H. Kambara

STS observations of Landau levels at graphite surfaces.

Scanning tunneling spectroscopy (STS) measurements were made on surfaces of two different kinds of graphite samples, Kish graphite and highly oriented pyrolytic graphite (HOPG), at very low temperatures and in high magnetic fields. We observed a series of peaks in the tunnel spectra associated with Landau quantization of the quasi-two-dimensional electrons and holes. A comparison with the calculated local density of states at the surface layers allows us to identify Kish graphite as bulk graphite and HOPG as graphite with a finite thickness of 40 layers. This explains the qualitative difference between the two graphites reported in the recent transport measurements which suggested the quantum-Hall effect in HOPG. This work demonstrates how powerful the combined approach between the high quality STS measurement and the first-principles calculation is in material science.

Edge states of Sr2RuO4 detected by in-plane tunneling spectroscopy.

Tunneling spectroscopy has been performed on Sr(2)RuO(4) searching for the edge states peculiar to topological superconductivity. Conductance spectra exhibit broad humps with three types of peak shape: domelike peak, split peak, and two-step peak. By comparing the experiments with predictions for unconventional superconductivity, these varieties are shown to originate from multiband chiral p-wave symmetry with weak anisotropy of pair amplitude. The broad hump in the conductance spectrum is a direct manifestation of the edge state due to chiral p-wave superconductivity.

Real-space imaging of alternate localization and extension of quasi-two-dimensional electronic states at graphite surfaces in magnetic fields

We measured the local density of states (LDOS) of a quasi-two-dimensional (2D) electron system near point defects on a surface of highly oriented pyrolytic graphite with scanning tunneling microscopy and spectroscopy. Differential tunnel conductance images taken at very low temperatures and in high magnetic fields show a clear contrast between localized and extended spatial distributions of the LDOS at the valley and peak energies of the Landau level spectrum, respectively. The localized electronic state has a single circular distribution around the defects with a radius comparable to the magnetic length. The localized LDOS is in good agreement with a spatial distribution of a calculated wave function for a single electron in 2D in a Coulomb potential in magnetic fields.

Anomalous Transport through the p-Wave Superconducting Channel in the 3-K Phase of Sr_2RuO_4

Using microfabrication techniques, we extracted individual channels of 3-kelvin (3-K) phase superconductivity in Sr2RuO4-Ru eutectic systems and confirmed odd-parity superconductivity in the 3-K phase, similar to pure Sr2RuO4. Unusual hysteresis in the differential resistance-current and voltage-current characteristics observed below 2 K indicates the internal degrees of freedom of the superconducting state. A possible origin of the hysteresis is current-induced chiral-domain-wall motion due to the chiral p-wave state.

Construction of a versatile ultralow temperature scanning tunneling microscope

We constructed a dilution-refrigerator (DR)-based ultralow temperature scanning tunneling microscope (ULT-STM) which works at temperatures down to 30 mK, in magnetic fields up to 6 T and in ultrahigh vacuum (UHV). Besides these extreme operation conditions, this STM has several unique features not available in other DR-based ULT-STMs. One can load STM tips as well as samples with clean surfaces prepared in an UHV environment to a STM head keeping low temperature and UHV conditions. After then, the system can be cooled back to near the base temperature within 3 h. Due to these capabilities, it has a variety of applications not only for cleavable materials but also for almost all conducting materials. The present ULT-STM has also an exceptionally high stability in the presence of magnetic field and even during field sweep. We describe details of its design, performance, and applications for low temperature physics.

C-axis critical current of a PrFeAsO0.7 single crystal

The c-axis transport properties of a high-pressure synthesized PrFeAsO0.7 single crystal are studied using s-shaped junctions. Resistivity anisotropy of about 120 detected at 50 K shows the presence of strong anisotropy in the electronic states. The obtained critical current density for the c-axis of 2.9×105 A/cm2 is two orders of magnitude larger than that in Bi2Sr1.6La0.4CuO6+δ. The appearance of a hysteresis in the current-voltage curve below Tc is the manifestation of the intrinsic Josephson effect similar to that in cuprate superconductors. The suppression of the critical current-normal resistance (IcRn) product is explained by an inspecular transport in s±-wave pair potential.

Localized distributions of quasi-two-dimensional electronic states near defects artificially created at graphite surfaces in magnetic fields.

We measured the local density of states of a quasi two-dimensional electron system (2DES) near defects, artificially created by Ar-ion sputtering, on surfaces of highly oriented pyrolytic graphite (HOPG) with scanning tunneling spectroscopy (STS) in high magnetic fields. At valley energies of the Landau level spectrum, we found two typical localized distributions of the 2DES depending on the defects. These are new types of distributions which are not observed in the previous STS work at the HOPG surface near a point defect [Y. Niimi, Phys. Rev. Lett. 97, 236804 (2006).10.1103/PhysRevLett.97.236804]. With increasing energy, we observed gradual transformation from the localized distributions to the extended ones as expected for the integer quantum Hall state. We show that the defect potential depth is responsible for the two localized distributions from comparison with theoretical calculations.

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.

Characterization of ZYX exfoliated graphite for studies of monolayer 3He below 1 mK

We have characterized ZYX exfoliated graphite (ZYX) by measuring the surface area and electrical resistivity perpendicular (ρ⊥) to the basal plane as a function of density. The purpose of the characterization is to seek possibilities to use this material as an adsorption substrate for studies of two dimensional (2D) 3He at ultralow temperatures (ULT) below 1 mK. A potential advantage of ZYX is its much larger single crystalline size (100–200 nm) than that of Grafoil (≈10 nm) which is an exfoliated graphite commonly used at ULT. Present results show that ZYX has sufficiently large surface area to be used in heat capacity or nuclear magnetic resonance experiments on 2D 3He samples and good thermal conductance to cool them well below 1 mK. We have demonstrated this by measuring nuclear magnetic susceptibility of monolayer solid 3He adsorbed on ZYX with a density of 1.1 g/cm3 down to 100 μK. Measured temperature dependencies of ρ⊥ and ρ∥ (resistivity parallel to the basal plane) below 70 K are discussed withi...

Construction of an ultra low temperature STM with a bottom loading mechanism

Abstract We have constructed and tested an ultra-low temperature scanning tunneling microscope that works with an atomic resolution at ultra low temperatures ( T⩾126 mK ) in magnetic fields ( B⩽6 T ) in ultra high vacuum (UHV). Clean sample surfaces can be prepared by several different methods and characterized by low energy electron diffraction in situ of UHV. A unique bottom loading mechanism enables us to cool back to the base temperature within 3 h after changing the sample and scanning tunneling microscopy tips.