Iron based superconductors: magnetism, superconductivity and electronic structure

Angle resolved photoemission spectroscopy (ARPES) reveals the features of the electronic structure of quasi-two-dimensional crystals, which are crucial for the formation of spin and charge ordering and determine the mechanisms of electron-electron interaction, including the superconducting pairing. The newly discovered iron based superconductors (FeSC) promise interesting physics that stems, on one hand, from a coexistence of superconductivity and magnetism and, on the other hand, from complex multi-band electronic structure. In this review I want to give a simple introduction to the FeSC physics, and to advocate an opinion that all the complexity of FeSC properties is encapsulated in their electronic structure. For many compounds, this structure was determined in numerous ARPES experiments and agrees reasonably well with the results of band structure calculations. Nevertheless, the existing small differences may help to understand the mechanisms of the magnetic ordering and superconducting pairing in FeSC.Comment: Invited Revie

A specialized isotope mass spectrometer for noninvasive diagnostics of Helicobacter pylori infection in human beings

A specialized isotope mass spectrometer for noninvasive diagnostics of Helicobacter pylori infection in human beings based on the carbon-13 isotope breath test has been designed and constructed. Important stages of the work included (i) calculating a low-aberration mass analyzer, (ii) manufacturing and testing special gas inlet system, and (iii) creating a small-size collector of ions. The proposed instrument ensures 13C/12C isotopic ratio measurement to within 1.7‰ (pro mille) accuracy, which corresponds to requirements for a diagnostic tool. Preliminary medical testing showed that the mass spectrometer is applicable to practical diagnostics. The instrument is also capable of measuring isotopic ratios of other light elements, including N, O, B (for BF2+ ions), Ar, Cl, and

PCT, spin and statistics, and analytic wave front set

A new, more general derivation of the spin-statistics and PCT theorems is presented. It uses the notion of the analytic wave front set of (ultra)distributions and, in contrast to the usual approach, covers nonlocal quantum fields. The fields are defined as generalized functions with test functions of compact support in momentum space. The vacuum expectation values are thereby admitted to be arbitrarily singular in their space-time dependence. The local commutativity condition is replaced by an asymptotic commutativity condition, which develops generalizations of the microcausality axiom previously proposed.Comment: LaTeX, 23 pages, no figures. This version is identical to the original published paper, but with corrected typos and slight improvements in the exposition. The proof of Theorem 5 stated in the paper has been published in J. Math. Phys. 45 (2004) 1944-195

Projective Hilbert space structures at exceptional points

A non-Hermitian complex symmetric 2x2 matrix toy model is used to study projective Hilbert space structures in the vicinity of exceptional points (EPs). The bi-orthogonal eigenvectors of a diagonalizable matrix are Puiseux-expanded in terms of the root vectors at the EP. It is shown that the apparent contradiction between the two incompatible normalization conditions with finite and singular behavior in the EP-limit can be resolved by projectively extending the original Hilbert space. The complementary normalization conditions correspond then to two different affine charts of this enlarged projective Hilbert space. Geometric phase and phase jump behavior are analyzed and the usefulness of the phase rigidity as measure for the distance to EP configurations is demonstrated. Finally, EP-related aspects of PT-symmetrically extended Quantum Mechanics are discussed and a conjecture concerning the quantum brachistochrone problem is formulated.Comment: 20 pages; discussion extended, refs added; bug correcte

Long quasi-periodic oscillations of the faculae and pores

Aims. The main goal of this work is to analyze the structural and temporal evolution of small-scale magnetic structures (SSMSs) observed in the solar atmosphere, such as solitary faculae and pores, and reveal long quasi-periodic oscillations of these structures.Methods. The statistical method of regression analysis and the wavelet transform were used to obtain the periods of oscillations and dependences between the parameters of magnetic structures and periods of oscillations.Results. Long-period oscillations with periods in the interval of 18-260 min are found for the structurally stable phase of SSMSs at the level of the solar photosphere. These long-period oscillations were interpreted as natural oscillations of the structurally stable long-lived magnetic structures around their equilibrium position. These oscillations, which are of similar nature, are observed in the chromospheric bright formations associated with photospheric SSMSs. Dependences between the magnetic field and the continuum intensity of the facula elements were found. It is shown that the continuum intensity of a SSMS decreases when its magnetic field increases

Magnetohydrodynamic equilibria of a cylindrical plasma with poloidal mass flow and arbitrary cross section shape

The equilibrium of a cylindrical plasma with purely poloidal mass flow and cross section of arbitrary shape is investigated within the framework of the ideal MHD theory. For the system under consideration it is shown that only incompressible flows are possible and, conscequently, the general two dimensional flow equilibrium equations reduce to a single second-order quasilinear partial differential equation for the poloidal magnetic flux function $\psi$, in which four profile functionals of $\psi$ appear. Apart from a singularity occuring when the modulus of Mach number associated with the Alfv\'en velocity for the poloidal magnetic field is unity, this equation is always elliptic and permits the construction of several classes of analytic solutions. Specific exact equlibria for a plasma confined within a perfectly conducting circular cylindrical boundary and having i) a flat current density and ii) a peaked current density are obtained and studied.Comment: Accepted to Plasma Physics & Controlled Fusion, 14 pages, revte

