In situ fabrication and thermoelectric properties of PbTe–polyaniline composite nanostructures

PbTe–polyaniline (PANi) composite nanopowders were in situ fabricated via an interfacial polymerization method at room temperature (~293 K). The phase structure, composition, and morphology of the powders were characterized by X-ray powder diffraction, infrared spectroscopy, transmission electron microscopy (TEM), and high-resolution TEM, respectively. The results show that the composite nanopowders consist of PbTe nanoparticles, PANi/PbTe core–shell nanostructure, and PbTe/PANi/PbTe three-layer sphere-like nanostructures. Formation mechanism of the PbTe–PANi composite nanostructures was proposed. The thermoelectric properties of the composite powders after being cold pressed into pellets were measured from 293 to 373 K. As the temperature increases from 293 to 373 K, the Seebeck coefficient of the composite decreases from 626 to 578 μV K⁻¹ and the electrical conductivity increases from 1.9 to 2.2 S m⁻¹.     

Validation of the Wiedemann–Franz law in a granular s-wave superconductor in the nanometer scale

The present study tries to evaluate the validity of the Wiedemann–Franz law in a granular s-wave superconductor in the presence of concentrated impurities. By using Green's function method and the Kubo formula technique, three distinct contributions of the Aslamazov–Larkin, the Maki–Thompson and, the density of states are calculated for both the electrical conductivity and the thermal conductivity in a granular s-wave superconductor. It is demonstrated that these different contributions to the fluctuation conductivity depend differently on the tunneling because of their different natures. This study examines the transport in a granular superconductor system in three dimensions in the limit of large tunneling conductance,which makes it possible to ignore all localization effects and the Coulomb interaction. We find that the tunneling is efficient near the critical temperature and that there is a crossover to the characteristic behavior of a homogeneous system.When it is far from the critical temperature, the tunneling is not effective and the system behaves as an ensemble of real zero-dimensional grains. The results show that the Wiedemann–Franz law is violated in both temperature regions.     

Fate of the Wiedemann–Franz Law near Quantum Critical Points of Electron Systems in Solids

We introduce and analyze two different scenarios for violation of the Wiedemann–Franz law in strongly correlated electron systems of solids, close to a topological quantum critical point (TQCP) where the density of states N(0) diverges. The first, applicable to the Fermi-liquid (FL) side of the TQCP, involves a transverse zero-sound collective mode that opens a new channel for the thermal conductivity, thereby enhancing the Lorenz number L(0) relative to the value L0 =π² k _B ²/3e ² dictated by conventional FL theory. The second mechanism for violation of the WF law, relevant to the non-Fermi-liquid (NFL) side of the TQCP, involves the formation of a flat band and leads instead to a reduction of the Lorenz number.     

Spin alignment and parity violation effects inρ 0 production in neutrino and antineutrino charged-current interactions

Most of the measured events on hydrogen, deuterium and neon targets from BEBC at CERN and the 15-foot Bubble Chamber at Fermilab are combined to measure spin-density matrix elements for ⁰ production in the current-fragmentation region in neutrino and antineutrino charged-current interactions. The spin-alignment parameter is found to be 0.08±0.11 for neutrino and 0.41±0.11 for antineutrino interactions. The parity-odd density matrix element Im(₁₀+_–10) is consistent with zero in both reactions. In the combined neutrino and antineutrino data an indication of parity violation is observed: Im _1–1=0.38±0.16 atx _B >0.2,Q ²>8 GeV² andz>0.6.     

The thermal and kinematic Sunyaev–Zel’dovich effects revisited

This paper shows that a simple convolution integral expression based on the mean value of the isotropic frequency distribution corresponding to photon scattering off electrons leads to useful analytical expressions describing the thermal Sunyaev–Zeldovich effect. The approach, to first order in the Compton parameter is able to reproduce the Kompaneets equation describing the effect. Second order effects in the parameter z = kT _e mc ² induce a slight increase in the crossover frequency.     

