How free are Composite Fermions at $\nu = 1/2$? A NMR investigation

TL;DR

This study uses NMR to investigate electron spin polarization at half-filled Landau level, confirming a Fermi sea of composite fermions and highlighting the significance of residual interactions, effective mass, and broken particle-hole symmetry.

Contribution

It provides experimental evidence for the existence of a composite fermion Fermi sea at ν=1/2 and quantifies residual interactions and symmetry breaking effects.

Findings

Confirmation of a composite fermion Fermi sea at ν=1/2

Measurement of CF effective mass, g-factor, and Fermi energy

Observation of broken particle-hole symmetry in spin polarization

Abstract

NMR measurements of the electron spin polarization have been performed on a 2D electron system at and around half-filled lowest Landau level. Comparing the magnetic field and the temperature dependence of the spin polarization to models of free and interacting composite fermions (CF), we confirm the existence of a Fermi sea and show that residual interactions are important. Independent measurements of the CF effective mass, g-factor and Fermi energy are obtained from the thermal activation of the spin polarization in tilted fields. The filling factor dependence of the spin polarization for $2/5 < \nu < 2/3$ reveals a broken particle-hole symmetry for the partially polarized CF Fermi sea.