Searching for Unconventional Superfluid in Excitons of Monolayer Semiconductors

TL;DR

This paper proposes that excitons in monolayer transition metal dichalcogenides can exhibit a form of 2D superfluidity with true long-range order and unconventional phase transition characteristics, differing from traditional BKT behavior.

Contribution

It introduces a novel 2D bosonic system with linear dispersion and nonzero critical temperature, challenging the conventional BKT paradigm for superfluid transitions.

Findings

Nonzero Bose-Einstein condensation temperature for excitons.

Goldstone mode with $oldsymbol{ ext{q}}$~$ o$~0 dispersion $ o$~$ ext{sqrt}(q)$.

Superfluid transition deviates from BKT type at large system sizes.

Abstract

It is well known that two-dimensional (2D) bosons in homogeneous space cannot undergo real Bose-Einstein condensation, and the superfluid to normal phase transition is Berezinskii-Kosterlitz-Thouless (BKT) type, associated with vortex-antivortex pair unbinding. Here we point out a 2D bosonic system whose low energy physics goes beyond conventional paradigm of 2D {\it homogeneous} bosons, i.e., intralayer excitons in monolayer transition metal dichalcogenides. With intrinsic valley-orbit coupling and valley Zeeman energy, exciton dispersion becomes linear at small momentum, giving rise to a series of novel features. The critical temperature of Bose-Einstein condensation of these excitons is nonzero, suggesting true long-range order in 2D homogeneous system. The dispersion of Goldstone mode at long wavelength has the form $\varepsilon(\boldsymbol{q})\sim\sqrt{q}$, in contrast to conventional linear phonon spectrum. The vortex energy deviates from the usual logarithmic form with respect to system size, but instead has an additional linear term. Superfluid to normal phase transition is no longer BKT type for system size beyond a characteristic scale, without discontinuous jump in superfluid density. With the recent experimental progress on exciton fluid at thermal equilibrium in monolayer semiconductors, our work points out an experimentally accessible system to search for unconventional 2D superfluids beyond BKT paradigm.