Long-range ferroelectric order in two dimensional excitonic insulators

TL;DR

This paper demonstrates that under specific conditions, two-dimensional bilayer semiconductors can exhibit true Bose-Einstein condensation of interlayer excitons, leading to coexisting superfluidity and ferroelectric order despite the Mermin-Wagner theorem.

Contribution

It reveals conditions enabling long-range ferroelectric order and Bose-Einstein condensation in 2D excitonic insulators, challenging conventional limitations.

Findings

Bose-Einstein condensation of interlayer excitons is possible in 2D bilayer semiconductors.

Superfluidity and ferroelectric order can coexist in these systems.

Specific conditions like electric field, band structure, and magnetic field are required.

Abstract

It is generally argued that Mermin-Wagner theorem excludes the possibility of long-range order in two dimensional bosonic systems at non-zero temperatures. In contrast, we show here that generic bilayer semiconductors could demonstrate true Bose-Einstein condensation of interlayer excitons. We show that the key requirements include (i) reduction of the interlayer band gap using an applied electric field so that excitons spontaneously appear in the ground state, (ii) band structure that allows for long-range electron-hole exchange interaction, and (iii) a finite magnetic field. Our results indicate that superfluidity and ferroelectric order can co-exist in two dimensional excitonic insulators.