Dynamical magnetic susceptibility in the lamellar cobaltate superconductor Na_xCoO_2⋅y\cdot yH_2O

We systematically analyze the influence of the superconducting gap symmetry and the electronic structure on the dynamical spin susceptibility in superconducting Na_xCoO_2$\cdot y$H_2O within a three different models: the single a_{1g}-band model with nearest-neighbor hoppings, the realistic three-band t_{2g}-model with, and without e'_g pockets present at the Fermi surface. We show that the magnetic response in the normal state is dominated by the incommensurate antiferromagnetic spin density wave fluctuations at large momenta in agreement with experimental temperature dependence of the spin-lattice relaxation rate. Also, we demonstrate that the presence or the absence of the e'_g-pockets at the Fermi surface does not affect significantly this conclusion. In the superconducting state our results for d_{x^2-y^2}- or d_{xy}-wave symmetries of the superconducting order parameter are consistent with experimental data and exclude nodeless $d_{x^2-y^2} + id_{xy}$-wave symmetry. We further point out that the spin-resonance peak proposed earlier is improbable for the realistic band structure of Na_xCoO_2$\cdot y$H_2O. Moreover, even if present the resonance peak is confined to the antiferromagnetic wave vector and disappears away from it.Comment: Published version, PACS: 74.70.-b; 75.40.Gb; 74.20.Rp; 74.25.J

Theory of magnetic excitations in iron-based layered superconductors

Based on the effective four-band model we analyze the spin response in the normal and superconducting states of the Fe-pnictide superconductors. While the normal state spin excitations are dominated by the continuum of the interorbital antiferromagnetic fluctuations and the intraband spin density wave fluctuations, the unconventional superconductivity yields different feedback. The resonance peak in form of the well-defined spin exciton occurs {\it only} for the interband scattering at the antiferromagnetic momentum ${\bf Q}_{AFM}$ for the $s_\pm$ (extended s-wave) superconducting order parameter and it disappears rapidly for ${\bf q} < {\bf Q}_{AFM}$. The resonance feature is extremely weak for the $d_{x^2 -y^2}$-wave order parameter due to specific Fermi surface topology of these compounds. The essential difference between $s_\pm$-wave and $d_{x^2 -y^2}$-wave symmetries for the magnetic excitations can be used for experimental determination of the superconducting wave function symmetry.Comment: 5 pages, 3 figures, published versio

Three-orbital model for Fe-pnictides

We formulate and study the three-orbital model for iron-based superconductors. Results for the band structure, Fermi surface, and the spin susceptibility in both normal and superconducting s ± states are presented. We also discuss the pairing interaction and show that the dominant part of it should come from the intraorbital scattering. © 2013 Springer Science+Business Media New York

Theory of Raman response of a superconductor with extended s -wave symmetry: Application to the iron pnictides

We argue that Raman study of Fe-pnictides is a way to unambiguously distinguish between various superconducting gaps proposed for these materials. We show that A1g Raman intensity develops a true resonance peak below 2Δ if the pairing gap has A1g symmetry in the folded Brillouin zone (Δ (k=0) =Δ,Δ (π,π) =-Δ). No such peak develops for a pure s -wave gap, a d -wave gap, and an extended s -wave gap with Δ (k) =Δcos kx 2 cos ky 2. We show that the peak remains quite strong for the values of interpocket impurity scattering used to fit NMR data. © 2009 The American Physical Society

Spin-orbit coupling in Fe-based superconductors

We study the spin resonance peak in recently discovered iron-based superconductors. The resonance peak observed in inelastic neutron scattering experiments agrees well with predicted results for the extended s-wave (s ±) gap symmetry. Recent neutron scattering measurements show that there is a disparity between transverse and longitudinal components of the dynamical spin susceptibility. Such breaking of the spin-rotational invariance in the spin-liquid phase can occur due to spin-orbit coupling. We study the role of the spin-orbit interaction in the multiorbital model for Fe-pnictides and show how it affects the spin resonance feature. © 2013 Springer Science+Business Media New York

Itinerant in-plane magnetic fluctuations and many-body correlations in Nax_xCoO2_2

