Large enhancement of thermoelectric effect by Majorana bound states coupled to a quantum dot

Abstract

This paper presents a theoretical study of the thermoelectric effect in a quantum dot (QD) side-coupled to Majorana bound states (MBSs) formed at the two ends of a Rashba nanowire. In the low-temperature regime favorable for the preparation of MBSs, the magnitude of the thermopower and figure of merit (FOM) are both enhanced by two or three orders of magnitude upon optimizing the QD-MBS coupling strength. In addition, the sign of the thermopower can be reversed by changing the QD-MBS coupling strength, a phenomenon that may be useful for detecting MBSs. The direct overlap between the MBSs also changes the sign of the thermopower and suppresses the magnitude of the FOM at low temperature. However, such an overlap has little influence on the thermopower and FOM at high temperature. In the presence of Zeeman splitting in the QD, the peaks corresponding to spin-up and spin-down thermopowers shift in opposite directions in the QD energy-level space, resulting in a large 100% spin polarization and pure spin thermopower that is crucial in spin caloritronics. Finally, the spin (charge) FOMs are enhanced (reduced) by increasing the Zeeman splitting.