Spin-density fluctuations and the fluctuation-dissipation theorem in 3d ferromagnetic metals

Spatial and time scales of spin density fluctuations (SDF) were analyzed in 3d ferromagnets using ab initio linear response calculations of complete wavevector and energy dependence of the dynamic spin susceptibility tensor. We demonstrate that SDF are spread continuously over the entire Brillouin zone and while majority of them reside within the 3d bandwidth, a significant amount comes from much higher energies. A validity of the adiabatic approximation in spin dynamics is discussed. The SDF spectrum is shown to have two main constituents: a minor low-energy spin wave contribution and a much larger high-energy component from more localized excitations. Using the fluctuation-dissipation theorem (FDT), the on-site spin correlator (SC) and the related effective fluctuating moment were properly evaluated and their universal dependence on the 3d band population is further discussed

Nature of bonding and electronic structure in MgB2, a boron intercalation superconductor

Chemical bonding and electronic structure of MgB2, a boron-based newly discovered superconductor, is studied using self-consistent band structure techniques. Analysis of the transformation of the band structure for the hypothetical series of graphite - primitive graphite - primitive graphite-like boron - intercalated boron, shows that the band structure of MgB2 is graphite-like, with pi-bands falling deeper than in ordinary graphite. These bands possess a typically delocalized and metallic, as opposed to covalent, character. The in-plane sigma-bands retain their 2D covalent character, but exhibit a metallic hole-type conductivity. The coexistence of 2D covalent in-plane and 3D metallic-type interlayer conducting bands is a peculiar feature of MgB2. We analyze the 2D and 3D features of the band structure of MgB2 and related compounds, and their contributions to conductivity.Comment: 4 pages in revtex, 3 figures in 4 separate EPS file

Experimental and theoretical analysis of the upper critical field in FSF trilayers

The upper critical magnetic field H_{c2} in thin-film FSF trilayer spin-valve cores is studied experimentally and theoretically in geometries perpendicular and parallel to the heterostructure surface. The series of samples with variable thicknesses of the bottom and of the top Cu_{41}Ni_{59} F-layers are prepared in a single run, utilizing a wedge deposition technique. The critical field H_{c2} is measured in the temperature range $0.4-8$ K and for magnetic fields up to 9 Tesla. A transition from oscillatory to reentrant behavior of the superconducting transition temperature versus F-layers thickness, induced by an external magnetic field, has been observed for the first time. In order to properly interpret the experimental data, we develop a quasiclassical theory, enabling one to evaluate the temperature dependence of the critical field and the superconducting transition temperature for an arbitrary set of the system parameters. A fairly good agreement between our experimental data and theoretical predictions is demonstrated for all samples, using a single set of fit parameters. This confirms adequacy of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) physics in determining the unusual superconducting properties of the studied Cu_{41}Ni_{59}/Nb/Cu_{41}Ni_{59} spin-valve core trilayers.Comment: 16 pages, 7 figures; published versio

Spin currents and spin dynamics in time-dependent density-functional theory

We derive and analyse the equation of motion for the spin degrees of freedom within time-dependent spin-density-functional theory (TD-SDFT). Results are (i) a prescription for obtaining many-body corrections to the single-particle spin currents from the Kohn-Sham equation of TD-SDFT, (ii) the existence of an exchange-correlation (xc) torque within TD-SDFT, (iii) a prescription for calculating, from TD-SDFT, the torque exerted by spin currents on the spin magnetization, (iv) a novel exact constraint on approximate xc functionals, and (v) the discovery of serious deficiencies of popular approximations to TD-SDFT when applied to spin dynamics.Comment: now includes discussion of OEP and GGA; to appear in Phys. Rev. Let

Superconducting and Normal State Properties of Neutron Irradiated MgB2

We have performed a systematic study of the evolution of the superconducting and normal state properties of neutron irradiated MgB$_2$ wire segments as a function of fluence and post exposure annealing temperature and time. All fluences used suppressed the transition temperature, Tc, below 5 K and expanded the unit cell. For each annealing temperature Tc recovers with annealing time and the upper critical field, Hc2(T=0), approximately scales with Tc. By judicious choice of fluence, annealing temperature and time, the Tc of damaged MgB2 can be tuned to virtually any value between 5 and 39 K. For higher annealing temperatures and longer annealing times the recovery of Tc tends to coincide with a decrease in the normal state resistivity and a systematic recovery of the lattice parameters.Comment: Updated version, to appear in Phys. Rev.

