DETERMINATION OF THE TRANSPORT-PROPERTIES OF COMPOSITE FERMIONS UNDERHIGH HYDROSTATIC-PRESSURE

We describe a series of low temperature magneto-transport measurements on two high mobility, low carrier density, GaAs/AlGaAs heterojunction s under conditions of reduced Lande spin g-factor. The magnitude of th e effective Lande spin g-factor was reduced with the application of hy drostatic pressure and under these experimental conditions we demonstr ate the existence of unpolarised spin states of composite fermions in the fractional quantum Hall effect regime down to filling factors nu s imilar to 1/5. The activation energies of states at filling factors, n u = 2/3 and 2/5 in the lowest composite fermion hierarchy show a Varia tion with effective Lande g-factor that is consistent with a spin unpo larised ground state at these filling factors in the dilute electron s ystem. However, this is less clear in a moderate carrier density syste m, with concentrations >1 x 10(11) cm(-2). We also show that under con ditions of reduced effective Lande g-factor and varying levels of diso rder, structure in rho(xx) absent at ambient pressure develops at even filling factor fractions corresponding to metallic states predicted w ithin the composite fermion model. The variation of disorder with hydr ostatic pressure in the fractional quantum Hall effect is discussed wi th respect to the underlying electronic scattering mechanisms.