Observation of even-denominator fractional quantum Hall states at ν = 3/4 and 5/4 in the lowest Landau level.

Two-dimensional electron systems (2DESs) confined to wide GaAs quantum wells (QWs) provide a unique platform to study exotic fractional quantum Hall states (FQHSs) because the 2DES has a bilayer charge distribution with significant interlayer tunneling. Precise control over the 2DES density allows the tuning of the interlayer tunneling over a wide range. Here, we present our discovery of new even-denominator FQHSs in the lowest Landau level (orbital indexN= 0) at filling factorsand 5/4 in an ultrahigh-quality 2DES confined to a 72.5 nm-wide GaAs QW. The ground states atand 5/4 both evolve from composite fermion (CF) Fermi seas to FQHSs as the density is raised so that interlayer tunneling is sufficiently reduced and the 2DES becomes two-component, signaled by the behavior of the FQHSs flankingand 5/4. The two-component nature of theand 5/4 FQHSs is also evident from their extreme sensitivity to the bilayer charge distribution symmetry: both states disappear quickly when the charge distribution is made asymmetric by only. We find a natural explanation for the 3/4 and 5/4 FQHSs in terms of two states linked by particle-hole symmetry, and using Scarola-Jain bilayer CF framework which is a generalization of the well-known, two-component, Halperin state (state). Our observations elucidate the crucial role of competing energy and length scales in wide QW in stabilizing new ground states.