Theory of Andreev Spectroscopy in Unconventional, Flat Band Superconductors - II

The dichotomy between tunneling and Andreev spectroscopy experiments on moiré superconductors (SC), such as twisted bi- and tri-layer graphene, raise important questions. Point contact spectroscopy shows an enhancement of zero-bias conductance in the SC state, indicating Andreev reflection. However, the associated energy scale is notably smaller than the energy gap observed by scanning tunneling microscopy (STM), which also reveals gap nodes characteristic of an unconventional SC. Another puzzling question is why one observes Andreev reflection in a system where there is a strong mismatch in the bare Fermi velocities (v_F’s) of the metallic tip and flat band SC. The classic work of Nozieres and Deutscher, and our own generalization of it, suggests that this bare v_F mismatch should effectively make a transparent interface into a tunnel barrier. To address these issues, we consider the conductance of the normal-SC interface in a 2D system consisting of a circular metal disc surrounded by an unconventional SC. We will show how we are able to reconcile the puzzles of the v_F mismatch and the different energy scales observed in Andreev and STM spectra.