Extremely slow spin relaxation in a spin-unpolarized quantum Hall system
S. Dickmann
Accepted
Cyclotron spin-flip excitation in a n=2 quantum Hall system, being separated from the ground state by a slightly smaller gap than the cyclotron energy and from upper magnetoplasma excitation by the Coulomb gap , cannot relax in a purely electronic way but only with the emission of a short-wave acoustic phonon (k ~ 3·107/cm). As a result, relaxation in a modern wide-thickness quantum well occurs very slowly. We calculate the characteristic relaxation time to be ~ 1 s. Extremely slow relaxation should allow the production of a considerable density of zero-momenta cyclotron spin-flip excitations in a very small phase volume, thus forming a highly coherent ensemble - the Bose-Einstein condensate. The condensate state can be controlled by short optical pulses ( <~1 ms), switching it on and off.