Effects of magnetism and doping on the electron-phonon coupling in BaFe2_{2}As2_{2}

We calculate the effect of local magnetic moments on the electron-phonon coupling in BaFe$_{2}$As$_{2}+\delta$ using the density functional perturbation theory. We show that the magnetism enhances the total electron-phonon coupling by $\sim 50$, up to $\lambda \lesssim 0.35$, 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.Comment: 4 pages, 3 figure

Расчет теплофизических характеристик выпускного газа дизельного комбинированного двигателя внутреннего сгорания

У дослідженні визначаються коефіцієнти динамічної та кінематичної в’язкості, теплопровідності випускного газу дизельного комбінованого двигуна внутрішнього згоряння (КДВЗ) при характерних для випускних систем станах. Методика реалізації дослідження ґрунтується на теоріях газових сумішей з використанням формул зведення Сутерленда, згоряння рідких вуглеводних палив з розрахунку стехіометричних співвідношень. Результати дослідження являють собою значення коефіцієнтів в’язкості і теплопровідності випускного газу суміші стехіометричного складу при характерних для випускної системи дизельних КДВЗ станах, залежності в’язкості і теплопровідності випускного газу від складу паливної суміші за атмосферних тиску і температури. Похибка встановлених характеристик не перевищує 0,5 %. Висновки відповідають класичним уявленням про досліджені властивості газів: в’язкість і теплопровідність продуктів згоряння дизельних КДВЗ із підвищенням температури і збільшенням змісту повітря збільшуються, причому при зміні вмісту повітря в’язкість змінюється інтенсивніше за теплопровідність.The purpose of research is to determine the coefficients of dynamic and kinematics viscosity, thermal conductivity of the exhaust gas of diesel CICE for typical states of exhaust systems. The research application methods are based on theories of gas mixtures using the Sutherland reduction formulas, combustion of liquid hydrocarbon fuels at the rate of stoichiometric proportions. Research results are quantities of viscosity and thermal conductivity coefficients of the exhaust gas by stoichiometric composition mixture with typical conditions for exhaust system of diesel CICE, dependencies of viscosity and thermal conductivity of exhaust gas from the combustible mixture composition by atmospheric pressure and temperature. The established performance error does not exceed 0,5 %. Our conclusions conform with classical ideas on studied properties of gases: viscosity and thermal conductivity of combustion products of the diesel CICE increase with rise of temperature and air content, as well as with the intense viscosity changes compared to thermal conductivity with the air content.В исследовании определяются коэффициенты динамической и кинематической вязкости, теплопроводности выпускного газа дизельного комбинированного двигателя внутреннего сгорания (КДВС) при характерных для выпускных систем состояниях. Методика реализации исследования основана на теориях газовых смесей с использованием формул приведения Сутерленда, сгорания жидких углеводородных топлив из расчета стехиометрических соотношений. Результаты исследования представляют собой значения коэффициентов вязкости и теплопроводности выпускного газа смеси стехиометрического состава при характерных для выпускной системы дизельных КДВС состояниях, зависимости вязкости и теплопроводности выпускного газа от состава горючей смеси при атмосферных давлении и температуре. Погрешность установленных характеристик не превышает 0,5 %. Выводы отвечают классическим представлениям об исследованных свойствах газов: вязкость и теплопроводность продуктов сгорания дизельных КДВС с повышением температуры и увеличением содержания воздуха увеличиваются, причем при изменении содержания воздуха вязкость изменяется интенсивнее теплопроводности

Fermi Surface Nesting and the Origin of the Charge Density Wave in NbSe2_2

We use highly accurate density functional calculations to study the band structure and Fermi surfaces of NbSe2. We calculate the real part of the non-interacting susceptibility, Re chi_0(q), which is the relevant quantity for a charge density wave (CDW) instability and the imaginary part, Im chi_0(q), which directly shows Fermi surface (FS) nesting. We show that there are very weak peaks in Re chi_0(q) near the CDW wave vector, but that no such peaks are visible in Im chi_0(q), definitively eliminating FS nesting as a factor in CDW formation. Because the peak in Re chi_0(q) is broad and shallow, it is unlikely to be the direct cause of the CDW instability. We briefly address the possibility that electron-electron interactions (local field effects) produce additional structure in the total (renormalized) susceptibility, and we discuss the role of electron-ion matrix elements.Comment: Replacement of Table II values, minor changes to tex

Comment on "First-principles calculation of the superconducting transition in MgB2 within the anisotropic Eliashberg formalism"

Choi et al. [Phys. Rev. B 66, 020513 (2002)] recently presented first principles calculations of the electron-phonon coupling and superconductivity in MgB2, emphasizing the importance of anisotropy and anharmonicity. We point out that (1) variation of the superconducting gap inside the sigma- or the pi-bands can hardly be observed in real samples, and (2) taking the anisotropy of the Coulomb repulsion into account influences the size of the small gap, Delta_pi.Comment: 3 pages, 2 color figure

Two-gap superconductivity in MgB2_{2}: clean or dirty?

