CDW patterns of two-dimensional electrons in strong magnetic fields

The phase diagram of a two-dimensional electron system in a strong magnetic field is studied in the Hartree-Fock approximation. Special attention is paid to the consequences of the electron-hole symmetry in the system. When the ground Landau level is half-filled, the high-temperature gaseous phase undergoes a second-order transition to a square CDW with a period dependent on temperature. This transition preserves the electron-hole symmetry in the sense that the square CDW is self-dual in contrast to a triangular CDW where the dual CDW has a honeycomb pattern. If the density is slightly less than the half-filled case, the gaseous phase first undergoes a first-order transition to a triangular CDW, followed by another first-order transition to a square CDW. We discuss the role of quantum effects which are responsible for this unusual phase diagram.