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Soutenance de thèse

Mercredi 12 avril à 14h00,
Salle Remy Lemaire, K223

Orateur : Kévin LE CALVEZ
"Signatures of a 4pi periodic Andreev bound state in topological Josephson junctions"


Three dimensional topological insulators (3D TI) are a new state of matter composed of an electrically insulating bulk covered by metallic surface states. A topological Josephson junction composed of these surface states is predicted to host an Andreev Bound state (ABS) with a 4pi-periodicity in the superconducting phase difference across the junction.
We have fabricated hybrid topological Josephson junctions made of flake of TI crystal grafted with Vanadium superconducting leads. We have studied the response to a microwave of the current-voltage characteristics of a series of such junctions. Depending on the frequency, the measured maps of Shapiro steps show specific features that can be related to topological effects. A local thermal balance for the electronic bath in the junction enables us to estimate an effective, bias-dependent electronic temperature. The latter drives both the critical current amplitude and the rate of quasi-particle poisoning. Experimental data and numerical simulation compare very well, in particular regarding the non-closure of the superconducting current at the transition between the first two zero-indexed steps and the delay in appearance order of the first steps. We are thus able to estimate the 4pi-periodic Josephson current, although the 2pi-periodic current dominates in amplitude the Josephson coupling through the junction.

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