Bipolar Thermoelectricity in Bilayer-Graphene–Superconductor Tunnel Junctions

Abstract

We investigate the thermoelectric properties of a hybrid nanodevice composed of a two-dimensional 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 bilayer graphene (BLG) and a Bardeen-Cooper-Schrieffer superconductor. In this scheme, the nonlinear thermoelectric effect, predicted and observed in superconductor-insulator-${\mathrm{superconductor}}^{\ensuremath{'}}$ 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 $1\phantom{\rule{0.2em}{0ex}}\mathrm{nW}/\text{\ensuremath{\mu}}{\mathrm{m}}^{2}$ for temperature gradients of tens of kelvin. 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 the presence of the 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 nanofabrication techniques.