P.J. King

Low-resistance electrical contacts to single crystal BSCCO whiskers

We have developed a method for preparing low-resistance contacts to single-crystal whiskers of the 2:2:1:2 phase and of mixed 2:2:1:2/2:2:2:3 phases of BSCCO, and have studied the contact resistivities over the temperature range from 50 K to 300 K. The measurement technique uses a simple three contact geometry requiring a single voltage measurement. The values of the contact resistivities obtained in this work are lower than the great majority of those previously reported for the BSCCO superconductors, making the method suitable for applications which include the nanofabrication of device structures based on micrometre-sized single crystals and single-crystal whiskers. The anomalous values of contact resistivity found in mixed phase whiskers at temperatures between the transition temperatures of the two phases is attributed to spreading resistance and anisotropy.

Investigations of the in-plane anisotropy and the critical behaviour of 10°-tilted YBa2Cu3O7−δ films grown upon (106) SrTiO3 substrates

Abstract We describe the electrical properties of 10°-tilted YBa 2 Cu 3 O 7− δ (YBCO) films grown epitaxially upon as-received (106) SrTiO 3 substrates. (106) substrates which have been annealed at sufficiently high temperatures may exhibit a regular “step-and-terrace” surface structure, while un-annealed substrates (UAS) have more irregular terracing. We show that both the normal state and the superconducting properties of films grown upon UAS films vary considerably from the reported properties of films grown upon annealed substrates (AS films). The normal state in-plane anisotropies of UAS films are close to that of a near-perfect single crystal, while the in-plane anisotropy falls smoothly towards unity as the temperature is lowered through the superconducting regime. The critical currents of UAS films are lower than those of AS films. The superconducting behaviour of UAS films is consistent with a reduced density of extended defects including anti-phase boundaries, defects to which many of the properties of AS films have been attributed. We have also studied the properties of the phase-transition-like electrical behaviour found in UAS films, and observed good scaling collapse of the data under an algorithm based on a continuous thermodynamic phase transition such as that between a vortex liquid and a vortex glass. We find that the transition temperature, T t , and the scaling exponents ν and z do not depend upon the direction of the measurement current, but that the scaling is not “universal” since the exponents depend upon magnetic field and upon film thickness. The values of the exponents are far higher than might be expected from a continuous thermodynamic phase transition. The scaling of data for the track of a film which has been etched shows that both the exponents and the scaling functions are strongly influenced by chemical treatment.

The attenuation of hypersonic surface waves at low temperatures

Abstract A general formula for the attenuation of a surface wave by interaction with thermal phonons is given. It is argued that the attenuation coefficient will often be qualitatively similar to that of the slow shear wave propagating in the same direction.

Four contact electrical measurements on the anisotropic high-Tc superconductors: voltage reversals

Abstract We describe three mechanisms for the anomalous voltage reversals which are occasionally observed during four contact current-voltage measurements on the high temperature superconductors. Voltage reversals have been frequently observed for whiskers of the bismuth strontium calcium copper oxide (BSSCO) compounds, and we describe measurements made on high quality whiskers of the 2:2:1:2 and 2:2:2:3 phases. A simple model based on extreme anisotropy plus offset contacts can explain the gross features of four contact measurements in the ohmic flux-flow regime, but not the wealth of reversal phenomena found for the IVVI configuration in the lower temperature irreversibility region.

The scaling of the current voltage characteristics of epitaxial high Tc thin films

Abstract The Kosterlitz-Thouless theory for the electrical dissipation in the high temperature superconductors predicts scaling of the current voltage characteristics above the phase transition temperature but not below. We give an expression for the universal scaling curve which provides an excellent fit to data for epitaxial YBa 2 Cu 3 O 7 thin films taken in zero applied magnetic field. Surprisingly, the expression also provides an excellent fit to data taken in finite magnetic fields.

