T. W. Jing

Gold contacts on superconducting crystals of YBa2Cu3O7 with very low contact resistivity

By cleaving crystals of YBa2Cu3O7 in high vacuum and evaporating Au in situ, we have fabricated Au‐YBa2Cu3O7 junctions with very low contact resistance. Below Tc, the specified contact resistivity drops by three decades to an estimated value 2×10−9 Ω cm2 at 87.3 K. We emphasize the importance of correcting for spreading resistance in such studies. We also discuss the ideal contact resistivity, and possible contributions to the measured resistance very near Tc.

Andreev reflection and the proximity effect in Auz.sbnd;YBa2Cu3O7 junctions with ultra-low interface resistance

Abstract Gold contacts to YBa 2 Cu 3 O 7 crystals with specific contact resistivity under 10 −10 ωcm 2 are described. A gap structure of 10 mV is seen in the Andreev reflection signal. We discuss the possibility of superconductivity induced in Au by the proximity effect.

The Hall Effect and Magnetoresistance of the High-Temperature Cuprate Superconductors

We show that underlying the unusual temperature dependence of R H is a Hall angle damping rate which varies as T 2, in contrast with the linear-T dependence in the transport scattering rate. The effect of impurities on this rate is discussed. The dephasing rate for carriers in Bi 2201 is measured by low temperature magnetoresistance, and found to vary as the cube root of T from 0.4 to 20 K. Spin dependent effects in the magnetoresistance are discussed.

Transition from activated to diffusive behavior in the vortex-liquid state in YBa2Cu3O7

Abstract In the vortex-liquid state in YBa 2 Cu 3 O 7 , there exist two distinct dissipative states separated by a field H > k . Below H k , the resistivity is strongly field-activated, with a prefactor that is anomalously enhanced over the normal-state resistivity ϱ N by ≈12 orders of magnitude. At H k , the prefactor collapses to a temperature-independent value equal to ϱ N , and the vortex motion becomes diffusive. The existence of the H k boundary is also reflected in the anomalous behavior of the Hall resistivity.

Carrier Scattering Rates Measured by Hall Effect and Magnetoresistance in the High-Tc Oxides

Transport measurements are a useful way to probe the electronic properties of the oxide superconductors because they couple directly to the relevant charge excitations. The unusual linear-temperature variation of the resistivity has received much attention. However, other transport properties are also anomalous when compared with conventional metal behavior. In this report, we survey some recent Hall effect and magnetoresistance measurements on single crystals of YBa2Cu3O7 (YBCO) and Bi2Sr2CuO6 (Bi 2201).

Andreev reflection and tunneling results on YBa2Cu3O7 and Nd2−xCexCuO4−y single crystals

We will describe some results from recent experiments on Andreev reflection and tunneling in the superconductors YBa2Cu3O7 (“123”) and Nd2−xCexCuO4−y (“NCCO”)1,2. The interface resistance between gold and 123 crystals has been greatly reduced using a technique3 which does not involve high temperature annealing. Gold is evaporated in situ onto a crystal cleaved in high vacuum. Using the specific contact resistivity p[] as a gauge of the interface quality, we find that p[] in these junctions approaches the ideal value p [] 0 derived by counting the number of conduction channels in 123. (p[] is the product of the contact resistance R c and the contact area.) With these junctions, we have studied the decrease of the interface resistance near T c . We have also observed4 Andreev reflection of injected carriers at low temperatures T in these junctions. The T depedence of the Andreev signal is studied from 4.2 to 80 K. In the “T’ phase” superconductor1,2 NCCO we have used the same cleaving technique to make Giaever tunneling junctions with Pb as the counter electrode. We observe rather clearly the gap in the tunneling spectrum5. The dependence of the spectrum on temperature and magnetic field has been studied. A number of anomalous features are observed. These are described below.

Tunneling and Transort Experiments on Single-Crystal Nd2-xCexCuO4-y and YBa2Cu3O7

Several anomalous features are observed in tunneling spectra obtained on cleaved crystals of Nd2-xCexCuO4-y. The conductance curve is strongly asymmetric in the bias, and the gap parameter △ increases as the temperature is wanned towards T c . In one sample a weak magnetic field completely suppress distinctive structures which appear at energies above the gap. Hall measurements in Nd2-xCexCuO4-y crystals at low temperatures provide strong evidence for an anomalous contribution to the Hall scattering in the normal state. The Andreev reflection spectrum in YBa2Cu3O7 crystals is also reported.

Andreev Reflection, Thermal Conductivity, Torque Magnetometry, and Hall Effect Studies on High-Tc Systems

We survey recent experiments which probe the interface between the high-T c oxides and Au (Andreev reflection, and proximity induced superconductivity). We also discuss experiments which measure the anisotropy of 123 (thermal conductivity, torque magnetometry) and the Hall effect in the Bi- and Tl-based superconductors.