Nonlinear optical and magneto-optical effects in non-spherical magnetic granular composite

The magnetization-induced nonlinear optical and nonlinear magneto-optical properties in a magnetic metal-insulator composite are studied based on a tensor effective medium approximation with shape factor and Taylor-expansion method. There is a weakly nonlinear relation between electric displacement D and electric field E in the composite. The results of our studies on the effective dielectric tensor and the nonlinear susceptibility tensor in a magnetic nanocomposite are surveyed. It is shown that such a metal-insulator composite exhibits the enhancements of optical and magneto-optical nonlinearity. The frequencies at which the enhancements occur, and the amplitude of the enhancement factors depend on the concentration and shape of the magnetic grains.     

Temperature-dependent thermal expansion of cast and hot-pressed LAST (Pb–Sb–Ag–Te) thermoelectric materials

The thermal expansion for two compositions of cast and hot-pressed LAST (Pb–Sb–Ag–Te) n-type thermoelectric materials has been measured between room temperature and 673 K via thermomechanical analysis (TMA). In addition, using high-temperature X-ray diffraction (HT-XRD), the thermal expansion for both cast and hot-pressed LAST materials was determined from the temperature-dependent lattice parameters measured between room temperature and 623 K. The TMA and HT-XRD determined values of the coefficient of thermal expansion (CTE) for the LAST compositions ranged between 20 × 10^?6 K^?1 and 24 × 10^?6 K^?1, which is comparable to the CTE values for other thermoelectric materials including PbTe and Bi₂Te₃. The CTE of the LAST specimens with a higher Ag content (Ag_0.86Pb₁₉Sb_1.0Te₂₀) exhibited a higher CTE value than that of the LAST material with a lower Ag content (Ag_0.43Pb₁₈Sb_1.2Te₂₀). In addition, a peak in the temperature-dependent CTE was observed between room temperature and approximately 450 K for both the cast and hot-pressed LAST with the Ag_0.86Pb₁₉Sb_1.0Te₂₀ composition, whereas the CTE of the Ag_0.43Pb₁₈Sb_1.2Te₂₀ specimen increased monotonically with temperature.     

Pressure effects on magnetic properties and martensitic transformation of Ni–Mn–Sn magnetic shape memory alloys

The mechanism for the effects of pressure on the magnetic properties and the martensitic transformation of Ni-Mn- Sn shape memory alloys is revealed by first-principles calculations. It is found that the total energy difference between paramagnetic and ferromagnetic austenite states plays an important role in the magnetic transition of Ni-Mn-Sn under pressure. The pressure increases the relative stability of the martensite with respect to the anstenite, leading to an increase of the martensitic transformation temperature. Moreover, the effects of pressure on the magnetic properties and the martensitic transformation are discussed based on the electronic structure.     

Solitary excitations in the α-helix: Viscous and thermal effects

We report a molecular dynamics study of the propagation of a solitary excitation in a one-dimensional Lennard-Jones lattice when thermal and frictional effects are included. We give to the parameters that characterize the system the values of a typical chain of hydrogen-bonded peptide groups in the α-helix and observe that at temperatures and energies of biological interest and with water as a solvent, potential energies can be transported over long distances.     

Analysis and compensation of thermal lens effects in Tm：YAP lasers

The thermal lens effects in Tm：YAP laser are analyzed by solving the Poisson equation with finite difference method. The thermal focal lengths measured are in the range of 40-90 mm at the pump power of 16-34 W, consistent with the simulation results. The temperature contribution coefficient （the linear coefficient between the maximum temperature in the laser crystal and the pump power） of 1.19 K/W is also obtained. The convex lens and plano-concave mirror thermal lens compensation methods are proposed and applied to a high power pumped Tm：YAP laser.     

Spin–orbit effects and superconductivity in oxide materials

In a variety of materials superconductivity is associated with the existence of a quantum critical point (QCP). In the case of the hole doped cuprates there is evidence which suggests that the important quantum degrees of freedom near the superconducting critical point are localized charge and spin density fluctuations. We argue that if these degrees of freedom are strongly coupled by spin–orbit interactions, a new type of quantum criticality arises with monopole-like quasi-particles as the important quantum degrees of freedom. In layered material this type of quantum criticality can be modeled using a 2-dimensional non-linear Schrodinger equation with an SU(N) gauge field. We exhibit a pairing wave function for quasi-particles that has topological order and anisotropic properties. The superconducting transition would in some respects resemble a KT transition.     

