Chen Chuan
Email Address
c2dchen@nus.edu.sg
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Publication Reproduction of the Charge Density Wave Phase Diagram in 1T-TiSe2 Exposes its Excitonic Character(AMER PHYSICAL SOC, 2018-11-29) CHEN CHUAN; Singh, Bahadur; Lin, Hsin; Pereira, Vitor M; Dr Chen Chuan; CENTRE FOR ADVANCED 2D MATERIALS; PHYSICS© 2018 American Physical Society. Recent experiments suggest that excitonic degrees of freedom play an important role in precipitating the charge density wave (CDW) transition in 1T-TiSe2. Through systematic calculations of the electronic and phonon spectrum based on density functional perturbation theory, we show that the predicted critical doping of the CDW phase overshoots the experimental value by 1 order of magnitude. In contrast, an independent self-consistent many-body calculation of the excitonic order parameter and renormalized band structure is able to capture the experimental phase diagram in extremely good qualitative and quantitative agreement. This demonstrates that electron-electron interactions and the excitonic instability arising from direct electron-hole coupling are pivotal to accurately describe the nature of the CDW in this system. This has important implications to understand the emergence of superconductivity within the CDW phase of this and related systems.Publication Antiferromagnetism and chiral d-wave superconductivity from an effective $t-J-D$ model for twisted bilayer graphene(2019-01-31) Gu, Xingyu; Chen, Chuan; Leaw, Jia Ning; Laksono, Evan; Pereira, Vitor M; Vignale, Giovanni; Adam, Shaffique; Dr Chen Chuan; CENTRE FOR ADVANCED 2D MATERIALS; YALE-NUS COLLEGE; PHYSICSStarting from the strong-coupling limit of an extended Hubbard model, we develop a spin-fermion theory to study the insulating phase and pairing symmetry of the superconducting phase in twisted bilayer graphene. Assuming that the insulating phase is an anti-ferromagnetic insulator, we show that fluctuations of the anti-ferromagnetic order in the conducting phase can mediate superconducting pairing. Using a self-consistent mean-field analysis, we find that the pairing wave function has a chiral d-wave symmetry. Consistent with this observation, we show explicitly the existence of chiral Majorana edge modes by diagonalizing our proposed Hamiltonian on a finite-sized system. These results establish twisted bilayer graphene as a promising platform to realize topological superconductivity.Publication Anomalous Quantum Metal in a 2D Crystalline Superconductor with Electronic Phase Nonuniformity.(2019-05-17) Li, Linjun; Chen, Chuan; Watanabe, Kenji; Taniguchi, Takashi; Zheng, Yi; Xu, Zhuan; Pereira, Vitor M; Loh, Kian Ping; Castro Neto, Antonio H; Dr Chen Chuan; CENTRE FOR ADVANCED 2D MATERIALS; PHYSICS; CHEMISTRYThe details of the superconducting to quantum metal transition (SQMT) at T = 0 are an open problem that invokes great interest in the nature of this exotic and unexpected ground state (Ephron et al., 1996; Mason and Kapitulnik, 1999; Chervenak and Valles, 2000). However, the SQMT was not yet investigated in a crystalline 2D superconductor with coexisting and fluctuating quantum orders. Here, we report the observation of a SQMT in 2D ion-gel-gated 1T-TiSe2 (Li et al., 2016) driven by a magnetic field. A field-induced crossover between Bose quantum metal and vortex quantum creeping with an increasing field is observed. We discuss the interplay between superconducting and CDW fluctuations (discommensurations) and their relation to the anomalous quantum metal (AQM) phase. From our findings, gate-tunable 1T-TiSe2 emerges as a privileged platform to scrutinize, in a controlled way, the details of the SQMT, the role of coexisting fluctuating orders and, ultimately, to obtain a deeper understanding of the fate of superconductivity in strictly two-dimensional crystals near zero temperature.Publication Discommensuration-driven superconductivity in the charge density wave phases of transition-metal dichalcogenides(AMER PHYSICAL SOC, 2019-03-18) Chen, Chuan; Su, Lei; Neto, AH Castro; Pereira, Vitor M; Dr Chen Chuan; CENTRE FOR ADVANCED 2D MATERIALS; PHYSICS© 2019 American Physical Society. We introduce a McMillan-Ginzburg-Landau theory to describe the cooperative coexistence of charge density and superconducting order in two-dimensional crystals. With a free energy that explicitly accounts for the competition between commensurate and incommensurate ground states, we are able to map the transition between these phases and monitor the development of discommensurations in the near-commensurate regime. Attributing the enhancement of superconducting order to density wave fluctuations, we propose a coupling scheme that yields a phase diagram in qualitative agreement with experiments in conducting transition-metal dichalcogenides. The model predicts the development of nonuniform superconductivity similar to that arising from a pair density wave, with a spatial texture driven by the underlying charge density wave fluctuations.