Gap function of hexagonal pnictide superconductor SrPtAs from quasiparticle interference spectrum

Abstract

-The pnictide superconductor SrPtAs has a hexagonal layered structure containing inversion symmetry. It is formed by stacking two inequivalent PtAs layers separated by Sr layers. The former have no local (in-plane) inversion symmetry and therefore a (layer-) staggered Rashba spin orbit coupling appears which splits the three Kramers degenerate bands into six quasi-2D bands. The symmetry of the superconducting state of SrPtAs is unknown. Three candidates, spinsinglet A1g and Eg as well as triplet A2u states have been proposed. We predict the quasiparticle interference (QPI) spectrum for these gap functions in t-matrix Born approximation. We show that distinct differences in the pattern of characteristic QPI wave vectors appear. These results may be important to determine the gap symmetry of SrPtAs by STM-QPI method.Transition metal pnictide superconductors (SC) in particular the Fe-based systems are all of the tetragonal (orthorhombic) structure. The layered Pt-pnictide SrPtAs [1] is the first superconductor (T c = 2.4 K) in that class with hexagonal structure composed of honeycomb Pt-As layers spaced by Sr layers. It may be viewed as a MgB 2 type structure with Mg sites ocuppied by Sr and B sites in an ordered fashion such that Pt-As alternate in the 2D honeycomb layers as well along the hexagonal c-axis. The resulting structure has an overall 3D inversion center whereas the individual layers lack 2D inversion symmetry which is not contained in the C 3v layer point group.Because the electronic states at the Fermi level are mostly of Pt(5d) type with strong spin orbit coupling this leads to a peculiar electronic band structure [2]. Firstly the two inequivalent Pt-As layers have only small interlayer hopping which results in a quasi-2D band structure consisting of three hole bands and associated Fermi surface (FS) columns. Secondly an effective 2D Rashba spin orbit coupling term leads to a large splitting of the three bands which depends on k z in such a way that overall 3D inversion symmetry is restored. This has consequences for the possible superconducting pair states. Due to essentially decoupled layers it is reasonable to assume only intra-layer pairing. Then one can expect features as in the non-centrosymmetric superconductors consisting of a mixture of spin-singlet and triplet pairing of the in-plane order parameter due to lack of local 2D inversion symmetry. For the overall 3D superconducting state even or odd parity classification is restored due to the two inequivalent Sr-Pt layers. The momentum dependence of these unconventional pair states was investigated theoretically by Goryo et al [3] and it was found that even A 1g , E g and odd A 2u states are viable candidates. However sofar there is only few experimental evidence to discriminate between them [4].One of the most powerful recent methods to determine the symmetry of the gap function is STM quasiparticle interference (QPI) technique [5]. The Fourier transform of the differential conductance scans as function of bias voltage give a fingerpr...

High-resolution magnetic penetration depth and inhomogeneities in locally noncentrosymmetric SrPtAs

“…2,4 Regarding the symmetry of the superconducting phase, three possible stable states have been suggested according to the point group D 3d : E g , A 1g , and A 2u . [6][7][8] These states allow parity mixing, despite the overall parity conservation in SrPtAs. The E g has a dominant chiral d-wave component and the A 2u possesses a prevailing f -wave component.…”

Symmetry and Gap Classification of Non-Symmorphic SrPtAs

“…Utilizing Tables II-IV, we can now construct order parameters belonging to any given irreducible representation of D 6h . In the following, we look at some examples for illustration, namely A 1g , B 1u , and E 2g , that have been discussed for SrPtAs [3][4][5][6][7][8][9] starting with A 1g : assuming intra-layer coupling, the gap function has a dominant…”

“…The pnictide superconductor SrPtAs 1) (T c = 2.4K) has attracted attention recently due to the time-reversalsymmetry (TRS) breaking occurring with the onset of superconductivity. [2][3][4][5][6][7][8][9][10] Unlike other pnictide superconductors, SrPtAs has a hexagonal crystal structure. This has important consequences for the unconventional superconducting order parameters which can be classified with respect to the irreducible representations of the hexagonal point group and are only allowed to mix within the same representation.…”

Possibility of chiral d -wave state in the hexagonal pnictide superconductor SrPtAs

“…We note that another possibility for the distinction using quasiparticle interference spectroscopy has also been pointed out. 17 FIG. 5.…”