Dynamics of vortex penetration, jumpwise instabilities, and nonlinear surface resistance of type-II superconductors in strong rf fields

Abstract

We consider nonlinear dynamics of a single vortex in a superconductor in a strong rf magnetic field B0 sin ωt. Using the London theory, we calculate the dissipated power Q(B0, ω), and the transient time scales of vortex motion for the linear Bardeen-Stephen viscous drag force, which results in unphysically high vortex velocities during vortex penetration through the oscillating surface barrier. It is shown that penetration of a single vortex through the ac surface barrier always involves penetration of an antivortex and the subsequent annihilation of the vortex antivortex pairs. Using the nonlinear Larkin-Ovchinnikov (LO) viscous drag force at higher vortex velocities v(t) results in a jump-wise vortex penetration through the surface barrier and a significant increase of the dissipated power. We calculate the effect of dissipation on nonlinear vortex viscosity η(v) and the rf vortex dynamics and show that it can also result in the LO-type behavior, instabilities, and thermal localization of penetrating vortex channels. We propose a thermal feedback model of η(v), which not only results in the LO dependence of η(v) for a steady-state motion, but also takes into account retardation of temperature field around rapidly accelerating vortex, and a long-range interaction with the surface. We also address the effect of pinning on the nonlinear rf vortex dynamics and the effect of trapped magnetic flux on the surface resistance Rs calculated as a function or rf frequency and field. It is shown that trapped flux can result in a temperature-independent residual resistance Ri at low T , and a hysteretic low-field dependence of Ri(B0), which can decrease as B0 is increased, reaching a minimum at B0 much smaller than the thermodynamic critical field Bc. We propose that cycling of rf field can reduce Ri due to rf annealing of magnetic flux which is pumped out by rf field from the thin surface layer of the order of the London penetration depth.

Vortex instability in molybdenum-germanium superconducting films

“…Thus overall η declines despite the shrinking in size with increasing E. Like the hot-electron case discussed earlier, this again leads to a non-monotonic j(E) curve and a vortex instability. The LO instability has been observed in previous experiments [14][15][16][17][18][19][20][21][22][23][24] and the combination of heating effects and the LO mechanism have been considered by various authors [25][26][27][28][29] .…”

Vortex-antivortex annihilation dynamics in a square mesoscopic superconducting cylinder

“…4, which represents the topology of the order parameter and the supercurrent distribution at the very moment when the collision takes place for the sample S 1 at T = 0.53. Very recently, Gurevich and Ciovati 20 have studied a V-AV collision in a semi-infinite sample in the context of rf fields. However, they were not able to observe the dramatic deformation of the V-AV cores since their analysis is based on rigid circular-shaped vortex cores.…”

Low temperature laser scanning microscopy of a superconducting radio-frequency cavity

“…As will be described in detail in a future publication, the experimental results showed evidence for changes in the intensity and location of the hotspots after laser scanning or RF cycling, which suggest that pinned vortices are one good candidate to explain the origin of "anomalous" RF losses. 26 The application of the LTLSM technique described in this article to map the surface resistance in the high-magnetic field (equator) region of a full single-cell elliptical cavity operating in the standard TM 010 mode is complicated by the need of bending the laser beam from the cavity axis toward the equatorial region. This might be accomplished, in principle, by using a small, low-loss dielectric wafer supported by a thin dielectric rod.…”