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Rayleigh scattering is a result of an interaction of photons with bound electrons. Rayleigh scattering is mostly neglected in calculations of hot star model atmospheres because most of the hydrogen atoms are ionized and the heavier elements have a lower abundance than hydrogen. In atmospheres of some chemically peculiar stars, helium overabundant regions containing singly ionized helium are present and Rayleigh scattering can be a significant opacity source. We evaluate the contribution of Rayleigh scattering by neutral hydrogen and singly ionized helium in the atmospheres of hot stars with solar composition and in the atmospheres of helium overabundant stars. We computed several series of model atmospheres using the TLUSTY code and emergent fluxes using the SYNSPEC code. These models describe atmospheres of main sequence B-type stars with different helium abundance. We used an existing grid of models for atmospheres with solar chemical composition and we calculated an additional grid for helium-rich stars with $N$(He)/$N$(H)=10. Rayleigh scattering by neutral hydrogen can be neglected in atmospheres of hot stars, while Rayleigh scattering by singly ionized helium can be a non-negligible opacity source in some hot stars, especially in helium-rich stars., Comment: Astronomy & Astrophysics, the second version, cross section (9) and (15) were corrected

We investigate the electronic dispersion of the high-T$_{\mathrm{c}}$ cuprate superconductors using the fully self-consistent version of the phenomenological model, where charge planar quasiparticles are coupled to spin fluctuations. The inputs we use ---the underlying (bare) band structure and the spin susceptibility $\chi$--- are extracted from fits of angle resolved photoemission and inelastic neutron scattering data of underdoped YBa$_{2}$Cu$_{3}$O$_{6.6}$ by T. Dahm and coworkers (T. Dahm et al., Nat. Phys. 5, 217 (2009)). Our main results are: (i) We have confirmed the finding by T. Dahm and coworkers that the main nodal kink is, for the present values of the input parameters, determined by the upper branch of the hour-glass of $\chi$. We demonstrate that the properties of the kink depend qualitatively on the strength of the charge-spin coupling. (ii) The effect of the resonance mode of $\chi$ on the electronic dispersion strongly depends on its kurtosis in the quasimomentum space. A low (high) kurtosis implies a negligible (considerable) effect of the mode on the dispersion in the near-nodal region. (iii) The energy of the kink decreases as a function of the angle $\theta$ between the Fermi surface cut and the nodal direction, in qualitative agreement with recent experimental observations. We clarify the trend and make a specific prediction concerning the angular dependence of the kink energy in underdoped YBa$_{2}$Cu$_{3}$O$_{6.6}$., Comment: 12 pages, 13 figures

With infrared spectroscopy we investigated the spin-reorientation transition from an orthorhombic antiferromagnetic (o-AF) to a tetragonal AF (t-AF) phase and the reentrance of the o-AF phase in the superconducting state of underdoped Ba$ _{1-x} $K$ _{x} $Fe$ _{2} $As$ _{2} $. In agreement with the predicted transition from a single-$\mathbf{Q}$ to a double-$\mathbf{Q}$ AF structure, we found that a distinct spin density wave (SDW) develops in the t-AF phase. The pair breaking peak of this SDW acquires much more low-energy spectral weight than the one in the o-AF state which indicates that it competes more strongly with superconductivity. We also observed additional phonon modes in the t-AF phase which likely arise from a Brillouin-zone folding that is induced by the double-$\mathbf{Q}$ magnetic structure with two Fe sublattices exhibiting different magnitudes of the magnetic moment., Comment: Supplementary information included

We report on results of our analysis of published experimental data of the in-plane infrared response of two representative underdoped high-$T_{\mathrm{c}}$ cuprate superconductors, focusing on a characteristic gap feature in the spectra of the real part of the conductivity and the corresponding structures of the memory function, that develop below a temperature $T^{\mathrm{ons}}$ considerably higher than $T_{\mathrm{c}}$. Several arguments based on comparisons of the data with results of our calculations are provided indicating that the features are due to superconductivity and that $T^{\mathrm{ons}}$ marks the onset of a precursor superconducting phase., Comment: 5 pages, 3 figures; supporting material: 14 pages, 6 figures

The formula frequently used to describe the c-axis infrared response of the coupled electron-phonon system of bilayer cuprate superconductors and providing important insights into the physics of these materials has been originally obtained at the level of the phenomenological multilayer model. Here we derive it using diagrammatic perturbation theory.

