Possible evidence of excitonic condensation in a topological insulator

Journal Article · Mon Apr 28 00:00:00 EDT 2025 · Proceedings of the National Academy of Sciences of the United States of America

DOI:https://doi.org/10.1073/pnas.2422667122· OSTI ID:2569047

Mori, Ryo ^[1]; Takasan, Kazuaki ^[2]; Ai, Ping ^[3]; Ciocys, Samuel ^[3]; Kawaguchi, Kaishu ^[4]; Kondo, Takeshi ^[4]; Morimoto, Takahiro ^[5]; Lanzara, Alessandra ^[3]

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1. Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States); University of California, Berkeley, CA (United States); Univ. of Tokyo, Chiba (Japan); Univ. of Maryland, College Park, MD (United States)
2. Materials Sciences Division; Lawrence Berkeley National Laboratory; Department of Physics; University of California; Department of Physics; The University of Tokyo
3. Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States); University of California, Berkeley, CA (United States)
4. Univ. of Tokyo, Chiba (Japan)
5. Univ. of Tokyo (Japan)

The transient excitonic condensate is a nonequilibrium electron–hole Bardeen–Cooper–Schrieffer state in a photoexcited semiconductor and semimetal, where electron–hole pairs undergo a phase transition and condense into a single coherent quantum state. Despite numerous experimental works to realize the predicted excitonic condensation phase, experimental evidence still remains elusive. This is largely due to the absence of direct measurements of a material’s transient momentum-dependent electronic structure and the excitonic state in the condensation regime. Here, using time and angle-resolved photoemission spectroscopy, we find direct evidence of a transient excitonic condensate in the spin-polarized spatially indirect excitonic topological states in Bi₂Te₃. Accompanying the formation of the excitonic topological states by photoexcitation, we reveal a splitting of the hole’s and electron’s quasi-equilibrium chemical potential followed by the band flattening and backbending of the transient topological surface state. Moreover, within the same momentum range, we report a reshaping of the bulk valence band in the form of a Mexican-hat-like Bogoliubov dispersion—hallmarks of the excitonic condensation, followed by the opening of an energy gap at the Fermi level. The fluence and temperature dependence of these renormalization effects are reminiscent of excitonic condensation within Bardeen–Cooper–Schrieffer (BCS)-like behavior. These results, together with theoretical simulation, point to the possible formation of a transient excitonic condensate and provide opportunities to manipulate topologically protected Bose condensates with light.

• Research Organization: Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES). Materials Sciences & Engineering Division (MSE); Japan Society for the Promotion of Science (JSPS); Japan Science and Technology Agency (JST); Gordon and Betty Moore Foundation
• Grant/Contract Number: AC02-05CH11231
• OSTI ID: 2569047
• Journal Information: Proceedings of the National Academy of Sciences of the United States of America, Journal Name: Proceedings of the National Academy of Sciences of the United States of America Journal Issue: 18 Vol. 122; ISSN 1091-6490; ISSN 0027-8424
• Publisher: National Academy of Sciences
• Country of Publication: United States
• Language: English

Related Subjects

ARPES
exciton
topological insulator