Signatures of a light-induced exciton condensate exhibiting BEC-BCS crossover

Khanh Duy Nguyen; Gabriele Berruto; Yunhe Bai; Thomas Marchese; Woojoo Lee; Haoran Lin; Jiangang Yang; Chong Liu; Y. Shirley Meng; Shuolong Yang

arXiv · 2608.20310

Signatures of a light-induced exciton condensate exhibiting BEC-BCS crossover

Khanh Duy Nguyen·Gabriele Berruto·Yunhe Bai·Thomas Marchese·Woojoo Lee·Haoran Lin·Jiangang Yang·Chong Liu·Y. Shirley Meng·Shuolong Yang

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Abstract

Exciton condensates provide a platform to study quasiparticle pairing, Bose-Einstein condensation-Bardeen-Cooper-Schrieffer (BEC-BCS) crossover, and excitonic topological phenomena. Achieving a nonequilibrium exciton condensate allows the ultimate tunability of these emergent phenomena. Yet, evidence of a light-induced, nonequilibrium exciton condensate and its BEC-BCS crossover remains elusive. Here, we use time- and angle-resolved photoemission spectroscopy to demonstrate signatures of a non-equilibrium exciton condensate and its BEC-BCS crossover in monolayer MnBi2Te4. Following optical excitation, a distinctive hole-like dispersion representing excitons emerges and persists for >20 ps. Strikingly, energy-domain sharpening in the valence band occurs 2 ps after time zero and exhibits a sharp onset at a threshold pump fluence of 0.84 mJ/cm2. The delayed and strongly nonlinear response is difficult to reconcile with transient field effects or conventional carrier-induced band shifts but is consistent with a model of exciton condensation governed by a Berezinskii-Kosterlitz-Thouless transition. The estimated threshold exciton density agrees quantitatively with the Nelson-Kosterlitz critical density. At higher fluences, the exciton feature develops a camel-back-shaped dispersion, consistent with the BEC-BCS crossover in the condensate framework. Our work establishes ultrathin MnBi2Te4 as a model system for studying nonequilibrium exciton condensates with a connection to superconductivity and exciton-driven topological phases.

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Khanh Duy Nguyen, Gabriele Berruto, Yunhe Bai, Thomas Marchese, Woojoo Lee, Haoran Lin, Jiangang Yang, Chong Liu, Y. Shirley Meng, Shuolong Yang. 2026-08-20. Signatures of a light-induced exciton condensate exhibiting BEC-BCS crossover. https://arxiv.org/abs/2608.20310

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