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We perform a systematic {\it ab initio} study of the work function and its uniform strain dependence for graphene and silicene for both tensile and compressive strains. The Poisson ratios associated with armchair and zigzag strains are also computed. Based on these results, we obtain the deformation potential, crucial for straintronics, as a function of the applied strain. Further, we propose a particular experimental setup with a special strain configuration that generates only the electric field, while the pseudomagnetic field is absent. Then, applying a real magnetic field, one should be able to realize experimentally the spectacular phenomenon of the collapse of Landau levels in graphene or related two-dimensional materials., 9 pages, 7 figures; final version published in PRB

International audience; The ionic liquid-based thermo-electrochemical cells receive increasing attention as an inexpensive alternative to solid-state thermo-electrics for waste heat harvesting applications. Recently, it has been demonstrated that magnetic nanoparticles (MNPs) in liquid-based thermoelectric materials result in enhancement of the Seebeck effect opening new perspectives to the design of a thermoelectric device with relatively high efficiency and cost effectiveness. Here, the role of an interacting assembly of MNPs in the thermoelectric signal is studied for the first time. Based on a thermodynamic approach, an analytic expression has been derived for the Seebeck coefficient that includes the inter-particle magnetic interactions in the assembly and the nanoparticle’s magnetic characteristics (saturation magnetization, magnetic anisotropy). Mesoscopic scale modelling with the implementation of the Monte Carlo Metropolis algorithm is performed to calculate their contribution to the Seebeck coefficient, for diluted assemblies of γ-Fe2O3 and CoFe2O4 nanoparticles, materials commonly used in ferrofluids. The results demonstrate the increase of the size and temperature range of the Seebeck coefficient with the increase of nanoparticles’ magnetic anisotropy paving the way for the detailed study of the magneto-thermal effects in high-performance thermoelectric materials based on ferrofluids.

Explicit analytical expressions for conductivity of a superconducting film above and below critical temperature in an arbitrary electric field are derived in the frameworks of the time dependent Ginzburg-Landau theory. It is confirmed that slightly below critical temperature the differential conductivity of superconducting film can become negative for small enough values of electric field. This fact may cause generation of electromagnetic oscillations if the superconducting film is appropriately coupled of with a resonator. Their maximal frequency is proportional to the value of critical temperature of superconducting transition. The obtained results can stimulate the development of Terahertz generators on the basis of high temperature superconducting films., Comment: 5 pages, 3 figures

Is it possible eternal existence, and there will inevitably destruction? The monograph for the first time describes the General theory of degradation. The theory is described in its practical application to multifunctional composite concrete and reinforced concrete structures. As you know, the basis of these structures is concrete. The monograph is composed of two parts. The first part of the monograph deals with various modern approaches to modeling the behavior of concrete. Concrete is considered as the most important industrial building material. A comprehensive analysis of the behavior of concrete as a composite material is performed. Structural, technological, statistical and phenomenological models of its behavior are reviewed and considered. Describes the modern theoretical model of structural concrete, including model structural mechanics and fracture mechanics, theory of similarity and theory of loose bodies. Reviewed various proposals for how to transform the graphs of the behavior of concrete under various conditions. The results of our own experimental studies are presented. The information base of concrete behavior is obtained and described. Examples of the processed results in the database. On the basis of factor, canonical and regression analyses statistical models of concrete behavior are proposed. Structural analysis of concrete as a composite material allowed to develop models of its elastic behavior. The influence of concrete composite structure on its deformation and strength is revealed. The effect of cracks on the relative deformation of concrete is shown. In the second part of the monograph describes the main provisions of the theory of degradation as a General theory of destruction of the object. The relative independence of energy and time in the destruction of objects is revealed. A new characteristic of the object – the energy potential-is proposed. The connection of the theory of degradation with durability of various objects is shown. For concrete, two main forms of constructing a model of its behavior are identified. On the basis of the developed models of concrete behavior, diagrams of standard tests of concrete and transformed diagrams of its behavior in time are constructed. The possibility of using the developed models in relation to other materials and structures: steel, pipe concrete, wood, structures made of polymeric materials.

