Pseudospin density wave instability in two-dimensional electron bilayers

Abstract

We investigate the instability of layer pseudospin paramagnetic (PSP) state to the formation of pseudospin density wave (PSDW) in two-dimensional (2D) electron bilayers, analogous to the formation of Overhauser spin density wave (SDW) in a single-layer 2D electron gas (2DEG) with spin 1/2. Our comprehensive study on phase diagrams, based on the self-consistent Hartree-Fock (HF) theory, reveals that the PSDW has a lower energy than both PSP and pseudospin ferromagnetic (PSF) states near the PSP-PSF phase transition boundary. When the two layers are populated by the same number of electrons, the PSDW momentum $Q_c \sim 2k_F$ near the PSP-PSDW boundary, where $k_F= (2\pi n)^{1/2}$ is the Fermi momentum characterized by the density in one of the two layers, and $Q_c$ decreases as the system transitions to the PSF regime. Extending the HF study to the case of unequal layer densities, the PSP phase is unstable to PSDW for small density imbalances, with momentum $Q_c \sim k_{F,t} + k_{F,b}$, where $k_{F,t}$ and $k_{F,b}$ are Fermi momenta of top and bottom layers, respectively. In PSDW regime, the ground state stability, defined by the energy difference between PSDW and the second lowest-energy state, is one order of magnitude lower than that in PSF regime, and decreases with increasing layer separation $d$. Furthermore, incorporating RPA static screening with the Hubbard-type local field correction leads to disappearance of both SDW and PSDW phases, and pushes the phase boundaries of paramagnetic to ferromagnetic transitions to larger $r_s$ values. Our study on PSDW in 2D electron bilayers is equally applicable to 2D hole bilayers. The idea of pursuing PSDW is, in general, relevant across various 2D bilayer systems, not limited to the parabolic model that we investigate in this paper, and provides a new possibility of exploring novel coherent phases.