Composite Fermions - Studies of effective masses and mixed spin systems.

R.J. Nicholas,

Clarendon Laboratory, Parks Rd, Oxford, OX1 3PU, U.K.

Studies have been made of the “Shubnikov-deHaas” oscillations of the Composite Fermions which are now usually used to described the Fractional Quantum Hall Effect. Two main phenomena have been studied:

1. The influence of carrier density.

The CF effective mass has been measured as a function of carrier density in GaAs/GaAlAs heteojunctions, from 0.6 up to 4.8 x 1⁰¹1 cm-2, using magnetic fields up to 50T. It is found to increase linearly with effective field B* up to 14T, until it reaches 1.6me. Data from all fractions are unified by the single parameter B* for all the samples over a wide range of densities. For values of B* above 2T, the effective CF gap is found to increase as ÖB*.

2. The importance of spin.

We have performed extensive studies of the CF oscillations in the region of occupancy around 3/2, where both spin states are occupied. We have varied the size of the Zeeman energy by both tilting the sample, which increases the size of the spin splitting relative to the CF Landau energies, and by applying hydrostatic pressure, which reduces the Zeeman energy due to the reduction of the band contribution to the electron g-factor. The relative strength of the different minima increase and decrease as a function of the Zeeman energy due to a crossing of the levels of the CF Landau levels arising separately from each spin. For very high values of hydrostatic pressure the Zeeeman energy approaches zero, and the Fractional Quantum Hall Effect doubles its degeneracy, with only even numerator fractions remaining, such as 2/3 and 4/3, and odd states such as 5/3 disappearing.

This work was done in collaboration with: C.T. Foxon, P.J. Gee, J.J. Harris, D.R. Leadley, J. Singleton and M. van der Burgt.

Proc. Mauntendorf Winter School 1996, to be published in Springer Series in Solid State Sciences.