Superconductivity In Weak Correlated Non- Centrosymmertric Lanthanide La₄RuAl、La₄IrAl And LaIrGe

One of the most interesting topics in physics is unconventional superconductivity. While superconductivity breaks U(1) gauge symmetry, additional symmetries are broken in unconventional superconductors. Non-centrosymmetric superconductors (NCS) are superconductors which lack a center of inversion. Anti-symmetric spin-orbit coupling (ASOC) lifts the degeneracy of the bands due to an inhomogeneous electric field caused by the lack of inversion center. Parity is no longer a good quantum number, which means that the wave function of the Cooper pairs contains a mixture of both even and odd parity. Nontrivial phenomena have been detected such as unconventional superconductivity, topological effects. Li2(PdxPt1-x)3B is a prototype system of weakly correlated NCS, giving direct experimental evidence that the presence of heavy elements strongly ASOC affect the properties. It is important to search for new NCS to investigate the effects due to the absence of inversion symmetry.In this dissertation, I present a systematic study on the superconducting properties of the following weakly-correlated non-centrosymmetric superconductors:La4RuAl, La4IrAl and LalrGe. All the samples were synthesized and characterized by myself. I found the presence of previously unreported superconductivity in La4RuAl, La4IrAl. Particular attention is paid to the sample synthesis and the nature of the superconducting order parameter.This dissertation contains the following four chapters:Chapter I introduces a theoretical background of superconductivity and recent progress of the study of non-centrosymmetric superconducting systems.Chapter 2 describes the experimental methods and instruments used in this dissertation, including techniques for sample growth and measurements of transport, magnetic and thermodynamic properties. Particular attention is paid to various methods for improving the experimental techniques.In Chapter 3 we show the evidence for the previously unreported superconductivity in the two lanthanide compounds La4RuAl and La4IrAl. Powder x-ray diffraction at room temperature suggests both compounds have a cubic structure. The space group is F43m(No.216) where inversion symmetry is broken. We have observed zero resistivity in transport measurements and the Meissner effect in magnetic susceptibility measurements. The critical temperature is Tc= 1.76K for La4RuAl and Tc= 1.6K for La4IrAl. A jump in the specific heat shows that the superconductivity is a bulk property. The Sommerfield coefficient is yN=20mJ/mol-K2 for La4RuAl and yN=22mJ/mol-K2 for La4IrAl. The fitting of the electronic specific heat for La4RuAl indicates that the superconducting gap is fully open. The upper critical field agrees with the WHH theory quite well which is below the Pauli limit, while the temperature and field dependence of the specific heat is consistent with a fully opened gap. This suggests that at present there is not yet evidence for significant singlet-triplet mixing in this system, arising from the effects of the anti-symmetric spin-orbit coupling.In Chapter 4, we present a detailed study of the sample preparation of LalrGe and measurements of the superconducting properties. We report that the critical temperature of LalrGe is 1.35K instead of the previously reported value of 4.5K, which probably can be attributed to an impurity phase La3Ir2Ge2.A summary is presented at the end of the dissertation.