Quantum phase transition of a two-dimensional quadrupolar system

Abstract

Ensembles with long-range interactions between particles are promising for revealing strong quantum collective effects and many-body phenomena. Here we study the ground-state phase diagram of a two-dimensional Bose system with quadrupolar interactions using a diffusion Monte Carlo technique. We predict a quantum phase transition from a gas to a solid phase. The Lindemann ratio and the condensate fraction at the transition point are γ = 0.269(4) and n0/n = 0.031(4), correspondingly. We observe the strong rotonization of the collective excitation branch in the vicinity of the phase transition point. Our results can be probed using state-of-the-art experimental systems of various nature, such as quasi-two-dimensional systems of quadrupolar excitons in transition metal dichalcogenide (TMD) tri-layers, quadrupolar molecules, and excitons or Rydberg atoms with quadrupole moments induced by strong magnetic fields.

Evidence for Exciton Crystals in a 2D Semiconductor Heterotrilayer

“…This can be understood by the addition of the linear Stark effect, , to the system Hamiltonian in eq : While the Stark term is zero for either quadrupolar or staggered dipolar lattices, it can overcome both stabilization energies ( and Δ Q ) and switch the interlayer excitons to one of the two heterojunctions to align with E⃗ (Figure d). From the experimental values of | | = 0.045 V·nm –1 , we have = −25 meV, which is within the range of the predicted stabilization energies of the exciton lattices. , …”

“…C | = 0.045 V•nm −1 , we have E p E = • = −25 meV, which is within the range of the predicted stabilization energies of the exciton lattices. 15,17 At n ex > n Mott (∼3 × 10 12 cm −2 in the heterobilayer 10,33 ), excitons in the trilayer driven by E ⃗ to one of the two heterobilayers is expected to undergo a Mott transition. This is confirmed in Figure 4e for the trilayer at n ex = 6.0 × 10 12 cm −2 , where the sharp PL peaks at E ⃗ = 0 V merge into broadened peaks as E ⃗ is swept in both directions, indicating field-driven phase transitions to electron/hole plasmas.…”

“…Interestingly, recent theoretical studies predicted that increasing the thickness from an asymmetric heterobilayer to a symmetric heterotrilayer (e.g., WSe 2 /MoSe 2 /WSe 2 ) results in the formation of robust crystalline phases of excitons. − Figure a illustrates the calculated band structures for MoSe 2 /WSe 2 and WSe 2 /MoSe 2 /WSe 2 . In the latter, there is an additional valence band splitting (Δ ± ) in the K valley due to hole hopping between the two WSe 2 layers.…”

Quadrupolar excitons and hybridized interlayer Mott insulator in a trilayer moiré superlattice

“…The interlayer excitons in the top and bottom bilayers have opposite polarities, which restores the symmetry. Their hybridization then forms a superposition state of interlayer excitons, canceling the dipolar moments and giving rise to a quadrupolar exciton, which has been predicted to enable intriguing quantum phase transition 26,[28][29][30] . In the presence of moiré coupling, this hybridization further gives rise to a new type of correlated electronic state, hybridized interlayer Mott insulator, in which the correlated holes are shared between the two WSe2 layers, and the layer population can be continuously tuned by an electric field.…”

Low-energy scatterings and pseudopotential of polarized quadrupoles

“…Huang et al found that quadrupolar Fermi gases in coupled one-dimensional tubes supported the triplet superfluid and spin-density wave phases [31]. More recently, using the diffusion Monte Carlo technique, Astrakharchik et al predicted a quantum phase transition from a gas to a solid phase in a two-dimensional Bose system with quadrupolar interactions [32].…”