Electron-Hole Pair Condensation and Liquefaction in Strong Magnetic Fields

Ground state properties of the electron-hole system in a strong magnetic field are investigated throughout the whole density range. By use of a variational formalism which takes account of the electron-hole pairing, explicity derived are the momentum distribution, the electron-hole pair amplitude and the single-particle excitation spectrum. The ground state energy is computed not only variationally but also in the random phase approximation (RPA). The result shows that there exist both gas and liquid excitonic phases each of which has characteristic nature of the electron-hole pairing. Qualitative discussions are given as to semimetals and highly excited semiconductors with emphasis on the presence of incoherent excitons.