Anisotropy of the incommensurate fluctuations in Sr2RuO4: a study with polarized neutrons

The anisotropy of the magnetic incommensurate fluctuations in Sr2RuO4 has been studied by inelastic neutron scattering with polarized neutrons. We find a sizeable enhancement of the out of plane component by a factor of two for intermediate energy transfer which appears to decrease for higher energies. Our results qualitatively confirm calculations of the spin-orbit coupling, but the experimental anisotropy and its energy dependence are weaker than predicted.Comment: 4 pages, 4 figure

Spin-Orbit-Induced Orbital Excitations in Sr2RuO4 and Ca2RuO4: A Resonant Inelastic X-ray Scattering Study

High-resolution resonant inelastic X-ray scattering (RIXS) at the oxygen K-edge has been used to study the orbital excitations of Ca2RuO4 and Sr2RuO4. In combination with linear dichroism X-ray absorption spectroscopy, the ruthenium 4d-orbital occupation and excitations were probed through their hybridization with the oxygen p-orbitals. These results are described within a minimal model, taking into account crystal field splitting and a spin-orbit coupling \lambda_{so}=200~meV. The effects of spin-orbit interaction on the electronic structure and implications for the Mott and superconducting ground states of (Ca,Sr)2RuO4 are discussed.Comment: accepted in PRB 201

Strong increase in ultrasound attenuation below T2 in Sr2RuO4 : possible evidence for domains

Funding information: B.J.R. and S.G. acknowledge support from the Office of Basic Energy Sciences of the U.S. Department of Energy under award No. DE-SC0020143. B.J.R. and S.G. acknowledge support from the Cornell Center for Materials Research with funding from the Materials Research Science and Engineering Centers program of the National Science Foundation (cooperative agreement No. DMR-1719875). T.G.K. acknowledges support from the National Science Foundation under Grant No. PHY-2110250. N.K. acknowledges support from Japan Society for the Promotion of Science (JSPS) KAKENHI (Grants No. JP17H06136, No. JP18K04715, and No. 21H01033) and Japan Science and Technology Agency Mirai Program (JPMJMI18A3) in Japan.Recent experiments suggest that Sr2RuO4 has a two-component superconducting order parameter (OP). A two-component OP has multiple degrees of freedom in the superconducting state that can result in low-energy collective modes or the formation of domain walls—a possibility that would explain a number of experimental observations including the smallness of the signature of time reversal symmetry breaking at Tc and telegraph noise in critical current experiments. We use resonant ultrasound spectroscopy to perform ultrasound attenuation measurements across the superconducting Tc of Sr2RuO4. We find that compressional sound attenuation increases by a factor of 7 immediately below Tc , in sharp contrast with what is found in both conventional (s-wave) and high-Tc (d-wave) superconductors. Our observations are most consistent with the presence of domain walls that separate different configurations of the superconducting OP. The fact that we only observe an increase in sound attenuation for compressional strains, and not for shear strains, suggests an inhomogeneous superconducting state formed of two distinct, accidentally degenerate superconducting OPs that are not related to each other by symmetry. Whatever the mechanism, a factor of 7 increase in sound attenuation is a singular characteristic that must be reconciled with any potential theory of superconductivity in Sr2RuO4.Publisher PDFPeer reviewe

Effects of In-Plane Impurity Substitution in Sr2RuO4

We report comparative substitution effects of nonmagnetic Ti^(4+) and magnetic Ir^(4+) impurities for Ru^(4+) in the spin-triplet superconductor Sr2RuO4. We found that both impurities suppress the superconductivity completely at a concentration of approximately 0.15%, reflecting the high sensitivity to translational symmetry breaking in Sr2RuO4. In addition, a rapid enhancement of residual resistivity is in quantitative agreement with unitarity-limit scattering. Our result suggests that both nonmagnetic and magnetic impurities in Sr2RuO4 act as strong potential scatterers, similar to the nonmagnetic Zn^(2+) impurity in the high-Tc cuprates.Comment: 4 pages, 2 figures. submitted to Journal of the Physical Society of Japa

Two-dimensional incommensurate magnetic fluctuations in Sr2_2(Ru0.99_{0.99}Ti0.01_{0.01})O4_4

