Takahiro Nishio

Superconductivity of nanometer-size Pb islands studied by low-temperature scanning tunneling microscopy

Using a low-temperature (1.2K) scanning tunneling microscopy, the tunneling spectra showing the superconducting gap was taken on Pb island structures, whose dimension ranges from 80 to 300nm in diameter and 7–12 monolayers in thickness. There is no considerable spatial variation in the tunneling spectra taken on a single island regardless of local geometry (center or peripheral) and thickness of the measured sites. The superconducting gap increases with the island size, and the size dependence is enhanced at higher temperature (3.9K). The behavior of the gap is explained qualitatively by considering the superconducting fluctuation in the small islands.

Atomically resolved imaging by low-temperature frequency-modulation atomic force microscopy using a quartz length-extension resonator.

Low-temperature ultrahigh vacuum frequency-modulation atomic force microscopy (AFM) was performed using a 1 MHz length-extension type of quartz resonator as a force sensor. Taking advantage of the high stiffness of the resonator, the AFM was operated with an oscillation amplitude smaller than 100 pm, which is favorable for high spatial resolution, without snapping an AFM tip onto a sample surface. Atomically resolved imaging of the adatom structure on the Si(111)-(7x7) surface was successfully obtained.

Supercurrent through InAs nanowires with highly transparent superconducting contacts

One-dimensional superconducting transistors have been fabricated with individual InAs nanowires (NWs) using radio-frequency sputter cleaning followed by in situ metal deposition. Because of the highly transparent contacts formed in between the InAs NWs and the metals, supercurrent, multiple Andreev reflections and Shapiro steps under microwave radiation have been observed. Near pinch-off gate regions, Fabry-Perot interference and a normal conductance quantization with resonant features have been observed, which were found to be correlated with a supercurrent flow.

Pressure-induced superconductivity in boron-doped Buckypapers

We show creation of paperlike thin film (Buckypaper) consisting of pseudo-two-dimensional network of boron-doped single-walled carbon nanotubes (B-SWNTs) within weakly intertube van der Waals coupling (IVDWC) state. It was formed by sufficiently dissolving as-grown ropes of B-SWNTs and densely assembling them on silicon substrate. We find that superconducting transition temperature (Tc) of 8 K under absent pressure can be induced up to 19 K by applying a small pressure to the film and that a frequency in the radial breathing phonon drastically increases with applying pressure. Discussion about IVDWC and distribution of B-SWNTs diameter imply the strong correlation.

Real-Space Observation of Screened Potential and Friedel Oscillation by Scanning Tunneling Spectroscopy

Using scanning tunneling spectroscopy at 5K electrostatic potential profile on the Si(111)• 3x• 3 Ag surface has been obtained with high spatial and energy resolutions. The potential profile measured around a step edge was explained with the Coulomb potential screened by a two-dimensional electron system which intrinsically exists on the surface. Friedel oscillation, which is characteristic to the screening, was also observed in the potential images. Its shape and phase were discussed in comparison with those of standing waves.

Tip-induced excitation of a single vortex in nano-size superconductors using scanning tunneling microscopy.

A single vortex is excited into nano-size Pb superconducting island structures by a local current injection from a probe tip of a scanning tunneling microscope. For the excitation, a sufficient amount and duration of the pulsed current are required. Injecting the current at peripheral sites is more effective than the center for the vortex excitation. Time-dependent Ginzburg-Landau calculations suggest that a current-induced normal-state area, which can be nucleated by the tunneling current exceeding the critical current and expanded by the Joule heating, reduces the required magnetic field for the vortex penetration and excites a vortex into the islands.

Thermally assisted penetration and exclusion of single vortex in mesoscopic superconductors

A single vortex overcoming the surface barrier in a mesoscopic superconductor with lateral dimensions of several coherence lengths and thickness of several nanometers provides an ideal platform to study thermal activation of a single vortex. In the presence of thermal fluctuations, there is nonzero probability for vortex penetration into or exclusion from the superconductor even when the surface barrier does not vanish. We consider the thermal activation of a single vortex in a mesoscopic superconducting disk of circular shape. To obtain statistics for the penetration and exclusion magnetic fields, slow and periodic magnetic fields are applied to the superconductor. We calculate the distribution of the penetration and exclusion fields from the thermal activation rate. This distribution can also be measured experimentally, which allows for a quantitative comparison.

Meissner effect in films of ropes of boron-doped single-walled carbon nanotubes; Correlation with applied pressure and boron-doped multi-walled nanotubes

Superconductivity in carbon nanotubes (CNTs) is attracting considerable attention. Recently, we have reported successful boron doping into single-walled CNTs (SWNTs) and also revealed its correlation with superconductivity. In the present study, we report results of pressure-applied magnetization measurements in films consisting of ropes of boron-doped SWNTs. It reveals that Tc and magnitude for Meissner effect is mostly independent of applied pressure, while magnitude of graphite diamagnetism drastically increases as pressure increases. We also report result of resistance measurements in the samples and also correlation of boron doped SWNTs with multi-walled CNTs, in which we reported superconductivity previously.

Size-Dependent Superconductivity of Pb Islands under Magnetic Fields Studied by Low-Temperature Scanning Tunneling Microscopy/Spectroscopy

Using a low-temperature scanning tunneling microscope, we investigated the superconducting gap of single-crystalline Pb islands, approximately 100 nm in diameter with 11–13 monolayers (3–4 nm) in thickness, under magnetic fields. The superconducting gap was observed under magnetic fields up to 2.2 T, higher than the critical field of bulk Pb. The critical fields of the Pb islands were evaluated from the magnetic field dependence of the zero-bias conductances. We found that the critical field depends on the volume V of the Pb islands as V-1/2.