L. R. Testardi

Anomalous laser‐induced voltages in YBa2Cu3Ox and ‘‘off‐diagonal’’ thermoelectricity

Large anomalous laser‐induced voltages observed in YBa2Cu3Ox are explained as the result of ‘‘off diagonal’’ thermoelectricity, an uncommon phenomenon which may only occur in low symmetry environments. The effect is accompanied by a large electrical impedance transformation and the coupling of electrical and thermal currents in orthogonal directions. It thus offers new thin film applications for power generation, cooling, heat pumping, heat flow measurement, and the fast self‐powered sensing of optical and other radiation energy absorbed in thin layers.

Characterization of the structural and magnetic ordering of Fe3O4/NiO superlattices grown by oxygen-plasma-assisted molecular-beam epitaxy

Oriented single‐crystalline thin films of NiO and Fe3O4 and Fe3O4/NiO superlattices have been grown on cleaved and polished substrates of MgO(001) using oxygen‐plasma‐assisted molecular‐beam epitaxy (MBE). We report the growth mode and structural characterization of these films using in situ RHEED and ex situ scanning electron microscopy and x‐ray diffraction, and their magnetic characterization using SQUID magnetometry. Also reported are preliminary results of magnetotransport measurements. MgO has a very small lattice mismatch to the cubic rocksalt structure of NiO and to the half‐unit‐cell dimension of the spinel structure of Fe3O4. Pseudomorphic growth of superlattices consisting of alternating layers of NiO and Fe3O4 with repeat wavelength down to 17 A and of single thick layers of either of these materials are readily obtained. The grown films exhibit cubic single‐crystalline symmetry in registry with the substrate, with sharp interfaces and strongly layer‐thickness‐dependent strain. RHEED pattern e...

Modulated electric conductivity in Fe3O4/NiO superlattices

High quality Fe3O4/NiO multilayered structures with nominal artificial periodicity 17 A/ 17 A and 34 A/34 A have been made by molecular beam epitaxy. The conductivity of the modulated structures is orders of magnitude higher for current parallel to the layers than for the transverse direction, reaching a ratio of 106−a value which is comparable with the largest known anisotropy for any material. This strong anisotropy shows that a like modulation of electrical conductivity has been achieved over distances as small as tens of A and also suggests the possibility of voltage controlled superlattice phenomena.

Anomalous behavior in transport critical current density of YBa2Cu3O7−x and YBa2Cu3O7−x‐Ag bulk samples

Transport critical current density Jc versus temperature measurements on bulk YBa2Cu3O7−x and YBa2Cu3O7−x‐Ag samples were carried out. An anomalous bifurcation in Jc‐T curves was observed at temperatures between 55 and 88 K. The bifurcation was introduced by application of a transient pulse. The x‐ray, scanning electron microscopy, and electron microprobe analysis results on these samples are presented.

Transport critical current densities of partially aligned bulk samples of YBa2Cu3O7−x superconductors

Temperature and magnetic field dependence of critical current densities (Jc) of partially oriented bulk YBa2Cu3O7−x samples are presented. A series of partially oriented samples has been fabricated by repeating the steps of the solid‐state reaction method. X ray and Raman studies detect the orientation of the grains on the surface and in the bulk of the samples. Jc increases with the degree of alignment of the microcrystals, up to a factor of three. The effect of partial bulk‐preferred orientation on the critical current anisotropy is also studied.

Magnetic ordering and electric polarizability of Zn‐doped La2CuO4+δ single crystals

Magnetization, magnetic susceptibility, and low‐frequency dielectric constant of La2Cu1−xZnxO4+δ (x=0, and 0.07) single crystals were measured. Results obtained in this work show that Zn doping eliminates spins from CuO planes, thus, reducing the correlation length, ξ2D. However, unlike Sr doping, magnetic dilution due to Zn doping is a very local effect, thus suppresses TN relatively slowly and shows little impact on metamagnetic behavior. Our data marks a difference from some results previously reported which claimed a stronger effect on Zn doping on the magnetic properties. It is interesting that Zn doping shows a large influence on the dielectric properties, and resulting in a significant change in the dielectric constant Kc. Most remarkably, the simultaneous polarization and magnetization ordering, described in this study, suggests a coupling of magnetic ordering and electric polarizability in the oxides.

Electronic Properties of OMVPE Grown films of YBa 2 Cu 3 O 7-δ on 1” LaAlO 3 Substrates

Films of YBa 2 CU 3 O 7-δ have been grown on 1” LaAlO 3 by OMVPE utilizing M(tmhd) n (M = Ba, Cu: n = 2; M = Y: n = 3; tmhd = 2,2,6,6-tetramethylheptane-3,5-dionato) as the source materials in a cold wall, vertical rotating disk reactor. The resultant films were characterized by SEM, XRD, T c , J c , and surface profilometry measurements. Relative to laser ablated thin films, the surface morphology was determined to be virtually featureless. In-situ depositions at substrate temperatures of

Measuring the homogeneity of ceramic superconductors by eddy current probing

The homogeneity of a bulk YBa2Cu3Ox sample prepared by standard methods has been determined. Measurements of the conductivity, Tc (by induction), and x‐ray analysis showed no deviation from bulk material previously characterized as ‘‘homogeneous’’ in the literature; the sample was shown to be inhomogeneous by the eddy current method. We use the sample’s response to multifrequency eddy current probing as a measure of homogeneity. This response is calculated from measurements of the complex impedance of a coil, which contains the sample, as a function of frequency. The homogeneity of the sample after being quenched in air from three successively higher temperatures (650, 700, 800 °C) has also been determined. A rough conductivity profile in one dimension is obtained for all four sample conditions.

Observation of a propagating normal/superconducting interface via I‐V analysis in YBa2Cu3Ox

Current‐voltage (I‐V) measurements for the high‐temperature superconducting phase of polycrystalline YBa2 Cu3 Ox show loop behavior for temperatures below Tc . A thermal model has been constructed which shows that the loop is caused by a normal/superconducting interface, created by the heat at the solder junction, propagating down the sample. The importance of this effect to device applications and to critical current measurements is briefly discussed.

Non-linear interfacial resistance in an indium-YBa2Cu3O7−x contact: Andreev-like behavior in a macroscopic interface

Abstract The current-voltage characteristics of an ultrasonically soldered indium contact made to polycrystalline YBa 2 Cu 3 O 7− x is reported. For temperatures below the superconducting transition temperature T c , the conductance is voltage dependent and has two distinct regimes, of high conductance σ 1 at low voltage and low conductance σ 2 at high voltage. Both the conductances and the break voltage increase rapidly for T T c with | dσ 1 dT| → ∞ at T c . The magnitude of the break voltage is roughly half the BCS superconducting energy gap. The excess high conductance is magnetic field dependent and can be eliminated in fields of ∼ 100 G. A field latched “off” state for the excess conductance has been observed. Much of the behavior suggests near ideal Andreev scattering, but under conditions where this would not be expected.