘリウム分子、ヘリウム４フィルム上のヘリウム３』
　２次元ヘリウム３が気液相転移するかどうかは興味深い問題である。多くの理論研究では、無限個のヘリウムに対して、２次元の気液相転移に対し、否定的な見方をとっている。一方、少数の原子から構成された系においては、例えばヘリウム３のダイマーは２次元においてのみ束縛エネルギーを持つことが知られている。また、ヘリウム４に関しては、２次元のダイマーは３次元よりも２０倍強く束縛する。少数系から多体系にいくに従って、ヘリウムがどのような振る舞いを見せるのか。これについて考えるために、今回は、ヘリウム分子に関する一連の理論研究を紹介し、その後、超流動状態のヘリウム４に浮いたヘリウム３の二次元系における比熱測定の実験を紹介する。

Oct/18/2011 (Tue) 16:30-18:30 @ Room431
Jan Raphael Bindel
『UHV-AFM/STM at 300mK and 14T』
Investigations of partial electrically conducting samples on an isolating substrate, like exfoliated graphene on SiO₂ or micro-structured graphene, with scanning tunneling microscopy (STM) lead to several challenges due to the limitation of STM to conducting surfaces. It gets even more challenging if the field of interest leads to ULT-STM, e.g. Superconductivity, Kondo-physics, Wigner-crystallization. I will present the development of an UHV-System for combined atomic force microscopy (AFM) and STM using a tuning fork. It is designed for temperatures down to 300mK and magnetic fields up to 14T. Additionally, it permits direct optical access to the microscope inside the cryostat. I will concentrate on the design of the AFM/STM and the Cryostat, which enables measurement periods of 100h at 300mK.

Oct/4/2011 (Tue) 16:30-18:30 @ Room431
中村 祥子
『Heats of Adsorption』
In order to know the heat capacity of the 2nd-layer Helium above 2 K, some corrections for desorption are needed. I will introduce how to derive the heat capacity of adsorbed films from the measured heat capacity with adsorption isotherms.

Jul/27/2011 (Mon) 13:00-15:00 @ Room431
成瀬 公暁
『New phase formation on solid surfaces』
　量子性の強い³Heや⁴Heが２次元平面上で自己凝縮するか否かは数多くの議論がなされてきた。我々の実験結果ではpuddle相の存在を示すγ値のキンクが見られている。しかし、 ³Heの純粋な二次元系については、自己凝縮しないことを示す理論計算や実験結果が大勢を占めているのが現状である。
　今回のコロキウムでは、まず表面における自己凝縮の基礎的なモデルを紹介したうえで、この問題に関連した一連の理論計算や実験結果の紹介を行いたい。

Jul/25/2011 (Mon) 16:30-18:30 @ Room431
武井 英人
『ナノスケールの超伝導におけるSTM観測』
　基板上に作成した擬2次元、あるいはナノスケールのアイランド状の超伝導については、超伝導の次元性やサイズ効果、または基板への 超伝導の染み出しなど興味深い物理がある。
　本コロキウムではそれら擬2次元、あるいはナノスケールの超伝導の先行研究を紹介し、最後に我々の研究しているグラファイト上のインジウムのナノスケールアイランドのSTMの結果を紹介したい。

Jul/08/2011 (Fri) 10:00-12:00 @ Room431
松井 幸太
『LEEDの動力学的取り扱い』
　LEEDの動力学解析では、運動学モデルでは解析できないような複雑な表面の構造を解析できる一方、 X線などと異なり散乱電子同士の干渉が非常に大きくその取り扱いは複雑である。
　本コロキウムではLEEDの動力学計算を紹介し、実際の実験でどのように利用されているか紹介する。

