Effect of Twin Structure on Strain Dependences of Critical Current
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Effect of Twin Structure on Strain Dependences of Critical Current Alexey V. Semenov1,2, Constantin G. Tretiatchenko1, Vassily L. Svetchnikov1, Volodymyr M. Pan1 and Harold Wiesmann3 1
G. V. Kurdyumov Institute for Metal Physics, 36 Vernadsky Blvd., Kyiv 03142, Ukraine Institute of Physics, 46 Nauki Avenue, Kyiv 03028, Ukraine 3 Brookhaven National Laboratory, 76 Cornell Avenue, Upton, NY 11973, U.S.A. 2
ABSTRACT We suggest a model explaining nonlinear dependences of critical current Jc in YBCO epitaxial films. Two features of YBCO are taken into account: twin domain structure in orthorhombic phase and the anisotropy of uniaxial strain dependence of Tc. Applied strain changes elementary pinning force of the defects located at low-angle dislocation boundaries between differently oriented twin domains. Account of Tc dependence on strain this leads to approximately parabolic strain behavior of Jc. We have obtained analytical expressions for the “initial strain”, which actually describes a natural misbalance between numbers of grain boundaries separating a and b oriented domains, as well as for “strain sensitivity”, which is determined by Tc dependence on uniaxial a and b strains and by the effective redistribution of vortices. Other experimentally observed effects, such as temperature, magnetic field and twopeak strain dependences of Jc, are shown to be described in the framework of suggested model. INTRODUCTION Reversible strain behavior of the critical current Jc in coated conductors and epitaxial PLD single crystal films is widely discussed in recent publications [1-5]. Reversible dependences of Jc on strain in BiSCO based tapes are quasi-linear and antisymmetric. They are explained by well established Tc dependence on uniaxial strain [1], whereas more complicated dependences in YBCO films are not comprehended, especially in applied magnetic field [2-5]. Reversible Jc dependences on strain ε in RABiTS coated conductors in self field were empirically described with a power function [2] B J c (ε ) = J c (ε m ) ⎛⎜1 − A ε − ε m ⎞⎟ ⎝ ⎠
(1)
with A = 5640 – 10500, B = 2.18 ± 0.02 and |εm| < 0.15%. εm was attributed to “initial strain” of specimens, A was mentioned as “strain sensitivity”, while neither interpretation was given to A and B values. The strain dependences of PLD epitaxial YBCO films on SrTiO3 single crystal substrates were measured only for the negative strain (compression) because even weak extension leads to substrate cracking and irreversible changes [2, 3]. The measured negative branch is well fitted by (1) with similar parameters. Critical currents through bi-crystal grain boundaries with various angles show qualitatively different behavior – monotonic increase at compression. Nevertheless, they were also fitted by (1) but with large negative εm exceeding the reversibility range [2, 3]. Jc(ε) in YBCO coated conductors in applied field were also measured and fitted with (1), while A and εm parameters were allowed to vary depending on the field H [4, 5]. Dependences A(H) and εm(H) turned out to be nonmo
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