Thermoelectric properties of magic angle twisted bilayer graphene-superconductor hetero-junction: effect of valley polarization and trigonal warping

TL;DR

This paper theoretically studies the thermoelectric properties of a magic-angle twisted bilayer graphene-superconductor hetero-junction, revealing significant effects of valley polarization and trigonal warping, with potential for thermoelectric applications.

Contribution

It introduces a detailed theoretical analysis of thermoelectric coefficients in MATBG-superconductor junctions, highlighting the impact of valley polarization and trigonal warping on thermoelectric performance.

Findings

High thermoelectric figure of merit ($zT$) up to 2.5 observed.

Violation of Wiedemann-Franz law near charge neutrality point.

Thermal conductance shows damped oscillations with barrier strength.

Abstract

We theoretically investigate the thermoelectric properties (electronic contribution) of a normal-superconductor (NS) hybrid junction, where the normal region consists of magic-angle twisted bilayer graphene (MATBG). The superconducting region is characterized by a common $s$-wave superconductor closely proximitized to the MATBG. We compute various thermoelectric coefficients, including thermal conductance, thermopower, and the figure of merit ($zT$), using the scattering matrix formalism. These results are further supported by calculations based on a lattice-regularized version of the effective Hamiltonian. Additionally, we explore the impact of trigonal warping and valley polarization on the thermoelectric coefficients. Notably, we find a significant variation in $zT$ as a function of these parameters, reaching values as high as 2.5. Interestingly, we observe a violation of the Wiedemann-Franz law near the charge neutrality point with the superconducting correlation, indicating that MATBG electrons behave as slow Dirac fermions in this regime. This observation is further confirmed by the damped oscillatory behavior of the thermal conductance as a function of the barrier strength when an insulating barrier is modelled at the interface of the NS junction. Beyond theoretical insights, our findings suggest new possibilities for thermoelectric applications using MATBG based NS junctions.