Jul/01/2011 (Fri) 10:00-12:00 @ Room431
Tomohiro MATSUI
『Band Gap Opening in Graphene』
Graphene exhibits a number of exotic electronic properties, such as unconventional integer quantum Hall effect, ultrahigh electron mobility, electron-hole symmetry, and ballistic transport even at room temperature. While many other properties of graphene are very promising for nanoelectronics, its zero-gap semiconductor nature is detrimental, since it prevents the pinch off of charge current as requested in conventional electronic devices. Thus, the opening and tuning band-gap of graphene are especially crucial to the practical application of graphene to electronic devices. Roughly speaking, there are two ways to lift the degeneracy of the two bands crossing at the Dirac points. One is to hybridize the electronic states at K and K', which requires breaking of the translational symmetry. The other is to break the equivalence between the A and B sublattice, which does not require any translation symmetry breaking. In this colloquium, I want to introduce some techniques to open and tune the band-gap of graphene, especially the way to functionalize the graphene by adsorbing atoms.

Jun/17/2011 (Fri) 10:00-12:00 @ Room431
佐藤 大輔
『希薄³He系におけるスピン緩和』
　本コロキウムでは、希薄な³He気体・液体における拡散、スピン緩和時間を題材とする。
　具体的には、3次元固体⁴He中にて準粒子として存在する希薄な³Heについての研究、3次元における 希薄な気体³Heについての研究結果を紹介し、最後に我々の研究しているグラファイト上2次元³Heの 低密度領域におけるNMRの結果を議論する予定である。

May/27/2011 (Fri) 10:15-12:00 @ Room431
中村 祥子
『ヘリウム４超流動転移における有限サイズスケーリング』
　有限サイズスケーリングは、臨界現象を調べる際に、有限系での計算結果を無限系に外挿する手段として一般的な方法である。 ヘリウム４超流動転移は、無限系とみなせる系における臨界現象の性質がよくわかっている上に、 マクロにクリーンな系が実現できるため、有限サイズスケーリングを実験的に検証できる希有な系である。 本コロキウムでは、スケーリングの概念と、ヘリウム４超流動転移における有限サイズスケーリングの実験の紹介を行いたい。

Apr/08/2011 (Fri) 10:00-12:00 @ Room431
中山 和貴
『超伝導量子干渉計による自己形成InAs量子ドットジョセフソン接合の位相測定』

■ セミナー (2010年度)

Feb/25/2011 (Fri) 10:00-12:00 @ Room431
成瀬 公暁
『２次元３ヘリウム系における核磁気共鳴測定法』
　核磁気共鳴測定法は凝縮系の磁気的性質を知る手段のひとつである。磁気共鳴では核・電子の磁気モーメントがおかれた局所的な場の情報を得ることができる点において、磁気モーメントによる統計量しか得られない比熱・帯磁率測定にまさる測定法である。今回のコロキウムでは核磁気共鳴測定法について説明した後、２次元３ヘリウム系における量子相転移に関するNMR実験を概観する。

Feb/04/2010 (Fri) 10:00-12:00 @ Room431
Tomohiro MATSUI
『STS Observation of Dirac Fermion Topologically appeared on Graphite』
Since the experimental success to fabricate Graphene, the uniquecharacters of Dirac fermion (DF) start to attract a big interest. Inaddition to Graphene, a typical material with massless DF, some othermaterials are also paid many attentions recently because of theirtopologically appeared DFs. Graphite is a well known quasitwo-dimensional (2D) material and one of the good candidates of suchtopological Dirac fermion (TDF). Here, we observed the TDF on thesurface of graphite with scanning tunneling microscopy and spectroscopy(STM/S) in magnetic fields. The observed surface density of states (DOS)show many peaks which are originated from the Landau levels on graphitesurfaces. The field (B) dependencies of the peaks are usually verycomplicated reflecting the irregular structure of graphite in zdirection with many stacking faults. However, the detailed analysissuggests that there certainly are peaks which have the √nB dependencelike massless DF in graphene. In this talk, I will show bothexperimentally and theoretically that massless DF can be localized onthe surface of graphite, in general.

