Z. Z. Wang

Transport and tunneling studies on single crystals of YBa2Cu3O7

Abstract Using a flux technique we have grown crystals of YBa 2 Cu 3 O 7 which are superconducting above 90 K as-grown. In 10% of these crystals large orthorhombic single domains exceeding 100×100 μm 2 are observed using polarised light. The conductivity anisotropy (≈100 at 290 K to ≈300 near T c ) indicates a strikingly two-dimensional electronic state. Anisotropy in the thermopower is also reported. Tunneling into these crystals provides direct evidence that the electronic spectrum is different from the BCS spectrum. A (possibly) novel spin-dependent gap which scales linearly with field is described. Finally, paraconductivity studies on crystals with transition widths δT ≈ 0.2 K are summarised.

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.

On the the crystal growth and chemistry of the new electron-type superconducting oxides

Abstract The effect on transport and superconducting properties produced by changes in x and y in the Nd2−xCexCuOy compound were studied in both polycrystalline ceramics and single crystals. Thermogravimetric analysis shows that the total oxygen content y for the as-prepared samples is always greater than 4 (i.e. presence of interstitial oxygen) whereas for the reduced sample y becomes equal to or smaller than 4 only when x is 0.15 or greater. This is the range of Ce content for which the material superconducts. For a material with Ce x=0.15 the superconducting properties can be varied reversibly by changing the oxygen content. In addition, we propose that the oxygen in these materials can be either ordered or disordered, thereby affecting the transport properties. Platelet-like crystals of Nd2−xCexCuOy with x=0 to 0.18, have been grown via a flux technique. Those having a Ce content between 0.14 and 0.17 are superconducting with the sharpest transitions (Tc = 21 ± 1K) for x = 0.14. Metallic-like behavior above Tc, with a linear temperature dependence above 150K was observed on all the crystals. Below 30K, the in-plane resistivity is independent of T. The Hall coefficient is sensitive to processing conditions but is usually negative above 100K. The upper critical field with field along the c-axis is 6T at 4.2K.

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.

Mid-infrared reflectivity of single crystal Nd2-xCexCu4O4-y

Abstract We have measured infrared reflectivity of single crystals of Nd 2−x Ce x Cu 4 O 4−y in the range between 0.05 eV and 1.5 eV. The superconducting or nearly superconducting crystals studied show a very high mid- IR reflectivity. Reflectivity of the samples is fitted much better with a Drude model, which yielded very similar fitting parameters (ω p ∼ 2.5 eV) for all samples, than YBa 2 Cu 3 O 7 or Bi 2 Sr 2 CuO 6 . However, a deviation from a Drude model was evident in the very low frequency and the fitting was considerably improved with a Drude-Lorentz model in both fittings of the reflectivity and the frequency dependent conductivity. Further, no temperature dependence of the reflectivity above 0.2 eV was observed between 25K and 200K. The present study suggests the inapplicability of the Drude model to this system and supports the existence of the mid- IR absorption.

Pinning modes and interlayer correlation in high-magnetic-field bilayer Wigner solids.

We report studies of pinning mode resonances in the low total Landau filling (nu) Wigner solid of a series of bilayer hole samples with negligible interlayer tunneling and with varying interlayer separation d. Comparison of states with equal layer densities (p,p) to single layer states (p,0) produced in situ by biasing, indicates that there is interlayer quantum correlation in the solid at small d. Also, the resonance frequency at small d is decreased just near nu = 1/2 and 2/3, indicating the importance in the solid of correlations related to those in the fractional quantum Hall effects.

Discontinuous jump in transverse magnetization of the flux lattice in YBa2Cu3O7 crystals observed by torque magnetometry

Abstract We have measured the torque in YBa2Cu3O7 crystals when the applied field is rotated at angle to the c-axis in the superconducting state. Below a characteristic field Hm we observe a novel discontinuity in the transverse magnetization when exceeds 90°. We interpret this jump as a giant reconfiguration of the flux lattice in response to the field rotation.

Unequal layer densities in bilayer Wigner crystal at high magnetic fields

We report studies of pinning mode resonances of magnetic field induced bilayer Wigner crystals of bilayer hole samples with negligible interlayer tunneling and different interlayer separations d, in states with varying layer densities, including unequal layer densities. With unequal layer densities, samples with large d relative to the in-plane carrier-carrier spacing a, two pinning resonances are present, one for each layer. For small d/a samples, a single resonance is observed even with significant density imbalance. These samples, at balance, were shown to exhibit an enhanced pinning mode frequency [Zhihai Wang et al., Phys. Rev. Lett. 136804 (2007)], which was ascribed to a one-component, pseudospin ferromagnetic Wigner solid. The evolution of the resonance frequency and line width indicates the quantum interlayer coherence survives at moderate density imbalance, but disappears when imbalance is sufficiently large.

Critical current in single crystalline Nd1.85Ce0.15CuO4 from AC susceptibility

Abstract We report measurements of the temperature dependence of the critical current in a single crystal of Nd 1.85 Ce 0.15 CuO 4 . This dependence is deduced from the variation versus the AC magnetic field of the real part of the magnetic susceptibility at different temperatures. In the framework of the critical state model, we deduce the variation of the critical current from 4.5 K to 15 K.

Temperature dependent Hall effect in Nd2−xCexCuO4 and “60 K” YBa2Cu3Oy single crystals

Abstract The Hall coefficient is studied in single crystals of the two superconductors Nd2−xCexCuO4 and “60 K” YBa2Cu3Oy. The strong temperature dependence is discussed. We also report the temperature dependence of the out-of-plane resistivity in the latter.