OBSERVATION OF A SPIN POLARIZATION PHASE-TRANSITION OF THE 4 3 FRACTIONAL QUANTUM HALL STATE IN A HIGH-MOBILITY 2D HOLE SYSTEM/

We report the observation of a spin polarization phase transition of t he 4/3 fractional quantum hall state of a high-mobility 2D hole system in a GaAs-(Ga, AI)As heterostructure. Mobilities in excess of 1 000 0 00 cm(2) V-1 s(-1) are achieved at a density of 0.86 x 10(11) cm(-2). The depth of the 4/3 minimum is seen to weaken as the carrier density is increased before disappearing completely. Upon further increase in carrier density the 4/3 minimum is reentrant aod proceeds to strengthe n. The 8/3 and 7/3 states are observed to show apparently anomalous be haviour.