Dec/10/2010 (Fri) 10:00-12:00 @ Room431
松尾 貞茂
『超伝導体で修飾した薄膜グラファイト』
　本コロキウムでは今年度から取り組んでいる超伝導体で修飾した薄膜グラファイトの輸送現象に関する実験結果をまとめて報告する。
　まず、薄膜超伝導系の特徴的な現象であるKT transitionを取り上げて、referenceとして参考にしている先行研究（超伝導体で修飾したグラフェン系のKT transitionなど）をいくつか紹介し、その後に今得られている我々の実験結果をまとめて報告する予定である。

Dec/3/2010 (Fri) 10:00-12:00 @ Room431
河合 直樹
『STM/STSによるグラフェンおよび関連物質の物性研究』
　グラフェンは2005年に劈開法による試料作成の成功が報告されて以来、世界中で精力的に研究が進められてきた。我々は研究室の先達によるグラファイトの研究を基礎として、主にSTM/STSをツールに国内で先駆的にグラフェンの研究に取り組んできた。
　本コロキウムではグラフェンおよび関連物質に対するこれまでの我々の取り組みを着想の段階から整理し検証するとともに、その後研究が各々の分野でどのように進展しているのかを報告する。併せて、我々がこれらの研究に際し行ったULT-STM装置自体に対する改良についても報告する。さらに、以上を踏まえ我々がこの後取り組もうとしている吸着系に対する試みについても説明する予定である。

Nov/26/2010 (Fri) 10:00-12:00 @ Room431
松井 幸太
『二次元ヘリウム薄膜の構造』
　グラファイト上二層目ヘリウムに4/7整合固相が存在することは、比熱や3Heでの磁性の測定などから肯定的な結果が得られていた。しかし近年行われた4HeのPIMC計算においては4/7相密度では4Heは固化せず低温で超流体になるという報告がなされており、4/7相でみられた比熱異常が固相の融解であるという解釈に対して否定的な結果となっている。
　そこで本コロキウムでは、過去に行われた二次元ヘリウム薄膜の構造解析の結果を比熱等の実験結果と比較し考察を行う。特に今回は中性子散乱を題材にとり詳しく見ていく。

Nov/09/2010 (Tue) 14:00-16:00 @ Room431
森下 将史（筑波大学大学院 数理物質科学研究科）
『カゴメ格子ヘリウム３単原子層膜の生成』
　三角格子やカゴメ格子などで、隣り合う反強磁性スピンを反平行に配置できないことに由来する幾何学的フラストレーションが理論・実験両面で盛んに研究され、量子スピン系においてはスピン液体状態などの新奇な状態が期待されている。このうち、グラファイト上の吸着ヘリウム３固相薄膜は２次元量子スピン系のモデル物質と考えられている。この系は三角格子を基調とするが、カゴメ格子を実現できると、理想的なカゴメ格子上量子スピン系も実現でき、興味深い。加えて、ヘリウム３固相薄膜の磁性を決定する多体交換相互作用の競合が、三角格子とカゴメ格子とでは大きく異なることが期待され、磁性がどのように異なるかにも興味がもたれる。
　グラファイト上ヘリウム３薄膜の吸着第２層に生ずる4/7整合固相においては、全ての吸着サイトのうち1/4が、吸着ポテンシャルが深い安定なサイトと考えられている。1/4のヘリウム３原子を非磁性なヘリウム４に置換すると、ヘリウム４がこの安定なサイトに吸着し、残るヘリウム３がカゴメ格子を組むことが期待される。ヘリウム４置換量を1/4、1/2、3/4と変えての熱容量測定の結果を基に、この可能性について議論する。併せて、この測定から、「ヘリウム４は4/7相には固化しない」との最近の理論予測についても議論する。

Nov/05/2010 (Fri) 10:00-12:00 @ Room431
中村 祥子
『グラファイト基板上の吸着実験における面密度』
　炭素系物質において一般的な比表面積の決定方法と細孔形状の解析法を紹介する。 また、過去の結果で我々がよく参照する吸着実験について、比表面積の決定方法と、物理量に特異性が観察された面密度の論文上での値をまとめ、今後の実験方針の指針としたい。