We investigate the imaginary part of the wave vector dependent dynamic spin susceptibility in Sr$_2$(Ru$_{0.99}$Ti$_{0.01}$)O$_4$ as a function of temperature using neutron scattering. At T=5 K, two-dimensional incommensurate (IC) magnetic fluctuations are clearly observed around $\mathbf{Q}_\text{c}=(0.3,0.3,L)$ up to approximately 60 meV energy transfer. We find that the IC excitations disperse to ridges around the $(\pi,\pi)$ point. Below 50 K, the energy and temperature dependent excitations are well described by the phenomenological response function for a Fermi liquid system with a characteristic energy of 4.0(1) meV. Although the wave vector dependence of the IC magnetic fluctuations in Sr$_2$(Ru$_{0.99}$Ti$_{0.01}$)O$_4$ is similar to that in the Fermi liquid state of the parent compound, Sr$_2$RuO$_4$, the magnetic fluctuations are clearly suppressed by the Ti-doping.Comment: 5 pages, 4 figure

Unrestricted Hartree-Fock Analysis of Sr3−x_{3-x}Cax_xRu2_2O7_7

We investigated the electronic and magnetic structure of Sr$_{3-x}$Ca$_x$Ru$_2$O$_7$ ($0 \leq x \leq 3$) on the basis of the double-layered three-dimensional multiband Hubbard model with spin-orbit interaction. In our model, lattice distortion is implemented as the modulation of transfer integrals or a crystal field. The most stable states are estimated within the unrestricted Hartree-Fock approximation, in which the colinear spin configurations with five different spin-quantization axes are adopted as candidates. The obtained spin structures for some particular lattice distortions are consistent with the neutron diffraction results for Ca$_3$Ru$_2$O$_7$. Also, some magnetic phase transitions can occur due to changes in lattice distortion. These results facilitate the comprehensive understanding of the phase diagram of Sr$_{3-x}$Ca$_x$Ru$_2$O$_7$.Comment: 16 pages, 7 figure

Magnetic fluctuations and itinerant ferromagnetism in two-dimensional systems with van Hove singularities

We use the quasistatic approach to analyze the criterion of ferromagnetism for two-dimensional (2D) systems with the Fermi level near Van Hove singularities (VHS) of the electronic spectrum. It is shown that the spectrum of spin excitations (paramagnons) is positively defined when the interaction I between electrons and paramagnons, which corresponds to the Hubbard on-site repulsion U, is sufficiently large. The critical interactions I_c and U_c remain finite at Van Hove filling and exceed considerably their values obtained from the Stoner criterion due to incommensurate spin fluctuations which are important near the ferromagnetic quantum phase transition. Combining the quasistatic approximation and the equation-of -motion method for the Green functions we obtain the results for the electronic self-energy to first order in the inverse number of spin components.Comment: This is an extended version of the paper in Sov. Phys. JETP, misprints are correcte

Infrared signatures of charge stripes in La(2-x)Sr(x)CuO(4)

The in-plane optical conductivity of seven La(2-x)Sr(x)CuO(4) single crystals with x between 0 and 0.15 has been studied from 30 to 295 K. All doped samples exhibit strong peaks in the far-infrared, which closely resemble those observed in Cu-O "ladders" with one-dimensional charge-ordering. The behavior with doping and temperature of the peak energy, width, and intensity allows us to conclude that we are observing charge stripes dynamics in La(2-x)Sr(x)CuO(4) on the fast time scale of infrared spectroscopy.Comment: 9 pages including figs. in pdf forma

Theory of Transport Properties in the p-wave Superconducting State of Sr2RuO4 - A Microscopic Determination of the Gap Structure -

We provide a detailed quantitative analysis of transport properties in the p-wave superconducting state of Sr2RuO4. Specifically, we calculate ultrasound attenuation rate and electronic thermal conductivity within the mean field approximation. The impurity scattering of the quasi-particles are treated within the self-consistent T-matrix approximation, and assumed to be in the unitarity limit. The momentum dependence of the gap function is determined by solving the Eliashberg equation for a three-band Hubbard model with the realistic electronic structure of Sr2RuO4. On the basis of the microscopic theory, we can naturally expect nodal structures along the c-axis on the cylindrical Fermi surfaces, even if we assume the chiral pairing state (i.e., \Delta(k) \sim k_x \pm {\rm i} k_y). Consequently, we obtain the temperature dependence of the transport coefficients in agreement with the experimental results. We can clarify that actually the thermal excitations on the passively superconducting bands contribute significantly to the thermal conductivity in a wide temperature range, in contrast to the case of other physical quantities.Comment: 12 pages, 7 figures, submitted to J. Phys. Soc. Jp