The article discusses the features of the application of the theory of degradation to the work of an integrated structure operating in a complex stress state. The analysis of the work of an integrated structure consisting of a steel shell filled with concrete (core structure). Based on the analysis of the construction work, we obtained the relations connecting the deformations of the steel shell and the polymer concrete core of the complex structure. The obtained relations made it possible to apply the diagrams of concrete work for uniaxial compression to analyze the possibility of using concrete as a core of an integrated structure. Experimental studies of the polymer concrete core of the structure were conducted. In total, ten concrete compositions were made and investigated. The compositions of concrete differed in cementitious: cement and polyester resin. As a filler used sand, gravel, ground clay, marble flour, soda and fine mineral fibers. Samples were tested for central and eccentric compression. During the tests used the methods used in testing cement concrete. As a result of the tests, complete schedules of the work of materials for uniaxial compression were obtained. The analysis of the energy characteristics of concrete schedules based on the theory of degradation is carried out. As a result of the discussion of the results obtained, conclusions are drawn about the possibility of using polymer concrete as the supporting core of an integrated structure with an external steel shell.

PRE; International audience; We discuss the details of ambipolar relaxation of the electric field in liquid asymmetric electrolytes to its stationary value. It is demonstrated that the account for finite boundary conditions modifies the existing concepts of this diffusion process. In particular, we succeeded to suggest a qualitatively correct explanation of the observed distribution of the electric fields over the bulk of the cuvette and its nonmonotonic behavior in measurements on the finite-size cuvette. We analyze the conditions of such an anomaly at the intermediate stages of the relaxation process

We propose a Ginzburg-Landau phenomenological model for the dependence of the critical temperature on microscopic strain in tetragonal high-Tc cuprates. Such a model is in agreement with the experimental results for LSCO under epitaxial strain, as well as with the hydrostatic pressure dependence of Tc in most cuprates. In particular, a nonmonotonic dependence of Tc on hydrostatic pressure, as well as on in-plane or apical microstrain, is derived. From a microscopic point of view, such results can be understood as due to the proximity to an electronic topological transition (ETT). In the case of LSCO, we argue that such an ETT can be driven by a strain-induced modification of the band structure, at constant hole content, at variance with a doping-induced ETT, as is usually assumed.

We express the link between conductivity and coefficients of Seebeck, Nernst-Ettingshausen, Peltier, and Thompson and Reghi-Leduc via the temperature derivative of the chemical potential of a system. These general expressions are applied to three-, two- and one-dimensional systems of charge carriers having a parabolic or Dirac spectrum. The method allows for predicting thermoelectric and thermomagnetic properties of novel materials and structures., Comment: 5 pages, 2 figures

We study Seebeck effect in liquid electrolytes, starting from its simple neutral analog---thermodiffusion (so-called Ludwig-Soret or Soret effect). It is observed that when two or more subsystems of mobile particles are subjected to the temperature gradient, various types of them respond to it differently. In the case when these fractions, with different mobility parameters (Soret coefficients), are oppositely charged (a case typical for electrolytes), the nonhomogeneous internal electric field is generated. The latter field prevents these fractions from space separation and determines the intensity of the appearing Seebeck effect.

The growth of the transverse resistivity just above the transition in Bi2Sr2CaCu2O8+x (BSCCO 2212) expitaxial films is studied in the framework of the recent fluctuation model proposed by Ioffe, Larkin, Varlamov and Yu Lu. The effects of different redox thermal treatments on the transverse resistive peak is investigated. The fit of the experimental curves with the theory is good and allows one to estimate the values of some physical parameters included in the theory itself (namely the in-plane Fermi velocity νF, the carrier lifetime τ and the hopping integral w). It is shown that the oxidizing thermal treatment gives rise to a strong decrease in the transverse resistance peak just above Tc. Such effect is ascribed to the increase in νF due to the growth of the carrier concentration induced by the oxidizing treatment. Effects on the carrier lifetime and the hopping integral are hindered by the large variation of νF.

