Functional Methods in Stochastic Systems

Field-theoretic construction of functional representations of solutions of stochastic differential equations and master equations is reviewed. A generic expression for the generating function of Green functions of stochastic systems is put forward. Relation of ambiguities in stochastic differential equations and in the functional representations is discussed. Ordinary differential equations for expectation values and correlation functions are inferred with the aid of a variational approach.Comment: Plenary talk presented at Mathematical Modeling and Computational Science. International Conference, MMCP 2011, Star\'a Lesn\'a, Slovakia, July 4-8, 201

Magnetic and crystal structures of the one-dimensional ferromagnetic chain pyroxene NaCrGe2O6

International audienceWe investigated the magnetic and structural properties of the ferromagnetic pyroxene NaCrGe2O6 by superconducting quantum interference device magnetometry and powder neutron diffraction. This material is the only ferromagnetic member of the recently reported multiferroic pyroxene family AMX2O6 A=Li,Na; M =Fe, Cr; X=Si,Ge . Below TC=6 K, the magnetic structure is characterized by one-dimensional magnetic chains with spins aligned along the c axis of the monoclinic cell. The magnetic moment of Cr3+ is significantly reduced by about 25%. We show that this is likely the result of the low dimensionality of the system. The associated magnetic space group is C2 /c . This symmetry does not allow a linear magnetoelectric effect. No structural phase transition was observed down to 1.8 K

Conductance of non-ballistic point contacts in hybrid systems "normal metal/superconductor" Cu/Mo-C and Cu/LaOFFeAs

We consider the shape of the curves of "Andreev" conductance of non-ballistic point contact NS heterosystems depending on the bias voltage at the contact. The obtained shape of those curves is caused by the contribution from the mechanism of coherent scattering by impurities which doubles the scattering cross section. The behavior of generalized and differential conductance is compared for ballistic and non-ballistic transport regimes. The criteria are considered allowing one to discriminate between those regimes with the corresponding conduction curves similar in appearance. The analysis is extended to the case of non-ballistic transport in NS point contacts with exotic superconductors, molybdenum carbide Mo-C and oxypnictide La[O$_{1-x}$F$_{x}$]FeAs from a group of iron-based superconductors

First study of radiation hardness of lead tungstate crystals at low temperatures

The electromagnetic calorimeter of PANDA at the FAIR facility will rely on an operation of lead tungstate (PWO) scintillation crystals at temperatures near -25 deg.C to provide sufficient resolution for photons in the energy range from 8 GeV down to 10 MeV. Radiation hardness of PWO crystals was studied at the IHEP (Protvino) irradiation facility in the temperature range from room temperature down to -25 deg.C. These studies have indicated a significantly different behaviour in the time evolution of the damaging processes well below room temperature. Different signal loss levels at the same dose rate, but at different temperatures were observed. The effect of a deep suppression of the crystal recovery process at temperatures below 0 deg.C has been seen.Comment: 10 pages 7 figure

Coexistence of Superconductivity and Charge Density Waves in Tantalum Disulfide : Experiment and Theory

The coexistence of charge density wave (CDW) and superconductivity in tantalum disulfide (2H-TaS2) at low temperature is boosted by applying hydrostatic pressures to study both vibrational and magnetic transport properties. Around Pc, we observe a superconducting dome with a maximum superconducting transition temperature Tc=9.1 K. First-principles calculations of the electronic structure predict that, under ambient conditions, the undistorted structure is characterized by a phonon instability at finite momentum close to the experimental CDW wave vector. Upon compression, this instability is found to disappear, indicating the suppression of CDW order. The calculations reveal an electronic topological transition (ETT), which occurs before the suppression of the phonon instability, suggesting that the ETT alone is not directly causing the structural change in the system. The temperature dependence of the first vortex penetration field has been experimentally obtained by two independent methods. While a d wave and single-gap BCS prediction cannot describe the lower critical field Hc1 data, the temperature dependence of the Hc1 can be well described by a single-gap anisotropic s-wave order parameter. © 2020 authors. Published by the American Physical Society

Экспериментальное исследование энергетической и экологической эффективности использования теплоты фазовых переходов поровой влаги древесины в теплонасосных сушильных камерах

