Possible evidence of excitonic condensation in a topological insulator

• DOI: 10.1073/pnas.2422667122
• Publication Date: 2025-04-28T19:00:36Z
• Authors: Ryo Mori; Kazuaki Takasan; Ping Ai; Samuel Ciocys; Kaishu Kawaguchi; Takeshi Kondo; Takahiro Morimoto; Alessandra Lanzara
• Keywords: Topological insulator

The transient excitonic condensate is a nonequilibrium electron–hole Bardeen–Cooper–Schrieffer state in photoexcited semiconductor and semimetal, where pairs undergo phase transition condense into single coherent quantum state. Despite numerous experimental works to realize the predicted condensation phase, evidence still remains elusive. This largely due absence of direct measurements material’s momentum-dependent electronic structure regime. Here, using time angle-resolved photoemission spectroscopy, we find spin-polarized spatially indirect topological states Bi 2 Te 3 . Accompanying formation by photoexcitation, reveal splitting hole’s electron’s quasi-equilibrium chemical potential followed band flattening backbending surface Moreover, within same momentum range, report reshaping bulk valence form Mexican-hat-like Bogoliubov dispersion—hallmarks condensation, opening an energy gap at Fermi level. fluence temperature dependence these renormalization effects are reminiscent (BCS)-like behavior. These results, together with theoretical simulation, point possible provide opportunities manipulate topologically protected Bose condensates light.