K. Oto

SUPERCONDUCTIVITY IN PTSI ULTRATHIN FILMS

Normal and superconducting properties of PtSi films with thicknesses of d=2–20 nm have been investigated. The superconducting transition has been observed on the thin films down to d=4 nm. The systematic reduction of the transition temperature with decreasing d (increasing the sheet resistance) has been explained by the localization and Coulomb interaction effects on superconductivity. The temperature dependence of the upper critical field indicates that the PtSi films behave as homogeneous superconductors.

Cyclotron resonance in double-layer two-dimensional electron system in tilted magnetic fields

Cyclotron resonance measurement of the double-layer two-dimensional electron system in tilted magnetic fields is made with far-infrared Fourier transform spectroscopy. Parallel magnetic field (B∥) dependence of the cyclotron mass is obtained by setting B∥ and the perpendicular magnetic field (B⊥) independently. We find a complicated behaviour of the cyclotron mass in tilted magnetic fields which is not explained by semiclassical theory.

Transport Properties in Band-Tails of High Mobility Poly-Si TFTs

Electrical properties of high-mobility polycrystalline silicon thin film transistors (poly-Si TFTs) are measured over the wide temperature range from 300 K to 1.5 K in order to clarify the role of band-tail states in the grain boundaries (GBs). Two distinct regimes of weak and strong localization of electrical transport have been identified below and above the carrier trap state density N st 1.0 x 10 12 cm -2 in the GBs for high-mobility poly-Si TFTs. Poly-Si TFTs with a smaller carrier trap state density N st ( 6 x 10 11 cm -2 ) show metallic behavior where the mobility increases as temperature decreases. Poly-Si TFTs with larger N st ( 1.5 x 10 12 cm -2 ) show semiconducting behavior of mobility. At low temperatures, on the former poly-Si TFT the resistance changes due to the weak-localization effect and on the latter poly-Si TFT due to the variable-range hopping (VRH) in the regime of strong localization. In both regimes, negative magnetoresistances have been observed and interpreted by the quantum interference effect. All of these features are attributed to the band-tail states decaying exponentially from the band-edge, based on the observations of irregular GB structure in the poly-Si film from transmission electron microscope images and the roughness at the SiO 2 /poly-Si interface seen on atomic force microscope images.

Transport properties in artificial lateral superlattice

We investigate commensurate oscillations in ordered and disordered artificial lateral superlattice (ALS) systems, in which the anti-dots are arranged in a square or triangular lattice. With increasing disorder of the anti-dot location, the peaks of the commensurate oscillations fade out. The peak heights are more strongly affected by the disorder along perpendicular direction to the current than by that along the parallel direction. In the square ALS system, the commensurate oscillations seem to be determined principally by the order along the perpendicular direction to the current, while in the triangular ALS system, the commensurate oscillations would be determined by the nearest neighbor distance between anti-dots and the order along the perpendicular direction. In addition, the appearance of each peak is determined by the ratio of the anti-dot diameter to the ALS period. The weak localization effect in very low magnetic field and the strongly temperature dependent conductance in the absence of magnetic field are also observed in the ALS systems.

Resistivity, Hall coefficient and transition temperatures in doped 80K and 110K BiSrCaCuO superconductors

Abstract The temperature dependence of Hall coefficient and electrical resistivity have been measured in Biue5f8Srue5f8Caue5f8Cuue5f8O superconductor system. The Hall coefficient (R H ) of the 110K phase is nearly proportional to 1/T, while the temperature dependence of R H in the 80K phase is smaller than those in the 110K phase. It seems that there is an optimum “carrier concentration” (1/eR H ) to get the maximum Tc in the 80K phase.

Transition between different types of breakdown of the quantum Hall effect by illumination, temperature change and the application of gate voltage

We investigate the breakdown of the quantum Hall effect (QHE) and observe a remarkable change in dependence of the critical current for the breakdown in sample width. Both linear and sub-linear dependences of the critical current on sample width are obtained for the same sample, depending on temperature, way of illumination, and gate bias voltage. The experimental results of the breakdown of the QHE are explained by comparing electron density fluctuations, which are characterized by the random potential from the ionized donors and by screening effects, with the thermal broadening of the electron energy distribution. It is suggested that the electron density fluctuations play an important role in the breakdown of the QHE.

Width of compressible strips in quantum Hall regime

We have investigated the width of the compressible strips in the edge state by magnetocapacitance between two-dimensional electron gas and a metal gate. The detail of Landau level filling factor dependence of the width is measured around the quantum Hall plateaux, and the influence of confinement potential on the width has been studied by tuning the side gate voltage. It is confirmed that the width is mainly governed by the depletion region at the sample boundary; the spatially gentle confinement potential forms wider compressible and incompressible strips. The nonlocal resistance has been also studied in the samples with tunable confinement potential by side gate, which supports the result of the capacitance measurement.

Hopping transport in band-tail of grain boundaries in poly-Si TFTs

Abstract The low-temperature transport in high-mobility poly-Si TFT fabricated using laser annealing of amorphous Si film has been studied in the two-dimensional variable-range hopping (VRH) regime near the crossover between the weak and strong localization. The resistance follows the Efros-Shklovskii (ES) VRH as In R α T−1/2 below ∼ 20 K in the presence of a Coulomb gap in the density of states. The negative magnetoresistance ΔR R varies below ∼ 4 K as ∼ T−3/2B2 in low-magnetic fields and as ∼T−3/4B in moderate fields both in agreement with the theoretical predictions based on quantum interference.

In-plane magnetic field dependence of cyclotron resonance in two-dimensional electron system

The in-plane magnetic field dependence of cyclotron resonance is measured both in single and double layer 2DES. In particular, we observe that the in-plane dependence of cyclotron mass in single-layer 2DES changes with carrier density and perpendicular magnetic field in the low electron density sample, which cannot be explained by the semiclassical calculation.

Effect of parallel high magnetic field on two-dimensional electron gas investigated by ballistic electron transport

Abstract We have studied the magnetic electron focusing effect (MEFE) and Weiss oscillations (WO) on two-dimensional electron gas (2DEG) in the magnetic fields parallel to the 2DEG plane ( B || ). The distortion of the Fermi loop in GaAs/AlGaAs 2DEG induced by the B || is observed in both MEFE and WO.