Features of reaction of coagulation hemostasis in children with pneumonia

The article presents data from a study of 161 child with pneumonia. Two groups of patients with uncomplicated pneumonia and pneumonia with pleural pulmonary complication were formed. The study of coagulation hemostasis was realized in the studied groups. It was found that in children with uncomplicated pneumonia levels of SFMC and D-dimer were significantly higher than in normal conditions that indicates activation of coagulation hemostasis. When complicated by pneumonia hemostasis matches the current intravascular coagulation: thrombocytosis, increased levels of fibrinogen, hypo-or hypercoagulation on aPTT, changing TV thrombinemia (SFMC is 5 times higher than normal), and increased level of D-dimer, inhibition of fibrinolysis, which indicates more pronounced activation of coagulation in children with pneumonia

Magnetic susceptibility, exchange interactions and spin-wave spectra in the local spin density approximation

Starting from exact expression for the dynamical spin susceptibility in the time-dependent density functional theory a controversial issue about exchange interaction parameters and spin-wave excitation spectra of itinerant electron ferromagnets is reconsidered. It is shown that the original expressions for exchange integrals based on the magnetic force theorem (J. Phys. F14 L125 (1984)) are optimal for the calculations of the magnon spectrum whereas static response function is better described by the ``renormalized'' magnetic force theorem by P. Bruno (Phys. Rev. Lett. 90, 087205 (2003)). This conclusion is confirmed by the {\it ab initio} calculations for Fe and Ni.Comment: 12 pages, 2 figures, submitted to JPC

Phenomenological Model of Longitudinal Spin Fluctuations in Itinerant Antiferromagnets

We present the phenomenological analysis of the spectrum of longitudinal spin fluctuations in isotropic itinerant electron antiferromagnets with account of spin anharmonicity giving rise to coupling of transverse and longitudinal normal modes. The spectrum consists of a quasielastic part forming a central peak or a dip, depending on temperature and the Landau relaxation rate. Effects of spin fluctuation coupling also give rise to an inelastic part of the spectrum which has a form of resonances or antiresonances near the magnon frequencies related to non-propagating longitudinal excitations

Electron-phonon interaction in ultrasmall-radius carbon nanotubes

We perform analysis of the band structure, phonon dispersion, and electron-phonon interactions in three types of small-radius carbon nanotubes. We find that the (5,5) can be described well by the zone-folding method and the electron-phonon interaction is too small to support either a charge-density wave or superconductivity at realistic temperatures. For ultra-small (5,0) and (6,0) nanotubes we find that the large curvature makes these tubes metallic with a large density of states at the Fermi energy and leads to unusual electron-phonon interactions, with the dominant coupling coming from the out-of-plane phonon modes. By combining the frozen-phonon approximation with the RPA analysis of the giant Kohn anomaly in 1d we find parameters of the effective Fr\"{o}lich Hamiltonian for the conduction electrons. Neglecting Coulomb interactions, we find that the (5,5) CNT remains stable to instabilities of the Fermi surface down to very low temperatures while for the (5,0) and (6,0) CNTs a CDW instability will occur. When we include a realistic model of Coulomb interaction we find that the charge-density wave remains dominant in the (6,0) CNT with $T_{\rm CDW}$ around 5 K while the charge-density wave instability is suppressed to very low temperatures in the (5,0) CNT, making superconductivity dominant with transition temperature around one Kelvin.Comment: 20 pages. Updated 7/23/0