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Liu Y, Lv H, Guo Y, Zhu H, Shang Z, Zhao Y, Lin Y, Tai X, Guo Z, Cui X, Zhao J, Yuan B, Liu Y, Zhang G, Sun Z, Wu X, Xie Y, Wu C. Interfacial Charge-Transfer Excitonic Insulator in a Two-Dimensional Organic-Inorganic Superlattice. J Am Chem Soc 2024. [PMID: 39022834 DOI: 10.1021/jacs.4c06216] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 07/20/2024]
Abstract
Excitonic insulators are long-sought-after quantum materials predicted to spontaneously open a gap by the Bose condensation of bound electron-hole pairs, namely, excitons, in their ground state. Since the theoretical conjecture, extensive efforts have been devoted to pursuing excitonic insulator platforms for exploring macroscopic quantum phenomena in real materials. Reliable evidence of excitonic character has been obtained in layered chalcogenides as promising candidates. However, owing to the interference of intrinsic lattice instabilities, it is still debatable whether those features, such as the charge density wave and gap opening, are primarily driven by the excitonic effect or by the lattice transition. Herein, we develop an intercalation chemistry strategy for obtaining a novel charge-transfer excitonic insulator in organic-inorganic superlattice interfaces that serves as an ideal platform to decouple the excitonic effect from the lattice effect. In this system, we observe a narrow excitonic gap, formation of a charge density wave without periodic lattice distortion, and metal-insulator transition, providing visualized evidence of exciton condensation occurring in thermal equilibrium. Our findings identify self-assembly intercalation chemistry as a new strategy for developing novel excitonic insulators.
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Affiliation(s)
- Yang Liu
- Key Laboratory of Precision and Intelligent Chemistry, CAS Key Laboratory of Mechanical Behavior and Design of Materials, University of Science & Technology of China, Hefei, Anhui 230026, P. R. China
| | - Haifeng Lv
- Key Laboratory of Precision and Intelligent Chemistry, CAS Key Laboratory of Mechanical Behavior and Design of Materials, University of Science & Technology of China, Hefei, Anhui 230026, P. R. China
| | - Yuqiao Guo
- Key Laboratory of Precision and Intelligent Chemistry, CAS Key Laboratory of Mechanical Behavior and Design of Materials, University of Science & Technology of China, Hefei, Anhui 230026, P. R. China
| | - Hongen Zhu
- National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui 230029, China
| | - Zhengmin Shang
- National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui 230029, China
| | - Yingcheng Zhao
- Key Laboratory of Precision and Intelligent Chemistry, CAS Key Laboratory of Mechanical Behavior and Design of Materials, University of Science & Technology of China, Hefei, Anhui 230026, P. R. China
| | - Yue Lin
- Key Laboratory of Precision and Intelligent Chemistry, CAS Key Laboratory of Mechanical Behavior and Design of Materials, University of Science & Technology of China, Hefei, Anhui 230026, P. R. China
| | - Xiaolin Tai
- Key Laboratory of Precision and Intelligent Chemistry, CAS Key Laboratory of Mechanical Behavior and Design of Materials, University of Science & Technology of China, Hefei, Anhui 230026, P. R. China
| | - Ziyang Guo
- Key Laboratory of Precision and Intelligent Chemistry, CAS Key Laboratory of Mechanical Behavior and Design of Materials, University of Science & Technology of China, Hefei, Anhui 230026, P. R. China
| | - Xuefeng Cui
- Key Laboratory of Precision and Intelligent Chemistry, CAS Key Laboratory of Mechanical Behavior and Design of Materials, University of Science & Technology of China, Hefei, Anhui 230026, P. R. China
| | - Jiyin Zhao
- Key Laboratory of Precision and Intelligent Chemistry, CAS Key Laboratory of Mechanical Behavior and Design of Materials, University of Science & Technology of China, Hefei, Anhui 230026, P. R. China
| | - Bingkai Yuan
- Key Laboratory of Precision and Intelligent Chemistry, CAS Key Laboratory of Mechanical Behavior and Design of Materials, University of Science & Technology of China, Hefei, Anhui 230026, P. R. China
| | - Yi Liu
- National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui 230029, China
| | - Guobin Zhang
- National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui 230029, China
| | - Zhe Sun
- National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui 230029, China
| | - Xiaojun Wu
- Key Laboratory of Precision and Intelligent Chemistry, CAS Key Laboratory of Mechanical Behavior and Design of Materials, University of Science & Technology of China, Hefei, Anhui 230026, P. R. China
