Persistence of pinning and creep beyond critical drive within the strong pinning paradigm

We study thermal effects on pinning and creep in type-II superconductors where vortices interact with a low density np of strong point-like defects with pinning energy ep and extension ξ, the vortex core size. Defects are classified as strong if the interaction between a single pin and an individual vortex leads to the appearance of bistable solutions describing pinned and free vortex configurations. Extending the strong pinning theory to account for thermal fluctuations, we provide a quantitative analysis of vortex depinning and creep. We determine the thermally activated transitions between bistable states using Kramer's rate theory and find the non-equilibrium steady-state occupation of vortex states. The latter depends on the temperature T and vortex velocity v and determines the current-voltage (or force-velocity) characteristic of the superconductor at finite temperatures. We find that the T = 0 linear excess-current characteristic v ∝ (j − jc) Θ(j − jc) with its sharp transition at the critical current density jc, keeps its overall shape but is modified in three ways due to thermal creep: a downward renormalization of jc to the thermal depinning current density j dp (T ) < jc, a smooth rounding of the characteristic around j dp (T ), and the appearance of thermally assisted flux flow (TAFF) v ∝ j exp(−U0/kBT ) at small drive j jc, with the activation barrier U0 defined through the energy landscape at the intersection of free and pinned branches. This characteristic emphasizes the persistence of pinning of creep at current densities beyond critical. arXiv:1903.09083v2 [cond-mat.supr-con]

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“…The new insights on the persistence of pinning and creep beyond the critical drive has been published in a short format in Ref. [33].…”

Section: Introductionmentioning

confidence: 99%

Strong pinning theory of thermal vortex creep in type-II superconductors

et al. 2019

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“…When including thermal fluctuations in the calculation of the pinning force density F pin , different jumps ∆e pin (t) in the pinning energy become relevant that depend on the time t evolution of the vortex state due to creep. While this relaxational time dependence leads to the decay of the persistent current density j(t), the corresponding velocity dependence leads to a rounding 14,15 of the transition 16 between pinned and dissipative states in the current-voltage characteristic; again the quantitative nature of the strong pinning description allows for a detailed comparison of the temperature-shifted and rounded excess-current characteristic predicted by theory with experimental data on superconducting films 17 .…”

Section: Introductionmentioning

confidence: 95%

Creep effects on the Campbell response in type II superconductors

Filippo¹,

Blatter²,

Geshkenbeǐn³

2021

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Applying the strong pinning formalism to the mixed state of a type II superconductor, we study the effect of thermal fluctuations (or creep) on the penetration of an ac magnetic field as quantified by the so-called Campbell length λC. Within strong pinning theory, vortices get pinned by individual defects, with the jumps in the pinning energy (∆epin) and force (∆fpin) between bistable pinned and free states quantifying the pinning process. We find that the evolution of the Campbell length λC(t) as a function of time t is the result of two competing effects, the change in the force jumps ∆fpin(t) and a change in the trapping area Strap(t) of vortices; the latter describes the area around the defect where a nearby vortex gets and remains trapped. Contrary to naive expectation, we find that during the decay of the critical state in a zero-field cooled (ZFC) experiment, the Campbell length λC(t) is usually nonmonotonic, first decreasing with time t and then increasing for long waiting times. Field cooled (FC) experiments exhibit hysteretic effects in λC; relaxation then turns out to be predominantly monotonic, but its magnitude and direction depends on the specific phase of the cooling-heating cycle. Furthermore, when approaching equilibrium, the Campbell length relaxes to a finite value, different from the persistent current which vanishes at long waiting times t, e.g., above the irreversibility line. Finally, measuring the Campbell length λC(t) for different states, zero-field cooled, field cooled, and relaxed, as a function of different waiting times t and temperatures T , allows to 'spectroscopyse' the pinning potential of the defects.

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“…While conceptually simpler than weak collective pinning, it has taken significantly longer to develop a strong pinning formalism. With its completion in the early 2000s, the formalism enabled computing numerous physical observables, including the critical current 53,54 , the excess-current characteristic [55][56][57][58][59][60] , and the ac Campbell response [11][12][13][14] .…”

Section: A Fundamentals Of Vortex Pinningmentioning

confidence: 99%

Perspective: Challenges and Transformative Opportunities in Superconductor Vortex Physics

Eley,

Glatz,

Willa

2021

Preprint

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In superconductors, the motion of vortices introduces unwanted dissipation that is disruptive to applications. Fortunately, material defects can immobilize vortices, acting as vortex pinning centers, which engenders dramatic improvements in superconductor material properties and device operation. This has motivated decades of research into developing methods of tailoring the disorder landscape in superconductors to increase the strength of vortex pinning. Yet efficacious materials engineering still alludes us. The electromagnetic properties of real (disordered) superconducting materials cannot yet be reliably predicted, such that designing superconductors for applications remains a largely inefficient process of trial and error. This is ultimately due to large gaps in our knowledge of vortex dynamics: the field is challenged by the extremely complex interplay between vortex elasticity, vortex-vortex interactions, and material disorder.In this Perspective, we review obstacles and recent successes in understanding and controlling vortex dynamics in superconducting materials and devices. We further identify major open questions and discuss opportunities for transformative research in the field. This includes improving our understanding of vortex creep, determining and reaching the ceiling for the critical current, advanced microscopy to garner accurate structure-property relationships, frontiers in predictive simulations and the benefits of artificial intelligence, as well as controlling and exploiting vortices in quantum information applications.

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Persistence of pinning and creep beyond critical drive within the strong pinning paradigm

Cited by 20 publications

References 35 publications

Strong pinning theory of thermal vortex creep in type-II superconductors

Strong pinning theory of thermal vortex creep in type-II superconductors

Creep effects on the Campbell response in type II superconductors

Perspective: Challenges and Transformative Opportunities in Superconductor Vortex Physics

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