Superconductivity and local noncentrosymmetricity in crystal lattices

Abstract

Symmetry of the crystal lattice can be a determining factor for the structure of Cooper pairs in unconventional superconductors. In this study we extend the discussion of superconductivity in non-centrosymmetric materials to the case when inversion symmetry is missing locally, but is present on a global level. Concretely, we investigate the staggered non-centrosymmetricity within a regular sublattice structure, in some analogy to the discussion of superconductivity in antiferromagnetic systems. Three crystal structures are analyzed in detail as illustrative examples for the extended classification of Cooper-pairing channels. One of the cases may be relevant for the class of ironpnictide superconductors.

Complex-Stripe Phases Induced by Staggered Rashba Spin–Orbit Coupling

“…18) In a number of theoretical studies, unusual features are predicted for systems that conserve inversion symmetry globally but not locally. [19][20][21][22][23][24] For instance, a pair-density wave (PDW) state could arise from the staggered antisymmetric spin-orbit coupling inherent to "local non-centrosymmetricity" in materials. 24) These works discuss multilayer superconductors, such as artificial superlattices of CeCoIn 5 , 25) multilayer high-T c cuprates, 26,27) and SrPtAs 28) as well as disordered phases of CePt 3 Si 29) and Sr 2 RuO 4 .…”

Role of strong spin-orbit coupling in the superconductivity of the hexagonal pnictide SrPtAs

“…Using the results of Ref. 26, we estimate that the size of the spin-triplet order parameter component is a factor α/W smaller than the spin-singlet order parameter (W is the bandwidth). Consequently, this spin-triplet pairing will not qualitatively change the results given above.…”

“…If the instability would occur in the spin-triplet channel, the SOC would force the d(k) vector to lie parallel to g(k). 26 A magnetic field in thê z direction would therefore be pair breaking, similar to the spin-singlet case. Thus, as is also the case in non-centrosymmetric superconductors, 11 measurements of spin-susceptibility and the critical field of SrPtAs do not suffice to distinguish spin-triplet and spin-singlet pairing.…”

“…We point out that in addition to a spin-singlet pairing order parameter, a spin-triplet order parameter component will also appear. 26 In particular, in a given layer a spin-triplet component with d(k) along the direction of g(k) exists, such that it has opposite sign in the two inequivalent layers of SrPtAs. Using the results of Ref.…”

Multigap superconductivity in locally noncentrosymmetric SrPtAs: AnAs75nuclear quadrupole resonance investigation

“…Thus, SrPtAs is a prime example for staggered non-centrosymmetricity. 3 A compound with a globally equivalent AlB 2 -type structure, MgB 2 , exhibits multigap superconductivity with T c ≈ 40 K. 4 The high spin-orbit coupling in SrPtAs opens the possibility for singlet-triplet mixing of the order parameter. 5 Interestingly, recent µSR experiments proved the development of a small static spontaneous internal field just below T c , evidencing time reversal symmetry (TRS) breaking in superconducting SrPtAs.…”