A new arXiv preprint reports a set of unusual, long-lived spectral features in one-layer films of MnBi₂Te₄ after optical pumping. The authors interpret the combined pattern as signatures of a nonequilibrium exciton condensate and a crossover between BEC and BCS regimes in their theoretical framework.
The experiment used time-resolved angle-resolved photoemission spectroscopy, or trARPES, to follow the material after it was pumped with 0.5-eV infrared pulses and probed with 6-eV pulses. The researchers examined one- and two-layer films grown on silicon and strontium titanate substrates.
A signal that outlasted the excitons
In the one-layer material, a localized, round-shaped feature at Γ lasted for at least 20 picoseconds after pumping. Its transient dispersion changed from electron-like at the initial 0-ps delay to hole-like at later delays, with representative spectra shown at 10 and 19 ps. The paper assigns the feature to excitons, but that assignment comes from its spectral behaviour and comparisons with models rather than a direct measurement of exciton coherence or population.
A separate signal appeared in the valence band. Comparing spectra taken 2 ps after the pump with those taken 2 ps before it revealed a peak around 0.6 to 0.7 eV below the Fermi level. The peak remained robust for at least 10 ps while the exciton-related feature decayed and a depletion between 0.2 and 0.6 eV below the Fermi level recovered.
The response changed abruptly
The authors’ strongest clue was the response to pump intensity. They used integrated exciton spectral intensity near the Fermi level as a proxy for exciton density. That signal was approximately linear at low fluences, while the valence-band peak was negligible below a threshold fluence of 0.84 mJ/cm² and emerged abruptly above it.
The dispersion changed with both intensity and delay. At 2 ps, the hole-like pattern developed a camel-back shape above the threshold. At 4.7 times the threshold and 10 ps, it became flatter, with a minimal dip at zero momentum. The authors link the sharpening of the valence-band peak to an increasing phase-correlation length within a phase-fluctuating BKT framework.
A compelling pattern with unanswered tests
The paper reports agreement between two model-based density estimates: about 1.6 × 10¹⁴ cm⁻² at the threshold using the exciton-density proxy, and 1.4 to 2.2 × 10¹⁴ cm⁻² from an NK-based estimate. In the authors’ phase diagram, densities much greater than the threshold are labelled BCS, 1.0 to 1.2 times the threshold is the crossover range, and 0.6 to 1.0 times the threshold is labelled pseudogap BEC, or BEC*. The density values and regime labels depend on the underlying assumptions rather than direct density measurements.
The study did not directly measure long-range phase coherence. Instead, coherence was inferred from the spectra within the BKT framework, and the crossover interpretation depends on the condensate model. The supplied analysis also notes that exciton-Floquet physics and exciton condensation can both explain some of the observations.
The findings are therefore evidence of spectroscopic signatures in optically excited MnBi₂Te₄ films, not a direct demonstration of a macroscopic condensate. Independent phase-sensitive or transport measurements, tests across films and substrates, and direct calibration of exciton density and the condensate order parameter would be needed to settle the interpretation.
Paper data and sources
Original title: Signatures of a light-induced exciton condensate exhibiting BEC-BCS crossover
Authors: Khanh Duy Nguyen, Gabriele Berruto, Yunhe Bai et al.
Journal/Repository: arXiv
Status: Preprint, not yet peer-reviewed
First online: 2026-08-20
DOI: Not available
Original paper · Full text