Nov/02/2010 (Thu) 13:00-14:00 @ Room445
Ya Yang (北京科技大学 材料科学与工程学院)
『SPM Studies of Single ZnO Nanomaterials』
We, for the first time, investigated the transverse electrical and electromechanical properties of single ZnO nanomaterials by using a scanning probe microscope [AFM, C-AFM, EFM, STM]. The measurement method is usually based on a single ZnO nanomaterial lying down on a metal substrate, as shown in Fig. 1. The conductive atomic force microscope tip was used as the top electrode. These investigations are very important because the carrier path through single transverse nanomaterials may be only several tens of nanometers. The physical mechanisms of the observed experimental results were discussed in detail. Based on these transverse physical properties, we have demonstrated several typical transverse nanodevices, such as hybrid diodes, piezoelectric switches, piezoelectric field effect transistors, and so on. Moreover, the service safety of the single ZnO nanomaterials and nanodevices was also characterized by SPM. These results support the applications of ZnO nanomaterials as the electrical and electromechanical nanodevices.
[*] 図はこちらのPDFファイルを参照

Oct/20/2010 (Wed) 11:00-12:00 @ Room414
戸田　亮（京都大学大学院理学研究科特定研究員GCOE）
『³He不純物のNMR測定による⁴He超流動固体現象の研究』
近年注目を集めている⁴He超流動固体現象は、系に含まれる³He不純物の量に大きく影響されることが知られている。 京大低温グループではこの現象の解明を目指し、捩れ振り子測定と同時に微量³Heに対するNMR測定を行ってきた。 セミナーでは実験結果を紹介するとともに、⁴He超流動固体現象への³He不純物の関わり方について議論を行う。

Oct/04/2010 (Fri) 10:00-12:00 @ Room431
佐藤 大輔
『2次元フェルミ流体の物理』
本コロキウムでは、最近、我々の主張しているグラファイト上に形成される低密度パドル相に関連して、2次元フェルミ流体における熱容量についてのトピックスを理論・実験の両面から取り扱う。はじめに理想フェルミガスの振る舞いを簡単に復習した後、3次元のパラマグノンモデルなどにも触れながら2次元系の熱容量におけるT²項の存在、パドル相における熱容量の密度変化の解釈などについて議論する予定である。

Sep/02/2010 (Thu) 16:00-18:00 @ Room414
Sander Otte (Delft University of Technology, The Netherlands & NIST, USA)
『The Graphene Landau Quartet Unveiled: an ultra-high resolution tunneling spectroscopy study』
With its focus shifting more and more from topography to spectroscopy,the STM has evolved into an incredibly versatile tool that can map boththe spatial and energy domains of an electronic quantum system. Whereasthe spatial resolution depends mostly on the stability of the tip-samplejunction, the energy resolution of tunneling spectroscopy measurementshas been limited by the temperature of the microscope. In this talk Iwill describe the design and development of a recently completeddilution refrigerator cooled STM system, in which tip and sample can bekept at a temperature of 13 mK in magnetic fields up to 15 T whilemaintaining picometer stability at the junction. The unparalleled energyresolution of this instrument is demonstrated very well by aspectroscopic study of Landau levels in epitaxial graphene: a perfecttwo-dimensional electron system. In this experiment the predictedfourfold degeneracy of the Landau levels is broken and their splittingas a function of the magnetic field measured for the first time. Inaddition, intriguing partial filling of the sublevels is observed,yielding access to promising electron correlation effects.

