Absence of Andreev Bound States in Noncentrosymmetric Superconductor PbTaSe$_2$ under Hydrostatic Pressures

Abstract

Noncentrosymmetric superconductor PbTaSe$_2$, hosting bulk nodal-line fermions (Phys. Rev. B. 89, 020505) and spin-helical surface states (Nature Communication 7, 10556), represents a prime candidate for realizing topological superconductivity and Majorana bound states (MBS). However, the definitive experimental signature of MBS in this system has thus far remained elusive. Here we provide a comprehensive investigation of its superconducting properties under hydrostatic pressure. Combining Andreev reflection spectroscopy and temperature-dependent resistance measurements, we identify a separated surface-like superconductivity from the bulk one at a critical pressure $P_c$. The superconducting surface state demonstrate an $s$-wave pairing state with a strong coupling strength. Under magnetic fields, the absence of zero-bias conductance peak in the pressurized point-contact Andreev reflection spectrum. Our findings imposes a constraint on the theoretical proposals for realizing Majorana bound states in noncentrosymmetric superconductors.