Nicola Pompeo

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.

Pinning analyses on epitaxial YBa2Cu3O7−δ films with BaZrO3 inclusions

The introduction of artificial pinning sites in YBa2Cu3O7−δ (YBCO) epitaxial thin films has been obtained by pulsed laser deposition (PLD) technique from YBCO-BaZrO3 (BZO) composite targets with BZO concentration ranging from 2.5 to 7 mol %. The typical critical temperature, Tc, drop observed in YBCO-secondary phase films grown by PLD has been successfully recovered by increasing the deposition temperature. Transport properties analyses revealed improved pinning force values for YBCO-BZO films for BZO concentration above 5 mol %. These films exhibited an anomalous dual peak structure which has been ascribed to the presence of two pinning contributions. The crossover field, Bm, separating these two pinning regimes has been observed to be temperature independent suggesting the occurrence of the matching field effect. The measured value of Bm agreed with the matching field value, BΦ=KndΦ0, as evaluated from transmission electron microscopy investigations. The mark of this phenomenon is retained down to 10 K,...

Transport Property Improvement by Means of BZO Inclusions in PLD Grown YBCO Thin Films

A detailed study on YBa₂Cu₃O_7-delta (YBCO) films with BaZrO₃ (BZO) inclusions (YBCO-BZO) deposited on SrTiO₃ substrate with different BZO contents is presented. X ray diffraction, SEM imaging and superconductive critical temperature (<i>T</i> _c) measurements were performed to evaluate film growth quality. Independently from the BZO content, all YBCO-BZO films showed <i>T</i> _c values close to that of pure YBCO indicating that the <i>T</i> _c reduction which is usually observed on YBCO-BZO films can be successfully recovered by changing the deposition conditions. All YBCO-BZO films showed an improved critical current density (<i>J</i> _c) dependence on magnetic field if compared to the typical <i>J</i> _c(<i>H</i>) of YBCO, resulting in higher irreversibility fields up to 9 T at 77 K. SEM observation on etched samples suggests a direct link between the defect density measured in YBCO-BZO samples and that calculated from the observed in field performances. The analysis of <i>J</i> _c(<i>B</i>) as a function of applied magnetic field direction revealed that the pinning due to isotropic pinning centers typical for YBCO is coupled with a <i>c</i>-axis correlated pinning in YBCO-BZO which can be ascribed to BZO induced columnar defects. <i>J</i> _c (<i>B</i>) recorded at several temperatures down to 10 K revealed matching effect features.

Dielectric resonators for the measurements of the surface impedance of superconducting films

Abstract We present the development, realization and setup of dielectric resonators, for the purpose of measuring the surface impedance at microwave frequencies of superconducting thin films. We focus on resonators designed to operate in dc magnetic fields, optimized for the measurements of the variation of the surface impedance with the applied field. Two resonators, operating at 8 and 48 GHz, are presented. We discuss different approaches to the measurement of the resonator parameters, with particular attention to the nonidealities of real setups in a cryogenic environment. Finally, we present some sample measurement of high-Tc and low-Tc superconducting films.

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.

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

In order to study the vortex pinning determined by artificially introduced pinning centers in the small-vortex displacement regime, we measured the microwave surface impedance at 47.7 GHz in the mixed state of YBa2Cu3O7−δ thin films, where submicrometric BaZrO3 particles have been incorporated. As a function of the BaZrO3 content, we observed that the absolute losses slightly decrease up to a BaZrO3 content of 5%, and then increase. We found that the magnetic-field-induced losses behave differently in that they are not monotonic with increasing BaZrO3 concentration. At small concentration (2.5%) the field-induced losses increase, but large reduction in the losses themselves, by factors up to 3, is observed upon further increasing the BaZrO3 concentration in the target up to 7%. Using measurements of both surface resistance and surface reactance, we estimate vortex pinning-related parameters. We found that BaZrO3 inclusions introduce deep and steep pinning wells. In particular, the minimum height of the en...

