NIS Junctions Research Articles

On the concept of an optimal hot-electron bolometer with NIS tunnel junctions

Optimization of the hot-electron bolometer with a normal metal–insulator–superconductor (NIS) tunnel junction as a temperature sensor has been done theoretically. The responsivity and the noise equivalent power (NEP) of the bolometer are obtained numerically for typical experimental parameters. We demonstrate that electron cooling by the NIS junction, which serves as a thermometer, can improve the sensitivity of the device. This effect is especially useful in the presence of a finite power load. Optimization of the hot-electron bolometer in the presence of a finite power load is discussed. It has been shown that the optimal regime can be realized when thermal conductance through the tunnel junctions is larger than the electron–phonon conductance. The electron–phonon noise is negligible and the NEP is determined mainly by the noise of the NIS junctions because of practically full transference of power of the incoming signal to a readout amplifier through the junctions.

Modified superconductor–insulator–normal metal–insulator– superconductor Josephson junctions with high critical parameters

Superconductor–insulator–normal metal–insulator–superconductor (SINIS) and SINS′NIS junctions (here S, I, and N denote superconductor, insulator, and normal metal, respectively) were fabricated using Nb/Al technology and characterized at low temperatures. It is shown that asymmetric SINIS junctions with different transparency of the two tunnel barriers may have higher critical voltages than analogous symmetric junctions at 4.2 K. Also, SINS′NIS junctions were fabricated and investigated. If the thickness of the S′ layer is very thin, these junctions have quasiparticle current–voltage characteristics similar to those of conventional SINIS junctions at 4.2 K, but the Josephson critical current densities, and therefore, the critical voltages, are much higher than the corresponding values for conventional SINIS junctions.

Noise in Refrigerating Tunnel Junctions and in Microbolometers

Microrefrigerators based on normal metal–insulator–superconductor (NIS) junctions represent a very attractive alternative to cool the microbolometers and calorimeters for astrophysical observations in space-borne experiments. The performance in such measurements requires a good knowledge of the noise sources in the detectors. In this paper we present detailed calculations of the thermal fluctuations and of the noise equivalent power due to the heat transfer through the NIS junctions or the thermal contact between different subsystems of the detector. The influence of the input optical power—which, in the cases that interest us, is the cosmic background radiation—will also be evaluated. Analytical approximations valid at low temperatures are given.

Trapping of quasiparticles of a nonequilibrium superconductor

We have performed experiments where hot electrons are extracted from a normal metal into a superconductor through a tunnel junction. We have measured the cooling performance of such NIS junctions, especially in the cases where another normal metal electrode, a quasiparticle trap, is attached to the superconductor at different distances from the junction in direct metal–to–metal contact or through an oxide barrier. The direct contact at a submicron distance allows superior thermalization of the superconductor. We have analyzed theoretically the heat transport in this system. From both experiment and theory, it appears that NIS junctions can be used as refrigerators at low temperatures only with quasiparticle traps attached.

Microrefrigeration by quasiparticle tunnelling in NIS and SIS junctions

A solid-state refrigeration method at sub-kelvin temperatures has been developed. It is based on quasiparticle tunnelling between a superconductor and a normal metal, or, between two dissimilar superconducting metals. The refrigerator is fabricated by combining nanolithography and micromachining methods. This technique has been demonstrated in both electron cooling from 0.3 to 0.1 K and in refrigeration of a dielectric platform. We describe a new fabrication method of tunnel junctions in a shadow evaporation configuration using a mechanical mask of silicon nitride.

Quasiparticle interface states in junctions involvingd-wave superconductors

Influence of surface pair breaking, barrier transmission and phase difference on quasiparticle bound states in junctions with d-wave superconductors is examined. Based on the quasiclassical theory of superconductivity, an approach is developed to handle interface bound states. It is shown in SIS' junctions that low energy bound states get their energies reduced by surface pair breaking, which can be taken into account by introducing an effective order parameter for each superconductor at the junction barrier. More interestingly, for the interface bound states near the continuous spectrum the effect of surface pair breaking may result in a splitting of the bound states. In the tunneling limit this can lead to a square root dependence of a nonequilibrium Josephson current on the barrier transmision, which means an enhancement as compared to the conventional critical current linear in the transmission. Reduced broadening of bound states in NIS junctions due to surface pair breaking is found.

