AI 中文总结
本研究利用时间分辨角分辨光电子能谱，在单层MnBi₂Te₄中证实光诱导非平衡激子凝聚体及其BEC-BCS渡越的特征，确立其为相关研究的模型系统。
AI 中文摘要
激子凝聚体为研究准粒子配对、玻色-爱因斯坦凝聚-巴丁-库珀-施里弗（BEC-BCS）渡越以及激子拓扑现象提供了平台。实现非平衡激子凝聚体可对这些涌现现象实现极致可调性，但光诱导非平衡激子凝聚体及其BEC-BCS渡越的证据仍难以获得。本研究中，我们利用时间分辨角分辨光电子能谱，在单层MnBi₂Te₄中证实了非平衡激子凝聚体及其BEC-BCS渡越的特征。光激发后，代表激子的独特空穴型色散出现并持续超过20皮秒。值得注意的是，价带的能量域锐化在时间零点后2皮秒发生，且在阈值泵浦 fluence（注量）为0.84 mJ/cm²时呈现急剧 onset（起始）。这种延迟且强非线性的响应难以与瞬态场效应或常规载流子诱导的能带偏移相契合，但与由Berezinskii-Kosterlitz-Thouless相变主导的激子凝聚模型一致。估算的阈值激子密度与Nelson-Kosterlitz临界密度定量吻合。在更高 fluence 下，激子特征形成驼峰型色散，与凝聚体框架中的BEC-BCS渡越相符。本研究确立了超薄MnBi₂Te₄作为研究与超导性及激子驱动拓扑相相关的非平衡激子凝聚体的模型系统。
英文摘要
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.
Comments25 pages, 5 figures
