Nonlocal Thermoelectricity in a topological Josephson junction

Foto: Sketch of the setup


Pisa - 10.06.2020 - Nonlocal Thermoelectricity in a topological Josephson junction
In a paper just published on Physical Review Letters, a NEST, CNR-NANO team identified a unique non-local thermoelectrical effect in a Quantum spin Hall system in 2-dimensional topological insulators, proximized with superconductors, also called topological Josephson junctions.

The quantum spin Hall state is characterised by Kramer paired helical edge states which propagate in opposite directions with opposite spin orientations (spin-momentum locking). Unambiguous identification of those edge state is fundamental to certify their topological nature and has prominent implications in condensed matter research and its applications in topological quantum computation and sensing. The team, composed by Gianmichele Blasi, Fabio Taddei, Matteo Carrega and coordinated by Alessandro Braggio from NEST laboratories (Scuola Normale Superiore and CnrNano) collaborating with Liliana Arrachea from ECyT-UNSAM (Argentina), investigated a three-terminal setup with helical edge states proximized by two superconductors and contacted with a normal-metal probe, in the presence of an external magnetic field. Since the whole system is particle-hole symmetric, nonlocality is the only way to generate linear thermoelectricity. Nonlocal thermoelectrical transport is generated in the probe by applying a thermal gradient between the superconductors.

The key to this response is the fact that a helical proximized edge state gives rise to a simultaneous flow of helical electrons and holes. Since they move in opposite directions, they thermalize with different reservoirs leading to a net thermoelectric current, tuneable by the magnetic field, flowing in the probe.

The researchers theoretically characterized the thermoelectric response of this hybrid junction in the linear-response regime, demonstrating that the nonlocal Seebeck coefficient can achieve high values, comparable with the best hybrid devices based on ferromagnetic elements.

Finally, researchers discuss how to exploit the external magnetic flux as a knob to control the thermoelectric response and the heat flow, thus opening a new route for thermoelectrical generation, thermal switches, sensors and applications based on topological Josephson junctions. The research has been funded by the bilateral project CNR-CONICET coordinated by Fabio Taddei.

Nonlocal Thermoelectricity in a Superconductor–Topological-Insulator–Superconductor Junction in Contact with a Normal-Metal Probe: Evidence for Helical Edge States
Gianmichele Blasi, Fabio Taddei, Liliana Arrachea, Matteo Carrega,and Alessandro Braggio
DOI: 10.1103/PhysRevLett.124.227701



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