Mixed-State Microwave Response in Superconducting Cuprates

Abstract

We investigate the physics of the microwave response in YBa 2 Cu 3 O 7−δ , SmBa 2 Cu 3 O 7−δ and MgB 2 in the vortex state. We first recall the theoretical basics of vortex-state microwave response in the London limit. We then present a wide set of measurements of the field, temperature, and frequency dependences of the vortex state microwave complex resistivity in superconducting thin films, measured by a resonant cavity and by sweptfrequency Corbino disk. The combination of these techniques allows for a comprehensive description of the microwave response in the vortex state in these innovative superconductors. In all materials investigated we show that flux motion alone cannot take into account all the observed experimental features, neither in the frequency nor in the field dependence. The discrepancy can be resolved by considering the (usually neglected) contribution of quasiparticles to the response in the vortex state. The peculiar, albeit different, physics of the superconducting materials here considered, namely two-band superconductivity in MgB 2 and superconducting gap with lines of nodes in cuprates, give rise to a substantially increased contribution of quasiparticles to the field-dependent microwave response. With careful combined analysis of the data it is possible to extract or infer many interesting quantities related to the vortex state, such as the temperature-dependent characteristic vortex frequency and vortex viscosity, the field dependence of the quasiparticle density, the temperature dependence of the σ-band superfluid density in MgB 2

Microwave Absorption by Vortices in Superconductors with a Washboard Pinning Potential

“…In the presence of a dc current j 0 = 1 and a series of dimensionless inverse temperatures g = U p /2T, as indicated, P(ω) demonstrates (i) an enhanced power absorption by vortices at low frequencies, (ii) a pronounced temperature-dependent minimum at intermediate frequencies and (iii) a sign change at certain conditions [27]. Experimentally, U 0 1000 ÷ 5000 K typically [10], and ω p usually ensues in the microwave range [16,30,42]. In the limit of zero dc current j 0 = 0, the curves coincide with the well-known results of Coffey and Clem (dashed line) [49].…”

“…Later on, the theory accounting also for vortex creep at non-zero temperature in a 1D cosine PP has been extended by Coffey and Clem (CC) [49]. In the following, the CC theory has been experimentally proved to be very successful [16] to describe the high-frequency electromagnetic properties of superconductors. However, it had been developed for a small microwave current and in the absence of a dc drive.…”

“…As a consequence of the guided vortex motion, an even-in-field reversal transverse resistivity component appears, unlike the ordinary Hall resistivity which is odd regarding the field reversal. A guiding of vortices can be achieved with different sorts of PP landscapes [15,16] though it is more strongly enhanced and can be more easily treated theoretically when using PPs of the washboard type (WPP).…”

Noise-Assisted Microwave Up-conversion by Vortices in Thin-Film Superconductors with a dc-Biased Washboard Pinning Potential

“…Quantitatively, an experimentally estimated value of ω p in the absence of a dc current at a temperature of 0.6T c is ω p ≈ 7 GHz for a 20 nm-thick [17] or a 40 nm-thick [18] Nb film. For comparison, in YBCO ω p < 15 GHz, typically [1]. While plotting the data, the dimensionless inverse temperature g = U p /2T will be used for convenience as it describes the relative intensity of thermal fluctuations with respect to the PP depth.…”

“…The understanding and optimization of the non-linear microwave properties of vortices are important both for basic physics and applications [1]. One of the most valuable experimental methods for the investigation of flux pinning mechanisms and vortex dynamics in type-II superconductors is the measurement of the complex ac response in the radiofrequency and microwave range [2][3][4][5].…”