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

Abstract

The experimental parameter ranges needed to generate superfluidity in optical and drag experiments in GaAs double quantum wells are determined, using a formalism that includes self-consistent screening of the Coulomb pairing interaction in the presence of the superfluid. The very different electron and hole masses in GaAs make this a particularly interesting system for superfluidity, with exotic superfluid phases predicted in the BCS-BEC crossover regime. We find that the density and temperature ranges for superfluidity cover the range for which optical experiments have observed indications of superfluidity, but that existing drag experiments lie outside the superfluid range. However we also show that for samples with low mobility with no macroscopically connected superfluidity, if the superfluidity survived in randomly distributed localized pockets, standard quantum capacitance measurements could detect these pockets.While Bose Einstein Condensation (BEC) and the BCS-BEC crossover phenomena in superfluidity have been extensively studied for ultracold Fermi atoms[1-3], it is probable that practical applications will instead be based on superfluidity in solid state devices. Existence of superfluidity in coupled atomically-flat layers in semiconductor heterostructures has been theoretically predicted [4,5], while recent observations of dramatically enhanced tunneling at equal densities in electron-hole double bilayer sheets of graphene [6,7] and in double monolayers of transition metal dichalcogenide monolayers [8,9] are strong experimental indications for electron-hole condensation [10].Electron-hole superfluidity and the BCS-BEC crossover was first proposed for an excitonic system in a conventional semiconductor heterostructure of double quantum-wells in GaAs[11]. This was based on extensions of earlier work on exciton condensation [12][13][14][15]. To block electron-hole recombination, Refs. 14, 15 proposed spatially separating the electrons and holes in a heterostructure consisting of two layers separated by an insulating barrier. 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]

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

“…T BKT is proportional to the density (see Equation (18)), so to achieve high transition temperature the density must be maximized. The threshold densities n 0 in Figure 2a are much larger than n 0 ∼ 8 × 10 11 cm −2 observed in DBG [15,16], n 0 ∼ 4 × 10 12 cm −2 for phosphorene [28], and n 0 ∼ 5 × 10 10 cm −2 for GaAs [5]. The reason is the large electron and hole effective masses in the TMDs [20] which lead to strong binding energies and thus large superfluid gaps ∆.…”

“…Since the average effective separation is large, there is superfluidity only for densities 10 10 cm −2 . The predicted transition temperatures for this system are relatively low, T BKT ∼ 1 K. To date, there have been no definitive observations of superfluidity in GaAs DQWs [5]. It would be particularly interesting to observe superfluidity in this system, because it is likely to have a rich phase diagram of exotic superfluid phases [7] due to the large difference in electron and hole effective masses in GaAs.…”

“…The strength of the electron-hole pairing is primarily controlled by the average effective separation between the electrons and the holes, determined by the thickness of the insulating barrier and by the widths of the quantum wells. It is challenging to identify the optimal configurations for the GaAs wells and the insulating barriers [5]. The wells cannot be too narrow or else interface roughness scattering, arising from insulating barrier Al atoms diffusing into the well regions, makes mobility impractically low.…”

Effect of Mismatched Electron-Hole Effective Masses on Superfluidity in Double Layer Solid-State Systems

“…To determine the screening in the presence of a superfluid, we evaluate the density response functions within the Random Phase Approximation [10]. Because of the different masses, there are distinct electron and hole normal polarizabilities [11]. The electron and hole normal polarizabilities in the presence of the superfluid are,…”

“…We have previously included the effect of mass imbalance on the electron-hole superfluid phase diagram in GaAs [11] and Ge/Si [12] double quantum well systems. Other examples of candidate systems with mass imbalance are Transition Metal Dichalcogenide heterostructures [13] and InAs/GaSb double quantum wells [14].…”