Momentum dependence of the electron-phonon coupling and self-energy effects in YBa_2Cu_3O_7 within the local density approximation

Using the local density approximation (LDA) and a realistic phonon spectrum we determine the momentum and frequency dependence of $\alpha^2F({\bf k},\omega)$ in YBa$_2$Cu$_3$O$_7$ for the bonding, antibonding, and chain band. The resulting self-energy $\Sigma$ is rather small near the Fermi surface. For instance, for the antibonding band the maximum of $Re \Sigma$ as a function of frequency is about 7 meV at the nodal point in the normal state and the ratio of bare and renormalized Fermi velocities 1.18. These values are a factor 3-5 too small compared to the experiment showing that only a small part of $\Sigma$ can be attributed to phonons. Furthermore, the frequency dependence of the renormalization factor $Z({\bf k},\omega)$ is smooth and has no anomalies at the observed kink frequencies which means that phonons cannot produce well-pronounced kinks in stoichiometric YBa$_2$Cu$_3$O$_7$, at least, within the LDA.Comment: Figure 1 slightly revised, text shortened, accepted as a PR

Minority-spin conduction in ferromagnetic Mn5_5Ge3_3Cx_x and Mn5_5Si3_3Cx_x films derived from anisotropic magnetoresistance and density functional theory

The anisotropic magnetoresistance (AMR) of ferromagnetic Mn$_5$Ge$_3$C$_x$ (0 $\le$ x $\le$ 1) and Mn$_5$Si$_3$C$_x$ (0.5 $\le$ x $\le$ 1) thin films was investigated and compared with density functional theory calculations from which the spin-split electronic density of states at the Fermi level and the spin polarization were obtained. The isostructural compounds exhibit different AMR behavior. While only Mn5Si3C0.5 shows a positive AMR ratio and a positive spin polarization, the negative AMR ratio of all other compounds is due to a negative spin polarization. The correlation between the sign of the AMR and the degree of spin polarization is in agreement with theoretical calculations of the AMR ratio indicating that the magnetoelectronic transport in both compounds is dominated by minority-spin conduction. The dominating role of minority-spin conduction remains unaffected even after incorporation of carbon into the crystalline lattice which weakens both AMR and spin polarization

First-principles calculations of the dispersion of surface phonons of the unreconstructed and reconstructed Pt(110)

We present result of calculations of the surface phonon dispersion curves for Pt(110) using density functional theory in the local density approximation and norm conserving pseudopotentials in a mixed-basis approach. Linear response theory is invoked and both the unreconstructed, and the missing row (1x2) reconstructed surfaces are considered. We find that the reconstruction is not driven by a phonon instability. Most of the observed phonon modes for the (1x2) structure can be understood in terms of simple folding of the (1x1) Brillouin zone onto that for the (1x2) surface. Largest changes in the phonon frequencies on surface reconstruction occur close to the zone boundary in the (001) direction. Detailed comparison of atomic force constants for the (1x1) and the (1x2) surfaces and their bulk counterparts show that the bulk value is attained after three layers. Our calculations reproduce nicely the Kohn anomaly observed along the (110) direction in the bulk. We do not find a corresponding effect on the surface

Properties of the phonon-induced pairing interaction in YBa2_2Cu3_3O7_7 within the local density approximation

The properties of the phonon-induced interaction between electrons are studied using the local density approximation (LDA). Restricting the electron momenta to the Fermi surface we find generally that this interaction has a pronounced peak for large momentum transfers and that the interband contributions between bonding and antibonding band are of the same magnitude as the intraband ones. Results are given for various symmetry averages of this interaction over the Fermi surface. In particular, we find that the dimensionless coupling constant in the d-wave channel $\lambda^d$, relevant for superconductivity, is only 0.022, i.e., even about ten times smaller than the small value of the s-wave channel. Similarly, the LDA contribution to the resistivity is about a factor 10 times smaller than the observed resistivity suggesting that phonons are not the important low-energy excitations in high-T$_c$ oxides.Comment: 6 pages, 7 figure

Pressure-induced topological phases of KNa2Bi

We report an ab initio study of the effect of hydrostatic pressure and uniaxial strain on electronic properties of KNa2Bi, a cubic bialkali bismuthide. It is found that this zero-gap semimetal with an inverted band structure at the Brillouin zone center can be driven into various topological phases under proper external pressure. We show that upon hydrostatic compression KNa2Bi turns into a trivial semiconductor with a conical Dirac-type dispersion of electronic bands at the point of the topological transition while the breaking of cubic symmetry by applying a uniaxial strain converts the compound into a topological insulator or into a three-dimensional Dirac semimetal with nontrivial surface Fermi arcs depending on the sign of strain. The calculated phonon dispersions show that KNa2Bi is dynamically stable both in the cubic structure (at any considered pressures) and in the tetragonal phase (under uniaxial strain)