Superconducting fluctuations in strongly anisotropic planar superconductors substantially affect the resistivity above the transition temperature. In a wide temperature range above T(c), the main effect of these fluctuations is to create a virtual gap in the electronic spectrum, causing an increase in resistivity. As a result the resistivity shows a maximum close to the transition temperature. Closer to the transition temperature the correction to the resistivity becomes negative and grows.as 1/(T - T(c))2, WhiCh is followed by the usual (T - T(c))1/2 law still closer to the transition temperature. This observation may help to understand the unusual behavior of the resistivity in the c direction in high-T(c) cuprates.

The experimentally observed increase of the transversal resistance near the edge of the transition in Bi 2 Sr 2 CaCu 2 O 8+δ epitaxial film is analyzed on the basis of a recent theoretical proposal on its fluctuation origin [L. Ioffe, A. I. Larkin, A. A. Varlamov, and L. Yu, Phys. Rev. B (to be published). It is shown that the excess resistance in a wide range of temperatures increases logarithmically as the superconducting transition temperature is approached from above, in agreement with the theoretical consideration. These results are also compared with the available data on YBa 2 Cu 3 O 7 compounds

Paraconductivity of MgB2 has been measured in homogeneous thin films grown by Hybrid physical chemical vapor deposition. In order to reduce the possible effects of spatial inhomogeneities, stripes of different widths were cut on the films. We measured several samples with different resistivity values; after subtracting the normal state resistivity, paraconductivity appears to be of the same order of magnitude in all the samples. The dependence on the reduced temperature epsilon = ln(T/Tc) is discussed and compared with the existing models. (c) 2007 Elsevier B.V. All rights reserved.

We develop a general approach to calculations of the electron spectrum of metallic multi-wall carbon nanotubes (MWNT) with arbitrary number of coaxial layers. It is based on the model with singular attractive potential of equidistant conductive cylinders. The knowledge of one-electron spectrum allows to construct the corresponding Green function and then to calculate the entropy and density of states for MWNT. We analyze the tunnelling between the nanotube and normal metal electrode. The possibility of direct determination of one-electron density of states by measurements of the tunnelling conductivity at low temperatures is proved and the necessary restrictions on temperature are formulated. We discuss briefly the conflicting experimental observations of electronic properties of MWNT., 14 pages, 1 figure

We report the calculation of paraconductivity in carbon nanotubes above the superconducting transition temperature. The complex behavior of paraconductivity depending upon the tube radius, temperature and magnetic field strength is analyzed. The results are qualitatively compared with recent experimental observations in carbon nanotubes of an inherent transition to the superconducting state and pronounced thermodynamic fluctuations above $T_{c}$. The application of our results to single-wall and multi-wall carbon nanotubes as well as ropes of nanotubes is discussed., Comment: 7 pages, 1 figure

We present paraconductivity measurements on YBCO thin films obtained by means of pulsed laser ablation. The films, grown under different oxygen pressures, exhibit critical temperatures ranging from 81 to 92 K. The excess conductivity has been analyzed in order to study the critical phenomena. We observe the Lawrence–Doniach crossover and, at higher temperatures, the crossover related to the breakdown of the Ginzburg–Landau (GL) approximation due to the increasing role of short wave fluctuations; the first crossover depends on the sample anisotropy and shifts to lower temperatures as the critical temperature of the film decreases, while for all the samples the second occurs roughly at the same reduced temperature. This proves the universality of the 2D paraconductivity behavior even out of the GL region. From the Lawrence–Doniach crossover values, we calculate the anisotropy factor which comes out to be nearly five when the critical temperature varies between 92–89 K, and increases up to 9.6 when the critical temperature is 81.5 K.

