We calculate the optical conductivity of an anisotropic two-dimensional system with Rashba spin-flip excitation within the Kubo formalism. We show that the anisotropic Rashba effect caused by an external field changes significantly the magnitude of the spin splitting. Furthermore, we obtain an analytical expression for the longitudinal optical conductivity associated with inter-band transitions as a function of the frequency for an arbitrary polarization angle. We find that the diagonal components of the optical conductivity tensor are direction-dependent and the spectrum of optical absorption is strongly anisotropic with an absorption window. The height and width of this absorption window are very sensitive to the system anisotropy. While the height of absorption peak increases with increasing effective mass anisotropy ratio, the peak intensity is larger when the light polarization is along the armchair direction. Moreover, the absorption peak width becomes broader as the density of state mass or Rashba interaction is enhanced. These features can be used to determine parameters relevant for spintronics through the optical absorption spectrum.

We theoretically study the Coulomb drag resistivity in a double-layer electron system with highly anisotropic parabolic band structure using Boltzmann transport theory. As an example, we consider a double-layer phosphorene on which we apply our formalism. This approach, in principle, can be tuned for other double-layered systems with paraboloidal band structures. Our calculations show the rotation of one layer with respect to another layer can be considered a way of controlling the drag resistivity in such systems. As a result of rotation, the off-diagonal elements of drag resistivity tensor have non-zero values at any temperature. In addition, we show that the anisotropic drag resistivity is very sensitive to the direction of momentum transfer between two layers due to highly anisotropic inter-layer electron-electron interaction and also the plasmon modes. In particular, the drag anisotropy ratio, ρ yy /ρ xx , can reach up to ∼ 3 by changing the temperature. Furthermore, our calculations suggest that including the local field correction in dielectric function changes the results significantly. Finally, We examine the dependence of drag resistivity and its anisotropy ratio on various parameters like inter-layer separation, electron density, short-range interaction and insulating substrate/spacer.

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]

Excitonic superfluidity in double phosphorene monolayers is investigated using the BCS mean-field equations. Highly anisotropic superfluidity is predicted where we found that the maximum superfluid gap is in the BEC regime along the armchair direction and in the BCS-BEC crossover regime along the zigzag direction. We estimate the highest Kosterlitz-Thouless transition temperature with maximum value up to ∼ 90 K with onset carrier densities as high as 4 × 10 12 cm −2 . This transition temperature is significantly larger than what is found in double electron-hole few-layers of graphene. Our results can guide experimental research towards the realization of anisotropic condensate states in electron-hole phosphorene monolayers.