Experimental evidence of anomalously large superconducting gap on topological surface state of β-Bi2Pd 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 beta-Bi2Pd film. We found that the superconducting gap of topological surface state (Delta(TSS) similar to 3.8 meV) is anomalously enhanced from its bulk value (Delta(b) similar to 0.8 meV). The ratio of 2 Delta TSS/k(B)Tc similar to 16.3, is substantially larger than the BCS value. By measuring beta-Bi2Pd 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 beta-Bi2Pd 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. (C) 2019 Science China Press. Published by Elsevier B.V. and Science China Press. All rights reserved.