Investigating The Order Parameter Of Several Exotic Superconductors

The superconducting order parameter is an important physical character to uncover the superconducting pairing state and the origin of superconductivity.We have studied the order parameters of several exotic superconductors by measuring the magnetic penetration depth and electronic specific heat,which contains the heavy fermion superconductor CeCu2Si2,noncentrosymmetric superconductor K2Cr3As3(quasi-one-dimensional crystal structure)and PbTaSe2(topologically nontrivial band structure)as well as the time reversal symmetry breaking superconductor Re6Zr and Re24Ti5.1.There is still a considerable debate about the superconducting order parameter of the first heavy fermion superconductor CeCu2Si2.The previous nuclear magnetic resonance,angle-dependent upper critical field and inelastic neutron scattering measurements have suggested CeCu2Si2 to be a nodal d wave superconductor,but recent specific heat measurements gave evidence for fully gapped superconductivity.In order to clarify this issue,we have precisely measured the magnetic penetration depth ??(T),which shows an exponential decay below 0.2Tc,suggesting fully gapped superconductivity in CeCu2Si2,consistent with the specific heat measurements.To reconcile the seemingly conflicting results,we have proposed a fully gapped d+d band-mixing pairing state for CeCu2Si2,which holds nodeless superconducting gaps and a sign-reversal order parameter in the Fermi surface.This model yields very good fits to both the superfluid density and specific heat,as well as explaining the observations from different experimental methods.2.K2Cr3As3 is a newly-discovered quasi-one-dimensional superconductor with a noncentrosymmetric crystal structure.To reveal its superconducting pairing state,we have measured the temperature dependence of the magnetic penetration depth of K2Cr3As3,which gives strong evidence for nodal superconductivity for the first time in experiments.Further analysis shows that the superfluid density can be accounted for by a previously proposed f wave model,which suggests unconventional superconductivity in K2Cr3As3.3.PbTaSe2 is a noncentrosymmetric superconductor with a topologically nontrivial surface state.To explore the role of broken inversion symmetry and nontrivial topological band structure on its superconducting pairing state,we have measured the temperature dependence of the lower critical field and magnetic penetration depth of PbTaSe2.It is shown that both the penetration depth and superfluid density can be well described by a single band isotropic s wave model,which suggests the absence of a mixture of spin singlet and triplet pairing,in spite of the noncentrosymmetric crystal structure.4.The occurrence of time reversal symmetry breaking in the superconducting state usually hints at unconventional superconductivity.While the previous research mainly focuses on the strongly correlated system,time reversal symmetry breaking has recently been found in the weakly correlated superconductors.To explore the superconducting pairing states of these exotic superconductors,we have measured the superconducting critical fields,magnetic penetration depth and electronic specific heat as a function of temperature of Re6Zr and Re24Ti5,which were reported to break time reversal symmetry below Tc.Our measurements show that both the superfluid density and specific heat can be well fitted by a single band nodeless s wave model,which exhibits distinct feature compared with the strongly correlated system.