Seizo Morita

Low-temperature characteristics of Matsushita carbon resistors with and without magnetic fields

The resistance versus temperature (RT) characteristics and magnetoresistance (MR) of 1/8 W nominal 68 Ω and 100 Ω Matsushita carbon resistors of grade ERC-18SG have been measured at temperatures between 8 and 0.05 K, in magnetic fields up to 84kOe (84 × 106/4 π Am−1). In both carbon resistors, the RT behaviour is expressed as R(T) = R2exp{(T2/T)1/2} in the high temperature region and as R(T) = R4exp{(T4/T)1/4} in the low temperature region. These relationships are explained as due to variable-range hopping conduction with and without the changing energy between isolated metallic grains, respectively. Negative and positive MRs have been observed in low and high magnetic fields, respectively. The negative MR increases with decreasing temperature at very low temperatures < 0.1 K. These phenomena are discussed in terms of the theory based on the strong Anderson localization.

Supported metal model catalyst surfaces examined by scanning tunnelling microscopy

Topographic and/or barrier‐height images of ultrafine Pt and Au metal particles supported on a vacuum‐deposited carbon film or titanium oxide thin films grown on titanium metal sheets were obtained. The topographic images of colloidal Au particles (5‐nm diameter) adsorbed on a titanium oxide thin film showed a structure elongated in the direction normal to the x scan, indicating their weak interaction with the support surface. The topographic images of Pt vacuum‐deposited on a carbon film showed c. 4‐nm diameter particles, larger than those observed in electron microscopy. The problems inherent to the STM observation of such dispersed metal systems are identified. In the case of Pt particles vacuum‐deposited on titanium oxide film, its barrier‐height image gave better indication of different phases on the surface than its topographic image. The significance of obtaining barrier‐height images along with topographic images for such sample systems is demonstrated.

Submillimeter wave response of tunnel junctions with an insulating barrier containing magnetic impurities

We have investigated PAT (Photon-Assisted-Tunneling) effect induced by SMMW (Submillimeter-Wave) in SIS point-contacts with an insulating barrier containing magnetic impurities: a magnetic barrier. The junctions turned out to have severely suppressed dc and ac Josephson currents, which makes them promising junctions for a high frequency mixer using PAT effect.

Effects of the Anderson localization on magnetoconductivity in metallic n-InSb and n-GaAs

Abstract The magnetoconductivity in metallic n-InSb and n-GaAs has been measured and analyzed with the microscopic theories. In accordance with the theoretical prediction, we have observed various kinks in the T 1 2 (T: temperature) and H 1 2 (H: magnetic fields) dependences of magnetoconductivity which are due to the change of strengths of the localization and Coulomb interaction effects. From these kinks, the values of physical parameters of the Anderson localization have been determined in n-InSb and n-GaAs. The experimental values of parameters are in good agreement with the theoretical prediction. In the regime where the usual microscopic theories on the Anderson localization cannot be applied, many interesting phenomena have been observed in magnetoconductivity.

Nonlinear response of tunnel-type Josephson junctions at submillimeter-wave frequencies

We have fabricated tunnel-type Josephson junctions with very small junction area by pressing the tip of a sharpened Nb wire onto a thinly oxidized Sn film, and studied their submillimeter-wave responses. It turns out that the junctions have a very high cutoff frequency, and are suitable for the investigation of the response to the submillimeter waves with the frequency higher than 1 THz. From the frequency and laser-power dependence of the Shapiro step, it is found that the tunnel-type junction shows much larger higher-order steps, and accordingly has much stronger nonlinearity than the bridge-type junction in the submillimeter-wave region. The large enhancement of the steps due to the Riedel singularity is also observed in the tunnel-type junction. In addition to the Shapiro steps, photon-assisted tunneling steps are observed clearly, reflecting the large and sharp gap structure in the dc IV curves of the tunnel-type junctions.

