Non-Fermi-liquid behavior in quantum impurity models with superconducting channels

Abstract

We study how the non-Fermi-liquid nature of the overscreened multichannel Kondo impurity model affects the response to a BCS pairing term that, in the absence of the impurity, opens a gap Δ. We find that the low-energy spectrum in the limit Δ→0 actually does not correspond to the spectrum strictly at Δ=0. In particular, in the two-channel Kondo model, the Δ→0 ground state is an orbitally degenerate spin singlet, while it is an orbital singlet with a residual spin degeneracy at Δ=0. In addition, there are fractionalized spin-1/2 subgap excitations whose energy in units of Δ tends toward a finite and universal value when Δ→0, as if the universality of the anomalous power-law exponents that characterize the overscreened Kondo effect turned into universal energy ratios when the scale invariance is broken by Δ≠0. This intriguing phenomenon can be explained by the renormalization flow toward the overscreened fixed point and the gap cutting off the orthogonality catastrophe singularities. We also find other non-Fermi-liquid features at finite Δ: the local density of states lacks coherence peaks, the states in the continuum above the gap are unconventional, and the boundary entropy is a nonmonotonic function of temperature. The persistent subgap excitations are characteristic of the non-Fermi-liquid fixed point of the model, and thus depend on the impurity spin and the number of screening channels.

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