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

Abstract

We study how the non-Fermi-liquid nature of the overscreened multi-channel Kondo impurity model affects the response to a BCS pairing term that, in the absence of the impurity, opens a gap $\Delta$. We find that non-Fermi liquid features do persist even at finite $\Delta$: the local density of states lacks coherence peaks, the states in the continuum above the gap are unconventional, and the boundary entropy is a non-monotonic function of temperature. Even more surprisingly, we also find that the low-energy spectrum in the limit $\Delta \to 0$ actually does not correspond to the spectrum strictly at $\Delta =0$. In particular, the $\Delta \to 0$ ground state is an orbitally degenerate spin-singlet, while it is an orbital singlet with a residual spin degeneracy at $\Delta =0$. In addition, there are fractionalized spin-1/2 sub-gap excitations whose energy in units of $\Delta$ tends towards a finite and universal value when $\Delta \to 0$; as if the universality of the anomalous power-law exponents that characterise the overscreened Kondo effect turned into universal energy ratios when the scale invariance is broken by $\Delta \ne 0$. This intriguing phenomenon can be explained by the renormalisation flow towards the overscreened fixed point and the gap cutting off the orthogonality catastrophe singularities.

5 pages, 4 figures

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Cited by in corpus (8)

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• Excitations in a superconducting Coulombic energy gap
• Frustration shapes multi-channel Kondo physics: a star graph perspective
• Yu-Shiba-Rusinov states, the BCS-BEC crossover, and the exact solution in the flat-band limit
• Superconducting quantum dot and the sub-gap states
• Subgap states in superconducting islands
• Qubit based on spin-singlet Yu-Shiba-Rusinov states