Electronic Structure near Impurities in the Chains of YBa₂Cu₃O₆₊ₓ

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We study the electronic structure near impurities in the one-dimensional chains of YBa₂Cu₃O₆₊ₓ. Assuming chain superconductivity below Tc due to a proximity coupling, we show that only a magnetic impurity induces resonance states in the local density of states. Its spatial structure reflects the particle-hole nature of chain superconductivity and thus distinguishes it from other broken symmetry phases. For a sufficiently large scattering strength, or due to interference effects arising from a second impurity, the chains undergo a quantum phase transition to a polarized state.