Aug/03/2010 (Tue) 14:00-16:00 @ Room431
Pedram Roushan (Princeton University)
『Visualizing Topological Surface States by Scanning Tunneling Microscopy』
Topological insulators are a new class of insulators in which a bulk gap for electronic excitations is generated because of strong spin-orbit coupling. These materials are distinguished from ordinary insulators by the presence of gapless metallic surface states, resembling chiral edge modes in quantum Hall systems, but with unconventional spin textures. We have performed scanning tunneling microscopy and spectroscopic studies of topological surface states on a range of different compounds Bi1-xSbx, Sb, Bi2Te3, MnxBi2-xTe3. I will describe how these experiments illustrate the importance of the spin-texture of these novel states for their scattering and quantum confinement. Experiments demonstrate that these states are protected from backscattering between opposite spin states due to their chiral spin textures [1]. Furthermore, we have measured the transmission and reflection probabilities of topological surface states of Sb through naturally occurring crystalline steps separating atomic terraces. In contrast to non-topological Shockley surface states of metals (such as those found on copper, silver and gold), which are either reflected or absorbed by atomic steps, we show that topological surface states of Sb penetrate such barriers with high probability [2]. Finally, I will also discuss the modification of the spin texture and the possibility of localization of these surface states in the presence of magnetic scattering centers.
[1] Roushan et al. Nature 460, 1106 (2009).
[2] J. Seo et al. Nature 466, 343 (2010).

Jul/16/2010 (Fri) 10:00-12:00 @ Room431
Tomohiro MATSUI
『Three Months Stay at NIST』
I have been in CNST/NIST (Center for Nanoscale Science and Technology/National Institute of Standards and Technology) as a CNST Visiting Fellow in the CNST Electron Physics Group for three months supported by the Excellent Young Researcher Oversea Visit Program (Japan Society for the Promotion of Science), where I have joined the research of Dr. Joseph A. Stroscio concering Graphene and other novel electronic properties obtained with scanning tunneling microscopy and spectroscopy (STM/STS). In this colloquium, I want to introduce some STM/STS studies about Graphene in NIST and show some pictures of other groups in eastern US.

Jul/09/2010 (Fri) 10:00-12:00 @ Room431
松井 幸太
『二次元水素系の構造転移』
　グラファイト上に吸着した2次元水素系はヘリウム同様高い量子パラメーターを持つ系であり、低温で様々な相が実現することが知られている。
　過去のコロキウムでは主に構造解析がどのように行われたかという点について扱ったが、今回のコロキウムではさらに比熱の結果や理論的背景などを踏まえてさらに詳しい二次元水素系の構造転移を見ていく。

Jun/25/2010 (Fri) 10:00-12:00 @ Room431
松尾 貞茂
『強磁性体超伝導体接合の近接効果』
　近年注目を集めている超伝導接合系の中でも特に興味深い特性を持つ強磁性体と超伝導体との接合に関連した話題を今回は取り上げたいと考えている。超伝導と強磁性体は通常は相容れない現象であるが、上記のような接合系を作り、クーパー対を侵入させることであたかも2つの現象が共存するかのような系を作ることができる。また、最近の微細加工技術の発展のおかげでこのような系に対する実験が可能になりこのような系に対する理解がすすんできている。
　今回のコロキウムでは前回のコロキウムの後半に少し触れた超伝導強磁性接合系の物理を概観し、その後、いくつかの実験を取り上げる。また、強磁性体を介したジョセフソン接合に関する実験も紹介するつもりである。

Jun/18/2010 (Fri) 10:00-12:00 @ Room431
佐藤 大輔
『量子スピン液体の物理』
　1973年にAndersonが初めて指摘した[1]ように、三角格子のような幾何学的フラストレーションのある格子上の局在スピン間に反強磁性的な相互作用が働くとき、その基底状態として絶対零度でも長距離秩序を持たない量子スピン液体状態が実現する可能性がある。
　長年にわたり、このスピン液体状態は理論上の産物であったが、ここ10年前後において我々の研究する3He吸着原子系や有機分子結晶などいくつもの物質について量子スピン液体が実現されているとの報告がなされてきた。しかも、興味深いことにこれらのうちのいくつかのものは励起エネルギーにギャップを持たないと考えられている。
　本コロキウムでは、Andersonが提案したResonating Valence Bondについての説明を行った後、ギャップレスの量子スピン液体が実現していると考えられている幾つかの物質を示し、それらについての理論の一部を紹介する。
[1] P.W. Anderson, Mater. Res. Bull. 8, 153 (1973).

