Long-living excited states of a 2D diamagnetic exciton

TL;DR

This paper investigates how weak magnetic fields affect the excited states of 2D diamagnetic excitons, revealing novel spectral features like a 'mexican hat' dispersion and linear crossings akin to spin-orbit effects.

Contribution

It demonstrates the magnetic field-induced coupling of degenerate exciton states and analyzes the resulting modifications in the exciton spectrum, including the 'mexican hat' shape and spectrum crossings.

Findings

Magnetic field couples S and P exciton states, altering their dispersion.

The lower exciton branch develops a 'mexican hat' shape with a finite momentum minimum.

Linear crossings in the spectrum are observed and analyzed.

Abstract

Hydrogenic excited states of a 2D exciton are degenerate. In the presence of a weak magnetic field, the $S$-states with a zero momentum of the center of mass get coupled to the $P$-states with finite momentum of the center of mass. This field-induced coupling leads to a strong modification of the dispersion branches of the exciton spectrum. Namely, the lower branch acquires a shape of a "mexican hat" with a minimum at a finite momentum. At certain magnetic field, exciton branches exhibit a linear crossing, similarly to the spectrum of a 2D electron in the presence of spin-orbit coupling. While spin is not involved, degenerate $S$ and $P$ states play the role of the spin projections. Lifting of degeneracy due to diamagnetic shifts and deviation of electron-hole attraction from purely Coulomb suppresses the linear crossing.