Spin phase diagram of the interacting quantum Hall liquid

Measurement of the ground-state spin polarization of quantum systems offers great potential for the discovery and characterization of correlated electronic states. However, spin polarization measurements have mainly involved optical 1 – 3 and NMR 4 , 5 techniques that perturb the delicate ground states and, for quantum Hall systems, have provided conflicting results 1 , 4 , 6 . Here we present spin-resolved pulsed tunnelling (SRPT) that precisely determines the phase diagram of the ground-state spin polarization as a function of magnetic field and Landau level (LL) filling factor ( ν ) with negligible perturbation to the system. Our phase diagram shows a variety of polarized, unpolarized and topological spin states in the lowest ( N = 0) LL, which can largely be described by a weakly interacting composite fermion (CF) model 7 . However, the phase diagram shows unexpected behaviour in the N = 1 LL. We observe fully polarized ν = 5/2 and 8/3 states but a partially depolarized ν = 7/3 state. This behaviour deviates from the conventional theoretical picture 7 , 8 of weakly interacting fractional quasiparticles, but instead suggests unusual electronic correlations and the possibility of new non-Abelian phases 9 – 11 . The results establish SRPT as a powerful technique for investigating correlated electron phenomena. The spin polarization of a quantum Hall system is determined by a spin-resolved tunnelling method. This technique shows a substantial regime where the weakly interacting composite fermion picture is not valid.