Jun/4/2010 (Fri) 10:00-12:00 @ Room431
中村 祥子
『超流動ヘリウム３のA-B転移における表面の効果』
　超流動ヘリウム３のA相からB相への相転移では、核生成に大きなエネルギー障壁が存在し、明瞭な過冷却状態が観測される。 しかし、観測される過冷却状態は、ほとんど相転移が実現しないほど安定であろうという予想に比べると、ずっと不安定である。 そのため、宇宙線によるエネルギー供与等、単純な計算では想定していない効果が、この相転移を励起していることが確実視されている。
　今回は、A-B転移に対する表面の効果を研究した Matt O'Keefe 氏の学位論文を中心に紹介する。

May/17/2010 (Mon) 10:00-12:00 @ Room431
河合 直樹
『グラフェン研究の出口とその周辺』
　これまで進められてきたグラフェン研究の出口の1つである「グラフェンの応用」について、グラフェンのトランジスタ応用の進展、またグラフェンの化学修飾体の代表例であるグラフェンオキサイドの特性とその作製からデバイス応用までを概観する。
　さらにグラフェンの研究の進展に伴い興味が深められている周辺領域の話題の例として電荷中性点での量子ホール効果の問題や冷却原子系でのディラック粒子の問題についても触れる予定である。

Apr/23/2010 (Fri) 16:00-18:00 @ Room431
武井 英人
『PbドープBiCo酸化物の電子状態の研究』
　BiCo酸化物Bi2Sr2Co2Oy(BC-2202)は高い熱起電力をもつ物質として注目を集め数多くの研究がなされてきた。構造においては、BC-2202は岩塩構造であるBi2Sr2O4層(絶縁層)と八面体であるCoO2層(伝導層)という二つの構造の違う層がクーロン力で結合している。周期性の違う構造が積み重なったmisfit構造をもち、modulationも確認されている。Biに対してPbを10％置換すると構造相転移がおこる。その際、electron diffractionの結果からmodulationが消えることがわかっている。
　私はBC-2202のmodulationの消失をSTM測定から確認することを目標とした。Biを20％ドープしたBC-2202の常温STM測定を行い、表面のトポグラフィーをとることに成功した。トポグラフィーから、modulationが消えていることが確認できた。またトポグラフィーにおいて、表面にランダムな隆起があることも確認できた。原因不明のランダムな隆起に関して仮説を出し、考察した。

成瀬 公暁
『渦の対消滅の性質』
　超伝導体にレーザー等で強力な光を照射すると瞬間的に超伝導が破壊され、光の照射をやめると回復する現象が報告されている。これまでは超伝導体内での準粒子の励起という視点で説明されてきたものだが、超伝導の現象論であるGL理論の枠組み内でこれを説明することを試みた。GL理論によれば、第二種超伝導の混合状態では渦が貫通し、その芯は超伝道が破壊される。光の照射によって渦・反渦が生成されるが、渦・反渦が対消滅する過程を数値計算によってシュミレーションを行い、渦数の減衰過程考察した。

Apr/9/2010 (Fri) 16:00-18:00 @ Room431
松尾 貞茂
『異方的超伝導体の近接効果』
　超伝導体のなかにはs波超伝導体とは違った対称性を持つ異方的超伝導体が存在し、その対称性に起因した特異な現象が起こることが知られている。
　今回のコロキウムでは前回取り上げた超伝導接合系について異方的超伝導体(主にd波)の場合の紹介を行う。特にd波超伝導体の接合において見られるゼロバイアスコンダクタンスピークやd波超伝導体のジョセフソン接合を用いたSQUIDの実験を取り上げる。時間が許せば、近年注目を集める超伝導強磁性接合を用いたSQUID実験例を取り上げたいと考えている。