Elastic scattering of hadrons

Colliding high energy hadrons either produce new particles or scatter elastically with their quantum numbers conserved and no other particles produced. We consider the latter case here. Although inelastic processes dominate at high energies, elastic scattering contributes considerably (18-25%) to the total cross section. Its share first decreases and then increases at higher energies. Small-angle scattering prevails at all energies. Some characteristic features are seen that provide informationon the geometrical structure of the colliding particles and the relevant dynamical mechanisms. The steep Gaussian peak at small angles is followed by the exponential (Orear) regime with some shoulders and dips, and then by a power-law drop. Results from various theoretical approaches are compared with experimental data. Phenomenological models claiming to describe this process are reviewed. The unitarity condition predicts an exponential fall for the differential cross section with an additional substructure to occur exactly between the low momentum transfer diffraction cone and a power-law, hard parton scattering regime under high momentum transfer. Data on the interference of the Coulomb and nuclear parts of amplitudes at extremely small angles provide the value of the real part of the forward scattering nuclear amplitude. The real part of the elastic scattering amplitude and the contribution of inelastic processes to the imaginary part of this amplitude (the so-called overlap function) at nonforward transferred momenta are also discussed. Problems related to the scaling behavior of the differential cross section are considered. The power-law regime at highest momentum transfer is briefly described.Comment: 72 pages, 11 Figures; modified Physics-Uspekhi 56 (2013)

Electronic Configuration of Yb Compounds

The total energy differences between divalent and trivalent configurations of Yb ions in a number of Yb compounds are studied. Two different band theoretical methods, which differ in the treatment of the localized f electrons, are used. The results show that in all Yb compounds the valence energy differences are equal to the energy needed to localize an f electron. These valence energy differences correlate with the number of f electrons hybridizing with the conduction bands in the trivalent configuration. For divalent YbS, the pressure induced f-electron delocalization implies an intermediate valency, as also indicated by experiment

Исследования термовольтаического эффекта в полупроводниках в среднетемпературном интервале

Introduction. A conversion of thermal energy into electrical energy is an urgent task. The thermovoltaic effect (TVE) found in samarium sulfide (SmS ) makes it possible to solve a higher efficiency problem as compared with classical thermoelectric generators operating on the basis of the Seebeck effect. TVE in SmS, is manifested in an appearance of voltage pulses of 0.05 V in continuous mode when a sample is heated to 470 K. Generation of electromotive force (EMF) is associated with a presence of a concentration gradient of defective Sm ions in sulfur sublattice vacancies, with a change in their valence (Sm2+ ® Sm3++e-) and with electronic Mott transitions. TVE discovered subsequently in ZnO, Ge, Si, and in some complex semiconductors, reached only 0.01 V.Aim. To increase the magnitude of the generated voltage and the operating temperature at TVE.Methods and materials. By the method of synthesis from simple substances, raw materials were obtained from which SmS/Sm1-xLnxS sandwich-type heterostructures were created, where Ln = Eu, Yb. In addition, samples based on a classical thermoelectric PbTe with different degrees of doping of the layers obtained by pressing at high temperature in a vacuum were investigated. On the unique equipment, excluding temperature gradients in samples, TVE was studied.Results. It was shown that an increase in the EMF value and in the operating temperature had been achieved due to the penetration of donor levels by doping. In doped samples on the basis of SmS, a generation of EMF up to 0.15 V was observed in continuous mode at T = 700 K. TVE detected in the n-type thermoelectric semiconductor heterostructure PbTe, make it possible to obtain the magnitude of the generated voltage of near 0.06 V in the medium temperature mode.Conclusion. The achieved results exceed the previously known ones and give grounds to continue research with the aim of developing semiconductor converters operating on the basis of TVE.Введение. Преобразование тепловой энергии в электрическую является актуальной задачей. Обнаруженный в сульфиде самария (SmS) термовольтаический эффект (ТВЭ) позволяет решать ее с большим КПД по сравнению с классическими термоэлектрическими генераторами, работающими на основе классического эффекта Зеебека. ТВЭ в SmS проявляется в спонтанном появлении импульсов электрического напряжения 0.05 В в непрерывном режиме при температуре образца 470 К. Генерация ЭДС связывается с наличием градиента концентрации дефектных ионов Sm, находящихся в вакансиях подрешетки серы, и с переменной валентностью ионов самария (Sm2+® Sm3++e-). При нагреве образца до определенной температуры концентрация носителей заряда скачкообразно увеличивается. Поскольку в образце дефектные ионы самария расположены неупорядоченно, происходит перенос заряда из области образца с большей концентрацией дефектных ионов в зону с меньшей концентрацией. Таким образом, создается ЭДС в направлении градиента концентрации этих ионов. Обнаруженный впоследствии в ZnO, Ge, Si и некоторых сложных полупроводниках ТВЭ достигал лишь 0.01 В.Цель работы. Решается проблема повышения генерируемого напряжения и рабочей температуры генерации при ТВЭ.Методы и материалы. Методом синтеза из простых веществ получены исходные материалы, из которых созданы гетероструктуры типа сэндвич SmS/Sm1-хLnхS, где Ln = Eu, Yb. Кроме того, исследованы образцы на основе классического термоэлектрика PbTe с различной степенью легирования слоев, полученных спрессовыванием при высокой температуре в вакууме. На уникальной установке, исключающей градиенты температуры в образцах, проведены исследования ТВЭ.Результаты. Показано, что повышение значений ЭДС и рабочей температуры достигается за счет заглубления донорных уровней путем легирования. В легированных образцах на основе SmS наблюдается генерация ЭДС до 0.15 В в непрерывном режиме при T = 700 K. Обнаруженный в гетероструктуре на основе термоэлектрического полупроводника n-типа PbTe ТВЭ позволяет получить значение генерируемого напряжения около 0.06 В в среднетемпературном режиме.Заключение. Достигнутые результаты превосходят известные ранее и дают основание продолжать исследования с целью разработки полупроводниковых преобразователей, работающих на основе ТВЭ