Non-centrosymmetric superconductors have received considerable attention because of their possible possession of unconventional spin-triplet pairing.For this reason,the non-centrosymmetric Re ₃W with α-Mn structure has been widely concerned.However,almost all the previous studies support that the non-centrosymmetric phase of Re ₃W is a conventional weak-coupling s-wave superconductor.Later on,it is proved that Re ₃W has two different superconducting phases,one is the non-centrosymmetric phase and the other has a centrosymmetric hexagonal structure.Thus,a comparative study of these two superconducting phases could provide more information about the effect of non-centrosymmetric structure on the pairing symmetry of Re ₃W.
In this paper,point-contact Andreev reflection experiments are carried out on Re ₃W/Au and the data can be well fitted by isotropic s-wave Blonder-Tinkham-Klapwijk (BTK) theory.In combination with our previous researches,we find that both centrosymmetric and non-centrosymmetric phases have similar temperature dependence of superconducting gap ( △) with almost the same gap ratio of △/ T _c.These results present strong evidence that both phases of Re ₃W are weak coupling Bardeen-Cooper-Schrieffer superconductors.
Another interesting finding is that both phases of Re ₃W could easily form an ideal point-contact junction (i.e.,inelastic scatterings at the interface can be ignored) with a normal metal tip.This is manifested as an extremely small broadening factor ( Γ) used in the fitting process,and indicates a clean (and possibly transparent) interface.Keeping this in mind,we can assume that the effective barrier ( Z) at the interface mainly comes from the mismatch between the Fermi velocity of the superconductor and that of the normal metal,which can be estimated from the formula Z ²=(1- r) ²/4 r,where ris the ratio between those two Fermi velocities.From this formula,we can obtain the Fermi velocity of Re ₃W by using the known value of Au's Fermi velocity and the fitting parameter Zfor the Re ₃W/Au point contacts.It is interesting to find that the chemical property of Re ₃W is stable in the atmospheric environment.Even if the samples are exposed to the atmospheric environment for nearly six months,the inelastic scatterings are still very weak,and the superconducting properties are unchanged.
Such an exceptional performance of Re ₃W can be utilized to study the physical properties of its counter electrode in a point contact.As an attempt,we build a point contact between Re ₃W and a ferromagnetic Ni tip,and measure its Andreev reflection spectra which are then fitted with a modified BTK model by considering spin polarization.The determined spin polarization of Ni is in good agreement with previously reported result. Moreover,using the Fermi velocities of Re ₃W and Ni,we can calculate the effective barrier to be around 0.3 in the Re ₃W/Ni interface,which coincides with the fitting parameter Z.These results self-consistently demonstrate the validity of the determination of Re ₃W's Fermi velocity and the cleanness/transparency of the studied point-contact interface.