Strongly correlated excitonic insulator in atomic double layers

Abstract

Excitonic insulators (EIs) arise from the formation of bound electron-hole pairs (excitons) 1, 2 in semiconductors and provide a solid-state platform for quantum many-boson physics 3-8 . Strong exciton-exciton repulsion is expected to stabilize condensed superfluid and crystalline phases by suppressing both density and phase fluctuations 8-11 . Although spectroscopic signatures of EIs have been reported 6, 12-14 , conclusive evidence for strongly correlated EI states has remained elusive. Here, we demonstrate a strongly correlated spatially indirect two-dimensional (2D) EI ground state formed in transition metal dichalcogenide (TMD) semiconductor double layers. An equilibrium interlayer exciton fluid is formed when the bias voltage applied between the two electrically isolated TMD layers, is tuned to a range that populates bound electron-hole pairs, but not free electrons or holes [15][16][17] . Capacitance measurements show that the fluid is exciton-compressible but chargeincompressibledirect thermodynamic evidence of the EI. The fluid is also strongly correlated with a dimensionless exciton coupling constant exceeding 10. We further construct an exciton phase diagram that reveals both the Mott transition and interaction-stabilized quasi-condensation. Our experiment paves the path for realizing the exotic quantum phases of excitons 8 , as well as multi-terminal exciton circuitry for applications 18-20 . 24 . We achieve these properties in atomic 2D semiconductor double layers, whose emergence has opened new paths to realize and control the many-exciton states [15][16][17][23][24][25][26][27][28][29][30][31][32] . Both the formation of dipolar excitons (i.e. excitons with a permanent dipole) and the reduced dielectric screening of the electrostatic interactions favor strong exciton-exciton repulsion. Separate electrical contacts to isolated electron and hole layers provide a reservoir for interlayer excitons with a conveniently tunable chemical potential that allows us to probe thermodynamic properties of the exciton fluid 15,16,24 .

Spatially indirect intervalley excitons in bilayer WSe2

“…Depending on whether the wave functions of electrons and holes are spatially separated, excitons can be divided into two types: spatially direct and indirect excitons. Because of the separation of the electrons and holes, spatially indirect excitons have a much longer lifetime than spatially direct excitons and are predicted to exhibit a wide spectrum of emergent physical phenomena, including but not limit to quantum-confined Stark effect [5][6][7], Bose-Einstein condensation [8][9][10][11][12][13][14], strongly correlated excitonic insulator states [15][16][17], high-temperature superconductivity [18], valley physics [19][20][21], and dissipationless exciton transistors [22][23][24]. The recent emergence of two-dimensional transition metal dichalcogenides (TMDCs) and their van der Waals (vdW) heterostructures offers an unprecedented platform to realize spatially indirect excitons.…”

Metal-insulator transitions in bilayer electron-hole systems in transition metal dichalcogenides

“…The total capacitance contains contributions from the self-capacitances due to the e-e and h-h interactions and the mutual capacitance from the e-h interaction. Such type of capacitance measurements have recently been carried out by Ma and co-workers [32] on similar structures. From where the solid lines cross the dashed lines we obtain the phase boundary of the transition in Fig.…”

Through the Lens of a Momentum Microscope: Viewing Light‐Induced Quantum Phenomena in 2D Materials

“…Beyond the appearance of tightly bound two-body excitons, the enhanced Coulomb interactions in 2D materials also result in the formation of the next level of complexity in a few-particle bound excitonic states, such as the three-particle trions and four-particle bi-excitons [9,114,115,120,121] . With increasing the exciton and free-carrier densities, one can generate increasingly complex multiexciton states and exciton condensed phases [122,123] , and corresponding multi-exciton phenomena such as exciton-exciton annihilation [71,72] and Mott dissociation [124,125] . These have been explored…”