Search

Doping dependence of the superconducting state structure and spin-fluctuation pairing mechanism in the $\mathrm{Ba}{(\mathrm{F}{\mathrm{e}}_{1\ensuremath{-}x}\mathrm{C}{\mathrm{o}}_{x})}_{2}\mathrm{A}{\mathrm{s}}_{2}$ family is studied. BCS-like analysis of experimental data shows that in the overdoped regime, away from the antiferromagnetic (AFM) transition, the spin-fluctuation interaction between the electron and hole gaps is weak, and $\mathrm{Ba}{(\mathrm{F}{\mathrm{e}}_{1\ensuremath{-}x}\mathrm{C}{\mathrm{o}}_{x})}_{2}\mathrm{A}{\mathrm{s}}_{2}$ is characterized by three essentially different gaps. In the three-gap state an anisotropic (nodeless) electron gap ${\mathrm{\ensuremath{\Delta}}}_{e}(x,\ensuremath{\varphi})$ has an intermediate value between the dominant inner ${\mathrm{\ensuremath{\Delta}}}_{2h}(x)$ and outer ${\mathrm{\ensuremath{\Delta}}}_{1h}(x)$ hole gaps. Close to the AFM transition the electron gap ${\mathrm{\ensuremath{\Delta}}}_{e}(x,\ensuremath{\varphi})$ increases sharply and becomes closer in magnitude to the dominant inner hole gap ${\mathrm{\ensuremath{\Delta}}}_{2h}(x)$. The same two-gap state with close electron and inner hole gaps ${\mathrm{\ensuremath{\Delta}}}_{2h}(x)\ensuremath{\approx}{\mathrm{\ensuremath{\Delta}}}_{e}(x,\ensuremath{\varphi})$ is also preserved in the phase of coexisting antiferromagnetism and superconductivity. The doping dependence of the electron gap ${\mathrm{\ensuremath{\Delta}}}_{e}(x,\ensuremath{\varphi})$ is associated with the strong doping dependence of the spin-fluctuation interaction in the AFM transition region. In contrast to the electron gap ${\mathrm{\ensuremath{\Delta}}}_{e}(x,\ensuremath{\varphi})$, the doping dependence of the hole gaps ${\mathrm{\ensuremath{\Delta}}}_{1,2h}(x)$ and the critical temperature ${T}_{c}(x)$, both before and after the AFM transition, are associated with a change of the density of states ${\ensuremath{\gamma}}_{nh}(x)$ and the intraband electron-phonon interaction in the hole bands. The nonphonon spin-fluctuation interaction in the hole bands in the entire Co concentration range is small compared with the intraband electron-phonon interaction and is not dominant in the $\mathrm{Ba}{(\mathrm{F}{\mathrm{e}}_{1\ensuremath{-}x}\mathrm{C}{\mathrm{o}}_{x})}_{2}\mathrm{A}{\mathrm{s}}_{2}$ family.

The terahertz and infrared spectra of the complex dynamic conductivity, as well as the temperature dependences of the density of a superconducting condensate and the electronic specific heat of superconducting Ba(Fe1 − x Co x )2As2 compounds, have been analyzed within a Bardeen-Cooper-Schrieffer-like model of a multiband superconductor with strong coupling. It has been shown that the superconducting state of these compounds is determined by three (one electronic and two hole) weakly interacting condensates. The order parameters of the condensates are: Δ1 ≈ 15 cm−1, Δ2 ≈ 21 cm−1, and Δ3 ≈ 30–35 cm−1. The results significantly refine the existing notions on the structure of the superconducting state of Co-doped BaFe2As2 multiband compounds.

Gap structure and spin-fluctuation pairing mechanism in the iron-based superconductors Ba(Fe1-xCox)2As2 is studied. We demonstrate that the overdoped compounds Ba(Fe1-xCox)2As2 have a three-gap structure with substantially different order parameters and a weak interband spin-fluctuation coupling. In the three-gap state of Ba(Fe1-xCox)2As2, the anisotropic electronic gap has an intermediate value between two isotropic hole gaps. During transition to the optimal regime, the electron gap strongly increases (by a factor of two) and gets close to the largest hole gap value, while the electron gap anisotropy sharply decreases pointing to a strong increase in the interband interaction, with spin fluctuations accompanying the antiferromagnetic transition. The same two-gap state with close values of the electron and largest hole gaps is preserved even with further decrease in doping. The performed study shows that the spin-fluctuation pairing mechanism has a significant effect on doping evolution of the electron gap only. The doping evolution of the dominant hole gap in Ba(Fe1-x Co x )2As2 is not directly related to the evolution of the spin fluctuations.

