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Kévin Le Calvez

Signatures of a 4pi periodic Andreev bound state in topological Josephson junctions

Published on 12 April 2017

Thesis presented April 12, 2017

Three dimensional topological insulators (3D TI) are a new state of matter composed of an electrically insulating bulk covered by metallic surface states. Theoretically, a topological Josephson junction composed of these surface states can host an Andreev Bound state (ABS) that has twice the periodicity of the conventional 2π periodic ABSs. The 4π periodic ABS is expected to be the building block of topological quantum computing. Therefore, we study the dynamic of this particular ABS by performing Shapiro measurement on Josephson junctions built with bismuth based 3D TI. To identify the effects of a 4π periodic ABS in a Shapiro measurement, we use a phenomenological model that simulates the voltage-current characteristics of a TJJ. We predict two signatures of the 4π periodic ABS and estimate their robustness against Joule heating and thermally activated quasiparticle poisoning of the 4π periodic mode. We study the Josephson junctions dynamics by performing Shapiro measurements on junctions built on Bi2Se3. We observe the two previously anticipated signatures, which are the non-conventional appearance order of the Shapiro steps and the remaining of a supercurrent at the closing of the Shapiro step n = 0. They prove the presence of a 4π periodic ABS. We also study the topological insulator BiSbTeSe2 that we have grown by using the melting growth method. By superconducting interferometric measurements, we show a superconducting surface transport without bulk electronic conduction.

Topological insulator, Superconductivity, Scanning tunneling microscopy

On-line thesis.