E. Polturak

Highly oriented as‐deposited superconducting laser ablated thin films of Y1Ba2Cu3O7−δ on SrTiO3, zirconia, and Si substrates

KrF excimer laser ablation of an Y1Ba2Cu3O7−δ pellet in 0.1–0.2 Torr of O2 ambient was used to deposit thin superconducting films onto SrTiO3, yttria‐stabilized zirconia (YSZ), and silicon substrates at 600–700 °C. The as‐deposited 1‐μm‐thick films at 650–700 °C substrate temperature were superconducting, without further high‐temperature annealing. All films had a similar Tc onset of ∼92 K but different zero‐resistance Tc of 90, 85, and 70 K for the films on SrTiO3, YSZ, and Si substrates, respectively. Angular x‐ray diffraction analysis showed that all the films were highly oriented with the c axis perpendicular to their surface. Critical current densities at 77 K were about 40 000 and 10 000 A/cm2 for the films on SrTiO3 and YSZ, respectively. Smooth surface morphology was observed in all films, with occasional defects and cracks in the films on YSZ, which seems to explain the lower critical current in these films.

Observation of Magnetic Flux Generated Spontaneously During a Rapid Quench of Superconducting Films

We report observations of spontaneous formation of magnetic flux lines during a rapid quench of YBa(2)Cu(3)O(7-delta) films through T(c). This effect is predicted according to the Kibble-Zurek mechanism of creation of topological defects of the order parameter during a symmetry-breaking phase transition. Our previous experiment, at a quench rate of 20 K/s, gave null results. In the present experiment, the quench rate was increased to >10(8) K/s. The amount of spontaneous flux increases weakly with the cooling rate.

Nonlinear surface impedance of YBCO thin films: Measurements, modeling, and effects in devices

High-Tc thin films continue to be of interest for passive device applications at microwave frequencies, but nonlinear effects may limit the performance. To understand these effects we have measured the nonlinear surface impedanceZs in a number of YBa2Cu3O7−x thin films as a function of frequency from 1 to 18 GHz, rf surface magnetic fieldHrf to 1500 Oe, and temperature from 4 K toTc. The results at lowHrf are shown to agree quantitatively with a modified coupled-grain model and at highHrf with hysteresis-loss calculations using the Bean critical-state model applied to a thin strip. The loss mechanisms are extrinsic properties resulting from defects in the films. We also report preliminary measurements of the nonlinear impedance of Josephson junctions, and the results are related to the models of nonlinearZS. The implications of nonlinearZS for devices are discussed using the example of a five-pole bandpass filter.

Nonlinear microwave surface impedance of YBCO films: latest results and present understanding

Abstract While passive high-temperature superconductors (HTS) microwave devices are finding application in wireless communication systems, the nonlinear surface impedance and intermodulation distortion (IMD) limit wider application. Understanding and controlling the nonlinear microwave response of HTS is important for narrow band, sharp cut-off receive filters, which are susceptible to intermodulation, and high-power transmit filters, which suffer from power-dependent absorption. The sources of these two related nonlinear microwave responses are not understood at present. We discuss the latest experimental results directed toward understanding the nonlinearities. Several experiments have ruled out some of the most likely origins. We discuss measurements of the impedance of films on bicrystals showing that low-angle (

Spontaneous macroscopic magnetization at the superconducting transitiontemperature of YBa2Cu3O7-|[delta]|

A noteworthy feature of the high-temperature superconductors is the unconventional symmetry of the superconducting order parameter. Several experiments have established that the order parameter has a four-fold dx2-y2 symmetry under rotation of the lattice (the order parameter of conventional superconductors is, in contrast, isotropic). An intriguing and much debated possibility is that, in certain cases, an additional imaginary component might be present, having an isotropic s-wave or d xy symmetry. A consequence of a complex order parameter of the form dx2-y2 + idxy is that it would break both reflection (parity, P) symmetry and time-reversal (T) symmetry, a clear signature of which would be the spontaneous appearance of a macroscopic magnetization at the superconducting transition temperature. Broken T symmetry has been reported, but searches for the effects of combined P and T symmetry breaking have so far yielded null results. Here we report the observation of a weak (∼10-5 gauss) magnetic field that appears spontaneously at the superconducting transition temperature of epitaxial thin films of YBa2Cu3O 7-δ. The magnetic signal originates near the edges of the samples. One interpretation for this observation is that the order parameter carries an intrinsic angular momentum, related to the breaking of P and T symmetries, but other possibilities cannot yet be excluded.

