We argue that the newly discovered superconductivity in a nearly magnetic, Fe-based layered compound is unconventional and mediated by antiferromagnetic spin fluctuations, though different from the usual superexchange and specific to this compound. This resulting state is an example of extended s-wave pairing with a sign reversal of the order parameter between different Fermi surface sheets. The main role of doping in this scenario is to lower the density of states and suppress the pair-breaking ferromagnetic fluctuations.

Abstract.The recently discovered Fe pnictide and chalcogenide superconductors display low temperature properties suggesting superconducting gap structures which appear to vary substantially from family to family, and even within family as a function of doping or pressure. We propose that this apparent nonuniversality can actually be understood by considering the predictions of spin fluctuation theory and accounting for the peculiar electronic structure of these systems, coupled with the likely "sign-changing s-wave" (s ± ) symmetry. We review theoretical aspects, materials properties, and experimental evidence relevant to this suggestion, and discuss which further measurements would be useful to settle these issues.

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Density-functional calculations of the phonon spectrum and electron-phonon coupling in MgB2 are presented. The E2g phonons, which involve in-plane B displacements, couple strongly to the px,y electronic bands. The isotropic electron-phonon coupling constant is calculated to be about 0.8. Allowing for different order parameters in different bands, the superconducting λ in the clean limit is calculated to be significantly larger. The E2g phonons are strongly anharmonic, and the non-linear contribution to the coupling between the E2g modes and the px,y bands is significant.The recent discovery of superconductivity near 40 K in MgB 2 has generated much interest in the properties of this simple intermetallic compound [1]. A significant B isotope effect strongly suggests phonon-mediated pairing [2]. To explain the large T c , an electron-phonon coupling (EPC) constant of λ ≈ 1 is needed. Yet estimates of the coupling strength based on the latest measurements of the low-temperature specific heat [3], combined with the density of states (DOS) from density-functional calculations [4], yield λ ≈ 0.6-0.7. Further, the measured temperature dependence of the electrical resistivity [5] is consistent with λ tr 0.6. First-principles calculations of the EPC give λ ≈ 0.7 − 0.9 [4,6,7]. Clearly there is a problem in reconciling all these numbers. Another puzzle involves tunneling measurements of the gap. Values of 2∆/k B T c ranging from 1.2 to 4 have been reported. The values below the BCS weak coupling limit of 3.5 have been attributed to surface effects, but the best-quality spectra [8] show a very clean gap with 2∆/k B T c = 1.25. Sharvin contact measurements [9] reveal a gap at 4.3 meV (2∆/k B T c = 2.6), and additional structures at 2∆/k B T c = 1.5 and 3, raising the possibility of multiple gaps. Careful analysis of the temperature and magnetic field dependence of the specific heat suggests anisotropic or multiple gap structure as well [3]. Thus, even if superconductivity in MgB 2 is phonon-mediated, it is likely that an analysis beyond the simple isotropic Eliashberg model is needed.The MgB 2 lattice consists of two parallel systems of flat layers. One layer contains B atoms in a honeycomb lattice, the other Mg atoms in a triangular lattice halfway between the B layers. First-principles calculations [4] find that the electronic states near the Fermi level are primarily B in character and the Fermi surface (FS) comprises four sheets: two nearly cylindrical hole sheets about the Γ-A line arising from quasi-2D p x,y B bands, and two tubular networks arising from 3D p z bonding and antibonding bands [4]. The difference in character between the sheets raises the possibility that each has a distinct gap that could be observed in the clean limit. Such interband anisotropy enhances the effective EPC constant relevant to superconductivity and decreases the coupling constant for transport, compared to the average values [10][11][12]. This could explain the discrepant values of λ deduced from different types of experiments.In this pa...

