In-gap states of the quantum dot coupled between a normal and superconducting lead

We study the in-gap states of the quantum dot hybridized to a conducting and superconducting electrode. The usual proximity effect suppresses electronic states over the entire subgap regime $|ω| < Δ$, where $Δ$ denotes the energy gap of superconductor. Owing to the Andreev scattering there can, however, emerge additional in-gap states whose line-broadening (inverse life-time) depends on the coupling to a normal electrode. We show that even number of such bound states appears in the quantum dot spectrum, depending on a competition between the Coulomb repulsion and the induced on-dot pairing. We discuss signatures of these in-gap states showing up in the tunneling conductance, especially in a low-bias regime dominated by the Andreev channel.

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