Exchange instabilities in semiconductor double-quantum-well systems

We consider various exchange-driven electronic instabilities in semiconductor double-layer systems in the absence of any external magnetic field. We establish that there is no exchange-driven bilayer-to-monolayer charge-transfer instability in the double-layer systems. We show that, within the unrestricted Hartree-Fock approximation, the low-density stable phase (even in the absence of any interlayer tunneling) is a quantum ``pseudospin-rotated'' spontaneous interlayer phase-coherent spin-polarized symmetric state rather than the classical Ising-like charge-transfer phase. The U(1) symmetry of the double-quantum-well system is broken spontaneously at this low-density quantum phase transition, and the layer density develops quantum fluctuations even in the absence of any interlayer tunneling. The phase diagram for the double-quantum-well system is calculated in the carrier density-layer separation space, and the possibility of experimentally observing various quantum phases is discussed. The situation in the presence of an external electric field is investigated in some detail using the spin-polarized-local-density-approximation-based self-consistent technique and good agreement with existing experimental results is obtained.