Based on the {\it ab-initio} band structure for Na$_x$CoO$_2$ we derive the single-electron energies and the effective tight-binding description for the $t_{2g}$ bands using projection procedure. Due to the presence of the next-nearest-neighbor hoppings a local minimum in the electronic dispersion close to the $\Gamma$ point of the first Brillouin zone forms. Correspondingly, in addition to a large Fermi surface an electron pocket close to the $\Gamma$ point emerges at high doping concentrations. The latter yields the new scattering channel resulting in a peak structure of the itinerant magnetic susceptibility at small momenta. This indicates dominant itinerant in-plane ferromagnetic fluctuations above certain critical concentration $x_m$, in agreement with neutron scattering data. Below $x_m$ the magnetic susceptibility shows a tendency towards the antiferromagnetic fluctuations. We further analyze the many-body effects on the electronic and magnetic excitations using various approximations applicable for different $U/t$ ratio.Comment: 10 page

Fluctuating charge density waves in the Hubbard model

The charge susceptibility of the two-dimensional repulsive Hubbard model is investigated using the diagram technique developed for the case of strong correlations. In this technique, a power series in the hopping constant is used. It is shown that once the Fermi level crosses one of the Hubbard subbands a sharp peak appears in the momentum dependence of the static susceptibility. With further departure from half-filling the peak transforms to a ridge around the $\Gamma$ point. In the considered range 0\leq|1-\bar{n}|\alt 0.2 of the electron filling $\bar{n}$ the static susceptibility is finite which points to the absence of the long-range charge ordering. However, for $|1-\bar{n}|\approx 0.12$ the susceptibility maxima are located halfway between the center and the boundaries of the Brillouin zone. In this case an interaction of carriers with tetragonal distortions can stabilize the charge density wave with the wavelength of four lattice spacings, as observed experimentally in the low-temperature tetragonal phase of lanthanum cuprates. In the range of parameters inherent in cuprate perovskites the character of the susceptibility evolution with $\bar{n}$ depends only weakly on the ratio of the nearest-neighbor hopping constant to the Hubbard repulsion and on details of the initial band structure. The location of the susceptibility maxima in the Brillouin zone is mainly determined by the value of $\bar{n}$.Comment: 8 pages, 4 figure

Specific features of spin, charge, and orbital ordering in cobaltites

Complex cobalt oxides known as cobaltites are reviewed, including LnCoO3-based perovskite-structured rare-earth cobaltites (where Ln is lanthanum or a lanthanide), quasi-two-dimensional and quasi-one-dimensional cobaltites of the types LnCo2O5+δ, La 2CoO4, and Ca3Co2O8, and NaxCoO2 yH2O superconducting compounds. Key experimental and theoretical results are presented, with emphasis on the interplay between charge, spin, and orbital degrees of freedom. Two problems of specific relevance to cobaltites - the spin state instability of Co3+ ions in LnCoO3, and the nature of superconductivity in Na xCoO2 yH2O - are also given significant attention. © 2009 Uspekhi Fizicheskikh Russian Academy of Sciences

Nonanalytic spin susceptibility of a fermi liquid: The case of Fe-based pnictides

We propose an explanation of the peculiar linear temperature dependence of the uniform spin susceptibility χ(T) in ferropnictides. We argue that the linear in T term appears to be due to the nonanalytic temperature dependence of χ(T) in a two-dimensional Fermi liquid. We show that the prefactor of the T term is expressed via the square of the spin-density-wave (SDW) amplitude connecting nested hole and electron pockets. Because of an incipient SDW instability, this amplitude is large, which, along with a small value of the Fermi energy, makes the T dependence of χ(T) strong. We demonstrate that this mechanism is in quantitative agreement with the experiment. © 2009 The American Physical Society

Incommensurate itinerant antiferromagnetic excitations and spin resonance in the FeTe0.6_{0.6}Se0.4_{0.4} superconductor

We report on inelastic neutron scattering measurements that find incommensurate itinerant like magnetic excitations in the normal state of superconducting FeTe$_{0.6}$Se$_{0.4}$ (\Tc=14K) at wave-vector $\mathbf{Q}_{inc}=(1/2\pm\epsilon,1/2\mp\epsilon)$ with $\epsilon$=0.09(1). In the superconducting state only the lower energy part of the spectrum shows significant changes by the formation of a gap and a magnetic resonance that follows the dispersion of the normal state excitations. We use a four band model to describe the Fermi surface topology of iron-based superconductors with the extended $s(\pm)$ symmetry and find that it qualitatively captures the salient features of these data.Comment: 7 pages and 5 figure