Tomohiro Matsui

Superconductivity in the quasi-two-dimensional conductor 2H-TaSe2

Abstract Superconductivity in 2H-TaSe2, a quasi-two-dimensional conductor, has been studied by the self-inductance method on single crystals. The transition temperature Tc in zero magnetic field is 133 mK which is very low because of nesting of a large portion of the Fermi surface caused by the charge density wave formation (TCDW=90 and 122 K ). The zero-temperature upper critical field is determined to be H c 2⊥ ≈1.4 mT and H c 2|| ≈4.1 mT with the field-oriented perpendicular and parallel to the c plane, respectively.

Development of a new ULT Scanning Tunneling Microscope at University of Tokyo

We describe design concepts and some technical details of a new ultra-low temperature scanning tunneling microscope (ULT-STM) which is now under construction at University of Tokyo. It is designed to work with an atomic resolution at temperatures down to 20 mK and in magnetic fields up to 6 T. It is possible to change samples and STM tips keeping ultra high vacuum and low temperature environments, which allows us to study almost all conducting materials and adsorbed samples.

Construction of a Dilution Refrigerator Based Ultra-Low Temperature Scanning Tunneling Microscope

We constructed a dilution refrigerator based ultra-low temperature scanning tunneling microscope (ULT-STM) which works at temperatures down to 20 mK, in magnetic fields up to 6 T and in ultrahigh vacuum (UHV). One can load samples/tips, which are prepared in a UHV chamber, to an STM head maintaining the low temperature and UHV conditions. After then they can be cooled back to the base temperature in several hours. We report results of a test measurement on a superconducting NbSe2 sample as well as recent STM/STS studies on graphite samples such as observations of the Landau quantization and visualization of the possible localized states in magnetic fields.

STM observations of 2D Kr and Xe adsorbed on graphite

Crystalline structures of two dimensional rare-gas solids physisorbed on a graphite surface are studied with a low-temperature scanning tunneling microscope (STM) at T=4 K. We have obtained atomically resolved STM images of monolayer krypton (Kr) for the first time as well as those of xenon (Xe). It was observed that the 2D structure of Xe is destroyed with small tip-sample separation. Distinct changes in the local density of states were observed in tunneling spectra after the adsorption. For a multi-layer Xe film, a characteristic telegraph pattern of the tunneling current was also observed, which probably indicates single atom dynamics.

Low temperature transport properties of pyrolytic graphite sheet

Abstract We have made thermal and electrical transport measurements of uncompressed pyrolytic graphite sheet (uPGS), a mass-produced thin graphite sheet with various thicknesses between 10 and 100xa0 μ m, at temperatures between 2 and 300xa0K. Compared to exfoliated graphite sheets like Grafoil, uPGS has much higher conductivities by an order of magnitude because of its high crystallinity confirmed by X-ray diffraction and Raman spectroscopy. This material is advantageous as a thermal link of light weight in a wide temperature range particularly above 60xa0K where the thermal conductivity is much higher than common thermal conductors such as copper and aluminum alloys. We also found a general relationship between thermal and electrical conductivities in graphite-based materials which have highly anisotropic conductivities. This would be useful to estimate thermal conductance of a cryogenic part made of these materials from its electrical conductance more easily measurable at low temperature.

Characterization of Pyrolytic Graphite Sheet: A New Type of Adsorption Substrate for Studies of Superfluid Thin Films

We have measured surface morphology and gas adsorption characteristics of uncompressed pyrolytic graphite sheet (uPGS) which is a candidate substrate for AC and DC superflow experiments on monolayers of He below T = 1 K. The PGS is a mass-produced thin graphite sheet with various thicknesses between 10 and 100μm. We employed a variety of measuring techniques such as imagings with optical microscope, SEM and STM, Raman spectroscopy, and adsorption isotherm. PGS has smooth and atomically-flat external surfaces with high crystallinity. Although the specific surface area (≤ 0.1 m/g) is rather small, by making use of its smooth external surface, the thinnest uPGS of 10μm thick is found to be suitable for the superflow experiments on the strictly two-dimensional helium systems.We have measured the surface morphology and gas adsorption characteristics of an uncompressed pyrolytic graphite sheet (uPGS) which is a candidate substrate for AC and DC superflow experiments on monolayers of

Development of an ultra-low temperature scanning tunneling microscope and applications for low temperature physics

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Pyrolytic Graphite Sheet, a New Adsorption Substrate for Superfluid Thin Films

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