Optically Pumped NMR Measurements of the Electron Spin Polarization in GaAs Quantum Wells near Landau Level Filling Factor nu=1/3

TL;DR

This study uses optically pumped NMR to directly measure electron spin polarization in GaAs quantum wells near the fractional quantum Hall state at nu=1/3, revealing small depolarizations not fully explained by existing theories.

Contribution

First direct measurement of electron spin polarization near nu=1/3 using optically pumped NMR, providing new insights into spin dynamics in fractional quantum Hall systems.

Findings

Measured P(nu,T) near nu=1/3, revealing neutral spin-flip excitations.

Observed small depolarization consistent with ~0.1 spin-flips per quasihole.

Depolarization behavior challenges current theoretical models.

Abstract

The Knight shift of Ga-71 nuclei is measured in two different electron-doped multiple quantum well samples using optically pumped NMR. These data are the first direct measurements of the electron spin polarization, P(nu,T)=<Sz(nu,T)>/max<Sz>, near nu=1/3. The P(T) data at nu=1/3 probe the neutral spin-flip excitations of a fractional quantum Hall ferromagnet. In addition, the saturated P(nu) drops on either side of nu=1/3, even in a Btot=12 Tesla field. The observed depolarization is quite small, consistent with an average of about 0.1 spin-flips per quasihole (or quasiparticle), a value which does not appear to be explicable by the current theoretical understanding of the FQHE near nu=1/3.