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Zeng Y, Crépel V, Millis AJ. Keldysh Field Theory of Dynamical Exciton Condensation Transitions in Nonequilibrium Electron-Hole Bilayers. PHYSICAL REVIEW LETTERS 2024; 132:266001. [PMID: 38996303 DOI: 10.1103/physrevlett.132.266001] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/22/2023] [Revised: 05/16/2024] [Accepted: 05/31/2024] [Indexed: 07/14/2024]
Abstract
Recent experiments have realized steady-state electrical injection of interlayer excitons in electron-hole bilayers subject to a large bias voltage. In the ideal case in which interlayer tunneling is negligibly weak, the system is in quasiequilibrium with a reduced effective band gap. Interlayer tunneling introduces a current and drives the system out of equilibrium. In this work we derive a nonequilibrium field theory description of interlayer excitons in biased electron-hole bilayers. In the large bias limit, we find that p-wave interlayer tunneling reduces the effective band gap and increases the effective temperature for intervalley excitons. We discuss possible experimental implications for InAs/GaSb quantum wells and transition metal dichalcogenide bilayers.
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Affiliation(s)
- Yongxin Zeng
- Department of Physics, Columbia University, New York, New York 10027, USA
- Center for Computational Quantum Physics, Flatiron Institute, New York, New York 10010, USA
- Department of Physics, University of Texas at Austin, Austin, Texas 78712, USA
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Verma N, Guerci D, Queiroz R. Geometric Stiffness in Interlayer Exciton Condensates. PHYSICAL REVIEW LETTERS 2024; 132:236001. [PMID: 38905692 DOI: 10.1103/physrevlett.132.236001] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/11/2023] [Revised: 03/28/2024] [Accepted: 05/06/2024] [Indexed: 06/23/2024]
Abstract
Recent experiments have confirmed the presence of interlayer excitons in the ground state of transition metal dichalcogenide bilayers. The interlayer excitons are expected to show remarkable transport properties when they undergo Bose condensation. In this Letter, we demonstrate that quantum geometry of Bloch wave functions plays an important role in the phase stiffness of the interlayer exciton condensate. Notably, we identify a geometric contribution that amplifies the stiffness, leading to the formation of a robust condensate with an increased Berezinskii-Kosterlitz-Thouless temperature. Our results have direct implications for the ongoing experimental efforts on interlayer excitons in materials that have nontrivial quantum geometry. We provide estimates for the geometric contribution in transition metal dichalcogenide bilayers through a realistic continuum model with gated Coulomb interaction, and find that the substantially increased stiffness may allow an interlayer exciton condensate to be realized at amenable experimental conditions.
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A compact device sustains a fluid of bosons. Nature 2021. [DOI: 10.1038/d41586-021-02876-x] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
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Ma L, Nguyen PX, Wang Z, Zeng Y, Watanabe K, Taniguchi T, MacDonald AH, Mak KF, Shan J. Strongly correlated excitonic insulator in atomic double layers. Nature 2021; 598:585-589. [PMID: 34707306 DOI: 10.1038/s41586-021-03947-9] [Citation(s) in RCA: 45] [Impact Index Per Article: 15.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/18/2021] [Accepted: 08/24/2021] [Indexed: 11/09/2022]
Abstract
Excitonic insulators (EIs) arise from the formation of bound electron-hole pairs (excitons)1,2 in semiconductors and provide a solid-state platform for quantum many-boson physics3-8. Strong exciton-exciton repulsion is expected to stabilize condensed superfluid and crystalline phases by suppressing both density and phase fluctuations8-11. Although spectroscopic signatures of EIs have been reported6,12-14, conclusive evidence for strongly correlated EI states has remained elusive. Here we demonstrate a strongly correlated two-dimensional (2D) EI ground state formed in transition metal dichalcogenide (TMD) semiconductor double layers. A quasi-equilibrium spatially indirect exciton fluid is created when the bias voltage applied between the two electrically isolated TMD layers is tuned to a range that populates bound electron-hole pairs, but not free electrons or holes15-17. Capacitance measurements show that the fluid is exciton-compressible but charge-incompressible-direct thermodynamic evidence of the EI. The fluid is also strongly correlated with a dimensionless exciton coupling constant exceeding 10. We construct an exciton phase diagram that reveals both the Mott transition and interaction-stabilized quasi-condensation. Our experiment paves the path for realizing exotic quantum phases of excitons8, as well as multi-terminal exciton circuitry for applications18-20.
