Abstract
Behavior of Andreev gap states in a quantum dot with Coulomb repulsion symmetrically attached to superconducting leads is studied via the perturbation expansion in the interaction strength. We find the exact asymptotic form of the spin-symmetric solution for the Andreev states continuously approaching the Fermi level. We thereby derive a critical interaction at which the Andreev states at zero temperature merge at the Fermi energy, being the upper bound for the 0-π transition. We show that the spin-symmetric solution becomes degenerate beyond this interaction, in the π phase, and the Andreev states do not split unless the degeneracy is lifted. We further demonstrate that the degeneracy of the spin-symmetric state extends also into the 0 phase in which the solutions with zero and non-zero frequencies of the Andreev states may coexist.
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Janiš, V., Pokorný, V. & Žonda, M. Spin-symmetric solution of an interacting quantum dot attached to superconducting leads: Andreev states and the 0-π transition. Eur. Phys. J. B 89, 197 (2016). https://doi.org/10.1140/epjb/e2016-70299-7
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DOI: https://doi.org/10.1140/epjb/e2016-70299-7