2023 APS March Meeting
Volume 68, Number 3
Las Vegas, Nevada (March 5-10)
Virtual (March 20-22); Time Zone: Pacific Time
Session K27: Superconductivity:Vortex & Magnetism
3:00 PM–6:00 PM,
Tuesday, March 7, 2023
Room: Room 219
Sponsoring
Unit:
DCMP
Chair: Joachim Wosnitza, Helmholtz-Zentrum Dresden-Rossendorf
Abstract: K27.00004 : Strong pinning transition with arbitrary defect potentials*
3:36 PM–3:48 PM
Abstract
Presenter:
Filippo Gaggioli
(ETH Zurich)
Author:
Filippo Gaggioli
(ETH Zurich)
Dissipation-free current transport in type II superconductors requires vortices, the topological defects of the superfluid, to be pinned by defects in the underlying material. The pinning capacity of a defect is quantified by the Labusch parameter κ ∼ fp/ξC ¯, measuring the pinning force fp relative to the elasticity C ¯ of the vortex lattice, with ξ denoting the coherence length (or vortex core size) of the superconductor. The critical value κ = 1 separates weak from strong pinning, with a strong defect at κ > 1 able to pin a vortex on its own. The onset of strong pinning at κ = 1+ exhibits a striking correspondence to the physics of a critical point terminating a thermodynamic first-order transition, with the Labusch parameter κ replacing the scaled temperature T/Tc. So far, this transition has been studied for isotropic defect potentials, resulting in a critical exponent μ = 2 for the onset of the strong pinning force density Fpin ∼ npfp(ξ/a0)2(κ−1)μ, with np denoting the density of defects and a0 the intervortex distance. This result is owed to the special rotational symmetry of the defect producing a finite trapping area Strap ∼ ξ2 at the strong-pinning onset. The behavior changes dramatically when studying anisotropic defects with no special symmetries: the strong pinning then originates out of isolated points with length scales growing as ξ(κ − 1)1/2, resulting in a different force exponent μ = 5/2. The strong pinning onset for arbitrary defect potentials is characterized by the appearance of unstable areas of elliptical shape whose boundaries mark the locations where vortices jump. The associated locations of asymptotic vortex positions define areas of bistable vortex states; these bistable regions assume the shape of a crescent with boundaries that correspond to the spinodal lines in a first-order transition and cusps corresponding to critical end- points. Both, unstable and bistable areas grow with κ > 1 and join up into larger domains; for a uniaxially anisotropic defect, they merge into the ring-shaped areas previously encountered for isotropic defects. Finally, we extend the analysis to the case of a random two-dimensional pinning landscape (or short, pinscape) and discuss the topological properties of unstable and bistable regions as expressed through the Euler characteristic.
*Swiss National Science Foundation.