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Feb/18/2010 (Tue) 10:00-12:00 @ Room431
松井 幸太
『グラファイト吸着水素分子系の構造解析と脱離評価』
　前回のコロキウムでは一層だけ吸着した二次元水素系の電子線散乱による構造解析について扱ったが、今回はより我々の目的に近い二層の重水素系に対する散乱実験の結 果について詳しく見ていく。また、これらの実験では輻射と電子線による水素の脱離を抑えるために様々な工夫がなされているが、水素の脱離に関しては分子の自由度に由来する光の吸収や電子線に よるイオン化の影響などが報告されている。我々の行おうとしているヘリウム系との比較材料としてこれらの結果をいくつか紹介する。

Dec/18/2009 (Fri) 10:00-12:00 @ Room431
河合 直樹
『Graphene edge properties and its control』
　グラフェンに現れる様々な物性を考えるにあたり、それが単に線型分散関係から導かれる性質であるのか、グラフェンのそもそものAB副格子トポロジーに起因するものであるのかを区別することは重要である。Geimらによるグラフェンの作製に約10年先だって藤田らによって提案されたグラフェンのzigzagエッジ局在状態は、その後者に属する代表的性質であると言えよう。
　本コロキウムでは、近年グラフェンナノリボン（GNR）応用の観点からも大きな注目を集めるグラフェンzigzagエッジ局在状態について、その波数空間・実空間での振る舞いやサイズ依存性をarmchairエッジとの比較を交えつつ概観する。続いて最近の話題から、磁場下での量子ホールエッジ状態との関係、およびエッジの積極的制御・評価法の紹介し、最後に我々の最近の試みにも触れる予定である。

Dec/2/2009 (Wed) 10:00-12:00 @ Room414
川端 一成
『Bi₂Sr₂CaCu₂O_8+δ超薄膜の輸送特性測定』
　本コロキウムでは、Micromechanical Cleavage法により作成されたBSCCO超薄膜試料の輸送特性測定に関する研究の成果を報告する。本研究において観測された、試料を薄膜化することによる超伝導転移温度の低下と、二次元系での特殊な相転移として知られているKosterlitz-Thouless転移の特徴を示すデータを中心に紹介し、この他に、先行研究の紹介や試料作成法、厚さの評価法など、研究の全体的な内 容についても話す予定である。

Nov/13/2009 (Fri) 10:00-12:00 @ Room431
中村 祥子
『グラファイト上 2次元ヘリウムに対する基板の効果』
　CorbozらのPIMC計算[1]の結果は、グラファイト上2次元ヘリウムについて、一層目の実験結果をよく説明している。そして、さらに粒子を増やして行って二層目に達した後のふるまいについても、続けて計算を行っている。 彼らは、4/7相が生じていると提案されてきた密度において生じているのは不整合相固体相であり、今までのPIMC計算で見られてきた4/7相は、一層目の零点振動を無視した結果として得られる虚像に過ぎないと結論付けている。そして、比熱の実験で見られているピーク構造は、この不整合固体相由来の何かでは、としている。 比熱の実験からは、この結果を検証することができないのだろうか。 整合相の存在が広く認められている一層目の1/3相における比熱測定[2]を調べ、検討したい。
[1] Corboz et al. PRB 78, 245414(2008)
[2] Campbell et al. PRB 32, 2861(1985) etc.

Oct/30/2009 (Fri) 10:00-12:00 @ Room431
佐藤 大輔
『グラファイト上単原子層ヘリウム3における基板の不均一性の影響』
　グラファイト上に吸着したヘリウムは理想的な2次元粒子系として振る舞うことが知られている。しかし、一方で、グラファイトの結晶表面は有限サイズの結晶子のあつまりで表わされ、その大きさは、我々の用いているGrafoilにおいては~10 nm×10 nm程度と十分に大きいとは言えない。その為に、熱容量や帯磁率の測定結果には基板の不均一部分の影響が現われてくることがある。本コロキウムでは、これまでの我々の測定結果や、他グループの結果から、不均一性の影響がどのような形で現われてきているかを理解し、我々がこれまで求めてきた2次元ヘリウム3の量子相図のより深い解釈を目指す。

