Low-temperature Ferromagnetism in a Weakly Doped Hubbard Magnet

In the framework of the diagrammatic method with self-consistent field, the maximum on the temperature dependence of the susceptibility of a weakly doped narrow-band Hubbard magnet below the Curie temperature TC is predicted. By numerical calculations, it is proved that it appears at small hole concentrations nh. In this case, the temperature dependence of the magnetization M(T) has a typical Fermi-like shape with the point of inflection at a temperature Tinf . The approximate solution of the system of equations for the mean spin and the chemical potential gives the Schottky susceptibility typical of a two-level system with a gap of order of nh(1 - nh)W, where W is the bandwidth. This behavior reflects the existence of two subbands with up- and down-spins. It may be observed experimentally in the surface layers of oxide metal nanoparticles with narrow bands and a weak oxygen nonstoichiometry.У межах теорiї ефективного самоузгодженого поля для слабко легованого хаббардовського магнетика передбачено появу низькотемпературного максимуму типу Шотткi на кривiй сприйнятливостi. Показано, що дану аномалiю зумовлено конкуренцiєю кiнематичних i теплових взаємодiй колективiзованих електронiв. Оцiнка величини молярної сприйнятливостi дозволяє зробити висновок про можливу реалiзацiю зазначеного явища в поверхневих шарах металооксидних наночастинок iз вузькими зонами

Electron dynamics in the normal state of cuprates: Spectral function, Fermi surface and ARPES data

An influence of the electron-phonon interaction on excitation spectrum and damping in a narrow band electron subsystem of cuprates has been investigated. Within the framework of the t-J model an approach to solving a problem of account of both strong electron correlations and local electron–phonon binding with characteristic Einstein mode ω ₀ in the normal state has been presented. In approximation Hubbard-I it was found an exact solution for the polaron bands. We established that in the low-dimensional system with a pure kinematic part of Hamiltonian a complicated excitation spectrum is realized. It is determined mainly by peculiarities of the lattice Green's function. In the definite area of the electron concentration and hopping integrals a correlation gap may be possible on the Fermi level. Also, in specific cases it is observed a doping evolution of the Fermi surface. We found that the strong electron–phonon binding enforces a degree of coherence of electron-polaron excitations near the Fermi level and spectrum along the nodal direction depends on wave vector module weakly. It corresponds to ARPES data. A possible origin of the experimentally observed kink in the nodal direction of cuprates is explained by fine structure of the polaron band to be formed near the mode −ω₀

Comparison of pressure, magnetic field and excess manganese effects on transport properties of film and bulk ceramic La–Ca manganites

The pressure, magnetic field and excess manganese effects on transport and magnetoresistance effect (MRE) have been studied in both the epitaxial films and bulk ceramics of manganites (La₀.₇Ca₀.₃)₁₋xMn₁₊xO₃₋y (x = 0–0.2). A comparison of electrical behavior in both kinds of samples of similar composition at hydrostatic pressures of up to 1.8 GPa and in a magnetic fields of up to 8 kOe has been performed. The pressure and magnetic field effects are shown to increase with increasing manganese content. Experimental data show that the pressure and magnetic field effects on temperatures of both metal–insulator transition (TMD) and MRE peak (TMR) are considerably stronger in the films than in ceramics. The hydrostatic pressure increases TMD and TMR. Magnetoresistance effect for both types of samples was shown to be favored by the pressure and magnetic field in an opposite way. A direct correlation is established between TMD and conductivity bandwidth as well as between MRE and concentration of charge carriers at applied pressure. The differences in the values of pressure effect on resistance, MRE and TMD temperature in the films and ceramics are connected with both granular structure of ceramics and the oxygen nonstoichiometry in ceramic and film samples of the same content as well as with the film strain induced by lattice mismatch between the film and the substrate. The origin of pressure–magnetic field effects is analyzed in the framework of double exchange interaction and small polaron hopping, and variable range hopping models

Quantum phase transitions and thermodynamic properties in highly anisotropic magnets

The systems exhibiting quantum phase transitions (QPT) are investigated within the Ising model in the transverse field and Heisenberg model with easy-plane single-site anisotropy. Near QPT a correspondence between parameters of these models and of quantum phi^4 model is established. A scaling analysis is performed for the ground-state properties. The influence of the external longitudinal magnetic field on the ground-state properties is investigated, and the corresponding magnetic susceptibility is calculated. Finite-temperature properties are considered with the use of the scaling analysis for the effective classical model proposed by Sachdev. Analytical results for the ordering temperature and temperature dependences of the magnetization and energy gap are obtained in the case of a small ground-state moment. The forms of dependences of observable quantities on the bare splitting (or magnetic field) and renormalized splitting turn out to be different. A comparison with numerical calculations and experimental data on systems demonstrating magnetic and structural transitions (e.g., into singlet state) is performed.Comment: 46 pages, RevTeX, 6 figure

The magnetic properties of potassium holmium double tungstate

The magnetic investigations of potassium holmium double tungstate KHo(WO⁴)² have been performed. The results of measurements of magnetic susceptibility and magnetization as a function of temperature (T from 0.3 K up to 100 K) and magnetic field (up to 1.5 T) are presented. A strong anisotropy of magnetic properties was found. The magnetic measurements data were used to calculate the interaction energy. It was shown that the interactions between nearest neighbors Ho³⁺ ions have antiferromagnetic character

Isolation and determination of the activity of IgA1 protease from Neisseria meningitidis

A method of the isolation and purification of IgA1 protease from a culture of Neisseria meningitidis serogroup A has been developed. Three inactivated intermediates of the production of the meningococcal vaccine, a culture liquid, as well as a supernatant and precipitate obtained by the precipitation of bacterial cells by cetavlon, served as a starting material. The purity of IgA1 protease was determined by SDS-PAGE. An immunoenzyme assay for determining the IgA1 protease activity has been developed. The yield of the enzyme with a specific activity of 0.5 to 4 million units/mg from 103 g of the cetavlon precipitate (40 l of culture liquid) was about 600 μg. It was shown that IgA1 protease isolated from serogroup A meningococcus is capable of protecting experimental animals (mice) infected with meningococcus of serogroup B. © 2010 Pleiades Publishing, Ltd

Isolation and determination of the activity of IgA1 protease from Neisseria meningitidis

A method of the isolation and purification of IgA1 protease from a culture of Neisseria meningitidis serogroup A has been developed. Three inactivated intermediates of the production of the meningococcal vaccine, a culture liquid, as well as a supernatant and precipitate obtained by the precipitation of bacterial cells by cetavlon, served as a starting material. The purity of IgA1 protease was determined by SDS-PAGE. An immunoenzyme assay for determining the IgA1 protease activity has been developed. The yield of the enzyme with a specific activity of 0.5 to 4 million units/mg from 103 g of the cetavlon precipitate (40 l of culture liquid) was about 600 μg. It was shown that IgA1 protease isolated from serogroup A meningococcus is capable of protecting experimental animals (mice) infected with meningococcus of serogroup B. © 2010 Pleiades Publishing, Ltd