FRACTIONAL QUANTUM HALL-EFFECT IN HIGHER LANDAU-LEVELS

We investigate, using finite size numerical calculations, the spin-pol arized fractional quantum Hall effect (FQHE) in the first excited Land au level (LL). We find evidence for the existence of an incompressible state at nu = 7/3 = 2 + 1/3, but not at nu = 2 + 2/5. Surprisingly, t he 7/3 state is found to be strongest at a finite thickness. The struc ture of the low-lying excited states is found to be markedly different from that in the lowest LL. This study also rules out FQHE at a large number of odd-denominator fractions in the lowest LL.