We report inelastic neutron scattering measurements on Na2IrO3, a candidate for the Kitaev spin model on the honeycomb lattice. We observe spin-wave excitations below 5 meV with a dispersion that can be accounted for by including substantial further-neighbor exchanges that stabilize zig-zag magnetic order. The onset of long-range magnetic order below TN = 15.3 K is confirmed via the observation of oscillations in zero-field muon-spin rotation experiments. Combining single-crystal diffraction and density functional calculations we propose a revised crystal structure model with significant departures from the ideal 90• Ir-O-Ir bonds required for dominant Kitaev exchange. [6,7], in which edge-sharing IrO 6 octahedra form a honeycomb lattice [see Fig. 1b)], have been predicted to display novel magnetic states for composite spin-orbital moments coupled via frustrated exchanges. The exchange between neighboring Ir moments (called S i,j , S=1/2) is proposed to be [2]where J K > 0 is an Ising ferromagnetic (FM) term arising from superexchange via the Ir-O-Ir bond, and J 1 > 0 is the antiferromagnetic (AFM) Heisenberg exchange via direct Ir-Ir 5d overlap. Due to the strong spin-orbital admixture the Kitaev term J K couples only the components in the direction γ, normal to the plane of the Ir-O-Ir bond [8,9]. Because of the orthogonal geometry, different spin components along the cubic axes (γ = x, y, z) of the IrO 6 octahedron are coupled for the three bonds emerging out of each site in the honeycomb lattice. This leads to the strongly-frustrated Kitaev-Heisenberg (KH) model [2], which has conventional Néel order [see Fig. 3a)] for large J 1 , a stripy collinear AFM phase [see Fig. 3c)] for 0.4 α 0.8, where α = J K / (J K + 2J 1 ) (exact ground state at α = 1/2), and a quantum spin liquid with Majorana fermion excitations [10] at large J K (α 0.8). Measurements of the spin excitations are very important to determine the overall energy scale and the relevant magnetic interactions, however because Ir is a strong neutron absorber inelastic neutron scattering (INS) experiments are very challenging. Using an optimized setup we here report the first observation of dispersive spin wave excitations of Ir moments via INS. We show that the dispersion can be quantitatively accounted for by including substantial further-neighbor in-plane exchanges, which in turn stabilize zig-zag order. To inform future ab initio studies of microscopic models of the interactions we combine single-crystal xray diffraction with density functional calculations to determine precisely the oxygen positions, which are key in mediating the exchange and controlling the spin-orbital admixture via crystal field effects. We propose a revised crystal structure with much more symmetric IrO 6 octahedra, but with substantial departures from the ideal 90• Ir-O-Ir bonds required for dominant Kitaev exchange [9], and with frequent structural stacking faults. This differs from the currentlyadopted model, used by several band-structure calculations [14,15], with asymme...

The layered honeycomb magnet α-RuCl3 has been proposed as a candidate to realize a Kitaev spin model with strongly frustrated, bond-dependent, anisotropic interactions between spin-orbit entangled j eff = 1/2 Ru 3+ magnetic moments. Here we report a detailed study of the three-dimensional crystal structure using x-ray diffraction on un-twinned crystals combined with structural relaxation calculations. We consider several models for the stacking of honeycomb layers and find evidence for a parent crystal structure with a monoclinic unit cell corresponding to a stacking of layers with a unidirectional in-plane offset, with occasional in-plane sliding stacking faults, in contrast with the currently-assumed trigonal 3-layer stacking periodicity. We report electronic band structure calculations for the monoclinic structure, which find support for the applicability of the j eff = 1/2 picture once spin orbit coupling and electron correlations are included. Of the three nearest neighbour Ru-Ru bonds that comprise the honeycomb lattice, the monoclinic structure makes the bond parallel to the b-axis non-equivalent to the other two, and we propose that the resulting differences in the magnitude of the anisotropic exchange along these bonds could provide a natural mechanism to explain the spin gap observed in powder inelastic neutron scattering [Banerjee et al.], in contrast to spin models based on the three-fold symmetric trigonal structure, which predict a gapless spectrum within linear spin wave theory. Our susceptibility measurements on both powders and stacked crystals, as well as magnetic neutron powder diffraction show a single magnetic transition upon cooling below TN ≈13 K. The analysis of our neutron powder diffraction data provides evidence for zigzag magnetic order in the honeycomb layers with an antiferromagnetic stacking between layers. Magnetization measurements on stacked single crystals in pulsed field up to 60 T show a single transition around 8 T for in-plane fields followed by a gradual, asymptotic approach to magnetization saturation, as characteristic of strongly-anisotropic exchange interactions.

The concept of a CDW induced by Fermi-surface nesting originated from the Peierls idea of electronic instabilities in purely 1D metals and is now often applied to charge ordering in real lowdimensional materials. The idea is that if Fermi surface contours coincide when shifted along the observed CDW wave vector, then the CDW is considered to be nesting-derived. We show that in most cases this procedure has no predictive power, since Fermi surfaces either do not nest at the right wave vector, or nest more strongly at the wrong vector. We argue that only a tiny fraction, if any, of the observed charge ordering phase transitions are true analogues of the Peierls instability because electronic instabilities are easily destroyed by even small deviations from perfect nesting conditions. Using prototypical CDW materials NbSe2, TaSe2, and CeTe3, we show that such conditions are hardly ever fulfilled, and that the CDW phases are actually structural phase transitions, driven by the concerted action of electronic and ionic subsystems, i.e., q-dependent electron-phonon coupling plays an indispensable part. We also show mathematically that the original Peierls construction is so fragile as to be unlikely to apply to real materials. We argue that no meaningful distinction between a CDW and an incommensurate lattice transition exists.

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