Narrow Results

This study investigates the vertex corrections of the optical conductivity in high-$T_c$ superconductors for hole-spin wave scattering in the superconducting state. The Feynman diagram technique is used to determine the mathematical expressions of the ladder diagrams in the calculation of the current–current correlation function. The sum of these vertex terms leads to a self-consistent form of the vertex function. By evaluating the frequency summation through contour integration, a general expression for the current–current correlation function is obtained, which allows us to determine the zeroth-order term and the first-order correction of the optical conductivity explicitly. Our findings for the dc conductivity in a clean superconductor indicate that the zeroth-order term is zero, a result attributed to the conservation of momentum and energy. Meanwhile, the first-vertex correction reveals the Drude behavior in the superconducting state at zero frequency.

Graphene/Ag composite aerogels (GA/Ag) with three-dimensional hierarchical porous microstructure were prepared by in-situ formation of silver nanoparticles obtained from AgNO₃ into the porous structure of graphene aerogel. When the used AgNO₃ concentration was 12wt.%, the specific surface area of the obtained GA/Ag was 128.6m²g${}^{-1}$, which was higher than that of GA. GA/Ag exhibited electrical resistivity of 0.63$\mathrm{\Omega }\cdot$cm and excellent microwave absorption property with the minimum reflection loss of −40dB at 10.9GHz with the corresponding effective absorption bandwidth of 4GHz (8.8–12.8GHz). The GA/Ag composite aerogels enabled multiple reflection loss and absorption of incident electromagnetic waves, realizing excellent microwave absorption effects based on impedance matching and efficient synergy. It could be predicted that GA/Ag with a hierarchical porous structure may show potential application prospects in the field of microwave absorption.

Interactions between planar Cu phthalocyanine molecules, CuPc, arranged in vertical stacking sequence are studied by first-principles density functional (DF) and semiempirical molecular dynamics (MD) methods. Dependence of electronic structure upon interplanar spacing and staggering angle is analyzed in order to interpret observed electrical conductivity and magnetic coupling behavior of partially oxidized Cu(pc)I. MD predictions of equilibrium structure of dimers and tetramers are in excellent agreement with experiment on the linear chain species. DF predictions of interaction energy of dimers are compared with MD and experiment.

This micro-review presents an overview of molecular materials derived from phthalocyanines which are substituted with crown ether rings. These flat disk-like molecules can self-assemble into ordered, well-defined structures with or without the aid of alkali metal ions. Highly ordered assemblies of crown ether phthalocyanines are capable of transporting electrons and ions, which is of interest for sensor applications.

The problem of computing the effective transport and mechanical properties of disordered materials, modeled by random or correlated percolation networks, is studied as an optimization problem. We show that calculating an optimal distribution of the potentials (voltages, displacements, etc.) throughout a disordered material that minimizes its total energy reduces the computation times for calculating the effective properties by a factor that depending on the morphology of the system, ranges from 3 to 73. Hence, this offers significant speed-up over the most efficient numerical methods currently available.

We analyze a random resistor–inductor–capacitor (RLC) lattice model of two-dimensional metal–insulator composites. The results are compared with Bruggeman's and Landauer's Effective Medium Approximations where a discrepancy was observed for some frequency zones. Such a discrepancy is mainly caused by the strong conductivity fluctuations. Indeed, a two-branches distribution is observed for low frequencies. We show also by increasing the system size that at p_c the so-called Drude peak vanishes; it increases for vanishing losses.

This paper confirms the applicability of a newly developed efficient algorithm, the direct electrifying method, for identifying backbone for 3D site and bond percolating networks. This algorithm is based on the current-carrying definition of backbone and carried out on the predetermined spanning cluster, which is assumed to be a resistor network. The scaling exponents so obtained for backbone mass, red bonds, and conductivity are in very good agreement with some existing results. The perfectly balanced bonds in 3D backbone structures are predicted first time to be 0.00179 ± 0.00009 and 0.00604 ± 0.00008 of the backbone mass for bond and site percolations, respectively.

By numerically solving the generalized Laplace equations by means of the finite difference method, we investigated isotropic electric conductivity of a three-dimensional continuum percolation model consisting of overlapped spheroids of revolution in continuum. Since the computational results strongly depend upon parameters in the discretization methods of the finite difference method, we explored the dependences in details to construct the computational scheme which can represent the continuum percolation model well. Using the discrete scheme, we obtained the conductivity curves, σ =c (p -p_c)^t, depending upon aspect ratio of the conductive spheroids for the volume fraction p. We found the fact that the critical exponent t is not universal, which depends upon the shape of spheroids with a range varying from 1.58 ± 0.08 to 1.94 ± 0.18 whereas 1.85 is reported as the standard one of cubic lattice case [A. B. Harris, Phys. Rev. B28, 2614 (1983)]. We also discussed its relation to the nonuniversality in the broad distribution continuum percolation models.