Ab initio lattice dynamics and electron-phonon coupling of Bi(111)

Under the terms of the Creative Commons Attribution License 3.0 (CC-BY).We present a comprehensive ab initio study of structural, electronic, lattice dynamical, and electron-phonon coupling properties of the Bi(111) surface within density functional perturbation theory. Relativistic corrections due to spin-orbit coupling are consistently taken into account. Changes of interatomic couplings are confined mostly to the first two bilayers, resulting in superbulk modes with frequencies higher than the optic bulk spectrum, and in an enhanced density of states at lower frequencies for atoms in the first bilayer. We give results for the momentum-dependent electron-phonon coupling of electronic states belonging to the two surface electronic bands along ΓM¯ which cross the Fermi energy. For larger momenta, the lower surface band exhibits a moderate electron-phonon coupling of about 0.45, which is larger than the coupling constant of bulk Bi. For momenta close to Γ¯, states of both surface bands show even stronger couplings because of interband transitions to bulk states near Γ¯ around the Fermi level. For these cases, the state-dependent Eliashberg functions exhibit pronounced peaks at low energy and strongly deviate in shape from a Debye-type spectrum, indicating that an extraction of the coupling strength from measured electronic self-energies based on this simple model is likely to fail.The work of M.A.O. and T.S.R. was partially funded by US Department of Energy, DE-FG02-07ER46354.Peer Reviewe

Effect of c(2x2)-CO overlayer on the phonons of Cu(001): a first principles study

We have examined the effect of a c(2x2) overlayer of CO on the surface phonons of the substrate, Cu(001), by applying the density functional perturbation theory with both the local (LDA) and the generalized-gradient (GGA) density approximations, through the Hedin-Lundqvist and the Perdew-Burke-Ernzerhof functionals, respectively. Our results (GGA) trace the Rayleigh wave softening detected by helium atom scattering (HAS) experiments to changes in the force constants between the substrate surface atoms brought about by CO chemisorption, resolving an ongoing debate on the subject. The calculated surface phonon dispersion curves document the changes in the polarization of some modes and show those of the modes originally along the $\bar{Y}$ direction of the clean surface Brillouin zone (SBZ) which are back-folded along the $\bar{\Delta}$ direction of the chemisorbed SBZ, to be particularly consequential. The vertical and shear horizontal section of $S_1$ in the SBZ of the clean surface, for example, is back-folded as a longitudinal-vertical mode, indicating thereby that $S_1$ $-$ predicted a long time back along $\bar{\Delta}$ for the clean surface $-$ may be indirectly assessed at $\bar{X}$ upon CO adsorption by standard planar scattering techniques. These findings further suggest that some of the energy losses detected by HAS along $\bar{\Delta}$, which were associated to multiphonon excitations of the adlayer frustrated translation mode, may actually correspond to the back-folded substrate surface modes

First-principles calculations of the phonon dispersion curves of H on Pt(111)

We have calculated the surface phonon dispersion curves for H on Pt(111), using first-principles, total energy calculations based on a mixed-basis set and norm-conserving pseudopotentials. Linear response theory and the harmonic approximation are invoked. For one monolayer of H in the preferred adsorption site (fcc hollow) vibrational modes polarized parallel and perpendicular to the surface are found, respectively, at 73.5 meV and 142.6 meV, at the Γ point of the surface Brillouin zone. The degeneracy of the parallel mode is lifted at the zone boundaries, yielding energies of 69.6 meV and 86.3 meV at the M point and 79.4 meV and 80.8 meV at the K point. The dispersion curves for H adsorption at the hcp hollow site differ only slightly from the above. In either case, H adsorption has considerable impact on the substrate modes; in particular the surface mode in the gap in the bulk phonon spectrum (around M point) is pushed into the bulk band. For on-top H adsorption, modes polarized parallel and perpendicular to the surface have respective energies of 47.4 meV and 277.2 meV, at the Γ point. The former disperses to 49.1 meV and 59.5 meV at the M point and to 56 meV and 56.7 meV at the K point. The H vibrational mode polarized perpendicular to the surface shows little dispersion, in all three cases considered. Insights are obtained from the hybridization of the H and Pt electronic states.Comment: 26 pages, 6 figure