A generalized α model for computing the superconducting parameters of real two-band superconductors is proposed based on an analysis of the properties of two-band equations in the theory of superconductivity. Using this model, we calculate the heat capacity and optical properties of Ba(Fe1 − xCox)2As2 superconducting compound and obtain the temperature dependences of the gaps and energies of the Leggett modes in the Mg1 − xAlxB2 superconducting system. Good quantitative coincidence of the calculated data and experimental results is demonstrated.

Terahertz and infrared spectra of the conductivity, σ(ν), and dielectric constant, ɛ(ν), of a Ba(Fe0.9Co0.1)2As2 film (T c = 20 K) have been analyzed together with previous specific-heat and angular resolved photoelectron spectroscopy data. It has been shown that the spectra σ(ν) and ɛ(ν) of Ba(Fe0.9Co0.1)2As2 in the superconducting phase at T = 5 K, as well as the magnetic field penetration depth, can be described well using the standard Bardeen-Cooper-Schrieffer (BCS) model with an additive contribution of electron and hole bands. It has been found that the measured temperature dependence of the magnetic field penetration depth in a wide temperature range 5 K < T < T c can be described only with the introduction of interband pairing interaction. The coupling constant of electron and hole bands, λ1, 2 = 0.1, as well as the temperature dependences of superconducting gaps in the electron and hole subsystems, has been determined using the model of two-band superconductivity developed earlier for MgB2.

A detailed theoretical analysis of the experimental data obtained earlier in the studies of the tunneling spectra in the MgB2 two-band superconducting system has been performed. It is shown that most these data are well described in the generalized two-band Bardeen-Cooper-Schrieffer theory with the constants of the intraband electron-phonon interaction that reasonably coincide with the ab initio calculation results. It is shown that the existence of specific collective excitation in this system induced by oscillations of the relative phase of two superconducting condensates (the Leggett mode) indicates the overestimation of the constants of the interband electron-phonon interaction in the ab initio calculations. The dependences of the superconducting gaps and the Leggett mode frequency on the temperature and the disorder degree in the Mg1 − x Al x B2 system have been thoroughly studied.

The properties of a superconductor featuring strong electron-phonon and electron-impurity interactions have been studied in a single-band approximation. It is shown that the elastic scattering of electrons on static impurities decreases the electron-phonon interaction constant in proportion to the ratio of the electron-impurity relaxation rate and the seeding band width. The optical spectral weight (integral of the real part of the optical conductivity) in various energy intervals of a metal in the normal and superconducting states has been calculated as a function of the temperature and the superconducting gap. In the region of large (compared to the phonon) energies, the dependence of the spectral weight on the superconducting gap is weak, while the dependence on the temperature is completely determined by the corresponding dependence of the relaxation rate due to the elastic electron-phonon scattering far from the Fermi surface. It is shown that a difference in behavior of the spectral weight between the normal and superconducting states at lower energies is determined by so-called Holstein’s shift of the feature in the optical conductivity spectrum (rather than by the gap width, as it is commonly believed) and sharply decreases upon the introduction of impurities.

We report ab initio linear-response calculations of the phonon spectra and the electron-phonon interaction for several transition metal carbides and nitrides in a NaCl-type structure. For NbC, the kinetic, optical, and superconducting properties are calculated in detail at various pressures and the normal-pressure results are found to agree well with the experiment. Factors accounting for the relatively low critical temperatures T c in transition metal compounds with light elements are considered and the possible ways of increasing T c are discussed.

The correlation between the density ρs(T→0) of superconducting condensate and the superconducting transition temperature Tc in underdoped HTSC systems is considered. It is shown that the linear relation between ρs(0) and Tc observed in some experiments can easily be interpreted in the framework of the conventional Bardeen-Cooper-Schrieffer (BCS) model without invoking any exotic superconductivity models.

The optical spectrum of the normal state of a doped two-dimensional antiferromagnet is analyzed in a Kondo lattice model with regard to the complex structure of a spin polaron. The optical properties are determined by sharply anisotropic scattering of spin-polaron excitations by antiferromagnetic fluctuations of a system of localized spins. It is shown that the relaxation of carriers in the infrared range is mainly attributed to the strong coupling between these carriers and the mode of low-frequency spin excitations with the quasimomentum close to the antiferromagnetic vector Q=(π,π). The latter coupling is associated with the fact that the regions of the Fermi surface of the lower polaron band are close to the boundary of the antiferromagnetic Brillouin zone. The calculated optical characteristics are in qualitative agreement with experimental data for the normal state of high-temperature superconductors (HTSCs).

