Particle-hole asymmetry of fractional quantum Hall states in the second Landau level of a two-dimensional hole system

We report the unambiguous observation of a fractional quantum Hall state in the Landau level of a two-dimensional-hole sample at the filling factor ν=8/3. We identified this state by a quantized Hall resistance and an activated temperature dependence of the longitudinal resistance and found an energy gap of 40 mK. Notably, the particle-hole conjugate state at filling factor ν=7/3 in our sample did not develop down to 6.9 mK. This observation is contrary to that in electron samples, in which the 7/3 state is typically more stable than the 8/3 state. We present evidence that the asymmetry between the 7/3 and 8/3 states in our hole sample is due to Landau-level mixing.