Sep/17/2009 (Thu) 10:00-12:00 @ Room431
松尾 貞茂
『Andreev反射と超伝導近接効果』
　超伝導近接効果はJosephson接合と同じくらいの長い歴史を持っているがいまだに超伝導研究において理学的、工学的に興味深い研究分野として発展を続けている。今回のコロキウムでは超伝導体と常伝導体の接合で起きる現象である超伝導近接効果とAndreev反射についての基本的な考え方、概念といくつかの実験の紹介を行う。

Sep/7/2009 (Mon) 14:00-16:00 @ Room431
松井 幸太
『LEEDによる物理吸着系の構造解析』
　LEEDの原理の簡単な説明を行い、実際の物理吸着系の構造解析においてLEEDがどのように用いられているのかをまとめた。さらに、具体的にグラファイト上の水素及び重水素分子吸着系に対する低温LEEDの先行研究を例に取り、実際にどのような解析によって構造が決定されているかをみる。

Jul/6/2009 (Mon) 14:00-16:00 @ Room431
Tomohiro MATSUI
『Superconductivity in nanometer scale』
　There are a lot of interesting problems in low dimensional and in "very small" superconductors. When the size of the system reaches to the coherence length ξ, they exhibit new aspects that cannot be seen in bulk superconductors. In this colloquium, I want to discuss about superconductivity in such nanometer size superconductors including superconducting films, islands and random network of Josephson junctions in FOMP (film of oxidized metal particles). Superconducting junctions which can be fabricated with STM will also be discussed, if time permitted. The language of the talk will be switched depending on the audience.

Jun/22/2009 (Mon) 14:00-16:00 @ Room431
佐藤 大輔
『グラファイト上単原子層ヘリウム3における熱容量測定』
　吸着第1層にヘリウム4を敷いた上に形成された第2層固体ヘリウム3、及び第3層液体ヘリウム3についての最新の実験結果を報告する。本コロキウムでは、我々の測定によって発見された第3層における2次元ヘリウム3の自己凝縮状態や、第2層の不整合固相領域において観測された交換相互作用の大きさの³He/³He/gr系との相違が下地の吸着ポテンシャルの寄与を考慮することで説明できることなどを示す。

Jun/8/2009 (Mon) 14:00-16:00 @ Room431
河合 直樹
『グラフェンをめぐる最近のトピック』
　前回のコロキウムではSiC上にエピタキシャルに合成されたグラフェンの話題を紹介した。そこではグラフェンが基板との相互作用から受ける影響の大きさが問題となっていた。一方、劈開法により絶縁体基板上に作成されたグラフェンは、試料サイズは小さいものの基板との相互作用が小さくグラフェン自身の性質が最もよく現れる系として盛んに研究が進められている。本コロキウムでは特にKlein tunnelingとedge stateというトピックを取り上げ、その理論的背景と最近の研究の進展を紹介したい。

May/25/2009 (Mon) 14:00-16:00 @ Room431
中村 祥子
『グラファイト上2次元希ガス結晶』
　グラファイト上2次元希ガス結晶を対象に、KTHNY(Kosterlitz-Thouless-Halperin-Nelson-Young) theoryとdomain wall 構造の理論を紹介し、それに適合する実験結果を参照した。

Apr/27/2009 (Mon) 14:00-16:00 @ Room431
川端　一成
『Kosteritz-Thouless theory』
　固体表面に吸着したHe-4薄膜などの二次元系における相転移では渦が本質的な役割を担う。 この渦の統計力学をくりこみ群の方法を用いて具体的に取り扱ったものが、Kosterlitz-Thouless 理論(KT理論)である。 本コロキウムでは、磁性体の転移点近傍での臨界現象（ゆらぎの発散）を例にとり、相関関数の定義、くりこみ変換の 考え方を紹介し、これらを二次元系に適用することで、KT理論の基本的な部分の導出を確認する。 また、残りの時間でKT理論を検証したいくつかの実験を紹介する予定である。

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