“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

“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

“Superconducting spintronic tunnel diode” published on Nature Communications

Superconducting spintronic tunnel diode

Most of our everyday electronic appliances, such as radios, logic components or solar panels, rely on diodes where current can flow primarily in one direction. Such diodes rely on the electronic properties of semiconductor systems which cease to work at the ultralow sub-Kelvin temperatures required in tomorrow’s quantum technology. Superconductors are metals whose electrical resistivity is usually zero but, when contacted with other metals, can exhibit high contact resistance.

This can be understood from the energy gap, which indicates a forbidden region for electronic excitations that form in superconductors. It resembles the energy gap in semiconductors but is typically much smaller. While the presence of such a gap has been known for decades, the diode-like feature has not been previously observed, because it requires breaking the usually robust symmetry of the contact’s current–voltage characteristics.

Our new work “Superconducting spintronic tunnel diode” published on Nature Communications demonstrates how this symmetry can be broken with the help of a ferromagnetic insulator suitably placed in the junction.

The research finding was made as part of the SUPERTED project, which is being funded under the EU’s Future and Emerging Technologies (FET Open). This project aims at creating the world’s first superconducting thermoelectric detector of electromagnetic radiation, based on superconductor/magnet heterostructures.

More information: E. Strambini et al, Superconducting spintronic tunnel diode, Nature Communications (2022). DOI: 10.1038/s41467-022-29990-2

Open post-doc position at SQEL

A one-year (eventually renewable) post-doc position (experimental) will be opened in summer 2021 to work in the low temperature nanotechnology labs at CNR-NANO & Scuola Normale Superiore in Pisa. High motivation and a basic education in solid-state physics or superconductivity or spintronics is welcome.

The work will be part of an ambitious ongoing European project (SUPERTED) aiming at the realization of novel highly sensitive quantum detectors based on superconductor/ferromagnetic insulator (S/FI) bi-layers. The candidate will participate to the low temperature (~10 mK) electrical measurements, to the design of the superconducting spintronic detectors, to the data analysis, and to the simulation of the hybrid nanodevices.

For additional information, feel free to contact us: Francesco Giazotto, Elia Strambini, Maria Spies.

Physical Review Letters features “Sauter-Schwinger Effect in a Bardeen-Cooper-Schrieffer Superconductor” as journal cover

Physical Review Letters features "Sauter-Schwinger Effect in a Bardeen-Cooper-Schrieffer Superconductor" as journal cover Physical Review Letters features “Sauter-Schwinger Effect in a Bardeen-Cooper-Schrieffer Superconductor” as journal cover

Since the 1960s a deep and surprising connection has followed the development of superconductivity and quantum field theory, where the Anderson-Higgs mechanism and the similarities between the Dirac and Bogoliubov–de Gennes equations are the most intriguing examples.

In a recent work “Sauter-Schwinger Effect in a Bardeen-Cooper-Schrieffer Superconductor” published on Physical Review Letters, we follow further this parallelism and show that it predicts an outstanding phenomenon: the superconducting Sauter-Schwinger effect. As in the quantum electrodynamics Schwinger effect, where an electron-positron couple is created from the vacuum by an intense electric field, we show that an electrostatic field can generate two coherent excitations from the superconducting ground-state condensate.

We are glad that our work has been selected for an Editors’ Suggestion and is featured as journal cover of this monthly issue.