| | - Yi Xie
- Key Laboratory of Precision and Intelligent Chemistry, CAS Key Laboratory of Mechanical Behavior and Design of Materials, University of Science & Technology of China, Hefei, Anhui 230026, P. R. China
| | - Changzheng Wu
- Key Laboratory of Precision and Intelligent Chemistry, CAS Key Laboratory of Mechanical Behavior and Design of Materials, University of Science & Technology of China, Hefei, Anhui 230026, P. R. China
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Kukreti S, Ramawat S, Singh N, Dixit A. Strain-engineered thermophysical properties ranging from band-insulating to topological insulating phases in β-antimonene. NANOSCALE 2023; 15:13997-14006. [PMID: 37455636 DOI: 10.1039/d3nr02255b] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 07/18/2023]
Abstract
The use of strain in semiconductors allows extensive modification of their properties. Due to their robust mechanical strength and flexibility, atomically thin 2D materials are very well suited for strain engineering to extract exotic electronic and thermophysical properties. We investigated the structural, electronic, thermal, and vibrational characteristics along with the phonon and carrier dynamics of β-Sb elemental monolayers for achieving the band-insulating phase at no strain and topological insulating phase at ∼15% biaxial strain. A reduction in stiffness was noticed due to the weakening of the π and σ bonds with strain, leading to anharmonicity in the system. This was further reflected by the drop in lattice thermal conductivity (κl) from 4.5 to 3.1 W m-1 K-1 at 15% strain, i.e., in the topological phase. The appearance of helical edge states at 15% strain and meeting the Z2 invariant criterion confirm the non-trivial topological state. The significant contribution of the out-of-plane A1g vibrational mode was noticed in the topological phase compared with the band-insulating phase. Further, the observed larger reduction in hole lifetime could be attributed to strong scattering near the valence band edge. Importantly, the dominance of the out-of-plane optical modes contributes significantly along the band edges to the topological phase, which is primarily due to the reduced buckling height under strain. Thus, this work emphasizes the microscopic origin of the onset of the topological phase in strained β-Sb monolayers and provides strain-engineered structure-property correlations for better insights.
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Affiliation(s)
- Sumit Kukreti
- Advanced Materials and Device (A-MAD) Laboratory, Department of Physics, Indian Institute of Technology Jodhpur, 342030, India.
| | - Surbhi Ramawat
- Advanced Materials and Device (A-MAD) Laboratory, Department of Physics, Indian Institute of Technology Jodhpur, 342030, India.
| | - Nirpendra Singh
- Department of Physics, Khalifa University of Science and Technology, Abu Dhabi-127788, United Arab Emirates
| | - Ambesh Dixit
- Advanced Materials and Device (A-MAD) Laboratory, Department of Physics, Indian Institute of Technology Jodhpur, 342030, India.
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Sethi G, Cuma M, Liu F. Excitonic Condensate in Flat Valence and Conduction Bands of Opposite Chirality. PHYSICAL REVIEW LETTERS 2023; 130:186401. [PMID: 37204894 DOI: 10.1103/physrevlett.130.186401] [Citation(s) in RCA: 3] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/14/2022] [Revised: 07/01/2022] [Accepted: 04/14/2023] [Indexed: 05/21/2023]
Abstract
Excitonic Bose-Einstein condensation (EBEC) has drawn increasing attention recently with the emergence of 2D materials. A general criterion for EBEC, as expected in an excitonic insulator (EI) state, is to have negative exciton formation energies in a semiconductor. Here, using exact diagonalization of a multiexciton Hamiltonian modeled in a diatomic kagome lattice, we demonstrate that the negative exciton formation energies are only a prerequisite but insufficient condition for realizing an EI. By a comparative study between the cases of both conduction and valence flat bands (FBs) versus that of a parabolic conduction band, we further show that the presence and increased FB contribution to exciton formation provide an attractive avenue to stabilize the excitonic condensate, as confirmed by calculations and analyses of multiexciton energies, wave functions, and reduced density matrices. Our results warrant a similar many-exciton analysis for other known and/or new candidates of EIs and demonstrate the FBs of opposite parity as a unique platform for studying exciton physics, paving the way to material realization of spinor BEC and spin superfluidity.
