Theory of Surface Spectroscopy for Noncentrosymmetric Superconductors. / Wennerdal, Niclas.

2016. 148 p.

Research output: ThesisDoctoral Thesis

Unpublished

Abstract

Most superconductors are classified having either spin-singlet or spin-triplet pairing. But noncentrosymmetric superconductors (NCSs) lack a centre of inversion. This allows for a large inversion-antisymmetric spin-orbit coupling (SOC), transforming under symmetry operations according to the crystal point group, throughout the Brillouin zone. The SOC splits the Fermi surface into two sheets having opposite helicity. Due to this splitting both singlet and triplet pairing coexist in NCSs, which has profound effects on the material properties.

In this thesis a comprehensive study of these effects is carried out for materials with tetrahedral Td, tetragonal C4v, and cubic point group O, as well as for several different singlet to triplet order parameter ratios. The superconducting order parameter is calculated using the quasiclassical method of superconductivity, both in the bulk and at a surface. The superconducting instability and the evolution of the gap below Tc are studied in the bulk, and it is shown that a second phase transition is possible for certain parameter values.

The spatially varying order parameters are calculated self-consistently for a range of different surface orientations in order to determine the orientation's effect on the order parameter suppression. For each self-consistent order parameter profile the surface density of states is calculated showing intricate structure of the Andreev bound states (ABS) as well as spin polarisation.

Furthermore, the tunnel conductance spectra are computed for different surface normals with two of them exhibiting zero-bias conductance peaks (ZBCPs). The zero-bias conductance and the ABS are studied in detail showing which quasiclassical trajectories contribute to the ZBCPs. In addition, the zero-bias conductance computed is for a range of surface orientations showing the sensitivity of the ZBCPs.

The large SOC in a NCS can endow the material with non-trivial topology, depending on Fermi surface geometry and singlet to triplet ratio. A topological phase diagram is constructed for open and closed Fermi surfaces showing a sharp transition between the two for the point group $O$. Lastly the topology's effect on the surface states is thoroughly investigated.
Original languageEnglish
QualificationPh.D.
Awarding Institution
Award date26 May 2016
Publication statusUnpublished - 2016

ID: 26480804