Composite fermion valley polarization energies: Evidence for particle-hole asymmetry

In an ideal two-component two-dimensional electron system, particle-hole symmetry dictates that the fractional quantum Hall states around ν=1/2 are equivalent to those around ν=3/2. We demonstrate that composite fermions (CFs) around ν=1/2 in AlAs possess a valley degree of freedom like their counterparts around ν=3/2. However, focusing on ν=2/3 and 4/3, we find that the energy needed to completely valley polarize the CFs around ν=1/2 is considerably smaller than the corresponding value for CFs around ν=3/2, thus betraying a particle-hole symmetry breaking.