Among all unconventional normal state properties of the high temperature super- conductors, the transport properties are the most puzzling. While the in-plane resistivity is almost linear with temperature in clean samples the transverse resis-tivity generally shows a peak just above T c . Such behavior was observed in all high T c compounds [1, 2, 3, 4] and even in conventional layered superconductors [5]. As noted by Anderson [6], the difference in the temperature dependence between the transverse and in-plane resistivity is very difficult to explain in a conventional Fermi liquid theory. It turns out that this peak is directly connected with the anisotropy of the sample, being very pronounced in highly anisotropic samples (BSCCO, YBCO annealed in reducing atmosphere) but almost absent in samples with low anisotropy (fully oxygenated BSCCO or YBCO). We identify here at least one source of this difference: the suppression of the positive paraconductivity along c-direction by the square of the interlayer transparency together with the growth of the normal resistance related to the fluctuation depression of the density of states at the Fermi level.

We have measured the differential optical reflectivity ( λ = 0.78 mm) of YBa 2 Cu 3 O 7 − x single crystals in the temperature range of 20–300 K using a thermal wave technique. A sharp signal maximum with a temperature width ∼ 10 K was observed below T c , and was attributed to the electronic contribution of the optical reflectivity. It is concluded that the two components of the differential reflectivity signal are related to accumulation of the superconducting phase and temperature variation of the electron-scattering time.

Measurements of the excess conductivity above the transition temperature on selected, high-quality, superconducting epitaxial films of the 2:2:1:2-phase Bi-Sr-Ca-Cu-O (BSCCO), obtained by liquid-phase epitaxy, are reported. The data confirm the two-dimensional nature of the thermodynamical fluctuations in BSCCO compounds in a wide range of temperature above ${\mathit{T}}_{\mathit{c}}$ and very good agreement is found with an extended version of the Aslamazov-Larkin theory [L. Reggiani, R. Vaglio, and A. A. Varlamov, Phys. Rev. B 44, 9541 (1991)].

Superconducting fluctuations in strongly anisotropic planar superconductors substantially affect the resistivity above the transition temperature. In a wide temperature range above Tc, the main effect of these fluctuations is to create a virtual gap in the electronic spectrum, causing an increase of transverse resistivity. This effect becomes especially competitive due to the suppression of paraconductivity in c-axis direction because of the hopping character of electron motion. The qualitative aspects of the phenomenon are discussed in detail.

The thermoelectric power behaviour is investigated in the vicinity of topological transition in the case of finite temperatures and impurity scattering. It is shown that the singularity in thermoelectric power early predicted by Vaks et al., is smeared out by those two factors. The results obtained are in good agreement with available experimental data.

It has been demonstrated that the thermocells, which contain ionically stabilized magnetic nanoparticles (ferrofluids) dispersed in aqueous electrolytes, show enhanced Seebeck coefficient. We have addressed this issue by deriving an analytic expression that relates the Seebeck coefficient to the chemical potential and its temperature derivative taking into account the nanoparticle’s intrinsic characteristics, i.e. magnetization and magnetic anisotropy. Mesoscopic scale modelling with the implementation of the Monte Carlo Metropolis algorithm has been performed on diluted assemblies of γ-Fe2O3 and CoFe2O4 nanoparticles – commonly used ferrofluid materials with different magnetic anisotropies and particle concentrations - to calculate the particle Seebeck coefficient at different temperatures. Importantly, our numerical results show enhancement of the Seebeck coefficient with the increase of the magnetic particle anisotropy and particle concentrations with a maximum that shifts to higher temperatures as the anisotropy increases. Our findings suggest that by tailoring the magnetic particle anisotropy, enhanced thermoelectric signal can be achieved in ferrofluid based thermoelectric materials.

We present paraconductivity measurements on YBCO thin films obtained by means of pulsed laser ablation in MBE condition. The films, grown in different oxygen pressure, exhibit critical temperatures ranging from 81 to 91K. The excess conductivity was analyzed in order to study critical phenomena. We observe the Lawrence-Doniach crossover and, at higher temperatures, the crossover related to the breakdown of the Ginzburg-Landau approximation, due to the importance of short wave fluctuations; the first crossover shifts to lower temperatures as the critical temperature of the film decreases, while the second occurs for all the samples roughly at the same reduced temperature, which proves the universality of the paraconductivity behaviour.