Theory of Andreev spectroscopy of multiband and flat-band superconductors across BCS-BEC crossover

We use our Green’s function framework for calculating metal–superconductor (SC) interfacial conductance [1] to systematically analyze Andreev and tunneling spectra in multiband SCs. We address the question of how the excitation spectra and their spectroscopic signatures evolve with attraction as the pair size becomes comparable to mean interparticle spacing. We first show that for a single-band SC the same characteristic energy scale appears in both the Andreev and tunneling spectra across the BCS–BEC crossover. We then extend this analysis to multiband SCs and investigate two models motivated by moiré graphene. We analyze (i) the honeycomb lattice with Dirac dispersion, and (ii) the Lieb lattice which harbors a flat band along with a Dirac cone that can be gapped by staggered hopping. The latter provides a platform to explore the effects of strong Fermi velocity mismatch between the metal and the SC in addition to multiband effects. We will present results on how the Andreev and tunneling spectra interpolate between the weak and strong pairing regimes in the multiband BCS-BEC crossover.

[1] S. Biswas, S. Suman, M. Randeria, R. Sensarma, arXiv:2503.07744, PNAS (to appear).