COMPOSITE-FERMION EFFECTIVE MASSES

Fractional quantum-Hall-effect features around filling factor nu = 1/2 have been analyzed using the composite-fermion approach. Effective ma sses deduced from the temperature dependence of the Shubnikov-de Haas (SdH) oscillations, in agreement with other measurements, show a diver gence as the filling factor approaches nu = 1/2 and scale as (density) (1/2). The magnetic-field dependence of the amplitude is explained qua ntitatively in terms of normal impurity scattering and a strong dephas ing term associated with density inhomogeneities of order 0.5%. It is pointed out that assumptions made in the derivation of the standard th eory used to analyze SdH oscillations are less likely to be satisfied for composite fermions and that some caution should therefore be used in interpreting effective-mass results obtained in this way.