Excitonic density wave and spin-valley superfluid in bilayer transition metal dichalcogenide

Abstract

Artificial moiré superlattices in 2d van der Waals heterostructures are a new venue for realizing and controlling correlated electronic phenomena. Recently, twisted bilayer WSe2 emerged as a new robust moiré system hosting a correlated insulator at moiré half-filling over a range of twist angle. In this work, we present a theory of this insulating state as an excitonic density wave due to intervalley electron–hole pairing. We show that exciton condensation is strongly enhanced by a van Hove singularity near the Fermi level. Our theory explains the remarkable sensitivity of the insulating gap to the vertical electric field. In contrast, the gap is weakly reduced by a perpendicular magnetic field, with quadratic dependence at low field. The different responses to electric and magnetic field can be understood in terms of pair-breaking versus non-pair-breaking effects in a BCS analog of the system. We further predict superfluid spin transport in this electrical insulator, which can be detected by optical spin injection and spatial-temporal imaging.

Dynamical mean field theory of moiré bilayer transition metal dichalcogenides: phase diagram, resistivity, and quantum criticality

“…In these plots the Fermi surfaces corresponding to half filling are indicated as white dashed lines. For a certain value of φ (φ = π/6 in the nearest neighbor model considered here), the three van-Hove points merge and form a higher order van-Hove singularity in which the density has a power-law divergence ρ( ) ∼ | | −1/3 [10,12,13]. This higher order van Hove point lies on the Fermi surfaces for half filling with carrier density n = 1, which are also such that the spin up and spin down Fermi surfaces are nested.…”

“…Moiré bilayers formed from transition metal dichalgocenide (TMDC) materials are now of intense interest [3][4][5][6][7][8][9][10][11]. In heterobilayer MoTe 2 /WSe 2 the bandwidth can be tuned over about an order of magnitude while in twisted homobilayer WSe 2 (tWSe 2 ) properties of the van Hove point including its Brillouin zone location and the degree of singularity are controlled by the displacement field [10,12,13]. tWSe 2 has recently been found to exhibit a reentrant metal-insulator transition that is controlled by the carrier concentration and displacement field, with regions of linear-T resistivity near the boundaries to the insulating phases [3,4].…”

Effect of disorder on density of states and conductivity in higher order van Hove singularities in two dimensional bands

“…[10][11][12][13][14][15][16][17][18][19] There is an even more recent surge of interest in higher order Van Hove singularities. [20][21][22][23][24] Some of the materials where they have been discovered include Sr 3 Ru 2 O 7 where a higher order (X 9 with n = 4) Van Hove singularity was shown to exist in the presence of an external magnetic field, 4,5 while a different higher order Van Hove saddle has been reported in highly overdoped graphene 25 and may be quite releVant for the recently observed phases of Bernal bilayer graphene. 26 Given the host of exotic physical phenomena that HOVHS promise, it is not a surprise that that HOVHS typically require delicate tuning of parameters in the system to obtain (effected through strain, pressure, twist, bias voltage, etc).…”

Two-dimensional Ferroelectric Ga2O3 Bilayers with Unusual Strain-engineered Interlayer Interactions

“…By vertically stacking two or more 2D vdW "Lego blocks" together, emergent applications can be realized in the fields of electrodes [11], transistors [12], optical sensors [13], and surface catalysis [14,15]. More fundamentally, 2D vdW multilayer structures serve as vital and active frontiers for several recent exciting advances in condensed matter physics, such as unconventional superconductivity [16], topological insulator [17], searching of Majorana zero modes [18], ultralow interlayer friction [19], interlayer excitons [20,21], and valleytronics [22,23].…”