We report Coulomb drag measurements on GaAs-AlGaAs electron-hole bilayers. The two layers are separated by a 10 or 25nm barrier. Below T≈1K we find two features that a Fermi-liquid picture cannot explain. First, the drag on the hole layer shows an upturn, which may be followed by a downturn. Second, the effect is either absent or much weaker in the electron layer, even though the measurements are within the linear response regime. Correlated phases have been anticipated in these, but surprisingly, the experimental results appear to contradict Onsager's reciprocity theorem.

We report an ambipolar device fabricated in undoped GaAs/AlGaAs quantum wells (widths 10 and 25 nm) with front and backgates that allow almost two orders of magnitude in density to be accessed in the same device (7×109cm−2 to 5×1011cm−2). By changing the well width, the relative electron and hole mobilities can be tuned, approaching similar velocities. We describe an approach to fully characterize the quantum well, including the impurity backgrounds and both the upper and lower interfaces, making use of the ability to control the carrier density and the position of the wavefunction independently over a wide range.

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]

Recently, it has been possible to design independently contacted electron-hole bilayers (EHBLs) with carrier densities  cm2in each layer and a separation of 10–20 nm in a GaAs/AlGaAs system. In these EHBLs, the interlayer interaction can be stronger than the intralayer interactions. Theoretical works have indicated the possibility of a very rich phase diagram in EHBLs consisting of excitonic superfluid phases, charge density waves, and Wigner crystals. Experiments have revealed that the Coulomb drag on the hole layer shows strong nonmonotonic deviations from a behaviour expected for Fermi-liquids at low temperatures. Simultaneously, an unexpected insulating behaviour in the single-layer resistances (at a highly “metallic” regime with ) also appears in both layers despite electron mobilities of above and hole mobilities over . Experimental data also indicates that the point of equal densities () is not special.

Semiconductor holes with strong spin-orbit coupling allow all-electrical spin control, with broad applications ranging from spintronics to quantum computation. Using a two-dimensional hole system in a gallium arsenide quantum well, we demonstrate a new mechanism of electrically controlling the Zeeman splitting, which is achieved through altering the hole wave vector k. We find a threefold enhancement of the in-plane g-factor g_{∥}(k). We introduce a new method for quantifying the Zeeman splitting from magnetoresistance measurements, since the conventional tilted field approach fails for two-dimensional systems with strong spin-orbit coupling. Finally, we show that the Rashba spin-orbit interaction suppresses the in-plane Zeeman interaction at low magnetic fields. The ability to control the Zeeman splitting with electric fields opens up new possibilities for future quantum spin-based devices, manipulating non-Abelian geometric phases, and realizing Majorana systems in p-type superconductor systems.