In the framework of the two dimensional Kondo lattice model the optical conductivity spectrum σ( ω) is investigated. The calculations are based on the realistic energy spectrum of spin–polaron excitations to fit photoemission experiments in cuprates. The memory function approach is used for deriving σ( ω). Obtained optical transport damping, plasma frequency and conductivity demonstrate qualitative agreement with experimental data. It is pointed out the crucial importance of nested Fermi surface scattering by vector Q = (π, π) due to strong carrier coupling to a resonance structure of spin excitations with momenta close to Q.  2001 Elsevier Science B.V. All rights reserved.

The nonadiabatic corrections to the self-energy part Σs(q, ω) of the phonon Green’s function are studied for various values of the phonon vectors q resulting from electron-phonon interactions. It is shown that the long-range electron-electron Coulomb interaction has no direct influence on these effects, aside from a possible renormalization of the corresponding constants. The electronic response functions and Σs(q, ω) are calculated for arbitrary vectors qand energy ω in the BCS approximation. The results obtained for q=0 agree with previously obtained results. It is shown that for large wave numbers q, vertex corrections are negligible and Σs(q, ω) possesses a logarithmic singularity at ω=2Δ, where Δ is the superconducting gap. It is also shown that in systems with nesting, Σs(Q, ω) (where Q is the nesting vector) possesses a square-root singularity at ω=2Δ, i.e., exactly of the same type as at q=0. The results are used to explain the recently published experimental data on phonon anomalies, observed in nickel borocarbides in the superconducting state, at large q. It is shown, specifically, that in these systems nesting must be taken into account in order to account for the emergence of a narrow additional line in the phonon spectral function S(q, ω)≈−π −1 Im D s (q, ω), where D s (q, ω) is the phonon Green’s function, at temperatures T

The optical conductivity and permittivity spectra of Ba(Fe${}_{0.9}$Co${}_{0.1}$)${}_{2}$As${}_{2}$ films (${T}_{c}=20 \text{K}$) are analyzed together with the literature data on photoemission and specific heat. In the superconducting state at all temperatures up to ${T}_{c}$ the temperature dependence of the magnetic field penetration depth is well described within the generalized two-band BCS model with intraband and interband pairing interactions considered. It is shown that the small superconducting energy gap $2{\ensuremath{\Delta}}^{(1)}=3.7\text{meV}$ develops in the electron subsystem while the larger gap $2{\ensuremath{\Delta}}^{(2)}=7.9\text{meV}$ opens in the hole subsystem. The normal state parameters, such as plasma frequency and scattering rate, of electron and hole conduction bands are determined.

Postoperative purulent mediastinitis was observed by the authors in 43 out of 1110 patients. Specific features of the clinical picture and course of this complication are described. The "closed" method of treatment was used in 20 patients. Five of them died from intoxication and arrosive bleeding. The "open" method of treatment of mediastinitis was used in 23 patients with no lethal outcomes.

Experimental and theoretical research on nonadiabatic effects in metals is briefly reviewed. It is shown that these effects are most manifest in optical phonon spectra at small wave vectors q satisfying the condition qvF < ω0(q), where vF is the electron Fermi velocity, and ω0(q) is the phonon frequency. Various theoretical approaches to calculating nonadiabatic effects are examined. Frohlich's phenomenological field-theoretical model, the most widely used in such calculations, is discussed in detail, some aspects of its application are considered, and its efficiency as a tool for studying nonadiabatic effects is demonstrated.

A new method of plastic surgery of the right ventricular outflow tract and the pulmonary artery without use of extracorporeal circulation (occlusion of main arteries) was tested in experiments carried out in 28 dogs. The method involves previous suture of a patch to the site selected for plastic surgery and, using an instrument designed by the authors, excision of the necessary part of the ventricular wall and pulmonary artery. The instrument consists of two handles, similar to those of pincers, and working on a pivot, with oval cavities on the inner sides of its elongated grasping jaws. The tip of lower jaw features a sloping edge to enable perforation of the outer wall of the heart. By pulling together the handles, connected with a spring, the jaws close excising the appropriate part of the heart. The depth of insertion of the jaws into the heart cavity's controlled by means of a graded scale on the outer side of the upper jaw. Using this technique it is possible to perform surgery even of extensive myocardial defects quickly, safely, with virtually no trauma and minimal loss of blood, and to remove large segments of the hypertrophic myocardium while retaining intracardiac circulation, without the need for extracorporeal circulation.