Nonequilibrium Andreev resonances in ballistic graphene Andreev interferometers

TL;DR

This paper investigates nonequilibrium Andreev resonances in a voltage-biased graphene three-terminal Josephson junction, revealing phase shifts and oscillation frequency doubling, with a model explaining the microscopic mechanisms involved.

Contribution

It introduces a model linking nonequilibrium Andreev bound states to phase-sensitive reflections, explaining the observed resistance oscillation crossovers in graphene JJs.

Findings

Resistance oscillations show maxima at flux quanta and half-flux quanta.

A transition point causes a $ ext{π}$ phase shift and frequency doubling.

The model explains the microscopic origin of phase-sensitive Andreev reflections.

Abstract

We study nonequilibrium Andreev resonances in a voltage-biased graphene three-terminal Josephson junction (JJ). We observe periodic oscillations of resistance with maxima at multiples of the magnetic flux quantum (noninversion regime). As we increase the bias voltage, we further observe a transition point beyond which oscillations exhibit a $\pi$ phase shift (inversion regime) with maxima of resistance occuring at multiples of half-flux quantum. At this transition point, the frequency of the oscillations is doubled. We develop a model based on the coupling of the static Andreev bound states (ABSs) to the nonequilibrium Fermi surface of graphene to explain the observed noninversion to inversion crossovers. Our model associates these crossovers to microscopic phase-sensitive Andreev reflections which couple the normal and superfluid components of the current. Our findings show that multiterminal JJs can be used to engineer unconventional energy-phase relations such as those expected in the $\pi$-shifted ABSs without relying on quartet and Floquet physics. These nonequilibrium ABSs could potentially find applications in superconducting $\pi$ qubits.