Bipolar Thermoelectricity in Bilayer-Graphene--Superconductor Tunnel Junctions

TL;DR

This paper explores a hybrid bilayer-graphene and superconductor tunnel junction that exhibits controllable bipolar thermoelectricity, with potential for enhanced thermoelectric performance and experimental realization.

Contribution

It demonstrates the emergence of nonlinear bipolar thermoelectricity in a graphene-superconductor junction due to broken particle-hole symmetry, with tunable properties via gating.

Findings

Achieves Seebeck coefficient up to 1 mV/K

Power density reaches 1 nW/μm² for tens of Kelvins

Thermoelectric effect is robust against Josephson coupling

Abstract

We investigate the thermoelectric properties of a hybrid nanodevice composed by a 2D carbon based material and a superconductor. This system presents nonlinear bipolar thermoelectricity as induced by the spontaneous breaking of the Particle-Hole (PH) symmetry in a tunnel junction between a BiLayer Graphene (BLG) and a Bardeen-Cooper-Schrieffer (BCS) superconductor. In this scheme, the nonlinear thermoelectric effect, predicted and observed in SIS' junctions is not affected by the competitive effect of the Josephson coupling. From a fundamental perspective, the most intriguing feature of this effect is its bipolarity. The capability to open and control the BLG gap guarantees improved thermoelectric performances, that reach up to 1 mV/K regarding the Seebeck coefficient and a power density of 1 nW/$\mu$m$^2$ for temperature gradients of tens of Kelvins. Furthermore, the externally controlled gating can also dope the BLG, which is otherwise intrinsically PH symmetric, giving us the opportunity to investigate the bipolar thermoelectricity even in presence of a controlled suppression of the PH symmetry. The predicted robustness of this system could foster further experimental investigations and applications in the near future, thanks to the available techniques of nano-fabrication.