Spin-charge Kondo effect for a quantum dot with side coupled Majorana zero mode

Abstract

We investigate a minimal system consisting of a quantum dot coupled to a Majorana zero mode and a normal lead. We identify the underlying screening process as a novel spin-charge Kondo effect, where the low-energy spin and charge degrees of freedom of the Majorana zero mode-quantum dot subsystem are fully screened by those in the normal lead, resulting in the formation of a spin-charge singlet. An effective low-energy model is derived, with charge fluctuations appropriately accounted for. This spin-charge Kondo effect is found to be consistent with the spin-dependent Andreev/normal boundary conditions induced by the Majorana zero mode. We demonstrate that the anomalous substructure in the spectrum and thermodynamic properties is closely tied to the proportion of the charge component in the screening cloud. The spin-charge screening cloud exhibits scaling behavior analogous to that of traditional Kondo systems, though the sub-leading even-odd effect is subtly modified by the boundary conditions. These findings enhance our understanding of Kondo physics and resolve key debates on quantum dot nanostructures with Majorana zero modes.