INFLUENCE OF POINT DEFECTS ON THE SHEAR ELASTIC COEFFICIENTS AND ON THE MELTING TEMPERATURE OF COPPER

We present molecular dynamics simulations of the influence of point defects on the shear elastic properties of copper. We find that vacancies do not influence these properties at all, while the introduction of interstitials causes a large reduction of the elastic coefficients. The simulations establish a phase diagram of the melting temperature as a function of the density of interstitials. A crystal having no free surface undergoes bulk mechanical melting as a result of the vanishing of C′ ≡ (C11 - C12)/2 once the specific volume reaches a critical value, equal to the experimental volume of liquid phase. This critical volume is history independent, in the sense that it does not depend on whether is it reached by heating the crystal or by adding defects at a constant temperature. These results generalize the Born model of melting for the case where point defects are present.

Molecular dynamics study of melting of the bcc metal vanadium. II. Thermodynamic melting

We present molecular dynamics simulations of the thermodynamic melting transition of a bcc metal, vanadium, using the Finnis-Sinclair potential. Our motivation was to answer the question whether premelting phenomena, extensively studied for fcc metals, are also present in bcc metals. We studied the structural, transport, and energetic properties of slabs made of 27 atomic layers with a free surface. We investigated premelting phenomena at the low-index surfaces of vanadium, V(111), V(001), and V(011), finding that as the temperature increases, the V(111) surface disorders first, then the V(100) surface, while the V(110) surface remains stable up to the melting temperature. Also, as the temperature increases, the disorder spreads from the surface layer into the bulk, establishing a thin quasiliquid film in the surface region. We conclude that the hierarchy of premelting phenomena is inversely proportional to the surface atomic density, being most pronounced for the V( 111) surface which has the lowest surface density.

Microwave power dependence of YBa2Cu3O7 thin‐film Josephson edge junctions

Using a stripline resonator technique, we measured the dependence on microwave current of the impedance of fabricated superconductor/normal‐metal/superconductor Josephson junctions in YBa2Cu3O7 thin films. The results are compared with predictions of a resistively shunted junction model that yields good agreement at low and intermediate values of the microwave current. At high currents deviations from the model predictions are observed, which we propose are due to flux penetration into the junction. The results are consistent with previously proposed explanations of the rf power dependence of the high‐Tc materials.

High temperature superconducting bolometer

Abstract Microbridges of thin films of YBa 2 Cu 3 O 7 were used to make infrared bolometers. Using a chopped cw-CO 2 laser at 10.6 μm, the peak responsivity of these films was measured as 0.5 V/W at a temperature of 90.3 K. The present experimental data agree well with a bolometric response of the irradiated films and calculations are presented to support this conclusion.

Search for spontaneous nucleation of magnetic flux during rapid cooling of YBa 2 Cu 3 O 7 − δ films through T c

We describe an experimental search for spontaneous formation of flux lines during a rapid quench of thin YBaCuO films through Tc. This effect is expected according to the Kibble-Zurek mechanism of a creation of topological defects of the order parameter during a symmetry breaking phase transition. Spontaneously formed vortices were previously observed in superfluid 3He, while a similar experiment in superfluid 4He gave negative results. Using a high Tc SQUID, we measured both the magnetic flux in the sample during a quench with a sensitivity of 20 phi-0/cm^2, and the field noise which one would expect from flux lines pinned in the film. The sensitivity was sufficient to detect spontaneous flux at a level corresponding to 10^(-3) of the prediction. Within our resolution, we saw no evidence for this effect.