■ Research Activities in FY2016

Our interests are in (i) novel quantum phases with strong correlations and frustration in two dimensional (2D) helium three (³He), four (⁴He) and their mixture, and (ii) novel electronic properties of low dimensional materials, particularly graphene and carbon nanotube. We are investigating these phenomena in a wide temperature range from room temperature down to microkelvins, using various experimental techniques such as NMR, calorimetry, torsional oscillator, scanning tunneling microscopy and spectroscopy (STM/S), and electronic transport measurement. Our efforts are also devoted to development of new experimental and cryogenic techniques.

1. Quantum Spin Liquid state in 2D ³He system:
Quantum spin liquid (QSL) is a noble magnetic ground state of S = 0 where the spin expectation value at each lattice site remains zero even at T = 0. 2D solid ³He is the first candidate material for QSL which was found experimentally by our group in 1997. Since then, many other candidate materials had been found in highly frustrated electronic spin systems, but 2D ³He is still one of the most promising candidates. Recently we have discovered a new QSL in 2D ³He on graphite preplated with a bilayer of HD. This has anomalous power-law temperature dependencies of specic heat (C) and magnetic susceptibility (χ) with fractional powers, while the previously known QSL in the second layer of ³He on graphite does a T-linear dependence for C and a T-independent χ.

2. Possible super fluid ``quantum liquid crystal'' phase in 2D ⁴He system:
So far, only three quantum phases, i.e., quantum gas, quantum liquid and quantum solid, have been known to exist in nature. Recently, we are proposing a new member to them, a quantum liquid crystal (QLC), which we found at densities in between the quantum liquid and solid phases below T = 1.4 K in the second layer of helium on graphite from heat capacity measurements. The most probable state for this is a quantum hexatic phase where the system possesses only the quasi-long range bond orientational ordering. In bosonic ⁴He, superfluid order can coexist in this phase below 0.4 K, which is considered as a member of supersolids.

3. Liquefaction of ³He in two dimensions:
Until our recent C measurement of monolayers of ³He of low densities, the ³He system has long been believed not to liquefy in 2D unlike in 3D. This is because of increasing fluctuations in lower dimensions. But we found ³He atoms form self-bound liquid paddles with a very low density of 0.6-0.9 nm^-2 below 1 K. Our finding stimulated theoreticians to reexamine this problem, and recent quantum Monte Carlo calculations seem to gave results much closer to the experiment than the previous attempts. We are now planning to start a new C measurement using a graphite substrate with a much longer coherence length than Grafoil used previously.

4. Zigzag edge state in graphene and its application to future nanoelectronics:
Graphene, a monatomic carbon sheet, has remarkable electronic and structural properties due to its unique linear dispersion relation. It is potentially applicable to a vast area of technology including graphene-based electronic devices. One of the most important topics in graphene studies is the zigzag edge state and its spin polarization. This localized electronic state was experimentally found for the first time by our group in 2005 using STM/S technique at naturally existing monatomic step edges on a graphite surface. It is expected that the spin degeneracy of the zigzag edge state can easily be lifted by a small perturbation like the electron-electron interaction. This will result in high spin polarization at the edge. Thus, in a nano ribbon with zigzag edges on both sides (zigzag nanoribbon), one can expect opening the band gap whose energy width is controllable by the ribbon width and applied magnetic field. Due to this, zigzag nanoribbon is considered to be highly useful in future electronics.
Fabrication of zigzag nanoribbons is highly demanding because of the difficulty of precise processing of zigzag edge in an atomic scale. We have recently succeeded in fabrication of hexagonal nanopits of monatomic depth with high-quality zigzag edges on a graphite surface by use of hydrogen plasma etching. The size and density of nanopit can be controlled by tuning the plasma excitation power, temperature, dose time, and partial pressure of hydrogen. Therefore, one can also fabricate a zigzag nanoribbon in between two adjacent hexagonal nanopits. We are now studying the local electronic state of the spin polarized zigzag edge state by STM/S with an atomic spatial resolution.

5. Random network of carbon nanotubes:
Carbon nanotube (CNT) is a rolled graphene with a nanometer size diameter and a high length/diameter ratio. It is a promising quasi one-dimensional (1D) material for future technology. Depending on the rolling chirality, CNT becomes either metallic or semiconducting. The electronic properties of metallic CNTs are expected to obey the Tomonaga-Luttinger liquid (TLL) theory which describes behaviors of interacting fermions in 1D. Recently we found that the electrical resistivity of random networks of high-purity double walled metallic CNTs show surprising behaviors expected from the TLL theory. Magneto-transport measurements gave us hints to solve the question why such a macroscopic 3D network can follow the TLL theory. Further research is now being undertaken to obtain more information and to seek for new functions of CNTs.

6. Development of new refrigeration systems:
Two projects are on going. One is to develop a compact and continuous nuclear demagnetization refrigerator (ccNDR) with which T = 0.8 mK can be kept continuously by use of two PrNi₅ nuclear spin stages connected in series. Such ultra-low temperatures are now required in many research disciplines including material science, quantum information, and astrophysics.
The other is to develop a refrigeration system for scanning probe microscope (SPM) based on helium circulation which enables us to take STS data at T = 2 - 4 K without being interrupted by liquid helium refilling which is now expensive and sometimes not available stably in industries or developing countries. We have successfully tested the high thermal shielding performance of the low-consumption and low vibration-transmission helium transfer tube we designed.