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Affiliation(s)
- Gurjyot Sethi
- Department of Materials Science and Engineering, University of Utah, Salt Lake City, Utah 84112, USA
| | - Martin Cuma
- Center for High Performance Computing, University of Utah, Salt Lake City, Utah 84112, USA
| | - Feng Liu
- Department of Materials Science and Engineering, University of Utah, Salt Lake City, Utah 84112, USA
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Signatures of the exciton gas phase and its condensation in monolayer 1T-ZrTe 2. Nat Commun 2023; 14:1116. [PMID: 36849499 PMCID: PMC9971207 DOI: 10.1038/s41467-023-36857-7] [Citation(s) in RCA: 3] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/12/2022] [Accepted: 02/16/2023] [Indexed: 03/01/2023] Open
Abstract
The excitonic insulator (EI) is a Bose-Einstein condensation (BEC) of excitons bound by electron-hole interaction in a solid, which could support high-temperature BEC transition. The material realization of EI has been challenged by the difficulty of distinguishing it from a conventional charge density wave (CDW) state. In the BEC limit, the preformed exciton gas phase is a hallmark to distinguish EI from conventional CDW, yet direct experimental evidence has been lacking. Here we report a distinct correlated phase beyond the 2×2 CDW ground state emerging in monolayer 1T-ZrTe2 and its investigation by angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy (STM). The results show novel band- and energy-dependent folding behavior in a two-step process, which is the signatures of an exciton gas phase prior to its condensation into the final CDW state. Our findings provide a versatile two-dimensional platform that allows tuning of the excitonic effect.
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Shi Q, Shih EM, Rhodes D, Kim B, Barmak K, Watanabe K, Taniguchi T, Papić Z, Abanin DA, Hone J, Dean CR. Bilayer WSe 2 as a natural platform for interlayer exciton condensates in the strong coupling limit. NATURE NANOTECHNOLOGY 2022; 17:577-582. [PMID: 35437321 DOI: 10.1038/s41565-022-01104-5] [Citation(s) in RCA: 16] [Impact Index Per Article: 8.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/12/2021] [Accepted: 02/21/2022] [Indexed: 06/14/2023]
Abstract
Exciton condensates (ECs) are macroscopic coherent states arising from condensation of electron-hole pairs1. Bilayer heterostructures, consisting of two-dimensional electron and hole layers separated by a tunnel barrier, provide a versatile platform to realize and study ECs2-4. The tunnel barrier suppresses recombination, yielding long-lived excitons5-10. However, this separation also reduces interlayer Coulomb interactions, limiting the exciton binding strength. Here, we report the observation of ECs in naturally occurring 2H-stacked bilayer WSe2. In this system, the intrinsic spin-valley structure suppresses interlayer tunnelling even when the separation is reduced to the atomic limit, providing access to a previously unattainable regime of strong interlayer coupling. Using capacitance spectroscopy, we investigate magneto-ECs, formed when partially filled Landau levels couple between the layers. We find that the strong-coupling ECs show dramatically different behaviour compared with previous reports, including an unanticipated variation of EC robustness with the orbital number, and find evidence for a transition between two types of low-energy charged excitations. Our results provide a demonstration of tuning EC properties by varying the constituent single-particle wavefunctions.
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Affiliation(s)
- Qianhui Shi
- Department of Physics, Columbia University, New York, NY, USA
- Department of Physics and Astronomy, University of California, Los Angeles, CA, USA
| | - En-Min Shih
- Department of Physics, Columbia University, New York, NY, USA
- Physical Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, MD, USA
- Department of Physics, Georgetown University, Washington, DC, USA
| | - Daniel Rhodes
- Department of Mechanical Engineering, Columbia University, New York, NY, USA
| | - Bumho Kim
- Department of Mechanical Engineering, Columbia University, New York, NY, USA
| | - Katayun Barmak
- Department of Applied Physics and Applied Mathematics, Columbia University, New York, NY, USA
| | - Kenji Watanabe
- Research Center for Functional Materials, National Institute for Materials Science, Tsukuba, Japan
| | - Takashi Taniguchi
- International Center for Materials Nanoarchitectonics, National Institute for Materials Science, Tsukuba, Japan
| | - Zlatko Papić
- School of Physics and Astronomy, University of Leeds, Leeds, UK
| | - Dmitry A Abanin
- Department of Theoretical Physics, University of Geneva, Geneva, Switzerland
| | - James Hone
- Department of Mechanical Engineering, Columbia University, New York, NY, USA
| | - Cory R Dean
- Department of Physics, Columbia University, New York, NY, USA.