Scaling properties of the electrical behaviour of 2:2:1:2 BSCCO whiskers

Abstract The electrical transport of single crystal whiskers of the 2:2:1:2 phase of Bismuth Strontium Calcium Copper Oxide (BSCCO), and of very small transverse dimensions, has been studied as a function of temperature and magnetic field. Over a very wide range of magnetic fields the current voltage characteristics obey a simple but powerful scaling relationship. This scaling has made possible a very detailed study of the similarities between that data and the predictions of the thermally activated flux creep (TAFC) model of Kim and Anderson. At lower fields and currents the temperature and magnetic field dependence of the barrier height displays typical behaviour for that class of model, but the flux-bundle parameters deduced are temperature dependent. At high fields and high currents the observed behaviour differs very significantly from the TAFC prediction.

The nature of the phase transition found in the electrical characteristics of high-Tc thin films

The current-voltage characteristics of high-quality YBCO thin films deposited onto MgO have been measured in both zero and finite applied magnetic fields. The wide dynamic range of the measurements, the high experimental precision and the observation of extremely well defined power law behaviour below the phase transition temperature have enabled the authors to demonstrate that vortex glass scaling does not occur, and that 2D vortex unbinding provides a better account of the results.

The scaling of the current voltage characteristics of the YBa2Cu3O7:Y2BaCuO5 composites

Abstract We have investigated features in the current voltage (I—V) characteristics of YBa2Cu3O7:Y2BaCuO5 composite sinters which indicate the occurrence of a second-order phase transition. For each sinter composition similar families of characteristics are found for a wide range of magnetic fields. The relation between V and I is seen to be of the form V = f(I, ζ(B, T)) where a single parameter ξ contains all of the field and temperature dependence. The locus of points in the B, T plane for which ζ(B, T) is a constant have been examined, and a low field and a high field regime have been identified. Samples of different composition show families of I—V characteristics which scale onto one another in a simple way, hence we are led to suppose the V = f(I, ζ(B, T, p)) where p is the dilution fraction. Data taken at temperatures below the phase transition agrees well with a simple scaling picture of dissipation close to a second order phase transition, with exponents which are independent of magnetic field an...

Helical current flow along twist boundaries in YBCO thin films

Current may flow in helical paths along the boundary between two tilted anisotropic conductors in circumstances when current flow in either conductor alone is restrained by the extreme anisotropy. We are able to explain number of features of the electrical properties of (103)/(013) oriented thin films deposited onto (110) substrates in terms of helical current paths involving the twist boundaries. These include the absence of weak link behaviour in the superconducting state for current flow along the substrate direction, the temperature dependence of the normal state resistivity for current flow in that direction, and the normal state electrical anisotropy. We predict that there are circumstances where currents will preferentially circulate around the boundaries of a tilt domain. The existence of these natural conduction loops may explain the period of recently observed quantum-interference behaviour in these films.

Anomalous dependence of the dissipation upon magnetic field angle in layered high Tc superconductors

Abstract We have investigated the dependence of the electrical dissipation upon the magnetic field angle in narrow single-crystal bismuth strontium calcium copper oxide (BSCCO) whiskers of the 2:2:1:2 phase. In the ohmic flux flow regime, which occurs at lower currents and for temperatures close to T co , we find unusual angular dependences. As the magnetic field angle is varied in the bc -plane close to H | b we observe a single minimum in the resistance, two minima with a central maximum, three minima and two maxima, or a single minimum with shoulders, depending upon the temperature. Scaling shows that these features depend only upon the magnitude of the magnetic field component parallel to the bic -axis. We propose the following explanation. At very low fields the dissipation is determined by thermally excited pancake vortices. As an applied bic -axis field is increased vortices enter in the very mobile vortex gas phase and the dissipation rapidly increases. As the field is further increased the vortices enter the solid phase and the dissipation falls due to reduced mobility. At far higher fields the dissipation increases with increasing vortex numbers.