Multicomponent screening and superfluidity in gapped electron-hole double bilayer graphene with realistic bands

Abstract

Superfluidity has recently been reported in double electron-hole bilayer graphene. The multiband nature of the bilayers is important because of the very small band gaps between conduction and valence bands. The long range nature of the superfluid pairing interaction means that screening must be fully taken into account. We have carried out a systematic mean-field investigation that includes (i) contributions to screening from both intraband and interband excitations, (ii) the lowenergy band structure of bilayer graphene with its small band gap and flattened Mexican hat-like low-energy bands, (iii) the large density of states at the bottom of the bands, (iv) electron-hole pairing in the multibands, and (v) electron-hole pair transfers between the conduction and valence band condensates. We find that the superfluidity strongly modifies the intraband contributions to the screening, but that the interband contributions are unaffected. Unexpectedly, the net effect of the screening is to suppress Josephson-like pair transfers and to confine the superfluid pairing entirely to the conduction band condensate even for very small band gaps, making the system behave similarly to a one-band superfluid.

Transition Metal Dichalcogenides as Strategy for High Temperature Electron-Hole Superfluidity

“…A recent experiment confirmed the prediction in Reference [14]: enhanced tunneling conductance was reported, which is a signature of electron-hole superfluidity, in a DBG system with a 1.4 nm WSe 2 insulating barrier [15]. This signature was observed only at lower densities and is in quantitative agreement with the theoretical predictions [16] of an upper limit of the carrier density for the superfluidity. Above this threshold density, screening kills the superfluidity.…”

“…The observed transition temperature is low, T c ∼ 1.5 K. Reference [14] had predicted a maximum transition temperature of 17 K in a DBG with a 1.4 nm hBN barrier. Reference [16] pointed out the importance of the strong interband screening from the valence band, a large effect here because of the very small band gap in bilayer graphene [17]. The effect of this additional screening is to reduce the threshold density and the maximum transition temperature.…”

Tunneling and fluctuating electron-hole Cooper pairs in double bilayer graphene

“…It has been argued that the most favorable conditions for observation of electron-hole condensation are reached near the mid-point of the BEC-BCS crossover [19-21, 39, 50]. In the opposite case the paired state is a multi-band BCS-like paired state [15][16][17][18]51] where pairing correlations also span to remote bands (valence band in the layer with excess of electrons and conduction band in the layer with excess of holes).…”

Experimental conditions for the observation of electron-hole superfluidity in GaAs heterostructures

“…Superfluidity in GaAs quantum-wells differs in significant ways from superfluidity in coupled atomically-flat layers. The large band gap in GaAs eliminates the multicondensate effects and multiband screening that are important in graphene [16], and the low-lying conduction and valence bands are nearly parabolic, and not dependent on gate potentials. arXiv:1910.06631v1 [cond-mat.supr-con]…”

“…For these reasons, experimental realization of superfluidity in GaAs quantum wells is of great interest. A major challenge facing experiments is that electron-hole superfluidity in double layer systems is exclusively a low density phenomenon, because strong screening of the long-range Coulomb pairing interactions suppresses superfluidity above an onset density n 0 , and this is low [4,16,26,27]. Nevertheless, there are reports suggesting possible experimental signatures of electronhole superfluid condensation in GaAs double quantum-wells.…”