Vortex depinning frequency inYBa2Cu3

M. Golosovsky

¹

,

M. I. Tsindlekht

²

,

H. Chayet

³

et al.

Abstract

We have used the parallel-plate resonator technique to study the microwave surface resistance R, and the penetration depth A, of YBazCu307 " thin epitaxial films at 5.5 GHz in the presence of magnetic field H &0.8 T and at 30 K & T & 80 K. The magnetic-field dependence of R, and A, allows determination of the vortex viscosity g, pinning constant a, and depinning frequency coo for dil'erent field orientations. To the best of our knowledge, this is among the first measurements of the angular dependence of g, a, and uo. We find that the angular dependences of g, a, and coo are fairly well described by the scaling model of Blatter, Geshkenbein, and Larkin. We demonstrate that {i)the temperature dependence of g is well described by the Bardeen-Stephen model with reduced normal-state resistivity, (ii} the pinning constant is close to its upper limit at lowest temperatures and decreases exponentially with increasing temperature, and (iii) the depinning frequency is of the order of 10 GHz, almost orientation independent and weakly depends on temperature. We analyze the values of the depinning frequency for difFerent type-II superconductors and demonstrate that it is directly related to the quasiparticle scattering rate.

Percolative vortex motion in high-temperature superconductors

“…16 A semiquantitative agreement was found for ac susceptibility measurements on Bi 2 Sr 2 CaCu 2 O 8 single crystals 17 and for vibrating reed measurements on YBa 2 Cu 3 O 7Ϫ␦ single crystals and films. 18 Reed et al 8 and Kötzler et al 9,10 showed that the extended TAFF model was in quantitative agreement with the frequency-dependent ac susceptibility of Y-1:2:3 single crystals and films near T c , whereas the ac susceptibility at lower temperatures was in agreement with the existence of critical fluctuations around a vortex glass transition temperature T g .…”

Critical-state effects on microwave losses in type-II superconductors

“…The sample, of dimensions 3.3 × 2.3 × 1.3 mm 3 , has been cut from a Tokyo Denkai batch with RRR = 300 and undergoes a superconducting transition at T c ≈ 9.2 K. We have studied the Nb because the depinning frequency [23] is small enough to suppose that the mw current induces the fluxon lattice moving in the flux-flow regime; furthermore, our experimental apparatus allows reaching DC magnetic fields of the order of H c2 .…”

Microwave response of thin niobium films under perpendicular static magnetic fields

“…When ω 0 ≈ ω, or higher, the restoring pinning force is comparable to the viscous drag force, and the response of the vortices is more complex. This is typical of HTSC where the depinning frequency is found to be of the order of 10 GHz, 16 but may be found also in low temperature superconductors such as in very thin Nb film analyzed in this paper below. In such cases, one needs both, microwave absorption and dispersion to determine the depinning frequency and flux flow parameters.…”