Lilia Boeri

Is LaFeAsO1-xFx an Electron-Phonon Superconductor?

In this Letter, we calculate the electron-phonon coupling of the newly discovered superconductor LaFeAsO1-xFx using linear response. For pure LaFeAsO, the calculated electron-phonon coupling constant lambda=0.21 and logarithmic-averaged frequency omegaln=206 K give a maximum Tc of 0.8 K, using the standard Migdal-Eliashberg theory. For the F-doped compounds, we predict even smaller coupling constants. To reproduce the experimental Tc, a 5-6 times larger coupling constant would be needed. Our results indicate that electron-phonon coupling is not sufficient to explain superconductivity in the whole family of Fe-As-based superconductors, probably due to the importance of strong-correlation effects.

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In this Letter, we calculate the electron-phonon coupling of the newly discovered superconductor LaFeAsO1-xFx using linear response. For pure LaFeAsO, the calculated electron-phonon coupling constant lambda=0.21 and logarithmic-averaged frequency omegaln=206 K give a maximum Tc of 0.8 K, using the standard Migdal-Eliashberg theory. For the F-doped compounds, we predict even smaller coupling constants. To reproduce the experimental Tc, a 5-6 times larger coupling constant would be needed. Our results indicate that electron-phonon coupling is not sufficient to explain superconductivity in the whole family of Fe-As-based superconductors, probably due to the importance of strong-correlation effects.

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First principles calculations of magnetic and, to a lesser extent, electronic properties of the novel LaFeAsO-based superconductors show substantial apparent controversy, as opposed to most weakly or strongly correlated materials. Not only do different reports disagree about quantitative values, there is also a schism in terms of interpreting the basic physics of the magnetic interactions in this system. In this paper, we present a systematic analysis using four different first principles methods and show that while there is an unusual sensitivity to computational details, well-converged full-potential all-electron results are fully consistent among themselves. What makes results so sensitive and the system so different from simple local magnetic moments interacting via basic superexchange mechanisms is the itinerant character of the calculated magnetic ground state, where very soft magnetic moments and long-range interactions are characterized by a particular structure in the reciprocal (as opposed to real) space. Therefore, unravelling the magnetic interactions in their full richness remains a challenging, but utterly important task.

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We substantiate by numerical and analytical calculations that the recently discovered superconductivity below 4 K in 3% boron-doped diamond is caused by electron-phonon coupling of the same type as in MgB2, albeit in three dimensions. Holes at the top of the zone-centered, degenerate sigma-bonding valence-band couple strongly to the optical bond-stretching modes. The increase from two to three dimensions reduces the mode softening crucial for T(c) reaching 40 K in MgB2. Even if diamond had the same bare coupling constant as MgB2, which could be achieved with 10% doping, T(c) would be only 25 K. Superconductivity above 1 K in Si (Ge) requires hole doping beyond 5% (10%).

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This paper explains the multi-orbital band structures and itinerant magnetism of the iron-pnictide and chalcogenide superconductors. We first describe the generic band structure of a single, isolated FeAs layer. Use of its Abelian glide-mirror group allows us to reduce the primitive cell to one FeAs unit. For the lines and points of high symmetry in the corresponding large, square Brillouin zone, we specify how the one-electron Hamiltonian factorizes. From density-functional theory, and for the observed structure of LaOFeAs, we generate the set of eight Fe d and As p localized Wannier functions and their tight-binding (TB) Hamiltonian, h (k) . For comparison, we generate the set of five Fe d Wannier orbitals. The topology of the bands, i. e. allowed and avoided crossings, specifically the origin of the d6 pseudogap, is discussed, and the role of the As p orbitals and the elongation of the FeAs4 tetrahedron emphasized. We then couple the layers, mainly via interlayer hopping between As pz orbitals, and give the formalism for simple tetragonal and body-centered tetragonal (bct) stackings. This allows us to explain the material-specific 3D band structures, in particular the complicated ones of bct BaFe2As2 and CaFe2As2 whose interlayer hoppings are large. Due to the high symmetry, several level inversions take place as functions of kz or pressure, and linear band dispersions (Dirac cones) are found at many places. The underlying symmetry elements are, however, easily broken by phonons or impurities, for instance, so that the Dirac points are not protected. Nor are they pinned to the Fermi level because the Fermi surface has several sheets. From the paramagnetic TB Hamiltonian, we form the band structures for spin spirals with wavevector q by coupling h (k) and h (k + q). The band structure for stripe order is studied in detail as a function of the exchange potential, Δ, or moment, m, using Stoner theory. Gapping of the Fermi surface (FS) for small Δ requires matching of FS dimensions (nesting) and d-orbital characters. The interplay between pd hybridization and magnetism is discussed using simple 4 × 4 Hamiltonians. The origin of the propeller-shaped Fermi surface is explained in detail. Finally, we express the magnetic energy as the sum over band-structure energies and this enables us to understand to what extent the magnetic energies might be described by a Heisenberg Hamiltonian, and to address the much discussed interplay between the magnetic moment and the elongation of the FeAs4 tetrahedron.

FeAs an electron-phonon superconductor ?

Abstract : We calculate the effect of local magnetic moments on the electron-phonon coupling in BaFe2As2+ using the density functional perturbation theory. We show that the magnetism enhances the total electron-phonon coupling by approximately 50%, up to lambda smaller or approximate to 0.35, still not enough to explain the high critical temperature, but strong enough to have a non-negligible effect on superconductivity, for instance, by frustrating the coupling with spin fluctuations and inducing order parameter nodes. The enhancement comes mostly from a renormalization of the electron-phonon matrix elements. We also investigate, in the rigid band approximation, the effect of doping, and find that lambda versus doping does not mirror the behavior of the density of states; while the latter decreases upon electron doping, the former does not, and even increases slightly.

Problems with reconciling density functional theory calculations with experiment in ferropnictides

The superconducting state of Ca-intercalated graphite CaC6 has been investigated by specific heat measurements. The characteristic anomaly at the superconducting transition (Tc = 11.4 K) indicates clearly the bulk nature of the superconductivity. The temperature and magnetic field dependence of the electronic specific heat are consistent with a fully gapped superconducting order parameter. The estimated electron-phonon coupling constant is lambda = 0.70 +/- 0.04, suggesting that the relatively high Tc of CaC6 can be explained within the intermediate coupling BCS approach.

Three-Dimensional MgB2-Type Superconductivity in Hole-Doped Diamond

The momentum and temperature dependence of the lifetimes of acoustic phonons in the elemental superconductors lead and niobium were determined by resonant spin-echo spectroscopy with neutrons. In both elements, the superconducting energy gap extracted from these measurements was found to converge with sharp anomalies originating from Fermi-surface nesting (Kohn anomalies) at low temperatures. The results indicate electron many-body correlations beyond the standard theoretical framework for conventional superconductivity. A possible mechanism is the interplay between superconductivity and spin- or charge-density-wave fluctuations, which may induce dynamical nesting of the Fermi surface.

On the multi-orbital band structure and itinerant magnetism of iron-based superconductors†

The pressure effect on the superconducting transition temperature (

Effects of Magnetism and Doping on the Electron-Phonon Coupling in BaFe2As2

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