Excitonic insulators and exciton condensates

5 problems

• Why counterflow dissipation stays finite in quantum Hall exciton condensates

  Open

  Two closely spaced electron layers in a strong magnetic field, each half filled, form an exciton condensate in which electrons of one layer bind to empty states (holes) of the other. Opposite currents in the two layers should then flow without resistance, yet a residual resistance persists at the lowest temperatures.

• Is the gap of Ta2NiSe5 excitonic or lattice driven

  Contested

  Ta2NiSe5 becomes insulating below about 326 K, and at the same temperature its crystal distorts slightly. It is unclear whether electron-hole attraction or the lattice distortion opens the gap.

• Is the charge-density wave of 1T-TiSe2 an exciton condensate

  Contested

  TiSe2 forms a charge-density wave (a periodic ripple of electron density and atom positions) below about 200 K. One experiment reported a softening electronic mode pointing to exciton condensation, while a 2025 calculation concluded that lattice fluctuations alone open the gap.

• Is the MoSe2/WSe2 dipolar excitonic insulator an exciton superfluid

  Open

  In two semiconductor layers separated by a thin insulator, electrons in one layer and holes in the other bind into long-lived excitons and make the system insulating. Drag experiments show these excitons move as units, but frictionless superflow has not been shown.

• Is the insulating state of monolayer WTe2 an excitonic insulator

  Contested

  Monolayer WTe2 is insulating at charge neutrality and conducts along its edges. Experiments suggest the bulk gap comes from electron-hole pairing, but a gap from ordinary band mixing or disorder has not been ruled out.