Hyperfine interactions in quantum Hall systems

Abstract

The hyperfine interaction between the nuclear and the electronic spins could be effectively used as a microscopic local probe for the properties of 2DEG, such as the electronic density of states, localization and interaction, phase transitions between the gaseous, quantum liquid and crystalline states in QHE systems. Since the electron Zeeman energy is orders of magnitude larger than the nuclear one, the energy gap in the spectrum of two-dimensional electrons in a strong magnetic field impose severe restriction on the flip-flop processes, which results in a very unsual magnetic field dependence of the nuclear spin relaxation and diffusion in such systems. This energy conservation problem is resolved, in real systems, by additional degrees of freedom, like impurities, phonons and electron interactions. In sufficiently pure systems the dipole-dipole interaction becomes operative in relaxation of the nuclear spins by conduction electrons. Electron interactions results in a new mechanism of indirect nuclear spin transport via the exchange of virtual electron-hole pairs (spin excitons). The virtual character of the spin-excitons, transfering the nuclear spin polarization, removes the problem of the energy conservation in the single flip-flop process.