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Rickhaus P, de Vries FK, Zhu J, Portoles E, Zheng G, Masseroni M, Kurzmann A, Taniguchi T, Watanabe K, MacDonald AH, Ihn T, Ensslin K. Correlated electron-hole state in twisted double-bilayer graphene. Science 2021; 373:1257-1260. [PMID: 34516786 DOI: 10.1126/science.abc3534] [Citation(s) in RCA: 23] [Impact Index Per Article: 7.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/19/2022]
Abstract
[Figure: see text].
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Affiliation(s)
- Peter Rickhaus
- Solid State Physics Laboratory, ETH Zürich, CH-8093 Zürich, Switzerland
| | | | - Jihang Zhu
- Department of Physics, University of Texas at Austin, Austin, TX 78712, USA
| | - Elías Portoles
- Solid State Physics Laboratory, ETH Zürich, CH-8093 Zürich, Switzerland
| | - Giulia Zheng
- Solid State Physics Laboratory, ETH Zürich, CH-8093 Zürich, Switzerland
| | - Michele Masseroni
- Solid State Physics Laboratory, ETH Zürich, CH-8093 Zürich, Switzerland
| | - Annika Kurzmann
- Solid State Physics Laboratory, ETH Zürich, CH-8093 Zürich, Switzerland
| | - Takashi Taniguchi
- National Institute for Material Science, 1-1 Namiki, Tsukuba 305-0044, Japan
| | - Kenji Watanabe
- National Institute for Material Science, 1-1 Namiki, Tsukuba 305-0044, Japan
| | - Allan H MacDonald
- Department of Physics, University of Texas at Austin, Austin, TX 78712, USA
| | - Thomas Ihn
- Solid State Physics Laboratory, ETH Zürich, CH-8093 Zürich, Switzerland.,Quantum Center, ETH Zürich, 8093 Zürich, Switzerland
| | - Klaus Ensslin
- Solid State Physics Laboratory, ETH Zürich, CH-8093 Zürich, Switzerland.,Quantum Center, ETH Zürich, 8093 Zürich, Switzerland
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Sethi G, Zhou Y, Zhu L, Yang L, Liu F. Flat-Band-Enabled Triplet Excitonic Insulator in a Diatomic Kagome Lattice. PHYSICAL REVIEW LETTERS 2021; 126:196403. [PMID: 34047585 DOI: 10.1103/physrevlett.126.196403] [Citation(s) in RCA: 15] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/21/2020] [Accepted: 04/15/2021] [Indexed: 06/12/2023]
Abstract
The excitonic insulator (EI) state is a strongly correlated many-body ground state, arising from an instability in the band structure toward exciton formation. We show that the flat valence and conduction bands of a semiconducting diatomic Kagome lattice, as exemplified in a superatomic graphene lattice, can possibly conspire to enable an interesting triplet EI state, based on density-functional theory calculations combined with many-body GW and Bethe-Salpeter equation. Our results indicate that massive carriers in flat bands with highly localized electron and hole wave functions significantly reduce the screening and enhance the exchange interaction, leading to an unusually large triplet exciton binding energy (∼1.1 eV) exceeding the GW band gap by ∼0.2 eV and a large singlet-triplet splitting of ∼0.4 eV. Our findings enrich once again the intriguing physics of flat bands and extend the scope of EI materials.
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Affiliation(s)
- Gurjyot Sethi
- Department of Materials Science and Engineering, University of Utah, Salt Lake City, Utah 84112, USA
| | - Yinong Zhou
- Department of Materials Science and Engineering, University of Utah, Salt Lake City, Utah 84112, USA
| | - Linghan Zhu
- Department of Physics, Washington University in St. Louis, St. Louis, Missouri 63130, USA
| | - Li Yang
- Department of Physics, Washington University in St. Louis, St. Louis, Missouri 63130, USA
- Institute of Materials Science and Engineering, Washington University in St. Louis, St. Louis, Missouri 63130, USA
| | - Feng Liu
- Department of Materials Science and Engineering, University of Utah, Salt Lake City, Utah 84112, USA
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