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

We report a detailed study performed on La2Zr2O7 pyrochlore material grown by the metal-organic decomposition method as buffer layers for YBa2Cu3O7-x (YBCO) coated conductors. High-quality epitaxial LZO thin films have been obtained on single crystal (SC) and Ni-5 %at.W substrates. In order to evaluate structural and morphological properties, films have been characterized by means of X-ray diffraction analyses, atomic force microscopy, and scanning electron microscopy. Precursor solutions and heat treatments have been studied by thermogravimetric analyses and infrared spectra with the aim of optimizing the annealing process. Thin films of YBCO have been deposited by pulsed laser ablation on this buffer layer. The best results obtained on SC showed YBCO films with critical temperature values above 90 K, high self-field critical current density values (Jc >; 1 MA/cm2) and high irreversibility field values (8.3 T) at 77 K together with a rather high depinning frequency νp (0.5 T, 77 K) >; 44 GHz as determined at microwaves. The best results on Ni-5% at.W has been obtained introducing in the heat treatment a pyrolysis process at low temperature in air in order to remove the residual organic part of the precursor solution.

Directional pinning and anisotropy in YBa2Cu3O7-x with BaZrO3 nanorods: intrinsic and nanorods-induced anisotropy

Abstract We present a study of the anisotropic vortex parameters as obtained from measurements of the microwave complex resistivity in the vortex state with a tilted applied magnetic field in YBa 2 Cu 3 O 7− x thin films with BaZrO 3 nanorods. We present the angular dependence of the vortex viscosity η , the pinning constant k p and the upper limit for the creep factor χ M . We show that the directional effect of the nanorods is absent in η , which is dictated by the mass anisotropy γ . By contrast, pinning-mediated properties are strongly affected by the nanorods. It is significant that the pinning and creep affected by the nanorods is detectable also at our very high operating frequency, which implies very short-range displacements of the vortices from their equilibrium position.

YBa 2 Cu 3 O 7−δ Films With BaZrO 3 Inclusions for Strong-Pinning in Superconducting Films on Single Crystal Substrate

The addition of artificially induced defects improves the pinning efficiency of magnetic flux vortices in YBa2Cu3O7-delta (YBCO) films. In YBCO films by pulsed laser ablation (PLD) technique this goal can be pursued by the introduction of epitaxial second phase nanoinclusions in the YBCO matrix. In the present work, the effect of BaZrO3 (BZO) inclusions in stoichiometric YBCO films on transport properties has been studied. Composite targets obtained by addition of BZO powder in molar percents ranging from 2.5 to 7 mol.% have been prepared with BZO mean grain size of about 1 mum. Structural and morphological characterizations of the films obtained by PLD from these targets have been carried out through X-ray diffractometry and Scanning Electron Microscopy (SEM) analysis. Dc electrical transport measurements at 77 K have revealed a better magnetic field retention of the critical current density, Jc, being at 3 T the 14.6% and the 16.4% of the zero field values for YBCO-BZO 5% and 7%, respectively. The complex surface impedance has been measured by means of a dielectric resonator at a high microwave frequency (50 GHz). In order to identify the potentially strong pinning induced by the inclusions, the field-dependent complex resistivity has been determined as a function of temperature in a moderate magnetic field (B < 1T). Very high depinning frequency are estimated in YBCO films with BZO inclusions indicating extremely high vortex pinning and steep potential wells.

Effect of

We perform measurements of high-frequency ( ~48 GHz) microwave surface impedance with an applied magnetic field in YBa2Cu3O7-delta (YBCO) laser-ablated films with various amounts of BaZrO3 (BZO) sub-micrometric inclusions, up to 7 mol % concentration. BZO inclusions are very effective in the reduction of the field-induced surface resistance in our experimentally accessible field range [0, 0.8] T. At temperatures low enough, the application of a moderate ( ~0.2 T) field makes the YBCO/BZO films markedly less dissipative than pure YBCO. This result, examined in the light of the very high measuring frequency (very small vortex oscillation amplitude) shows that BZO inclusions are even more effective pinning centers than columnar defects. We study the dependence of the vortex parameters (vortex viscosity, pinning constant) on the BZO concentration. We examine the correlation between the reduction of the microwave dissipation and the areal density of BZO-induced defects. We argue that the very improved performances in a magnetic field are due to individual pinning of vortices on BZO inclusions.