Tip-induced superconductivity enhancement in single-crystalline PdSb by point-contact spectroscopy

We report our mechanical point-contact spectroscopy (MPCS) study on single-crystalline superconductor PdSb with T-c similar to 1.32 K and mu H-0(c2)(0) similar to 900 Oe, whose T-c changes very slightly against hydrostatic pressure below 2.5 GPa. For MPCS with a soft Au tip, the Andreev reflection signals suggest the same superconducting temperature T-c as in the pristine PdSb crystal and the conductance curves show a fully gapped behavior, yielding a superconducting gap value of Delta(0)= 0.23 meV with 2 Delta(0)/k(B)T(c) similar to 4.25 in the strong-coupling regime. However, for MPCS with a hard W tip, an enhanced superconducting T-c can be observed with a concomitant increase in the superconducting gap Delta in a linear relation of 0.109 meV/kelvin, whereas 2 Delta(0)/k(B)T(c) decreases from 4.25 to 3.5 in a negative correlation with T-c. From first-principles calculations, we observe a Lifshitz transition related to Pd-4d(z)2 orbitals under uniaxial pressure, which is absent under hydrostatic pressure. Such Lifshitz transition results in an enhanced density of states near the Fermi level n(E-F), which is in line with the enhanced T-c. In addition, a soft phonon mode at H is hardened under uniaxial pressure, suggesting a reduced coupling strength in this case. Such local strain effect provides a natural explanation for our MPCS results on PdSb.