Direct observation of composite fermions and their fully spin-polarized Fermi sea near $ u=5/2$

The enigmatic even-denominator fractional quantum Hall state at Landau level filling factor $ u=5/2$ is arguably the most promising candidate for harboring Majorana quasi-particles with non-Abelian statistics and thus of potential use for topological quantum computing. The theoretical description of the $ u=5/2$ state is generally believed to involve a topological p-wave pairing of fully spin-polarized composite fermions through their condensation into a non-Abelian Moore-Read Pfaffian state. There is, however, no direct and conclusive experimental evidence for the existence of composite fermions near $ u=5/2$ or for an underlying fully spin-polarized Fermi sea. Here, we report the observation of composite fermions very near $ u=5/2$ through geometric resonance measurements, and find that the measured Fermi wavevector provides direct demonstration of a Fermi sea with full spin polarization. This lends crucial credence to the model of $5/2$ fractional quantum Hall effect as a topological p-wave paired state of composite fermions.