Theoretical Analysis of the Symmetry of the Order Parameter in Unconventional Superconductors: Singlet Versus Triplet Pairing
The novel superconductivity with Tc ≈ 1.5K observed recently in layered Sr2RuO4 seems to be a new example of unconventional superconductivity . Its structure is isomorphic to the high-Tc cuprate family La2-x ;Sr x CuO4 which, however, has a Tc ≈ 30K and is a singulet (but unconventional) superconductor. This makes the theoretical comparison of ruthenates and high-Tc cuprates very interesting. In Sr2RuO4 the presence of incommensurate antiferromagnetic and ferromagnetic spin fluctuations has been confirmed recently by inelastic neutron scattering (INS)  and NMR 17O Knight shift , respectively, suggesting also a pairing mechanism for Cooper-pairs due to spin fluctuations like in cuprates. This is further supported by the observed non s-wave symmetry of the order parameter. In contrast to cuprates, NMR  and polarized neutron scattering  measurements indicate spin-triplet state Cooper-pairing. In analogy to 3He this led theorists to conclude that p-wave superconductivity is present . However, by fitting the specific heat and the ultrasound attenuation Maki and co-workers doubted p-wave superconductivity  and propose an f-wave symmetry of the superconducting order parameter. A similar conclusion was drawn in Ref. . Recently it has been reported that also thermal conductivity measurements are most consistent with f-wave symmetry .
KeywordsFermi Surface Spin Fluctuation Spin Susceptibility Superconducting Order Parameter Pairing Symmetry
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