Shin-ichi Kaneko

First observation of the fourfold-symmetric and quantum regime vortex core in YNi2B2C by scanning tunneling microscopy and spectroscopy

The local quasi-particle density of states, N s ( E , r ), in the isolated vortex of YNi 2 B 2 C has been measured at 0.46 K by low-temperature scanning tunneling microscopy and spectroscopy. The vortex core is found to be fourfold-symmetric star-shaped in real space: Near E = 0 meV, N s ( E , r ) extends toward (the a -axis) and has a peak at the center of the vortex core. With increasing energy, N s ( E , r ) comes to extend toward and the peak of N s ( E , r ) splits into four peaks toward . N s ( E , r ) in the vortex core exhibits electron–hole asymmetry with the intensity, indicating the first observation of the vortex core in a quantum regime.

Observations of Slow Particles and Stars in Nuclear Emulsions Exposed at 17 m.w.e. Underground

Observations were carried out at a depth of 17 m.w.e. underground by nuclear plates made from Ilford G5 emulsions in gel form. The frequencies of µ-mesons and protons coming to rest in the emulsions were found to be 0.262±0.008 cm -3 day -1 and 0.049±0.004 cm -3 day -1 , respectively. The angular distribution of these slow protons was found to be isotropic and so they are regarded as being of secondary origin from fast µ-mesons underground. In addition to the stars of the types of N h +1 p with \(\text{N}_{h}{\geqslant}3\), several stars of the types of 1+1 p and 2+1 p were found with a cross section for production of a few times 10 -30 cm 2 per nucleon. Some of the stars of the type of 1+1 p may possibly be explained by photoproton production through the giant resonance absorption by emulsion nuclei of virtual photons associated with high energy µ-mesons.

Scanning tunneling spectroscopy of Bi2Sr2CaCu2O8+δ

Abstract Cleaved surfaces of Bi 2 Sr 2 CaCu 2 O 8+ δ ( T c =86 K) have been investigated by low temperature scanning tunneling spectroscopy. The clean surface has been prepared by cleaving the sample at liquid helium temperature in a vacuum. The intrinsic superconducting density of states has been obtained in the tunneling spectra exhibiting a superconducting gap with a metallic background; tunneling spectra with semiconducting backgrounds are not intrinsic. The tunneling spectra have a broad bump just outside a superconducting gap. The background conductance is slightly asymmetric with respect to the Fermi energy E F ; the density of states above E F is smaller than that below E F . Because of the broad bump, the spectra cannot be fitted by the theoretical bulk density of states of s-wave or d-wave superconductors. The broad bump in the tunneling spectra can be seen to consist of two peaks at bias voltages of ∼30 meV and ∼40 meV. The shape of measured spectra is similar to the spectra calculated in the 2D Hubbard model.

Quantum Limiting Behaviors of a Vortex Core in an Anisotropic Gap Superconductor

Quantized bound states at a vortex core are discretized in YNi 2 B 2 C. By using scanning tunneling spectroscopy with an unprecedented 0.1 nm spatial resolution, we find and identify the localized spectral structure, where in addition to the first main peak with a positive low energy, a second subpeak coming from the fourfold symmetric gap structure is seen inside the energy gap. Those spectral features are understood solving the Bogoliubov–de Gennes equation for a fully three-dimensional gap structure. A particle–hole asymmetric spectrum at the core site and quantum oscillation in the spectra are clearly observed.

Observation of the High-Energy Proton-Proton Collision in Photographic Emulsion

The tracing-back method has been applied to the detection of showers which could be regarded as those produced by single nucleon-nucleon collisions in a stack of stripped emulsions exposed at the stratosphere. The method consists in tracing star-producing tracks back along their passage through successive emulsion strips and in finding primary interactions. It enabled us to detect the interactions without any bias irrespective of the number of accompanying evaporation prongs. A shower of the type 1+5 p detected by this method in a preliminary observation was analysed in terms of the single proton-proton collision. By the assumption of balance of transverse momenta for the charged outgoing particles alone, the incoming proton energy was estimated to be about 25 Gev and the inelasticity to be about 0.4. In this paper these results and another possible estimates has been presented together with a brief description of some features of the shower.

Ubiquitous V-Shape Density of States in a Mixed State of Clean Limit Type II Superconductors

It is demonstrated theoretically and experimentally that the low energy density of states N(E) is described by a singular V-shape form N(E)=N(0)(H)+alpha|E|+O(E2) for all clean superconductors in a vortex state, irrespective of the underlying gap structure. The linear term alpha|E| which has not been recognized so far is obtained by exactly evaluating the vortex contribution. Based on microscopic Eilenberger theory N(E) is evaluated for the isotropic gap, line, and point-node gaps to yield a V-shape N(E). Scanning tunneling spectroscopy-STM experiments on NbSe2 and YNi2B2C give direct evidence for this. We provide arguments on the significance of this finding and on the relevance to other experiments.

Frequency of Slow Cosmic-ray µ-Mesons Stopped in the Photographic Emulsions at 23 m.w.e. Underground

The absolute frequency of slow µ-mesons stopped in the photographic emulsions at a depth of 23 m.w.e. underground was measured. The result is 0.22±0.03 particles cm -3 day -1 , which is greater than the value published by George and Evans by a factor of about 2.5.

Local electronic density of states near steps on the c-plane of Bi2Sr2CaCu2Ox studied using low temperature scanning tunneling spectroscopy and microscopy

The local electronic density of states near steps on the c-plane of Bi 2 Sr 2 CaCu 2 O x running along the [110] direction in the CuO 2 plane was measured by low temperature scanning tunneling spectroscopy and microscopy. The electronic states were found to be insulating near a step edge on the upper terrace and an insulator-superconductor boundary was formed 7 nm away from the step edge. The superconducting tunneling spectra on the upper terrace depend on the distance from the boundary and the tunneling conductance inside the superconducting gap increases in the region of 10 nm from the boundary. It is speculated that the changes of the superconducting spectra are due to the boundary effects of d x2-y2 -wave superconductors.

A Calibration Measurement of Multiple Scattering of Slow µ-Mesons by the Constant Sagitta Method

Multiple scattering observations have been carried out on 103 slow cosmic-ray µ-mesons coming to rest in G5 emulsions. The emulsions were coated, exposed and processed at a depth of about 17 m.w.e. underground. It is shown that the constant sagitta scheme recommended by the Gottingen group gives satisfactorily constant mean sagittae with increasing residual range at least in the region of velocity \(0{\lesssim}\beta{\lesssim}0.46\). Observations of 81 µ-mesons are also described which were done by the scheme based upon the Williams theory of scattering and upon the range-energy relation given by Bradner et al. The error introduced by the use of schemes other than that of Gottingen is discussed, The results are compared with Glassers calibration experiment on 101 artificially produced slow protons.

Random organization of vortices under an anisotropic condition

Many colliding particles that are periodically sheared by ac drive self-organize to avoid future collisions, which is known as random organization. Recently, we have observed the random organization in the vortex system of a strip-shaped amorphous Mo x Ge1−x film, where the vortices experience periodic local shear from ac drive and the random pinning potential. In this work, we study how random organization changes in the vortex system under the tilted field, where an anisotropic vortex-vortex interaction is introduced. We find that characteristic times of random organization for the vortices driven in the tilted direction are significantly smaller than those in the untilted field.