Hyperfine-interaction-induced critical exponents in the quantum Hall effect

Abstract

We study localization-delocalization transition in quantum Hall systems with a random field of nuclear spins acting on two-dimensional (2D) electron spins via hyperfine contact (Fermi) interaction. We use the Chalker-Coddington network model, which corresponds to the projection onto the lowest Landau level. The inhomogeneous nuclear polarization acts on the electrons as an additional confining potential and, therefore, introduces additional parameter $p$ (the probability to find a polarized nucleus in the vicinity of a saddle point of random potential) responsible for the change from quantum to classical behavior. In this manner we obtain two critical exponents corresponding to quantum and classical percolations. We also study how the 2D extended state develops into the one-dimensional critical state.