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Affiliation(s)
- Liguo Ma
- School of Applied and Engineering Physics, Cornell University, Ithaca, NY, USA
| | - Phuong X Nguyen
- School of Applied and Engineering Physics, Cornell University, Ithaca, NY, USA
| | - Zefang Wang
- School of Applied and Engineering Physics, Cornell University, Ithaca, NY, USA
| | - Yongxin Zeng
- Department of Physics, University of Texas at Austin, Austin, TX, USA
| | - Kenji Watanabe
- National Institute for Materials Science, Tsukuba, Japan
| | | | - Allan H MacDonald
- Department of Physics, University of Texas at Austin, Austin, TX, USA
| | - Kin Fai Mak
- School of Applied and Engineering Physics, Cornell University, Ithaca, NY, USA. .,Laboratory of Atomic and Solid State Physics, Cornell University, Ithaca, NY, USA. .,Kavli Institute at Cornell for Nanoscale Science, Ithaca, NY, USA.
| | - Jie Shan
- School of Applied and Engineering Physics, Cornell University, Ithaca, NY, USA. .,Laboratory of Atomic and Solid State Physics, Cornell University, Ithaca, NY, USA. .,Kavli Institute at Cornell for Nanoscale Science, Ithaca, NY, USA.
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Kaneko T, Sun Z, Murakami Y, Golež D, Millis AJ. Bulk Photovoltaic Effect Driven by Collective Excitations in a Correlated Insulator. PHYSICAL REVIEW LETTERS 2021; 127:127402. [PMID: 34597083 DOI: 10.1103/physrevlett.127.127402] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/17/2020] [Accepted: 08/10/2021] [Indexed: 06/13/2023]
Abstract
We investigate the bulk photovoltaic effect, which rectifies light into electric current, in a collective quantum state with correlation driven electronic ferroelectricity. We show via explicit real-time dynamical calculations that the effect of the applied electric field on the electronic order parameter leads to a strong enhancement of the bulk photovoltaic effect relative to the values obtained in a conventional insulator. The enhancements include both resonant enhancements at sub-band-gap frequencies, arising from excitation of optically active collective modes, and broadband enhancements arising from nonresonant deformations of the electronic order. The deformable electronic order parameter produces an injection current contribution to the bulk photovoltaic effect that is entirely absent in a rigid-band approximation to a time-reversal symmetric material. Our findings establish that correlation effects can lead to the bulk photovoltaic effect and demonstrate that the collective behavior of ordered states can yield large nonlinear optical responses.
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Affiliation(s)
- Tatsuya Kaneko
- Department of Physics, Columbia University, New York, New York 10027, USA
| | - Zhiyuan Sun
- Department of Physics, Columbia University, New York, New York 10027, USA
| | - Yuta Murakami
- Department of Physics, Tokyo Institute of Technology, Meguro, Tokyo 152-8551, Japan
| | - Denis Golež
- Center for Computational Quantum Physics, Flatiron Institute, New York, New York 10010, USA
- Faculty of Mathematics and Physics, University of Ljubljana, Jadranska 19, SI-1000 Ljubljana, Slovenia
- Jozef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia
| | - Andrew J Millis
- Department of Physics, Columbia University, New York, New York 10027, USA
- Center for Computational Quantum Physics, Flatiron Institute, New York, New York 10010, USA
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