A monolayer transition metal dichalcogenide as a topological excitonic insulator

Abstract

Monolayer transition metal dichalcogenides in the T' phase promise to realize the quantum spin Hall (QSH) effect at room temperature, because they exhibit a prominent spin-orbit gap between inverted bands in the bulk. Here we show that the binding energy of electron-hole pairs excited through this gap is larger than the gap itself in MoS2, a paradigmatic material that we investigate from first principles by many-body perturbation theory (MBPT). This paradoxical result hints at the instability of the T' phase against the spontaneous generation of excitons, and indeed we find that it gives rise to a recostructed `excitonic insulator' ground state. Importantly, we show that in this system topological and excitonic order cooperatively enhance the bulk gap by breaking the crystal inversion symmetry, in contrast to the case of bilayers where the frustration between the two orders is relieved by breaking time reversal symmetry. The excitonic topological insulator departs distinctively from the bare topological phase as it lifts the band spin degeneracy, which results in circular dichroism. A moderate biaxial strain applied to the system leads to two additional excitonic phases, different in their topological character but both ferroelectric as an effect of electron-electron interactions.

arXiv commentfive colour figures

References in corpus (33)

• Quantum ESPRESSO: a modular and open-source software project for quantum simulations of materials
• Quantum Spin Hall Effect in Graphene
• Quantum Spin Hall Effect and Topological Phase Transition in HgTe Quantum Wells
• $Z_2$ Topological Order and the Quantum Spin Hall Effect
• Quantum Spin Hall Insulator State in HgTe Quantum Wells
• MoS_2 as an ideal material for valleytronics: valley-selective circular dichroism and valley Hall effect
• Excitons in atomically thin transition metal dichalcogenides
• Quantum Spin Hall Effect and Topological Field Effect Transistor in Two-Dimensional Transition Metal Dichalcogenides
• Optical Spectrum of MoS${}_2$: Many-body Effects and Diversity of Exciton States
• Yambo: an \textit{ab initio} tool for excited state calculations
• Evidence for Helical Edge Modes in Inverted InAs/GaSb Quantum Wells
• Observation of the Quantum Spin Hall Effect up to 100 Kelvin in a Monolayer Crystal
• Quantum Spin Hall Effect in Inverted Type II Semiconductors
• Dielectric screening in two-dimensional insulators: Implications for excitonic and impurity states in graphane
• Edge conduction in monolayer WTe2
• Many-body perturbation theory calculations using the yambo code
• An exact Coulomb cutoff technique for supercell calculations
• Signatures of exciton condensation in a transition metal dichalcogenide
• Electrically Tunable Low Density Superconductivity in a Monolayer Topological Insulator
• Gate-induced superconductivity in a monolayer topological insulator
• Excitonic superfluid phase in Double Bilayer Graphene
• Quantum Hall Drag of Exciton Superfluid in Graphene
• Exciton condensation and charge fractionalization in a topological insulator film
• Gate-Tuned Spontaneous Exciton Insulator in Double-Quantum Wells
• Direct Visualization of 2D Topological Insulator in Single-layer 1T'-WTe2
• Theory of Electronic Ferroelectricity
• Coherent Order Parameter Oscillations in the Ground State of the Excitonic Insulator Ta2NiSe5
• Interplay of exciton condensation and quantum spin Hall effect in InAs/GaSb bilayers
• Carbon nanotubes as excitonic insulators
• Time Reversal Symmetric Topological Exciton Condensate in Bilayer HgTe Quantum Wells
• Gate tuning from exciton superfluid to quantum anomalous Hall in van der Waals heterobilayer
• Time-Reversal Symmetry-Breaking Nematic Insulators near Quantum Spin Hall Phase Transitions
• Emergence of excitonic superfluid at topological-insulator surfaces