Unconventional superconductivity in two dimensional time reversal symmetric noncentrosymmetric superconductors

Abstract

In this thesis we study unconventional pairing observed in noncentrosymmetric superconductors by using spin dependent pairing potentials under the maximal C∞v and the time reversal symmetries (Θ-phase). The lack of inversion symmetry in these materials induces spin orbit interaction which removes the spin degeneracy and splits the Fermi surface into two branches. We demonstrated that in such systems mixed parity in the superconducting order parameter is present. Recently a large number of noncentrosymmetric superconductors appeared with anomalous behavior of a double and isotropic full energy gap present indicating a large inversion symmetry breaking, at the same time displaying an exponentially suppressed low temperature thermodynamic response pointing at a BCS like s-wave pairing. We clarify in this thesis that an isotropic energy gap can accommodate a parity mixed condensate with a comparably strong singlet and triplet pairings. The topology of such a configuration can be nontrivial although the system can have an exponentially suppressed temperature dependence in the thermodynamic response. We investigate other implications of such a behavior and suggest that some of the recent controversial experiments can be explained by the existence of nodal structure in the superconducting pair potential. We also investigate tunneling spectroscopy of different type of pairs by calculating differential conductance through a normal metal superconductor junctions. It is shown that each type of pairs have distinct behavior in the picture of Andreev reflection spectroscopy which is an effective tool to clarify order parameter especially for superconductors having nodal structure in superconducting gap. The zero bias conductance is observed for d wave superconductors and its origin is discussed. Furter it is showed that the neglected term in the theory of Andreev reflection in Josephson junctions with d-wave superconductors can change drastically the appearance of zero bias anomalies.