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“Bipolar Thermoelectric Josephson Engine” published in Nature Nanotechnology

Bipolar thermoelectric Josephson engine Bipolar thermoelectric Josephson engine

Members of the SQEL team have developed a new type of thermoelectric engine that converts heat into electricity through the Josephson effect. The engine, which the team dubbed the “Bipolar thermoelectric Josephson Engine” is based on the effect of spontaneous particle-hole symmetry breaking in superconducting systems and has been published in the prestigious journal Nature Nanotechnology.

The work, carried out by G. Germanese and coworkers, experimentally demonstrates that superconducting tunnel junctions develop very large bipolar thermoelectricity in the presence of a considerable thermal gradient due to the spontaneous breaking of particle-hole symmetry, a novel concept already studied by other members of SQEL.

Our study is then pivotal for groundbreaking investigations of nonlinear thermoelectric effects in different systems ranging from semiconductors and low-dimensional electronic materials to high-temperature superconductors and topological insulators.

More information: Germanese, G., Paolucci, F., Marchegiani, G. et al. Bipolar thermoelectric Josephson engine. Nat. Nanotechnol. (2022). https://doi.org/10.1038/s41565-022-01208-y

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“A thermal superconducting quantum interference proximity transistor ” published on Nature Physics

Thermal superconducting quantum interference proximity transistor Thermal superconducting quantum interference proximity transistor

Researchers at SQEL have recently developed a transistor that takes advantage of this specific quality of superconductors, a thermal superconducting quantum interference proximity transistor (T-SQUIPT) published on Nature Physics.

T-SQUIPT was first theoretically proposed by some of the authors of our recent paper several years ago, although without a concrete realization yet. Our implementation of the T-SQUIPT exploits a long superconducting nanowire as proximitized element thus allowing us to demonstrate the possibility to phase-tune the thermal transport properties of a superconductor and to realize the first thermal memory cell as well.

The core concept of T-SQUIPT is a nanoscopic island of aluminum (Al) that can be made superconducting- or normal metal-like with quantum interference induced by two superconducting leads defining a ring and placed in good metallic contact with the island. For integer values of the flux quantum piercing the superconducting loop, superconductivity is reinforced and the island behaves as a good thermal insulator. For semi-integer values of the flux quantum, superconductivity is ideally suppressed, and the island behaves as a good thermal conductor.

As part of their recent study, SQEL researchers demonstrated this ability of their transistor by sinking heat from a metallic electrode into it, which was also coupled to the aluminum island through a tunnel contact. Overall, our findings demonstrate the feasibility of phase-coherently manipulating the energy transport qualities of quantum devices.

In the future, the T-SQUIPT transistor could pave the way towards the realization of a variety of new devices. The recent paper also enhances the current understanding of energy transfer at the nanoscale, thus potentially improving its management.

More information: Nadia Ligato et al, Thermal superconducting quantum interference proximity transistor, Nature Physics (2022). DOI: 10.1038/s41567-022-01578-z

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“Nonlocal thermoelectricity in a topological Josephson junction” just published on “Physical Review Letters”

Sketch of the setup used to probe the nonlocal Thermoelectricity in a topological Josephson junction Sketch of the setup used to probe the nonlocal Thermoelectricity in a topological Josephson junction

In a paper just published on Physical Review Letters, a NEST-CNR-NANO team lead by SQEL member Alessandro Braggio 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 SQEL and NEST laboratory (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.

Blasi, G., Taddei, F., Arrachea, L., Carrega, M., & Braggio, A. (2020). Nonlocal Thermoelectricity in a S-TI-S Junction in Contact with a N-Metal Probe: Evidence for Helical Edge States. Physical Review Letters, 124(22), 227701. DOI: https://doi.org/10.1103/PhysRevLett.124.227701. arXiv: http://arxiv.org/abs/1911.04367

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“Nonlinear thermoelectricity with electron-hole symmetric systems” published on Physical Review Letters

“Nonlinear thermoelectricity with electron-hole symmetric systems” published on Physical Review Letters

Recently thermoelectric systems have been extensively investigated since the growing interests in the field of quantum thermodynamics and in studying of thermal transport at the nanoscale.

In a two-terminal system, a necessary condition for thermoelectricity in the linear regime – i.e., for a small voltage V and a small temperature bias ∆T – is breaking the electron-hole symmetry which results in the transport property I(V, ∆T) ≠ −I(−V, ∆T), where I is the charge current flowing through the two-terminal system.

In a new research paper “Nonlinear thermoelectricity with electron-hole symmetric systems” published on “Physical Review Letters” by G. Marchegiani, A. Braggio e F. Giazotto, we demonstrate that this condition is no longer required outside the linear regime. Even a prototype system like a tunnel junction between two different superconductors, can exhibit nonlinear thermo-electric effect based on the spontaneous breaking of electron-hole symmetry in the system.

“The next step will be the experimental realization of this effect and further theoretical investigations of this new discovered mechanism”.

commented F. Giazotto

Check out also the Italian press release!