Non equilibrium transport and dynamics in conventional and topological superconducting junctions

• Applicant: Gianluca Rastelli, Ph.D.
• Subject Area: Theoretical Condensed Matter Physics
• Term: from 2016 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 326061083

This project concerns the theoretical study of quantum transport and non-equilibrium dynamics in superconducting junctions coupled to an electromagnetic environment and microwave irradiation, taking into account quasiparticle effects.Recent experiments have focused on superconducting junctions in various kinds of nanostructures (such as atomic contacts and topological materials) with the objective of creating a novel type of versatile superconducting junction beyond the standard Josephson tunnel junctions. These systems are promising for the potential realization of new types of qubits that are based on single Andreev levels with some envisioned protection against dissipation and decoherence. Indeed, junctions using topological materials are expected to give rise to special Andreev levels that correspond to Majorana states whose unambiguous detection is currently the subject of intense study.One important and common problem in mesoscopic superconducting junctions is the non-equilibrium population of long-lived quasiparticles. The underlying mechanisms for the relaxation dynamics and non-equilibrium properties of these quasiparticles are not fully understood. The presence of quasiparticle excitations degrades or even disrupts the operation of the systems, and thus such particles represent an intrinsic source of decoherence and dissipation. Understanding the non-equilibrium dynamics of quasiparticles is therefore a crucial issue.This project sets out to study the effects of quasiparticles in state-of-the-art novel superconducting junctions including atomic point contacts, short nanowires, and topological junctions. Specifically, motivated by recent results in Andreev spectroscopy experiments, this project will study the properties of junctions when they are irradiated by microwaves to reveal the Andreev excitation spectrum and its dynamics. For such systems, we will provide a theoretical framework for the non-equilbrium dynamics of the quasiparticles based on the interplay between the time-dependent external field and the fluctuations of the electromagnetic environment.Ongoing experiments in atomic junctions call for a theoretical analysis that includes all these aspects simultaneously. As this is theoretically challenging, this project represents pioneering work in the field.Understanding these effects is a crucial prerequisite for fully exploiting this new class of device in the form of novel, dissipation-free, non-linear elements in quantum circuits or in qubits architectures.Finally, the greatest innovation of the project is its extension of the theoretical investigation to include topological junctions. Anticipating future experiments, the project aims at a major breakthrough by analyzing a theoretical proposal to experimentally reveal Majorana states using Andreev spectroscopic measurementsin topological superconducting junctions.

• DFG Programme: Research Grants
• International Connection: USA
• Co-Investigator: Professor Dr. Wolfgang Belzig
• Cooperation Partners: Dr. Gianluigi Catelani; Dr. Leonid Glazman