Impurity-modulated Aharonov–Bohm oscillations and intraband optical absorption in quantum dot–ring nanostructures

In this work we study the electronic states in quantum dot–ring complex nanostructures with an on-center hydrogenic impurity. The influence of the impurity on Aharonov–Bohm energy spectra oscillations and intraband optical absorption is investigated. It is shown that in the presence of a hydrogenic donor impurity the Aharonov–Bohm oscillations in quantum dot–ring structures become highly tunable. Furthermore, the presence of the impurity drastically changes the intraband absorption spectra due to the strong controllability of the electron localization type.     

Frustrated magnetic interactions,giant magneto–elastic coupling,and magnetic phonons in iron–pnictides

We present a detailed first-principles study of Fe-pnictides with particular emphasis on competing magnetic interactions, structural phase transition, giant magneto–elastic coupling and its effect on phonons. The exchange interactions J_i,j(R) are calculated up to ≈12 Å from two different approaches based on direct spin-flip and infinitesimal spin-rotation. We find that J_i,j(R) has an oscillatory character with an envelop decaying as 1/R³ along the stripe-direction while it is very short range along the diagonal direction and antiferromagnetic. A brief discussion of the neutron scattering determination of these exchange constants from a single crystal sample with orthorhombic-twinning is given. The lattice parameter dependence of the exchange constants, dJ_i,j/da are calculated for a simple spin-Peierls like model to explain the fine details of the tetragonal–orthorhombic phase transition. We then discuss giant magneto–elastic effects in these systems. We show that when the Fe-spin is turned off the optimized c-values are shorter than experimental values by 1.4 Å for CaFe₂As₂, by 0.4 Å for BaFe₂As₂, and by 0.13 Å for LaOFeAs. We explain this strange behavior by unraveling surprisingly strong interactions between arsenic ions, the strength of which is controlled by the Fe-spin state through Fe–As hybridization. Reducing the Fe-magnetic moment, weakens the Fe–As bonding, and in turn, increases As–As interactions, causing a giant reduction in the c-axis. These findings also explain why the Fe-moment is so tightly coupled to the As-z position. Finally, we show that Fe-spin is also required to obtain the right phonon energies, in particular As c-polarized and Fe–Fe in-plane modes that have been recently observed by inelastic X-ray and neutron scattering but cannot be explained based on non-magnetic phonon calculations. Since treating iron as magnetic ion always gives much better results than non-magnetic ones and since there is no large c-axis reduction during the normal to superconducting phase transition, the iron magnetic moment should be present in Fe-pnictides at all times. We discuss the implications of our results on the mechanism of superconductivity in these fascinating Fe-pnictide systems.     

Equilibrium and non-equilibrium effects in nucleus–nucleus collisions

Local thermal and chemical equilibration is studied for central A+A collisions at 10.7–160 AGeV in the Ultrarelativistic Quantum Molecular Dynamics model (UrQMD). The UrQMD model exhibits strong deviations from local equilibrium at the high density hadron–string phase formed during the early stage of the collision. Equilibration of the hadron–resonance matter is established in the central cell of volume V=125 fm³ at later stages, t≥10 fm/c, of the resulting quasi-isentropic expansion. The thermodynamical functions in the cell and their time evolution are presented. Deviations of the UrQMD quasi-equilibrium state from the statistical mechanics equilibrium are found. They increase with energy per baryon and lead to a strong enhancement of the pion number density as compared to statistical mechanics estimates at SPS energies.     

Microstructure investigations and magnetic after-effect in amorphous and nanocrystalline Fe–Zr–Ti–B–Cu alloy

The microstructure evolution and low field magnetic properties i.e. initial magnetic susceptibility, stabilization field and magnetic after-effect as disaccommodation of the amorphous and nanocrystalline Fe₈₀Zr₄Ti₃B₁₂Cu₁ alloy were investigated. The heat treatment of the as-quenched Fe₈₀Zr₄Ti₃B₁₂Cu₁ alloy at 773 K for 1 h leads to its nanocrystallization. It was stated that initial magnetic susceptibility increases and intensity of disaccommodation decreases with increasing of annealing temperature. The magnetic after-effect of the investigated nanocrystalline samples is connected with relaxation processes that occur in the amorphous matrix.