Electromagnetic Response of a kx±ikyk_x\pm ik_y Superconductor: Effect of Order Parameter Collective Modes

Effects of order parameter collective modes on electromagnetic response are studied for a clean spin-triplet superconductor with $k_x\pm ik_y$ orbital symmetry, which has been proposed as a candidate pairing symmetry for Sr$_2$RuO$_4$. It is shown that the $k_x \pm ik_y$ superconductor has characteristic massive collective modes analogous to the clapping mode in the A-phase of superfluid $^3$He. We discuss the contribution from the collective modes to ultrasound attenuation and electromagnetic absorption. We show that in the electromagnetic absorption spectrum the clapping mode gives rise to a resonance peak well below the pair breaking frequency, while the ultrasound attenuation is hardly influenced by the collective excitations.Comment: 4 pages RevTex, 1 eps figur

Anisotropy in the Antiferromagnetic Spin Fluctuations of Sr2RuO4

It has been proposed that Sr_2RuO_4 exhibits spin triplet superconductivity mediated by ferromagnetic fluctuations. So far neutron scattering experiments have failed to detect any clear evidence of ferromagnetic spin fluctuations but, instead, this type of experiments has been successful in confirming the existence of incommensurate spin fluctuations near q=(1/3 1/3 0). For this reason there have been many efforts to associate the contributions of such incommensurate fluctuations to the mechanism of its superconductivity. Our unpolarized inelastic neutron scattering measurements revealed that these incommensurate spin fluctuations possess c-axis anisotropy with an anisotropic factor \chi''_{c}/\chi''_{a,b} of \sim 2.8. This result is consistent with some theoretical ideas that the incommensurate spin fluctuations with a c-axis anisotropy can be a origin of p-wave superconductivity of this material.Comment: 5 pages, 3 figures; accepted for publication in PR

Linear Field Dependence of the Normal-State In-Plane Magnetoresistance of Sr2RuO4

The transverse and longitudinal in-plane magnetoresistances in the normal state of superconducting Sr2RuO4 single crystals have been measured. At low temperatures, both of them were found to be positive with a linear magnetic-field dependence above a threshold field, a result not expected from electronic band theory. We argue that such behavior is a manifestation of a novel coherent state characterized by a spin pseudo gap in the quasi-particle excitation spectrum in Sr2RuO4.Comment: 4 pages + 5 figure

Magnetic ordering in Sr2RuO4 induced by nonmagnetic impurities

We report unusual effects of nonmagnetic impurities on the spin-triplet superconductor Sr2RuO4. The substitution of nonmagnetic Ti4+ for Ru4+ induces localized-moment magnetism characterized by unexpected Ising anisotropy with the easy axis along the interlayer c direction. Furthermore, for x(Ti) > 0.03 magnetic ordering occurs in the metallic state with the remnant magnetization along the c-axis. We argue that the localized moments are induced in the Ru4+ and/or oxygen ions surrounding Ti4+ and that the ordering is due to their interaction mediated by itinerant Ru-4d electrons with strong spin fluctuations.Comment: 5 pages, 4figure

Magnetic Phase Diagram of Ca2-xSrxRuO4 Governed by Structural Distortions

We constructed, by the first-principles calculations, a magnetic phase diagram of Sr$_{2}$RuO$_4$ in the space spanned by structural distortions. Our phase diagram can qualitatively explain the experimental one for Ca$_{2-x}$Sr$_x$RuO$_4$. We found that the rotation and the tilting of RuO$_6$ octahedron are responsible for the ferro- and antiferro-magnetism, respectively, while the flattening of RuO$_6$ is the key factor to stabilize those magnetic ground states. Our results imply that the magnetic and the structural instabilities in Sr$_2$RuO$_4$ are closely correlated cooperatively rather than competitively.Comment: 3 figures; accepted by PRB as rapid communicatio

Low temperature electronic properties of Sr_2RuO_4 III: Magnetic fields

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 function of field or temperature, the anisotropy of the two spatial components of the order parameter, and the fluctuation field H_p.Comment: 8 pages REVTEX, 4 figure

Impurities and orbital dependent superconductivity in Sr_2RuO_4

There now exists a wealth of experimental evidence that Sr_2RuO_4 is an odd-parity superconductor. Experiments further indicate that among the bands stemming from the Ru {xy,xz,yz} orbitals, the portion of the Fermi surface arising from the xy orbitals exhibits a much larger gap than the portions of the Fermi surface arising from the {xz,yz} orbitals. In this paper the role of impurities on such an orbital dependent superconducting state is examined within the Born approximation. In contrast to expected results for a nodeless p-wave superconductor the unique nature of the superconducting state in Sr_2RuO_4 implies that a low concentration of impurities strongly influences the low temperature behavior.Comment: 5 pages 3 figure

Vortex lattice structures and pairing symmetry in Sr2RuO4

Recent experimental results indicate that superconductivity in Sr2RuO4 is described by the p-wave E_u representation of the D_{4h} point group. Results on the vortex lattice structures for this representation are presented. The theoretical results are compared with experiment.Comment: 4 pages, 3 figures, M2S-HTSC-VI proceeding

Detailed study of the ac susceptibility of Sr2RuO4 in oriented magnetic fields

We have investigated the ac susceptibility of the spin triplet superconductor Sr$_2$RuO$_4$ as a function of magnetic field in various directions at temperatures down to 60 mK. We have focused on the in-plane field configuration (polar angle $\theta \simeq 90^{\circ}$), which is a prerequisite for inducing multiple superconducting phases in Sr$_2$RuO$_4$. We have found that the previous attribution of a pronounced feature in the ac susceptibility to the second superconducting transition itself is not in accord with recent measurements of the thermal conductivity or of the specific heat. We propose that the pronounced feature is a consequence of additional involvement of vortex pinning originating from the second superconducting transition.Comment: Accepted for publication in Phys. Rev.