Experimental evidence of anomalously large superconducting gap on topological surface state of $\beta$-$\rm\mathbf{Bi_2Pd}$ film

TL;DR

This study provides experimental evidence that the topological surface state of $eta$-$ m{Bi_2Pd}$ film exhibits an anomalously large superconducting gap, suggesting potential for higher-temperature Majorana mode stabilization.

Contribution

It demonstrates the anomalous enhancement of the superconducting gap on the topological surface state of $eta$-$ m{Bi_2Pd}$ film, a key step in realizing high-temperature topological superconductivity.

Findings

Superconducting gap of TSS is ~3.8 meV, much larger than bulk (~0.8 meV).

The gap ratio 2Δ/Tc is ~14.2, exceeding BCS predictions.

Chemical potential shifts upward in the film compared to bulk.

Abstract

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 $\beta$-$\rm{Bi_2Pd}$ film. We found that the superconducting gap of topological surface state ($\Delta_{TSS} \sim$ 3.8 meV) is anomalously enhanced from its bulk value ($\Delta_b \sim$ 0.8 meV). The ratio of $2\Delta_{TSS}/k_BT_c \sim14.2$, is substantially larger than the BCS value. By measuring $\beta$-$\rm{Bi_2Pd}$ 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 $\beta$-$\rm{Bi_2Pd}$ 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.