Direct observation of room-temperature exciton condensation

TL;DR

This study demonstrates the first direct observation of exciton condensation at room temperature in monolayer tungsten diselenide, revealing a phase transition with extended spatial coherence, paving the way for quantum technologies.

Contribution

The paper reports the first room-temperature exciton condensation in a monolayer material using advanced microscopy and nanoscale trapping techniques.

Findings

Observation of a sharp degeneracy threshold indicating phase transition

Extended first-order spatial coherence with algebraic decay

Condensation signatures consistent across over 30 samples

Abstract

Exciton condensation--an interaction-driven, macroscopically coherent paired-fermion state--offers the prospect for dissipationless energy transport in solids, akin to that in superconductivity. Although their light effective mass and strong Coulomb binding favour high transition temperatures, convincing demonstrations of pure-exciton condensation have hitherto been limited to cryogenic conditions. Here, we report the direct observation of quasi-equilibrium condensation of dark excitons in monolayer tungsten diselenide at 300 K and ambient pressure. We achieve this by creating nanoscale spacing-graded Stark traps to confine free excitons, setting the finite-size scale, non-resonant off-axis optical injection to control the local density-temperature trajectory, and employing surface plasmon polariton-enhanced microsphere-assisted microscopy to boost dark-exciton emission and directly image first-order spatial coherence with sub-diffraction resolution. We observe a sharp degeneracy threshold and a clear phase transition, evidenced by extended first-order spatial coherence with algebraic decay and a critical exponent consistent with the universal Berezinskii-Kosterlitz-Thouless criterion. Identical condensation signatures are observed in over 30 independent samples. Our work establishes a room-temperature excitonic platform for exploring strongly correlated many-body physics and advancing near-dissipationless, coherent quantum technologies.