CPM Seminar

Quantum Hall ferromagnet at high filling factors:
A magnetic field induced Stoner transition

B.A. Piot

Grenoble High Magnetic Field Laboratory
CNRS

The integer quantum Hall effect (QHE) observed in the magnetotransport properties of a two-dimensional electron gas is usually described within the framework of a single electron picture. While this picture works well for the even integer QHE, this is not the case for the odd integer QHE, where it misses most, if not all, of the essential physics. Here, we present an experimental investigation of spin splitting in the integer quantum Hall effect for a series of Al_xGa1-_xAs / GaAs heterojunctions and quantum wells. Magnetoresistance measurements are performed at mK temperature to characterize the electronic density of states and estimate the strength of many body interactions. We propose a simple model with no free parameters which correctly predicts the magnetic field required to observe spin splitting confirming that the appearance of spin splitting is a result of a competition between the disorder induced energy cost of flipping spins and the exchange energy gain associated with the polarized state. In this model, the single particle Zeeman energy plays no role, so that the appearance of this quantum Hall ferromagnet in the highest occupied Landau level can also be thought of as a magnetic field induced Stoner transition.