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Superconducting proximity effect on a two-dimensional Dirac electron system

The superconducting proximity effect on two-dimensional massless Dirac electrons is usually analyzed using a simple model consisting of the Dirac Hamiltonian and an energy-independent pair potential. Although this conventional model is plausible, it is questionable whether it can fully describe the proximity effect from a superconductor. Here, we derive a more general proximity model starting from an appropriate microscopic model for the Dirac electron system in planar contact with a superconductor. The resulting model describes the proximity effect in terms of the energy-dependent pair potential and renormalization term. Within this framework, we analyze the density of states, the quasiparticle wave function, and the charge conservation of Dirac electrons. The result reveals several characteristic features of the proximity effect, which cannot be captured within the conventional model.