Quantum-electrodynamical approach to the exciton spectrum in Transition-Metal Dichalcogenides

TL;DR

This paper uses quantum-field theory to accurately determine exciton energy spectra and lifetimes in transition-metal dichalcogenides, revealing significant valley-dependent energy splitting due to electromagnetic interactions.

Contribution

It introduces a quantum-electrodynamical approach employing Bethe-Salpeter and Schwinger-Dyson equations to analyze excitons in TMDs, accounting for full electromagnetic interactions.

Findings

Exciton energy splitting of about 170 meV between valleys.

Effective Zeeman magnetic field of approximately 1400 T.

Valley selection mechanism via dynamical TR symmetry breaking.

Abstract

Manipulation of intrinsic electron degrees of freedom, such as charge and spin, gives rise to electronics and spintronics, respectively. Electrons in monolayer materials with a honeycomb lattice structure, such as the Transition-Metal Dichalcogenides (TMD's), can be distinguished according to the region (valley) of the Brillouin zone to which they belong. Valleytronics, the manipulation of this electron's property, is expected to set up a new era in the realm of electronic devices. In this work, we accurately determine the energy spectrum and lifetimes of exciton (electron-hole) bound-states for different TMD materials, namely WSe$_2$, WS$_2$ and MoS$_2$. For all of them, we obtain a splitting of the order of 170 meV between the exciton energies from different valleys, corresponding to an effective Zeeman magnetic field of 1400 T. Our approach, which employs quantum-field theory (QFT) techniques based on the Bethe-Salpeter equation and the Schwinger-Dyson formalism, takes into account the full electromagnetic interaction among the electrons. The valley selection mechanism operates through the dynamical breakdown of the time-reversal (TR) symmetry, which originally interconnects the two valleys. This symmetry is spontaneously broken whenever the full electromagnetic interaction vertex is used to probe the response of the system to an external field.