Determination of the transport properties of composite fermions under high hydrostatic pressure

Abstract

We describe a series of low temperature magneto-transport measurements on two high mobility, low carrier density, GaAs/AlGaAs heterojunctions under conditions of reduced Landé spin g-factor. The magnitude of the effective Landé spin g-factor was reduced with the application of hydrostatic pressure and under these experimental conditions we demonstrate the existence of unpolarised spin states of composite fermions in the fractional quantum Hall effect regime down to filling factors v ~ 1 5 . The activation energies of states at filling factors, v = 2 3 and 2 5 in the lowest composite fermion hierarchy show a variation with effective Landé g-factor that is consistent with a spin unpolarised ground state at these filling factors in the dilute electron system. However, this is less clear in a moderate carrier density system, with concentrations > 1 × 10 11 cm −2. We also show that under conditions of reduced effective Landé g-factor and varying levels of disorder, structure in ϱ xx absent at ambient pressure develops at even filling factor fractions corresponding to metallic states predicted within the composite fermion model. The variation of disorder with hydrostatic pressure in the fractional quantum Hall effect is discussed with respect to the underlying electronic scattering mechanisms.