Enrico Silva

Reliable determination of vortex parameters from measurements of the microwave complex resistivity

Dipartimento di Fisica “E. Amaldi” and Unit`a CNISM,Universit`a Roma Tre, Via della Vasca Navale 84, I-00146 Roma, ItalyWe discuss and propose a complete data treatment, in close contact to typical microwave ex-perimental data, in order to derive vortex parameters, such as pinning constant and viscous dragcoefficient (also referred to as “vortex viscosity”), in a way as model-independent as possible. Weshow that many of the accepted models for the complex resistivity can be described by a single,very general analytical expression. Using typical measurements of real and imaginary resistivity asa function of the applied field, we show that, even for single-frequency measurements, it is alwayspossible to obtain (a) estimates of viscous drag coefficient and pinning constant with well-definedupper and lower bounds and (b) quantitative information about thermal creep. It turns out thatneglecting thermal creep, in particular and counterintuitively at low temperatures, might result ina severe overestimation of the viscous drag coefficient. We also discuss the impact of thermal creepon the determination of the pinning constant. The present results might lead to a reconsiderationof several estimates of the vortex parameters.

A cavity system for the measurement of the surface resistance at 48 GHz in high- superconductors

We describe a system for the measurement of the surface resistance of high- superconductors (HTCSs) at 48 GHz as a function of the temperature, the external magnetic field and its orientation with respect to the crystallographic axes of the sample. The set-up is based upon a resonant cavity. Thin films, bulk ceramics and single crystals can be accommodated in the cavity. A geometry in which there is no c axis microwave current flow, or c axis microwave field penetration, is chosen, so that unwanted effects depending on the anisotropy of HTCSs are ruled out. Geometrical factors for samples smaller than the cavity endplate are calculated. Some measurements on HTCS thin films and single crystals are presented, as well as some of the items of physical information obtainable from the measurements.

Measurements and removal of substrate effects on the microwave surface impedance of YBCO films on SrTiO3

We reconsider the problem of the measurements of the microwave complex surface impedance in thin superconducting films deposited on SrTiO3 substrates. We perform measurements of the complex surface impedance Zs = Rs+i?Xs of thin YBa2Cu3O7?? films deposited by laser ablation on SrTiO3 substrates. The typical oscillations due to the strong temperature variation of the SrTiO3 permittivity are confirmed in Rs and observed in ?Xs. The effects of the SrTiO3 substrate are evident even well below the superconducting transition temperature of YBa2Cu3O7??. Similarly to previous works, we describe the overall response in terms of impedance transformations. We extend the known results by (i)?considering the measurements of the imaginary part, (ii)?comparing the measurements to the absolute dc resistivity measured on the same sample, and (iii)?suggesting a method for measuring the intrinsic thin film surface impedance by adjusting the substrate impedance. To demonstrate the feasibility of microwave measurements of intrinsic properties of films grown onto SrTiO3 substrates, we check the proposed method by measuring the field dependent surface impedance before and after removal of the substrate resonance.

Wideband microwave measurements in Nb/Pd84Ni16/Nb structures and comparison with thin Nb films.

We present wideband microwave measurements (1–22xa0GHz) taken on superconductor/ferromagnet heterostructures with the Corbino disc technique. We apply the technique to Nb/PdNi/Nb trilayers in the vortex state and we compare the results to data taken with the same technique on a Nb thin film. We show that it is possible to directly extract the genuine flux-flow resistivity from our frequency-dependent measurements without overcoming the (depinning) critical current. The characteristic frequency for vortex relaxation can also be estimated, without resorting to a specific model. We find that the F layer determines a weakening of pinning and an enhancement of the flux-flow resistivity with respect to the Nb film.

New aspects of microwave properties of Nb in the mixed state

We present a study of the frequency dependence of the vortex dynamics in a conventional superconductor. We have employed a swept frequency, Corbino disk technique to investigate the temperature (3.6 K -T c ) and high-field (from H c2 /2 to H c2 ) microwave complex resistivity in Nb thin (20-40 nm) films as a function of the frequency (1-20 GHz). We have found several previously unnoticed features: (i) a field-dependent depinning frequency in the GHz range; (ii) deviations from the accepted frequency dependence [1], than can be ascribed to some kind of vortex creep; and (iii) the presence of switching phenomena, reminiscent of vortex instabilities. We discuss the possible origin of the features here reported.

Excess conductivity of overdoped Bi 2 Sr 2 CaCu 2 O 8 + x crystals well above T c

We have used a multiterminal technique in order to measure the

Reduction in the field-dependent microwave surface resistance in YBa2Cu3O7−δ with submicrometric BaZrO3 inclusions as a function of BaZrO3 concentration

(mathit{a},mathit{b})

MOD Derived Pyrochlore Films as Buffer Layer for All-Chemical YBCO Coated Conductors

plane excess conductivity

ANISOTROPIC RENORMALIZED FLUCTUATIONS IN THE MICROWAVE RESISTIVITY IN YBA2CU3O7-DELTA

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Magnetic 3D-2D crossover in Bi2Sr2CaCu2O8+x

in several