Models of Superconductivity in Sr_2RuO_4

Is the mechanism of unconventional superconductivity in Sr$_2$RuO$_4$ closer in spirit to superfluid $^3$He, or to the cuprates, pnictides, and organic superconductors? We challenge prevailing assumptions in this field and using well-controlled perturbative renormalization group calculations, we suggest that superconductivity in Sr$_2$RuO$_4$ resembles more closely the quasi-one dimensional organic superconductors. Our theory has certain phenomenological consequences that are...

Recent thermal conductivity measurements on $\mathrm{Sr_2 Ru O_4}$ [E. Hassinger et al., Phys. Rev. X 7, 011032 (2017)] were interpreted as favoring a pairing gap function with vertical line nodes while conflicting with chiral $p$-wave pairing. Motivated by this work we study the effects of deep superconducting gap minima on impurity induced quasiparticle thermal transport in chiral $p$-wave models of $\mathrm{Sr_2 Ru O_4}$. Combining a self-consistent T-matrix analysis and s...

In the last few years the gap symmetries of many new superconductors,including Sr$_2$RuO$_4$, CeCoIn$_5$, $\kappa$-(ET)$_{2}$Cu(NCS)$_{2}$, YNi$_{2}$B$_{2}$C and PrOs$_{4}$Sb$_{12}$, have been identified via angle-dependent magnetothermal conductivity measurements. However, a controversy still persists as to the nature of the superconductivity in Sr$_2$RuO$_4$. For PrOs$_{4}$Sb$_{12}$, spin-triplet superconductivity has recently been proposed. Here, we also propose g-wave sup...

Many seemingly contradictory experimental findings concerning the superconducting state in Sr$_2$RuO$_4$ can be accounted for on the basis of a conjectured accidental degeneracy between two patterns of pairing that are unrelated to each other under the $(D_{4h})$ symmetry of the crystal: a $d_{x^2-y^2}$-wave $(B_{1g})$ and a $g_{xy(x^2-y^2)}$-wave $(A_{2g})$ superconducting state. In this paper, we propose a generic multi-band model in which the $g$-wave pairing involving the...

Based on the microscopic model introduced previously the observed specific heat and ac-susceptibility data in the superconducting phase in Sr_2RuO_4 with applied magnetic fields are described consistently within a phenomenological approach. Discussed in detail are the temperature dependence of the upper critical fields H_{c2} and H_2, the dependence of the upper critical fields on the field direction, the linear specific heat below the superconducting phase transition as a fu...

One key feature of the multi-orbital superconducting Sr$_2$RuO$_4$ is the presence of nodal or near-nodal quasiparticle excitations revealed in a wide variety of experiments. Typically, a nodal gap structure in a two-dimensional model would be inconsistent with the chiral or helical p-wave interpretations. However, we demonstrate that true gap nodes may emerge along certain high-symmetry directions on the quasi-one-dimensional Fermi surfaces, if the multi-orbital chiral or he...

Symmetry arguments based on the point group of a system and thermodynamic measurements are often combined to identify the order parameter in unconventional superconductors. However, lattice translations, which can induce additional momenta with vanishing order parameter in the Brillouin zone, are neglected, especially in gap functions otherwise expected to be constant, such as in chiral superconductors. After a general analysis of the symmetry conditions for vanishing gap fun...

We investigate the order parameter of Sr$_2$RuO$_4$ in an exchange-only magnetic field. A Ginzburg-Landau symmetry analysis implies three possibilities: a pure $^3$He A phase, a $^3$He A$_1$ or a $^3$He A$_2$ phase. We explore the exchange field dependence of the order parameter and energy gap in a one-band model of Sr$_2$RuO$_4$. The numerical solutions show no A$_1$ phase and that the A$_2$ phase is lower in free energy than the A phase. We explore heat capacity as a functi...

We use functional renormalization group method to study a three-orbital model for superconducting Sr$_2$RuO$_4$. Although the pairing symmetry is found to be chiral $p$-wave, the atomic spin-orbit coupling induces near-nodes for quasiparticle excitations. Our theory explains a major experimental puzzle between $d$-wave-like feature observed in thermal experiments and the chiral $p$-wave triplet pairing revealed in nuclear-magnetic-resonance and Kerr effect.

This paper gives a detailed numerical study of the superconducting electronic heat transport in the unconventional multiband superconductor Strontium Ruthenate Sr$_2$RuO$_4$. The study demostrates that a model with different nodal structures on different sheets of the Fermi surface is able to describe quantitatively experimental heat transport data. The contribution of the density of states DOS is given for each sheet of the Fermi surface and the total contribution is also ca...