Scanning tunneling microscopy of metal surfaces in air

Abstract We have constructed a scanning tunneling microscope (STM) for operation in air. We obtained surface images of a commercial titanium (Ti) plate, a barrier layer of anodic aluminum oxide (Al 2 O 3 ) and platinum (Pt) fine particles supported on a TiO 2 /Ti plate. We found the following results: (1) The resolution of our STM is better than 10 A in air. (2) The surface image of the barrier layer has a somewhat complicated structure compared with hexagonal cells observed by SEM. (3) The surface of Pt fine particles is flat and shows a sharp and deep cliff down to the TiO 2 /Ti plate.

Time domain simulation of Shapiro steps in Josephson junctions

Using the time domain formulation of the theory of the tunnel junction, we investigated Shapiro steps by digital simulation, under the condition of a constant current source given by I_{dc} + I_{rf} \sin \omegat . The integral kernels for the Josephson and the quasiparticle current were computed assuming a nonzero pair breaking parameter \delta=0.1 , and T = 0K. We obtained I rf dependence of the zeroth and first Shapiro steps, I 0 and I 1 , and the frequency dependence of I_{1}^{\MAX} , the maximum of the first Shapiro step as a function of I rf , for a few values of the junction capacitance. we found the following results. (1) I_{1}^{\MAX}/I_{c} (I c :the critical current), showed the Riedel peak at \omega/\omega_{g} \simeq 1 , where ω g is the gap frequency 4 Δ/h. (2) For \omega/\omega_{g} >1\cdot I_{1}^{\MAX}/I_{c} agrees well with that for the constant voltage bias. (3) As the frequency becomes smaller below \omega_{g} \cdot I_{1}^{\MAX}/I_{c} is severely depressed compared to that for the constant voltage bias.

Characteristic features of the ac Josephson effect above the gap voltage in niobium point contacts

Abstract We have investigated the strength of the ac Josephson effect, IMAX1/IC, above the gap voltage Vg in Nb point contacts. We found that its dependence on voltage is empirically formulated as IMAX1/IC = exp{-α(V-β)}, where the damping constant α has a strong correlation with the magnitude of Vg. From the existence of the threshold at ≈Vg in the damping of IMAX1/IC, we conclude that the exponential damping of the strength of the ac Josephson effect above the gap voltage is caused by the injection of quasiparticles. By finding of the correlation between the gap voltage and the cooling efficiency of the point contact, we confirm that the damping constant α is determined by the cooling efficiency of the point contact.

Anomalous frequency dependence of giant quantum attenuation in Bismuth below 1 K

Abstract Giant quantum attenuation of longitudinal sound waves in Bi has been measured at temperatures down to 8 mK for the case that the electron (n = 0, s = + 1) and hole (n = 1, s = − 1) Landau levels cross simultaneously Fermi level at a field of about 90 kOe. The temperature and frequency dependences of the peak attenuation due to these two levels are expressed by αp ∝ T-μων with μ ⋍ 1 and ν ⋍ 1 at T > 1 K. When T ≲ 1 K the value of μ decreases by decreasing temperature and below 0.1 K, αp takes almost constant value. At T ≲ 0.5 K, ν becomes larger than 1 and maximum value of ν observed is 2 at T ∼ 0.05 K. These features of the attenuation peak at very low temperatures are consistent with what are expected in the fluctuation region of the gas-liquid type transition of the electron-hole system.

Scanning tunnelling microscopy images of noble metal fine particles supported on thin titanium oxide film

Abstract Scanning tunnelling microscopy images of Pt fine particles supported on a thin film of titanium oxide spontaneously formed on a titanium metal surface were obtained, for the first time, using a simple STM unit operating under ambient conditions. The examined samples constitute a double-barriered system for STM: one barrier consists of the probe tip-to-metal particle air gap and the other the titanium oxide thin film. The sample also serves as a model for supported metal catalysts. Problems in obtaining and interpreting STM images of fine metal particles supported on thin oxide films are identified and discussed.