Studies Of The Order Parameter Of Several Novel Superconductors

The study of superconducting order parameter plays a crucial role in understanding the origin of superconductivity.The symmetry of superconducting gap structure can be obtained by studying the temperature dependence of magnetic penetration depth.In this thesis,we have studied the order parameter of several novel superconductors by measuring the magnetic penetration depth based on tunnel diode osccilator(TDO)and muon spin rotation/resonance(μSR)techniques,which contains the iron-germanide superconductor YFe2Ge2,charge density wave superconductor LaPt2Si2,noncentrosymmetric superconductor LaRhSn and the time-reversal-symmetry breaking superconductor La4RuAl.1.YFe2Ge2 is one of the newly discovered iron-germanide superconductor,which shows similar properies with KFe2 As2 and unconventional superconductivity.In order to investigate the superconducting order parameter of YFe2Ge2,we have measured the magnetic penetration depth based on TDO method,which shows exponential activated temperature dependence at low temperatures,indicating fully gapped superconductivity in YFe2Ge2.The converted superfluid density can be well described by a nodeless twogap s-wave model,providing strong evidence for multiband superconductivity in YFe2Ge2.The study of impurity/disorder effect suggests that the pairing state might be the sigh-changing s±type,similar to many iron-based supercondutors.2.LaPt2Si2 is a charge density wave superconductor,the pairing state of which hasn’t been clearly understood based on the earlier specifc heat and μSR studies.Therefore,we have performed the measurement of magnetic penetration depth of single-crystalline LaPt2Si2 using the TDO-based method.Both of the magnetic pentration depth and the converted superfluid density can be well described by a single-gap s-wave model,giving strong evidence for the s-wave pairig state in LaPt2Si2.3.LaRhSn is a noncentrosymmetric superconductor,in this thesis,we have measured the magnetic penetration depth of LaRhSn using TDO based method and μSR technique.The results of specific heat,s penetration depth and superfluid density can be consistently described by a nodeless single gap s-wave model,indicating negligible contributions of any spin triplet pairing component.In addition,zero-field μSR measurements indicate that the time-reversal-symmetry is preserved when entering the superconducting state.4.La4RuAl is a new noncentrosymmetric superconductor,the order parameter study of which would be helpful for understanding the peculiar physics in noncentrosymmetric systems.We have measured the temperature dependence of magnetic penetration depth,which can be well described by nodeless s-wave pairing state.However,the zero-field μSR measurements indicate that the time-reversalsymmetry is breaking when entering the superconducting state,indicating the superconductivity in La4RuAl is not simple BCS-type,which definetly need further experimental and theoretical studies.