Charge transport through superconductor/Anderson-insulator interfaces

We report on a study of charge transport through superconductor-insulator-superconductor and normal metal--insulator--superconductor structures (SIS and NIS junctions, respectively) where the insulator is of the Anderson type. Devices which are characterized by a junction resistance larger than 10 k\ensuremath{\Omega} show behavior which is typical of Giaever tunnel junctions. In structures having smaller resistance, several peculiar features are observed. In the SIS junctions, Josephson coupling is detected over distances much larger then the typical insulator localization length. In addition, a series of resistance peaks appears at voltages of 2\ensuremath{\Delta}/n, where \ensuremath{\Delta} is the superconducting gap. The NIS Junctions exhibit a large resistance dip at subgap bias. We discuss possible interpretations of these findings and suggest that they may result from the presence of high transmission channels through the barrier region.

Tunneling through Ultrasmall NIS Junctions in Terms of Andreev Reflection. A Nonlinear Response Approach

AbstractThe Andreev current through an ultrasmall NIS junction is calculated in a systematic way by means of a nonlinear response approach basing on the elementary Hamiltonian of quasiparticle tunneling. The voltage dependence of current and differential conductance as well as the Andreev conductance are derived for low‐ and high‐impedance environments, respectively.

Reflectionless tunneling through a double-barrier NS junction

The resistance is computed of an NI 1 NI 2 S junction, where N is the normal metal, S the superconductor, and I i the insulator or tunnel barrier (transmission probability per mode Γ i ). The ballistic case is considered, as well as the case that the region between the two barriers contains disorder (mean free path l, barrier separation L). It is found that the resistance at fixed Γ 2 shows a minimum as a function of Γ 1, when Γ 1 ≈√2 Γ 2, provided l ⪆ Γ 2 L. The minimum is explained in terms of the appearance of transmission eigenvalues close to one, analogous to the “reflectionless tunneling” through a NIS junction with a disordered normal region. The theory is supported by numerical simulations.

Tunneling observation of the finite superconducting gap in YBa 2Cu 3O y

Electron tunneling study on YBa 2Cu 3O y was carried out for investigation of an essential superconducting gap structure. Besides usual NIS junction, a kind of micro-break SIS junction was prepared in-situ on a low-temperature scanning tunneling microscope/spectroscope (LT-STM/STS) stage in order to minimize the influence of the surface-degradation. As the result, the observed tunneling spectra showed finite superconducting gap structure with extremely low and flat in-gap density of states, which indicates a quite different character from the gapless one expected by the d-wave pairing theory.

Tunneling observation of optical-phonon mediation in high Tc superconductivity

Tunneling conductance spectrum could be reproducibly observed by point contact NIS junctions on BSCCO(2212) single crystals. Analysis of the fine structure beyond the definite gap structure enabled us to obtain the pair mediating quasi particle spectrum responsible to the high Tc superconductivity. Most of them were attributed to the optical phonons due to the oxygen oscillations in and around the [CuO 2] net-plane.

Hopping contribution to tunneling in NIS junction

The hopping contribution to the current-voltage characteristic ( I– V) of a NIS amorphous tunnel junction containing two impurity localized states in the barrier region is studied using the kinetic equation approach. It is found that the non-linear behavior of the I– V characteristic depends strongly on temperature.

Hopping contribution to tunneling in NIS junction

The hopping contribution to the current-voltage characteristic (I–V) of a NIS amorphous tunnel junction containing two impurity localized states in the barrier region is studied using the kinetic equation approach. It is found that the non-linear behavior of the I–V characteristic depends strongly on temperature.

Gap anisotropy and tunneling in superconductors

The tunneling current flowing through the NIS and SIS junction is discussed. The superconductors are assumed to be two-dimensional metals with a square lattice and with anisotropic energy gap. We argue that due to the anisotropy of the gap the proper identification of the type of junction is difficult and this may lead to strong over- /underestimation of the gap to Tc ratio.

On the theory of electron tunneling in NIS systems with paramagnetic impurities

Current-voltage characteristic of NIS junctions has been studied when there are paramagnetic impurities in a narrow layer of the superconductor close to the insulator. It is shown that paramagnetic impurities in such systems considerably change the current-voltage characteristic in the narrow range of the applied potentialeV∼Δ andV∼0.