Impure AdS/CFT correspondence

We study momentum relaxation due to dilute, weak impurities in a strongly coupled CFT, a truncation of the M2 brane theory. Using the AdS/CFT correspondence, we compute the relaxation time scale as a function of the background magnetic field B and charge density ρ. The theory admits two different types of impurities. We find that for magnetic impurities, momentum relaxation due to the impurity is suppressed by a background B or ρ. For electric impurities, due to an underlying instability in the theory toward an ordered phase, the inverse relaxation time scale increases dramatically near √B²+ρ²/σ₀²~21T². We compute the Nernst response for the impure theory, and comment on similarities with recent measurements in superconductors.