Point-contact Spectroscopy Studies On Several Noncentrosymmetric Superconductors

Studies on noncentrosymmetric superconductors have been one of the focus of unconventional superconductors,where the inversion symmetry is broken in its crystal structure and the associated antisymmetric spin-orbit coupling(SOC)splits the degenerate spins on the Fermi surface into two bands.The pairing state is not pure even-or odd-parity anymore,but a mixture of spin-singlet and-triplet states,yielding some exotic properties such as topological superconductivity.It is generally believed that spin-orbit coupling is the key parameter to tune the relative weight of the spin-triplet component,where stronger SOC will make the spin-triplet dominant and induce gaps nodes on the Fermi surface.As a powerful tool to study the superconducting gap and order parameter symmetry,point-contact spectroscopy has been applied to study several Re-based noncentrosymmetricsuperconductors,including Re6Zr,Re6Hf and Re24Ti5,to systematically explore the effect of SOC on superconductivity and the underlying pairing mechanism.This thesis is organized as following:In the first chapter,we briefly introduce the history of superconductivity and the related theoretical model.Special attention has been given to the background and recent theoretical development of noncentrosymmetric superconductors.In the second chapter,we focus to introduce the experimental method of point-contact spectroscopy and its theoretical modeling,including Andreev reflection,Blonder-Tinkham-Klapwijk(BTK)model.The experimental techniques for point-contact spectroscopy under extreme conditions are discussed and its application on various superconductors are demonstrated.In the third chapter,point-contact spectroscopy results on single crystalline Re6Zr are discussed.The noncentrosymmetic superconductor Re6Zr is reported to break the time reversal symmetry by some μ SR studies.However,our measured point-contact conductance curves for Re6Zr suggest a fully gapped pairing symmetry(probably s-wave)in Re6Zr and the gap evolution as a function of temperature or magnetic field follows the standard BCS behaviors.In the fourth chapter,we present our point-contact spectroscopy study on polycrystalline Re6Hf.As a sister compound of Re6Zr,Re6Hf is also reported to break inversion and time reversal symmetry.Moreover,Re6Hf is generally believed to have an enhanced antisymmetric spin-orbital coupling and thus a larger weight of spin-triplet component,due to the heavier atomic mass of Hf element than Zr.Point-contact spectroscopy on the superconducting gap inRe6Hf,however,suggests a similar gap behavior between Re6Zr and Re6Hf,which are both likely a fully gapped s-wave pairing symmetry.In the fifth chapter,point-contact spectroscopy study on noncentrosymmetric Re24Ti5 under pressure is discussed.PCS has been extended in a pressure environment so that pressure can be a tuning parameter of the electronic states with a simultaneous spectroscopic characterization.Our PCS pressure studies on the noncentrosymmetric superconductor Re24Ti5 indicate that its superconducting transition temperature Tc and the extracted gap don’t change with pressure.