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Posts tagged quasiparticles

PRL quasiparticles

The attempt to unveil quasiparticle mysteries has been finally published in PRL

Theoretical Model to Explain Excess of Quasiparticles in Superconductors
Anton Bespalov, Manuel Houzet, Julia S. Meyer, and Yuli V. Nazarov
Phys. Rev. Lett. 117, 117002 – Published 9 September 2016

ABSTRACT: Experimentally, the concentration of quasiparticles in gapped superconductors always largely exceeds the equilibrium one at low temperatures. Since these quasiparticles are detrimental for many applications, it is important to understand theoretically the origin of the excess. We demonstrate in detail that the dynamics of quasiparticles localized at spatial fluctuations of the gap edge becomes exponentially slow. This gives rise to the observed excess in the presence of a vanishingly weak non-equilibrium agent.

Unveiling mysteries

with my Grenoble collaborators. Fresh submission.
How many quasiparticles can be in a superconductor?

Anton Bespalov, Manuel Houzet, Julia S. Meyer, Yuli V. Nazarov

Experimentally and mysteriously, the concentration of quasiparticles in a gapped superconductor at low temperatures always by far exceeds its equilibrium value. We study the dynamics of localized quasiparticles in superconductors with a spatially fluctuating gap edge. The competition between phonon-induced quasiparticle recombination and generation by a weak non-equilibrium agent results in an upper bound for the concentration that explains the mystery.

Strong effects of weak ac driving in short superconducting junctions

has been published in PRB couple of days ago.
DOI: 10.1103/PhysRevB.91.104522

You can find the preprint here.

We study a short superconducting junction subject to a dc and ac phase bias. The ac modulation changes the occupation of the Andreev bound states formed at the constriction by transitions between bound states and the continuum. In a short junction, the non-equilibrium Andreev bound state population may relax through processes that conserve parity of the occupation number on the same bound state and processes that do not conserve it. We argue that the parity conserving processes occur on a much faster time scale. In this case, even a weak driving may lead to a large deviation of the supercurrent from its equilibrium value. We show that this effect is accompanied by a quasiparticle current which may lead to a measurable charge imbalance in the vicinity of the junction. Furthermore, we study the time evolution of the supercurrent after switching off the ac drive. On a time scale where parity relaxation is negligible, the supercurrent relaxes to a stationary non-equilibrium state. Finally, we briefly outline the regime of ultraweak driving where the ac-induced processes occur on a time scale comparable to parity relaxation.

Eduard Driessen

who is presently a postoc in Grenoble with Claude Chapelier has given a talk yesterday. He reported very recent results on low-temperature STM experiments with superconducting nanowires. It is a tremendous technical challenge he has overcame.

The results were many and various. Some were pretty familiar and irritating like the incomplete gap in this disordered superconductor: the thing that has an explanation, yet the explanation so primitive and far-fetched that you feel yourself ashamed that you can’t do it better. Some were generation-binding: the hot spots in superconducting wires have been hot topic in 70s when Eduard’s former supervisors Hans Mooij and Teun Klapwijk were at his stage of the career, and STM experiments of Eduard just neatly visualized the intimate structure of these spots.

Some were teasingly interesting. Eduard pushed a 1nA – tiny – current to the STM and observed a gigantic suppression of the critical current in the nanowire. It is like you add an extra droplet to a river stream – and it does not flow anymore. These droplets must be quasiparticles.
The result suggests that the quasiparticles accumulate being added at a rate of 10^7 per second. This is thus about slow quasiparticle dynamics – a topic of my long interest

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