C.J. Muller

Experimental observation of the transition from weak link to tunnel junction

Abstract An extension to Morelands break junction technique is developed in order to obtain a clean and stable, mechanically adjustable junction. As a function of an externally applied force the coupling of two electrodes can be varied in vacuum. Experiments are described of a junction with niobium electrodes at 4.2 K which undergo a continuous change in normal resistance R N , from 1 to 10 9 Ω upon applying an increasing force. In this resistance range we discern a transition from a weak link regime to a tunnel regime. The current voltage ( I–V ) curves are reproducible upon adjustment changes in the whole resistance range. In the weak link regime the two electrodes of the junction are in physical contact with each other. The product of the critical current and normal resistance is compared with predictions of Ambegaokar-Baratoff and Kulik-Omelyanchuk. The product of the excess current and normal resistance shows a logarithmic increase for low R N values and decreases for the highest R N values in the weak link regime. Subharmonic gap structure, originating from multiple Andreev reflections is observed over a wide range of R N . In the transition regime the two electrodes are not in contact but there is still a large overlap of the superconducting and quasiparticle wave functions. In this regime a finite slope of the “critical current part” in the current voltage curve is observed. The I–V curves show features characteristics for both a weak link and a tunnel junction. In the tunnel regime there exists a vacuum gap between the electrodes and the Josephson coupling is suppressed. A considerable subgap current is observed, where the product of the subgap current and normal resistance is constant over almost four orders of magnitude of R N . A decreasing conductance near zero bias shows up in this regime. The normal resistane exhibits an exponential behaviour upon variations in the vacuum gap. The absolute stability of the distance between the two junction electrodes is estimated to be better than 0.5 pm over a 100 mV voltage range.

The magnetic field dependence of the low temperature specific heat in single crystals of YBa2Cu3O7

Abstract Specific heat measurements in magnetic fields up to 5 T and at temperatures below 1 K are performed on single crystals of YBa 2 Cu 3 O 7−δ with a transition temperature of 89 K. The specific heat goes through a maximum with increasing magnetic field strength. This behavior and the temperature dependence of the zero field data, C = aT n + bT 3 and with n = 0.42, are characteristic for localized spins coupled via random Heisenberg antiferromagnetic exchange. The results are analyzed with the semi-empirical random exchange model of Bulaevskii. Only two free parameters are available, which are completely determined by the zero-field data. Using these values the field behavior is correctly predicted. Representative data of other groups are analyzed with the same model. Defect states that are not related to the high- T c mechanism are seen as an origin of this excess contribution to the lattice specific heat.

Reduction of the Andreev scattering processes in ultra-small superconducting point contacts with direct conductivity

Abstract We report on measurements using ultra-small superconducting point contacts, variable in size, with electrodes formed with the elements Pb, Nb, Sn, In, and Ta. We focus on the behavior of the critical current-normal resistance product as a function of constriction size. When normalized to the gap value, a material-independent, uniform reduction of this product is found when the constriction size is reduced towards atomic dimensions. We present a description, where the destruction of interference from Andreev reflected particles in the contact leads to a reduction of the critical current, thus explaining the reduction of the critical current-normal resistance product.

Observation of the “skew supercurrent” in Josephson vacuum barrier tunnel junctions as a function of the junction normal resistance

Abstract A mechanically controllable break junction is used to obtain the smallest possible Josephson junction, formed by two superconducting electrodes, which are separated by a thin subatomic vacuum barrier. The current voltage characteristics show a “skew supercurrent”, a supercurrent-like feature with a finite resistance and a “critical skew current value” It at a finite voltage Vt. The skew supercurrent is examined over a large normal resistance range by adjustment of the vacuum barrier thickness. In the studied range of six orders of magnitude of the junction normal resistance a crossover from a clear zero voltage supercurrent to a skew supercurrent is observed, for which Vt remains constant.

Conductance and supercurrent discontinuities in atomic size point contacts

Abstract A technique is described, which uses the adjustable stress in a bending beam to control the constriction diameter between two electrodes. The controllability of the constriction diameter is useful when a large range in the normal resistance of different junctions is required for a systematic study. In the high resistance point contact regime quantum size effects are studied in metallic and superconducting constrictions. When charging the constriction diameter reproducible steps in the conductance of the order of 2e 2 h are observed. For superconducting point contacts concominant jumps in the supercurrent of order eΔ h are seen. For contacts adjusted at a step the conductance switches in time between two values, which are interpreted as “two level fluctuations” in the site of a single atom in the constriction.

Anomalies in the superconducting properties of the misfit layer compound (BiS)1.08TaS2

Superconducting weak link junctions of (BiS)1.08TaS2 single crystals are investigated using a novel weak link production technique. Previous and present measurements of the superconducting properties of (BiS)1.08TaS2 indicate the presence of two different critical temperatures at which transitions take place. Experimental observations are presented supporting the assertion that a transition from bulk (T<0.75 K) to 1D superconductivity (0.75 K<T<1.05 K) takes place. Differential resistance measurements of the (BiS)1.08TaS2 weak link junctions indicate the presence of superconducting fluctuations up to temperatures as high as 2.4 K, three times the bulk transition temperature. Furthermore, in these measurements a rich peak structure is observed at low temperatures.

One atom point contacts.

Abstract Using the recently developed mechanically controllable break junction technique, the transition between vacuum tunneling and metallic contact is studied for copper, aluminium and platinum.