Experimental evidence of anomalously large superconducting gap on topological surface state of β-BiPd film.

Connate topological superconductor (TSC) combines topological surface states with nodeless superconductivity in a single material, achieving effective p-wave pairing without interface complication. By combining angle-resolved photoemission spectroscopy and in-situ molecular beam epitaxy, we studied the momentum-resolved superconductivity in β-BiPd film. We found that the superconducting gap of topological surface state (Δ ∼ 3.8 meV) is anomalously enhanced from its bulk value (Δ ∼ 0.8 meV). The ratio of 2Δ/kT ∼ 16.3, is substantially larger than the BCS value. By measuring β-BiPd bulk single crystal as a comparison, we clearly observed the upward-shift of chemical potential in the film. In addition, a concomitant increasing of surface weight on the topological surface state was revealed by our first principle calculation, suggesting that the Dirac-fermion-mediated parity mixing may cause this anomalous superconducting enhancement. Our results establish β-BiPd film as a unique case of connate TSCs with a highly enhanced topological superconducting gap, which may stabilize Majorana zero modes at a higher temperature.