Is There an Exchange--Driven Bilayer to Monolayer Charge Transfer Instability in Semiconductor Double Quantum Well Systems ?

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  • 19 September 1996
Abstract
A definitive negative answer to the question posed in the title of this paper is given within the framework of the Hartree-Fock theory, showing in the process that earlier work on the subject which incorrectly concluded that such a charge transfer instability exists at low electron density and small layer separation is a feature of the restricted Hartree-Fock approximation. It is also argued, based on approximate inclusion of correlation effects, that the non-existence of a double-layer to single-layer charge transfer instability in semiconductor double quantum well structures is, in fact, a general result. In particular, within the Hartree-Fock approximation, the low density symmetry broken stable phase even in the absence of any interlayer tunneling is a quantum ``pseudospin rotated'' spontaneous interlayer phase coherent 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 various approximations (within the Hartree-Fock theory and including approximate correlation effects) in the carrier density--layer separation space, and the possibility of experimentally observing various quantum phases e.g. spin polarized ferromagnetic phase, spontaneous interlayer coherent phase) 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. The nature of the low density symmetry-broken spin- and pseudospin- polarized phase is critically discussed.

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