Possible evidence of excitonic condensation in a topological insulator

• 成果类型: Article
• 署名作者: Mori, Ryo; Takasan, Kazuaki; Ai, Ping; Ciocys, Samuel; Kawaguchi, Kaishu; Kondo, Takeshi; Morimoto, Takahiro; Lanzara, Alessandra
• 署名单位: United States Department of Energy (DOE); Lawrence Berkeley National Laboratory; University of California System; University of California Berkeley; University of Tokyo; University System of Maryland; University of Maryland College Park; University of Tokyo; University of Tokyo
• 刊物名称: PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
• ISSN/ISSBN: 0027-14545
• DOI: 10.1073/pnas.2422667122
• 发表日期: 2025-05-06
• 关键词: bose-einstein condensation; electron-hole gas; interlayer excitons; TRANSITION; state

摘要

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 quasiequilibrium 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 Mexicanby 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.