Evidence for Localized Minority Spins in the Half-filled Lowest Landau Level

Yong Qing Li

Max-Planck-Institut für Festkörperforschung, Germany

At half filling of the lowest Landau level, correlations among the electrons give rise to remarkable Fermi liquid behavior. This has been successfully explained with the composite fermion picture: a metallic state of weakly interacting quasi-particles forms and these quasi-particles effectively experience no magnetic field at half filling. Theoretical descriptions of the transport in this regime remain, however, unsatisfactory. Calculated values of the longitudinal resistance are one to two orders of magnitude larger than those observed in state-of-the-art samples. Moreover, all existing transport models assume full spin polarization. In experiments however the composite fermion sea is often partially polarized. By tuning the ratio between the Zeeman energy and the Coulomb energy, it is possible to induce a transition from a partially to a fully polarized composite fermion sea. The importance of the spin degree of freedom has been suggested in recent nuclear magnetic resonance measurements, but it is still unclear how the change in spin polarization influences the transport properties at half filling.
Here, we present a systematic study of the transport at half filling in tilted magnetic fields and, with the help of a backgate, for a wide range of carrier densities. Both parameters allow to go from a partially to a fully polarized Fermi liquid. The spin polarization is verified with all-electrical nuclear spin relaxation experiments. They utilize current induced nuclear spin depolarization as well as resistive detection. Since these experiments rely exclusively on quasi dc-transport, they can be performed at ultralow temperatures (T<20 mK). A strong deviation from a Korringa type temperature dependence of the nuclear spin relaxation rate (1/T1) is observed. The anomalously large spin relaxation rate at the low temperatures is mainly attributed to the localization of minority spins. This may help to understand the spin-dependent transport properties at half filling.
This work is in collaboration with V. Umansky, K. von Klitzing, and J.H. Smet.