Anomalous Fano factor as a signature of Bogoliubov Fermi surfaces

TL;DR

This paper predicts distinctive noise signatures, especially in the Fano factor, that can serve as experimental evidence for Bogoliubov Fermi surfaces in superconductor hybrid devices under magnetic fields.

Contribution

It introduces a theoretical framework to identify Bogoliubov Fermi surfaces through noise spectroscopy and Fano factor measurements in hybrid superconducting devices.

Findings

Fano factor drops below two with many Bogoliubov quasiparticles.

Discontinuities in Fano factor occur at phase transition lines.

Magnetic field controls the number and orientation of Bogoliubov Fermi surfaces.

Abstract

Noise spectroscopy is a key technique to investigate the nature and dynamics of charge carriers in superconductors. The recently discovered superconducting hybrids with Bogoliubov Fermi surfaces exhibit a particularly intriguing and rich charge dynamics, as their charge carriers consist of both Cooper pairs and an extensive number of Bogoliubov quasiparticles. Motivated by this, we compute the noise spectra of Bogoliubov Fermi surfaces and identify their key signatures in the differential conductance and the Fano factor. Specifically, we consider a semiconductor/superconductor hybrid device with an in-plane magnetic field, which exhibits several Bogoliubov Fermi surfaces. The number and orientation of the Bogoliubov Fermi surfaces in this device can be readily controlled by the applied magnetic field, which in turn alters the noise signal. In particular, we find that the Fano factor exhibits a reduced value, substantially lower than two, whenever the charge dynamics is governed by a large number of Bogoliubov quasiparticles. Using experimentally relevant parameters, we make a number of specific predictions for the noise spectra, that can be used as direct evidence of Bogoliubov Fermi surfaces. In particular, we find that the Fano factor as a function of magnetic field and spin-orbit coupling exhibits characteristic discontinuities at the transition lines that separate phases with different number of Bogoliubov Fermi surfaces.