Researches On Topological Semimetals And Superconductors With Point-contact Measurements

As an important issue in condensed matter physics,superconductivity had received intense attentions since it was found.For more than one hundred years,researches on superconductivity have never been stopped.The most studied superconductors are conventional superconductors,cuprate superconductors,iron-based superconductors and heavy fermion superconductors.The pairing mechanism of conventional superconductors is closely related to electron-phonon coupling,i.e.,an s-wave BCS pairing,while that of high-T_c superconductors including both cuprates and iron-based materials is still unclear.It has been expected that spin triplet pairing might exist in non-centrosymmetric superconductors,which have thus been studied extensively.In recent years,with the introduction of topology into condensed matter physics,people have made great progress in the researches on topological materials.Topological semimetals often have special band structures and potential applications,thus attract a lot of interests.Most importantly,some novel physical phenomena,e.g.,topological superconductivity could emerge base on these materials if some appropriate scheme could be designed.In this thesis,we have studied the pairing symmetry of Re₃W with point-contact measurements.We find that both the non-centrosymmetric phase and centrosymmetric phase of Re₃W are s-wave superconductors.It was also found that Re₃W and metal tips can easily form stable point-contact junctions with weak inelastic scatterings,suggesting its possible applications.Another work in this thesis is about the point-contact measurements on topological semimetal WC.We successfully induced superconductivity on WC with metal tips in point-contact measurements,and found strong evidence for its unconventional nature.The first chapter briefly introduces the background knowledge of superconductivity and topological semimetals.Compared with other topological semimetals,WC hosts symmetry-protected three-band crossings（three-fold degeneracy）near the Fermi level,which is different from Dirac semimetal Cd₃As₂hosting four-fold degenerate points and Weyl semimetal TaAs hosting two-fold degenerate points.So it can be regarded as an intermediate state between Dirac semimetal and Weyl semimetal.Detailed investigations of WC will help to get further insight into the properties of topological semimetals and explore new quantum phenomena.The second chapter gives a introduction of Andreev reflection and point-contact measurements.Andreev reflection is a special transport process that occurs at the interface between a normal metal and a superconductor,and it can be quantitatively described by the Blonder–Tinkham–Klapwijk（BTK）theory.Based on Andreev reflection,we could detect the superconducting order parameters by point-contact measurements.The third chapter presents the results of point-contact measurements performed on Re₃W alloy.By comparative measurements,we found that both non-centrosymmetric phase and centrosymmetric phase of Re₃W are conventional superconductors with s-wave pairing symmetry.Moreover,both phases can form high quality point-contact junctions with very weak inelastic scatterings.In order to examine the stability of Re₃W’s surface,the experiments were repeated after Re₃W had been exposed to the atmosphere for nearly six months.The results demonstrated that the inelastic scatterings around the surface of Re₃W could still be neglectable,indicating that Re₃W is a good candidate to study the physical properties of its counter electrode in point-contact experiments.As an attempt,we successfully measure the spin polarization of Ni using Ni/Re₃W point contacts,which is in good agreement with that determined by other methods.The fourth chapter details the point-contact measurements on topological semimetal WC.We successfully induced superconductivity on WC with metal tips,including both magnetic tips and non-magnetic tips.The obtained highest T_c is 11.75K,which is so far the highest value induced on topological semimetals by point-contact measurements.Most surprisingly,the obtained T_c distributions are insensitive to the tips’magnetism,indicating the unconventional nature of the induced superconductivity.Based on experimental results and theoretical calculations,we could exclude tip’s pressure or significant changes of DOS（density of states）as the dominant origin of such tip induced superconductivity.In the end,we summarize the works of this thesis,and give some proposals for our future research.We would like to study other interesting materials by using Re₃W as an electrode in point contacts.We would also do more point contact experiments on WC and related topological materials to get further understanding of the mechanism of tip-induced superconductivity.We believe these studies would provide an avenue to find new quantum phenomena such as unconventional superconductivity.