Evidence for a Monolayer Excitonic Insulator

Abstract

The interplay between topology and correlations can generate a variety of quantum phases, many of which remain to be explored. Recent advances have identified monolayer WTe2 as a promising material for doing so in a highly tunable fashion. The ground state of this two-dimensional (2D) crystal can be electrostatically tuned from a quantum spin Hall insulator (QSHI) to a superconductor. However, much remains unknown about the gap-opening mechanism of the insulating state. Here we report evidence that the QSHI is also an excitonic insulator (EI), arising from the spontaneous formation of electron-hole bound states (excitons). We reveal the presence of an intrinsic insulating state at the charge neutrality point (CNP) in clean samples and confirm the correlated nature of this charge-neutral insulator by tunneling spectroscopy. We provide evidence against alternative scenarios of a band insulator or a localized insulator and support the existence of an EI phase in the clean limit. These observations lay the foundation for understanding a new class of correlated insulators with nontrivial topology and identify monolayer WTe2 as a promising candidate for exploring quantum phases of ground-state excitons.

39 pages; Nat. Phys. (2021)

References in corpus (8)

• Quantum Spin Hall Effect and Topological Field Effect Transistor in Two-Dimensional Transition Metal Dichalcogenides
• Observation of the Quantum Spin Hall Effect up to 100 Kelvin in a Monolayer Crystal
• Evidence for an excitonic insulator phase in 1T-TiSe${}_2$
• Semimetal to semimetal charge density wave transition in 1T-TiSe${}_2$
• Strain Tunable Semimetal-Topological-Insulator Transition in Monolayer 1T'-WTe2
• Signatures of phonon and defect-assisted tunneling in planar metal-hexagonal boron nitride-graphene junctions
• Possible Excitonic Insulating Phase in Quantum-Confined Sb Nanoflakes
• Exciton topology and condensation in a model quantum spin Hall insulator

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• Atomically Thin Quantum Spin Hall Insulators
• Bilayer WSe${}_2$ as a natural platform for interlayer exciton condensates in the strong coupling limit
• Evidences for the exciton gas phase and its condensation in monolayer 1T-ZrTe2
• Excitonic Condensate in Flat Valence and Conduction Bands of Opposite Chirality
• Second order Josephson effect in excitonic insulators
• Signature of spin-triplet exciton condensations in LaCoO${}_3$ at ultrahigh magnetic fields up to 600 T
• Unveiling the underlying interactions in Ta2NiSe5 from photo-induced lifetime change
• Quantum oscillations in interaction-driven insulators
• Theory of competing excitonic orders in insulating WTe${}_2$ monolayers
• Metastable polymorphic phases in monolayer TaTe2
• Topological charge pumping in excitonic insulators
• Exciton condensation in strongly correlated quantum spin Hall insulators
• Synergistic correlated states and nontrivial topology in coupled graphene-insulator heterostructures
• Fermi edge singularity in neutral electron-hole system
• Excitonic Insulator to Superconductor Phase Transition in Ultra-Compressed Helium
• Theory of Glide Symmetry Protected Helical Edge States in WTe${}_2$ Monolayer
• Reshaped Weyl fermionic dispersions driven by Coulomb interactions in MoTe2
• Interlayer electron-hole friction in tunable twisted bilayer graphene semimetal
• Interplay of quantum spin Hall effect and spontaneous time-reversal symmetry breaking in electron-hole bilayers I: Transport properties
• Low-energy effective theory and anomalous Hall effect in monolayer ${\mathrm{WTe}}_2$
• Excitonic insulators and superfluidity in 2D bilayers without external fields
• Interplay of quantum spin Hall effect and spontaneous time-reversal symmetry breaking in electron-hole bilayers II: Zero-field topological superconductivity
• Thermal robustness of the quantum spin Hall phase in monolayer WTe${}_2$
• Excitonic Metal and Non-Fermi Liquid Behaviour in Twisted Double Bilayer Graphene near Charge Neutrality
• Realization of graphene logics in an exciton-enhanced insulating phase
• Optimizing Floquet engineering for non-equilibrium steady states with gradient-based methods
• Transitions in Xenes between excitonic, topological and trivial insulator phases: influence of screening, band dispersion and external electric field
• Excitonic instability in transition metal dichalcogenides