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Que Y, Chan YH, Jia J, Das A, Tong Z, Chang YT, Cui Z, Kumar A, Singh G, Mukherjee S, Lin H, Weber B. A Gate-Tunable Ambipolar Quantum Phase Transition in a Topological Excitonic Insulator. ADVANCED MATERIALS (DEERFIELD BEACH, FLA.) 2024; 36:e2309356. [PMID: 38010877 DOI: 10.1002/adma.202309356] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/11/2023] [Revised: 10/26/2023] [Indexed: 11/29/2023]
Abstract
Coulomb interactions among electrons and holes in 2D semimetals with overlapping valence and conduction bands can give rise to a correlated insulating ground state via exciton formation and condensation. One candidate material in which such excitonic state uniquely combines with non-trivial band topology are atomic monolayers of tungsten ditelluride (WTe2 ), in which a 2D topological excitonic insulator (2D TEI) forms. However, the detailed mechanism of the 2D bulk gap formation in WTe2 , in particular with regard to the role of Coulomb interactions, has remained a subject of ongoing debate. Here, it shows that WTe2 is susceptible to a gate-tunable quantum phase transition, evident from an abrupt collapse of its 2D bulk energy gap upon ambipolar field-effect doping. Such gate tunability of a 2D TEI, into either n- and p-type semimetals, promises novel handles of control over non-trivial 2D superconductivity with excitonic pairing.
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Affiliation(s)
- Yande Que
- School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore, 637371, Singapore
| | - Yang-Hao Chan
- Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, 106319, Taiwan
- Physics Division, National Center of Theoretical Physics, Taipei, 10617, Taiwan
| | - Junxiang Jia
- School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore, 637371, Singapore
| | - Anirban Das
- Department of Physics, Indian Institute of Technology Madras, Chennai, Tamil Nadu, 600036, India
- Center for Atomistic Modelling and Materials Design, Indian Institute of Technology Madras, Chennai, Tamil Nadu, 600036, India
| | - Zhengjue Tong
- School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore, 637371, Singapore
| | - Yu-Tzu Chang
- Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, 106319, Taiwan
| | - Zhenhao Cui
- School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore, 637371, Singapore
| | - Amit Kumar
- School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore, 637371, Singapore
| | - Gagandeep Singh
- School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore, 637371, Singapore
| | - Shantanu Mukherjee
- Department of Physics, Indian Institute of Technology Madras, Chennai, Tamil Nadu, 600036, India
- Center for Atomistic Modelling and Materials Design, Indian Institute of Technology Madras, Chennai, Tamil Nadu, 600036, India
- Quantum Centre for Diamond and Emergent Materials, Indian Institute of Technology Madras, Chennai, Tamil Nadu, 600036, India
| | - Hsin Lin
- Institute of Physics, Academia Sinica, Taipei, 115201, Taiwan
| | - Bent Weber
- School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore, 637371, Singapore
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Crépel V, Guerci D, Cano J, Pixley JH, Millis A. Topological Superconductivity in Doped Magnetic Moiré Semiconductors. PHYSICAL REVIEW LETTERS 2023; 131:056001. [PMID: 37595206 DOI: 10.1103/physrevlett.131.056001] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/06/2023] [Accepted: 07/06/2023] [Indexed: 08/20/2023]
Abstract
We show that topological superconductivity may emerge upon doping of transition metal dichalcogenide heterobilayers above an integer-filling magnetic state of the topmost valence moiré band. The effective attraction between charge carriers is generated by an electric p-wave Feshbach resonance arising from interlayer excitonic physics and has a tunable strength, which may be large. Together with the low moiré carrier densities reachable by gating, this robust attraction enables access to the long-sought p-wave BEC-BCS transition. The topological protection arises from an emergent time reversal symmetry occurring when the magnetic order and long wavelength magnetic fluctuations do not couple different valleys. The resulting topological superconductor features helical Majorana edge modes, leading to half-integer quantized spin-thermal Hall conductivity and to charge currents induced by circularly polarized light or other time-reversal symmetry-breaking fields.