The in-plane infrared response of the high-Tc cuprate superconductors was studied using the spin-fermion model, where charged quasiparticles of the copper-oxygen planes are coupled to spin fluctuations. First, we analyzed structures of the superconducting-state conductivity reflecting the coupling of the quasiparticles to the resonance mode observed by neutron scattering. The conductivity computed with the input spin susceptibility in the simple form of the mode exhibits two prominent features: an onset of the real part of the conductivity starting around the frequency of the mode omega_{0} and a maximum of a related function W(omega), roughly proportional to the second derivative of the scattering rate, centered approximately at omega=omega_{0}+Delta_{0}/hbar, where Delta_{0} is the maximum value of the superconducting gap. The two structures are well known from earlier studies. Their physical meaning, however, has not been sufficiently elucidated thus far. Our analysis involving quasiparticle spectral functions provides a clear interpretation. Second, we explored the role played by the spin-fluctuation continuum. Third, we investigated the temperature dependence of the conductivity, of the intraband spectral weight, and of the effective kinetic energy. The changes of the latter two quantities below Tc are determined by the formation of the gap, by a feedback effect of the spin fluctuations on the quasiparticles, and by a significant shift of the chemical potential., Comment: 20 pages, 18 figures, submitted to Physical Review B

Changes of the 400 cm^{-1} peak in the c-axis conductivity of underdoped YBa2Cu3O6.6 upon application of a parallel magnetic field reported by Kojima et al. are shown to be consistent with the model where the peak is due to the superfluid. Results of our calculations of the c-axis response of bilayer compounds with well defined bilayer-split bands are presented and discussed. For moderate values of the bilayer splitting (Delta E comparable to 2Delta_{max}) the spectra of the superconducting state exhibit an additional mode which is due to the condensate and similar to the one of earlier phenomenological approaches., Comment: 4 pages, 2 figures, submitted to the Proceedings of SNS 2004 (J. Phys. Chem. Sol.)

We present an extension of the model proposed recently to account for dramatic changes below Tc (anomalies) of some c-axis polarized infrared-active phonons in bilayer cuprate superconductors, that applies to trilayer high-Tc compounds. We discuss several types of phonon anomalies that can occur in these systems and demonstrate that our model is capable of explaining the spectral changes occurring upon entering the superconducting state in the trilayer compound Tl2Ba2Ca2Cu3O10. The low-temperature spectra of this compound obtained by Zetterer and coworkers display an additional broad absorption band, similar to the one observed in underdoped YBa2Cu3O7-delta and Bi2Sr2CaCu2O8. In addition, three phonon modes are strongly anomalous. We attribute the absorption band to the transverse Josephson plasma resonance, similar to that of the bilayer compounds. The phonon anomalies are shown to result from a modification of the local fields induced by the formation of the resonance. The spectral changes in Tl2Ba2Ca2Cu3O10 are compared with those occurring in Bi2Sr2Ca2Cu3O10, reported recently by Boris and coworkers., Comment: submitted to Physica C, 21 pages, 11 figures

We present an extension of the c-axis tight-binding sum rule discussed by Chakravarty, Kee, and Abrahams [Phys. Rev. Lett. 82, 2366 (1999)] that applies to multilayer high-Tc cuprate superconductors (HTCS) and use it to estimate--from available infrared data--the change below Tc of the c-axis kinetic energy, Hc, in YBa2Cu3O(7-delta) (delta=0.45,0.25,0.07), Bi2Sr2CaCu2O8, and Bi2Sr2Ca2Cu3O10. In all these compounds Hc decreases below Tc and except for Bi2Sr2CaCu2O8 the change of Hc is of the same order of magnitude as the condensation energy. This observation supports the hypothesis that in multilayer HTCS superconductivity is considerably amplified by the interlayer tunnelling mechanism., Comment: 6 pages, 2 figures

We review some previous studies concerning the intra-bilayer Josephson plasmons and present new ellipsometric data of the c-axis infrared response of almost optimally doped Bi_{2}Sr_{2}CaCu_{2}O_{8}. The c-axis conductivity of this compound exhibits the same kind of anomalies as that of underdoped YBa_{2}Cu_{3}O_{7-delta}. We analyze these anomalies in detail and show that they can be explained within a model involving the intra-bilayer Josephson effect and variations of the electric field inside the unit cell. The Josephson coupling energies of different bilayer compounds obtained from the optical data are compared with the condensation energies and it is shown that there is a reasonable agreement between the values of the two quantities. We argue that the Josephson coupling energy, as determined by the frequency of the intra-bilayer Josephson plasmon, represents a reasonable estimate of the change of the effective c-axis kinetic energy upon entering the superconducting state. It is further explained that this is not the case for the estimate based on the use of the simplest ``tight-binding'' sum rule. We discuss possible interpretations of the remarkable agreement between the Josephson coupling energies and the condensation energies. The most plausible interpretation is that the interlayer tunneling of the Cooper pairs provides the dominant contribution to the condensation energy of the bilayer compounds; in other words that the condensation energy of these compounds can be accounted for by the interlayer tunneling theory. We suggest an extension of this theory, which may also explain the high values of T_{c} in the single layer compounds Tl_{2}Ba_{2}CuO_{6} and HgBa_{2}CuO_{4}, and we make several experimentally verifiable predictions., Comment: 16 pages (including Tables) and 7 figures; accepted for publication in Physical Review B