Thermal creep induced by cooling a superconducting vortex lattice

Abstract

A perturbed system relaxes towards an equilibrium given by a minimum in the potential energy landscape. This often occurs by thermally activated jumps over metastable states. The corresponding dynamics is named creep and follows Arrhenius' law. Here we consider the situation where the equilibrium position depends on temperature. We show that this effect occurs in the vortex lattice of the anisotropic superconductor 2H-NbSe2 when the magnetic field is tilted away from the principal axes, and that it leads to the peculiar appearance of creep when cooling the sample. Temperature determines the system's ground state and at the same time brings the system back to equilibrium, playing a dual and antagonistic role. We expect that cooling induced creep occurs in correlated systems with many degrees of freedom allowing to tune the equilibrium state via heat treatment.Superconducting vortices are lines of quantized magnetic flux Φ 0 = hc/2e where the superconducting order parameter is depressed at a length scale of order of the superconducting coherence length ξ and circular supercurrents are flowing at a length scale of order of the penetration depth λ. Vortices arrange in a lattice, which can be triangular, square or disordered due to the interactions of vortices with the crystalline environment 1-5 . Vortices tend to be pinned on material defects or inclusions consisting of places where superconductivity is depressed on length scales of order ξ. When varying the applied magnetic field, its strength or direction, vortices enter or exit the sample 1-3 . This produces vortex motion, which is often counteracted by pinning. The action of the pinning landscape results in long-lived out-of-equilibrium vortex distributions that relax through thermal creep over a manifold of barriers 1-3,6-10 . The phenomenon of creep has been observed in interacting systems of particles, such as colloids, polymers, solids consisting of mixtures or in lattices of entities formed through electronic interactions (domain walls or skyrmions) [11][12][13][14][15] . Vortex lattices are often considered as a model system. Most efforts to understand vortex creep have focused on trying to immobilize vortices and thereby increase the critical current j c for applications 16 . But the interaction between vortices and the underlying superconducting material is very rich and can produce counterintuitive phenomena. Here we find that the equilibrium state towards which the system creeps can be modified with the temperature, leading to a phenomenon that we term self-imposed creep. We study vortex creep in the the superconductor 2H-NbSe 2 and we observe that, as a consequence of self-imposed creep, the vortex lattice starts creeping when cooling, see Fig. 1. The layered system 2H-NbSe 2 shows weak pinning and small creep rates. Vortex properties depend crucially on random disorder potential cooling creep fast relaxation warming ξ U 0 x V(x) self-imposed FIG. 1. Schematic response of a particle confined in a potential subject to random disorder. The disorder pote...

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“…scanning tunneling microscopy). 55 Moreover, at sufficiently high temperatures, the vortex state undergoes a melting transition in which pinning becomes ineffective. [56][57][58][59][60] A.…”