A large number of experimental facts and theoretical arguments favor a two-gap model for superconductivity in MgB$_{2}$. However, this model predicts strong suppression of the critical temperature by interband impurity scattering and, presumably, a strong correlation between the critical temperature and the residual resistivity. No such correlation has been observed. We argue that this fact can be understood if the band disparity of the electronic structure is taken into account, not only in the superconducting state, but also in normal transport

Correlated metals and the LDA+U method

While LDA+U method is well established for strongly correlated materials with well localized orbitals, its application to weakly correlated metals is questionable. By extending the LDA Stoner approach onto LDA+U, we show that LDA+U enhances the Stoner factor, while reducing the density of states. Arguably the most important correlation effects in metals, fluctuation-induced mass renormalization and suppression of the Stoner factor, are missing from LDA+U. On the other hand, for {\it moderately} correlated metals LDA+U may be useful. With this in mind, we derive a new version of LDA+U that is consistent with the Hohenberg-Kohn theorem and can be formulated as a constrained density functional theory. We illustrate all of the above on concrete examples, including the controversial case of magnetism in FeAl.Comment: Substantial changes. In particular, examples of application of the proposed functional are adde

"Chain scenario" for Josephson tunneling with pi-shift in YBa2Cu3O7

We point out that all current Josephson-junction experiments probing directly the symmetry of the superconducting state in YBa2Cu3O7, can be interpreted in terms of the bilayer antiferromagnetic spin fluctuation model, which renders the superconducting state with the order parameters of extended $s$ symmetry, but with the opposite signs in the bonding and antibonding Cu-O plane bands. The essential part of our interpretation includes the Cu-O chain band which would have the order parameter of the same sign as antibonding plane band. We show that in this case net Josephson currents along and perpendicular to the chains have the phase shift equal to pi.Comment: 4 pages, revtex, 1 figure uuencoded (POSTSCRIPT figure replaced - the previous file did not print Greek letters correctly

A valence bond liquid on the honeycomb lattice

The honeycomb lattice material Li2RuO3 undergoes a dimerization of Ru4+ cations on cooling below 270C, where the magnetic susceptibility vanishes. We use density functional theory calculations to show that this reflects the formation of a 'valence bond crystal', with a strong bond disproportionation. On warming, x-ray diffraction shows that discrete three-fold symmetry is regained on average, and the dimerization apparently disappears. In contrast, local structural measurements using high-energy x-rays, show that disordered dimers survive at the nanoscale up to at least 650C. The high temperature phase of Li2RuO3 is thus an example of a valence bond liquid, where thermal fluctuations drive resonance between different dimer coverages, a classic analogue of the resonating valence bond state often discussed in connection with high T$_c$ cuprates.Comment: 5 pages, 4 figures, References correcte

Andreev spectra and subgap bound states in multiband superconductors

The theory of Andreev conductance is formulated for junctions involving normal metals (N) and multiband superconductors (S) and applied to the case of superconductors with nodeless extended $s_{\pm}$-wave order parameter symmetry, as possibly realized in the recently discovered ferro pnictides. We find qualitative differences from tunneling into s-wave or d-wave superconductors that may help to identify such a state. First, interband interference leads to a suppression of Andreev reflection in the case of a highly transparent N/S interface and to a current deficit in the tunneling regime. Second, surface bound states may appear, both at zero and at non-zero energies. These effects do not occur in multiband superconductors without interband sign reversal, though the interference can still strongly modify the conductance spectra.Comment: 4 pages, 4 figure

The monoclinic crystal structure of α\alpha-RuCl3_3 and the zigzag antiferromagnetic ground state

The layered honeycomb magnet alpha-RuCl3 has been proposed as a candidate to realize a Kitaev spin model with strongly frustrated, bond-dependent, anisotropic interactions between spin-orbit entangled jeff=1/2 Ru4+ magnetic moments. Here we report a detailed study of the three-dimensional crystal structure using x-ray diffraction on untwinned crystals combined with structural relaxation calculations. We consider several models for the stacking of honeycomb layers and find evidence for a 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 jeff=1/2 picture once spin orbit coupling and electron correlations are included. We propose that differences in the magnitude of anisotropic exchange along symmetry inequivalent bonds in the monoclinic cell could provide a natural mechanism to explain the spin gap observed in powder inelastic neutron scattering, 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 neutron powder diffraction show a single magnetic transition at TN ~ 13K. The analysis of the neutron 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 60T show a single transition around 8T for in-plane fields followed by a gradual, asymptotic approach to magnetization saturation, as characteristic of strongly anisotropic exchange interactions.Comment: 13 pages, 9 figures, published in Physical Review