The article deals with one of the most energetically and ecologically effective technologies of wood drying. Special construction polystyrene wall blocks are used as frame fillings of experimental the drying chamber according to the technology “Plastbau”, known with its ability to improved heat-insulating properties. The results of the experimental study of the thermal regime show the high energy and ecological efficiency of this wood drying technology.В статье рассматривается одна из наиболее энергетически и экологически эффективных технологий сушки древесины, базирующихся на применении тепловых насосов. Приведенные результаты экспериментальных исследований теплового режима подтверждают высокую энергетическую и экологическую эффективность рассматриваемой технологии сушки древесины

Principles of statistical mechanics of random networks

We develop a statistical mechanics approach for random networks with uncorrelated vertices. We construct equilibrium statistical ensembles of such networks and obtain their partition functions and main characteristics. We find simple dynamical construction procedures that produce equilibrium uncorrelated random graphs with an arbitrary degree distribution. In particular, we show that in equilibrium uncorrelated networks, fat-tailed degree distributions may exist only starting from some critical average number of connections of a vertex, in a phase with a condensate of edges.Comment: 14 pages, an extended versio

Analysis of nonlinear conductivity of point contacts on the base of FeSe in the normal and superconducting state

Nonlinear conductivity of point contacts (PCs) on the base of FeSe single crystals has been investigated. Measured dV/dI dependencies demonstrate the prevailing contribution to the PC conductivity caused by the degraded surface. Superconducting (SC) feature in dV/dI like a sharp zero-bias minimum develops for relatively low ohmic PCs, where the deep areas of FeSe are involved. Analysis of dV/dI has shown that the origin of the zero-bias minimum is connected with the Maxwell part of the PC resistance, what masks energy dependent spectral peculiarities. Even so, we have found the specific features in dV/dI — the sharp side maxima, which may have connection to the SC gap, since their position follows the BCS temperature dependence. Exploring the dV/dI spectra of the rare occurrence with Andreev-like structure, the two gaps with Δ = 2.5 and 3.5 meV were identified

Effect of pressure on the magnetic properties of CrB₂

Magnetic susceptibility c of the itinerant antiferromagnet CrB₂ with TN ≃ 87K was studied as a function of the hydrostatic pressure up to 2 kbar at fixed temperatures 78 and 300 K. The pressure effect on c is found to be negative in sign and weakly dependent on the magnetic state of the compound. In addition, the measured pressure dependence of the Néel temperature, dTN / dP = (0.1 ± 0.1) K/kbar, is roughly two orders of magnitude smaller than the corresponding value for the pure chromium. The main contributions to c and their volume dependence are calculated ab initio within the local spin density approximation, and appeared to be in close agreement with the experimental data

Magnetic properties and electronic structure of LaFeAsO₀.₈₅F₀.₁

The magnetic properties of LaFeAsO₀.₈₅F₀.₁ compound were investigated by measurements of dc magnetization at different values of the magnetic field, H = 0.02, 1.0 and 2.0 T, in the temperature range 4.2–300 K. The superconducting behavior was found below 26 K, whereas a distinct peculiarity in the low-field dependence of the magnetic susceptibility χ(T) was clearly observed at TM ≈ 135 K, which resembles a weak ferromagnetic (FM) response with saturation magnetic moment of about 10⁻⁴ μB per formula unit at 50 K. The transition at TM is presumably not governed by magnetic impurities but rather correlated with the antiferromagnetic (AFM) transition in the undoped LaFeAsO compound at about the same temperature. We suggest, that observed magnetic properties of the LaFeAsO₀.₈₅F₀.₁ sample are due to an interplay of FM and AFM transitions, and presumably related to an intrinsic feature of a small portion of the undoped LaFeAsO phase inherent in our sample. In order to shed light on the problem of magnetic instabilty of the LaFeAsO, the ab initio DFT calculations of electronic structure and paramagnetic susceptibility were performed within the local spin density approximation. It is shown, that a V-shaped peculiarity in the density of electronic states in a close vicinity of the Fermi level can govern magnetic properties of LaFeAsO under fluorine doping and/or oxygen deficiency