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Spin textures, screening and excitations in dirty quantum Hall ferromagnets

We study quantum Hall ferromagnets in the presence of a random electrostatic impurity potential. Describing these systems with a classical non-linear sigma model and using analytical estimates supported by results from numerical simulations, we examine the nature of the ground state as a function of disorder strength, $Δ$, and deviation, $δν$, of the average Landau level filling factor from unity. Screening of an impurity potential requires distortions of the spin configuration, and in the absence of Zeeman coupling there is a disorder-driven, zero-temperature phase transition from a ferromagnet at small $Δ$ and $|δν|$ to a spin glass at larger $Δ$ or $|δν|$. We examine ground-state response functions and excitations.