Point-contact Andreev reflection spectroscopy on Re<sub>3</sub>W superconductor

Abstract

Non-centrosymmetric superconductors have received considerable attention because of their possible possession of unconventional spin-triplet pairing.For this reason,the non-centrosymmetric Re<sub>3</sub>W with <i>α</i> -Mn structure has been widely concerned.However,almost all the previous studies support that the non-centrosymmetric phase of Re<sub>3</sub>W is a conventional weak-coupling s-wave superconductor.Later on,it is proved that Re<sub>3</sub>W 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<sub>3</sub>W.</br>In this paper,point-contact Andreev reflection experiments are carried out on Re<sub>3</sub>W/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 (<i>△</i>) with almost the same gap ratio of <i>△</i>/<i>T</i><sub>c</sub>.These results present strong evidence that both phases of Re<sub>3</sub>W are weak coupling Bardeen-Cooper-Schrieffer superconductors.</br>Another interesting finding is that both phases of Re<sub>3</sub>W 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 (<i>Γ</i>) used in the fitting process,and indicates a clean (and possibly transparent) interface.Keeping this in mind,we can assume that the effective barrier (<i>Z</i>) 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 <i>Z</i><sup>2</sup>=(1-<i>r</i>)<sup>2</sup>/4<i>r</i>,where <i>r</i> is the ratio between those two Fermi velocities.From this formula,we can obtain the Fermi velocity of Re<sub>3</sub>W by using the known value of Au's Fermi velocity and the fitting parameter <i>Z</i> for the Re<sub>3</sub>W/Au point contacts.It is interesting to find that the chemical property of Re<sub>3</sub>W 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.</br>Such an exceptional performance of Re<sub>3</sub>W 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<sub>3</sub>W 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<sub>3</sub>W and Ni,we can calculate the effective barrier to be around 0.3 in the Re<sub>3</sub>W/Ni interface,which coincides with the fitting parameter <i>Z</i>.These results self-consistently demonstrate the validity of the determination of Re<sub>3</sub>W's Fermi velocity and the cleanness/transparency of the studied point-contact interface.