Yosihisa Takeuti

Damping Factor of the Riedel Peak in Bridge-Type Josephoson Junctions

The Riedel peak in the Josephson current in bridge-type point-contact Josephson junctions has been studied by measuring the frequency dependence of the maximum height of the fundamental ( n =1) Shapiro-step in the energy range near \(\hbar\omega{\sim}4\varDelta_{0}\) with a submillimeter-wave laser, where Δ 0 is the real part of the energy gap. The imaginary part of the energy gap, Δ 2 , has been estimated from the observed rounding of the Riedel singularity. As a result, the damping factor \(\delta{=}\varDelta_{2}/\varDelta_{0}{\gtrsim}0.05\) has been obtained. The value of δ∼0.05 seems to be the lowest value which is intrinsic to bridge-type junctions. It has been also found that the value of δ is large in the junctions which show strong heating effects.

On the Reflection and Transmission Coefficients Associated with a Lowest Exciton Level (in the Case of Weak Coupling between Polariton and Phonon)

With use of the photon Greens function, the reflection and transmission coefficient associated with a lowest exciton level based on the “polariton” picture was calculated. To consider the effects of the spatial dispersion due to the translational motion of excitons, the “specular” and “diffuse” boundary conditions are adopted. The calculation based on a simple model for polaritons shows that the minimum of the transmission coefficient arises in the neighbourhood of the transverse exciton energy, and the hump may be expected at the longitudinal exciton energy.

Frequency Dependence of the Submillimeter-Wave Response in the Tunnel-Type and Bridge-Type Josephson Junctions

We have fabricated ultrasmall tunnel-type Josephson junctions with Nb–SnOx–Sn film point-contacts. Using these junctions, we have measured for the first time the Shapiro steps in the submillimeter-wave region and have compared the strength of steps of this junction with that of the bridge-type Nb–Sn ingot point-contacts. We found that the tunnel-type junction shows a sharper Riedel peak than the bridge-type junction.

Correlation between Types of Junction and Submillimeter-Wave Responses in Point-Contact Josephson Junctions

It has been found that point-contact Josephson junctions can be clearly classified into bridge-type and tunnel-type by introducing new parameters characterizing their I-V curves. Nearly-ideal tunnel-type junctions with Nb–SuOx–Sn (Film) point-contacts were obtained. Tunnel-type junctions showed larger gap energies than bridge-type junctions in Nb–SnOx–Sn and Nb–NbOx–NbN point-contacts. The submillimeter-wave responses were measured in Nb–Nb point-contacts, and it was found that the contacts with the best performance were bridge-type junctions with a sharp gap structure; tunnel-type junctions showed rather bad performance. On irradiation with laser light, the center of the Shapiro step moved to the lower-current side in bridge-type junctions but to the higher-current side in tunnel-type junctions. These differences can be explained by differences in the heating effects between bridge-type and tunnel-type junctions.

Submillimeter-Wave Response of Nb-YBaCuO Point-Contact Josephson Junctions

Shapiro steps in the submillimeter-wave (SMMW) range were observed in junctions made of a high-Tc superconductor, YBa2Cu3Ox (YBaCuO), and niobium. Junctions were made by pressing a Nb needle on a sintered YBaCuO disk. Shapiro steps up to the fifth order were observed for SMMWs of wavelength 871.6 µm (frequency 344.0 GHz). The dependence of the Shapiro step width on the SMMW power is in accordance with the Bessel function dependence. For SMMWs of wavelength 570.6 µm (525.4 GHz), Shapiro steps up to the second order were observed.

Frequency Dependence of the ac Josephson Effect in Nb Point Contacts in the Submillimeter-Wave Region

We have measured the strength of the ac Josephson effect in Nb point contacts using an optically-pumped submillimeter-wave laser. We found that the frequency (i.e., corresponding step-voltage) dependence of the ac Josephson effect is clearly classified into three regions. In region I (V\lesssim1 mV) the voltage dependence is consistent with the prediction of the resistively-shunted junction model, while it is similar to the prediction of the frequency-dependent self-coupling theory in region II (1 mV\lesssimV\lesssimVg (gap voltage)). In region III (V\gtrsim Vg), we found an exponentially damping behavior with increasing voltage.

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.

Simulation of Shapiro Steps in a Josephson Junction Based on the Microscopic Theory of the Tunnel Junction

Shapiro steps in a Josephson tunnel junction were investigated by the simulation based on the microscopic theory of the tunnel junction. A nonzero pair-breaking parameter, δ=0.1, was assumed to make the computation tractable. The dependence of the step width of the zeroth- and the first-order Shapiro steps on the rf power was calculated for the frequency range, 0.2fg≦f≦1.5fg, where fg is the energy gap frequency. The steps are unstable at low frequencies, f\lesssim0.7fg, and for large values of the junction capacitance. They become stable for larger frequencies. The steps are stable, on the whole, for small capacitance. The frequency dependence of the first maximum of the first-order Shapiro step, I1MAX, was obtained. It shows the Riedel peak, with even larger height than the prediction of the theory assuming a constant voltage source for a small capacitance.

Metallic impurity conduction and metal-nonmetal transition in n-InSb at low temperature

Abstract The resistivities of n-InSb in the metallic impurity conduction range were investigated down to 20 mK at zero and 100 Oe of applied magnetic field. Samples in the metallic impurity conduction range were divided into two groups. The resistivities of samples with high carrier concentration take maximum values at several hundreds mK. Below this temperature, the resistivities decrease when the temperature is lowered. On the other hand, the resistivities of samples with low carrier concentration increase when the temperature is lowered. We found that the dominant mechanisms which determine the sign of the temperature coefficient of the resistivities at zero magnetic field are different from each other in the two groups of the samples.