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Affiliation(s)
- Valentin Crépel
- Center for Computational Quantum Physics, Flatiron Institute, New York, New York 10010, USA
| | - Daniele Guerci
- Center for Computational Quantum Physics, Flatiron Institute, New York, New York 10010, USA
| | - Jennifer Cano
- Center for Computational Quantum Physics, Flatiron Institute, New York, New York 10010, USA
- Department of Physics and Astronomy, Stony Brook University, Stony Brook, New York 11794, USA
| | - J H Pixley
- Center for Computational Quantum Physics, Flatiron Institute, New York, New York 10010, USA
- Department of Physics and Astronomy, Center for Materials Theory, Rutgers University, Piscataway, New Jersey 08854, USA
| | - Andrew Millis
- Center for Computational Quantum Physics, Flatiron Institute, New York, New York 10010, USA
- Department of Physics, Columbia University, New York, New York 10027, USA
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Disorder-dominated quantum criticality in moiré bilayers. Nat Commun 2022; 13:7469. [DOI: 10.1038/s41467-022-35103-w] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/27/2022] [Accepted: 11/16/2022] [Indexed: 12/04/2022] Open
Abstract
AbstractMoiré bilayer materials have recently attracted much attention following the discovery of various correlated insulating states at specific band fillings. Here we discuss the metal-insulator transitions (MITs) that have been observed in the same devices, but at fillings far from the strongly correlated regime dominated by Mott-like physics, displaying many similarities to other examples of disorder-dominated MITs. We propose a minimal theoretical model describing the interplay of interactions and disorder, which is able to capture all the universal aspects of quantum criticality, as observed in experiments performed on several devices.
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Morales-Durán N, Hu NC, Potasz P, MacDonald AH. Nonlocal Interactions in Moiré Hubbard Systems. PHYSICAL REVIEW LETTERS 2022; 128:217202. [PMID: 35687431 DOI: 10.1103/physrevlett.128.217202] [Citation(s) in RCA: 5] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/16/2021] [Revised: 01/19/2022] [Accepted: 05/02/2022] [Indexed: 06/15/2023]
Abstract
Moiré materials formed in two-dimensional semiconductor heterobilayers are quantum simulators of Hubbard-like physics with unprecedented electron density and interaction strength tunability. Compared to atomic scale Hubbard-like systems, electrons or holes in moiré materials are less strongly attracted to their effective lattice sites because these are defined by finite-depth potential extrema. As a consequence, nonlocal interaction terms like interaction-assisted hopping and intersite exchange are more relevant. We theoretically demonstrate the possibility of tuning the strength of these coupling constants to favor unusual states of matter, including spin liquids, insulating ferromagnets, and superconductors.
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Affiliation(s)
| | - Nai Chao Hu
- Department of Physics, University of Texas at Austin, Austin, Texas 78712, USA
| | - Pawel Potasz
- Institute of Physics, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University, Grudziadzka 5, 87-100 Toruń, Poland
| | - Allan H MacDonald
- Department of Physics, University of Texas at Austin, Austin, Texas 78712, USA
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Abstract
We present a mechanism for unconventional superconductivity in doped band insulators, where short-ranged pairing interaction arises from Coulomb repulsion due to virtual interband or excitonic processes. Remarkably, electron pairing is found upon infinitesimal doping, giving rise to Bose–Einstein condensate (BEC)–Bardeen–Cooper–Schrieffer (BCS) crossover at low density. Our theory explains puzzling behaviors of superconductivity and predicts spin-triplet pairing in electron-doped ZrNCl and WTe2. Despite being of fundamental importance and potential interest for topological quantum computing, spin-triplet superconductors remain rare in solid state materials after decades of research. In this work, we present a three-particle mechanism for spin-triplet superconductivity in multiband systems, where an effective attraction between doped electrons is produced from the Coulomb repulsion via a virtual interband transition involving a third electron [V. Crépel, L. Fu, Sci. Adv. 7, eabh2233 (2021)]. Our theory is analytically controlled by an interband hybridization parameter and explicitly demonstrated in doped band insulators with the example of an extended Hubbard model. Our theory of exciton-mediated pairing reveals how, as a matter of principle, a two-particle bound state can arise from the strong electron repulsion upon doping, opening a viable path to Bose–Einstein condensate (BEC)–Bardeen–Cooper–Schrieffer (BCS) physics in solid state systems. In light of this theory, we propose that recently discovered dilute superconductors such as ZrNCl, WTe2, and moiré materials can be spin-triplet and compare the expected consequences of our theory with experimental data.
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