Gap and mass measurements of composite fermions at $\nu$=5/3 in a 2D electron system with tunable valley occupation

In the composite fermion (CF) picture, the fractional quantum Hall state appearing at the filling factor of 5/3 is analogous to the integer quantum Hall state at $\nu$=1. We report energy gap measurements at $\nu$=5/3 in AlAs quantum wells which reveal a persistent gap even when the two anisotropic in-plane valleys are degenerate. This is reminiscent of the finite gap for the integer quantum effect for electron systems at $\nu$=1 in the g*=0 limit. As a symmetry breaking strain is controllably applied to the system, we observe a linear increase of gap followed by saturation, both of which are qualitatively consistent with a simple CF fan diagram. We also report mass measurements for the same state which suggest a mass enhancement for CFs by a factor of three over the electron band mass at a magnetic field of about 15T.