Pressure-induced surface superconductivity in the noncentrosymmetric superconductor ${\mathrm{PbTaSe}}_2$: Pressure-dependent point-contact Andreev spectroscopy

A notable characteristic of ${\mathrm{PbTaSe}}_2$, a prototypical noncentrosymmetric superconductor (NCS), is that its superconductivity can be modulated through a structural transition under hydrostatic pressure [Kaluarachchi et al. Phys. Rev. B 95, 224508 (2017)]. Here we report on a combined pressure-sensitive point-contact Andreev reflection (PCAR) spectroscopy with bulk resistance measurements on ${\mathrm{PbTaSe}}_2$, to elucidate the nature of the surface and bulk superconductivity and their evolution with hydrostatic pressure. It is found that in a high-pressure region the superconducting gap opening temperature $T_c^A$ is significantly lower than the bulk resistive transition temperature $T_c^R$, revealing a clear experimental signature of surface-bulk separation associated with enhanced antisymmetric spin-orbit coupling (ASOC). The PCAR spectra, reflecting the superconducting surface state, are analyzed with two-dimensional Blonder-Tinkham-Klapwijk theory, yielding an isotropic $s$-wave full BCS gap in the strong-coupling regime. These results suggest the coexistence of full gap $s$-wave superconductivity and topological surface states in ${\mathrm{PbTaSe}}_2$, indicating that this NSC superconductor with significantly enhanced ASOC may offer a solid platform to investigate the topological aspect in the superconducting condensate.