We present here polarization operator of non-relativistic fermions with spin-orbit (SO) Rashba interaction. The spectrum of this fermions is moat type having minimum on a circle. Contrary to Dirac or non-relativistic fermions, Fermi sea here has a geometry of Corbino disk which reflects in a transport properties of excitation’s.

Using the AdS/CFT correspondence, we study the anisotropic charge transport properties of both supersymmetric and nonsupersymmetric matter fields on (2+1)-dimensional defects coupled to a (3+1)-dimensional "heat bath." We focus on the cases of a finite external background magnetic field, finite net charge density and finite mass and their combinations. In this context, we also discuss the limitations due to operator mixing that appears in a few situations and that we ignore in our analysis. At high frequencies, we discover a spectrum of quasiparticle resonances due to the magnetic field and finite density and at small frequencies, we perform a Drude-like expansion around the DC limit. Both of these regimes display many generic features and some features that we attribute to strong coupling, such as a minimum DC conductivity and an unusual behavior of the "cyclotron" and plasmon frequencies, which become related to the resonances found in the conformal case in an earlier paper. We further study the hydrodynamic regime and the relaxation properties, from which the system displays a set of different possible transitions to the collisionless regime. The mass dependence can be cast in two regimes: a generic relativistic behavior dominated by the UV and a nonlinear hydrodynamic behavior dominated by the IR. In the massless case, we furthermore extend earlier results from the literature to find an interesting selfduality under a transformation of the conductivity and the exchange of density and magnetic field.

Samples with the nominal composition Bi₂Sr₂GdCu₂O_y in Bi-2212 where Gd replaces Ca as well as samples without Gd were prepared by solid-state reaction method. From the room temperature X-ray diffraction data, the samples were found to be similar to the single phase Bi-2212 structure. Impedance studies were performed from room temperature to 423 K at different frequencies in the range of 10 to 700 KHz. The AC conductivity increases with temperature and frequency, exhibiting frequency dispersion at low temperature region. The activation energy from AC conductivity in the high temperature region is found to be 0.432 eV. The permitivity increases with the increase in temperature and at 373 K it shows a maximum value exhibiting a dielectric loss. Complex impedance spectra are analyzed in terms of bulk relaxation and interfacial effects. The activation energy of the dipoles involved in the relaxation was estimated to be 0.482 eV. The universal power law of Jonscher is verified in the present system.

Optical conductivity spectra of conducting oxides including high-T_C cuprates - compared to non-superconducting nickelates in particular - deduced from infrared reflectivity spectroscopy in polarized light and their doping and temperature dependence, are discussed with emphasis on the signatures of intrinsic inhomogeneities of charge carrier concentration. The model of disordered metal is shown to be compatible with the general loss of spectral weight in the optical conductivity upon decreasing frequency, instead of a maximum conductivity at zero frequency as found within a Drude description. In high-T_C multilayer cuprates, the discussion focuses upon the c-axis response and the inhomogeneity of the electronic concentration related to the conducting planes alternating with insulating sheets. The onset of stripes that are intrinsic inhomogeneities of the charge carrier distribution is discussed in the example of nickelates.

Ferroelectric, hysteresis, impedance spectroscopy parameters, AC conductivity, and piezoelectric properties in the ceramics of Pb_0.74K_0.52Nb₂O₆ and Pb_0.74K_0.13Sm_0.13Nb₂O₆ have been studied. X-ray diffraction study reveals single phase with the orthorhombic structure. The samples were characterized for ferroelectric and impedance spectroscopy properties from room temperature to 600°C. Cole–Cole plots (Z″ versus Z′) are drawn at different temperatures. The results obtained are analyzed to understand the conductivity mechanism in both the samples. The piezoelectric constant d₃₃ has been found to be 96 × 10^-12 C/N in PKN.

Over 10 years pass after experimental discovery Josephson plasma resonance (JPR) in high-T_c superconductors. JPR spectroscopy became the powerful tool to study vortex phase diagram and interlayer electrodynamics in Josephson coupled layered superconductors like certain of high-T_c cuprates and molecular superconductors. We describe main principles of the JPR spectroscopy for measurement longitudinal plasma oscillations at the fundamental frequency ω_p. That frequency and linewidth of resonance is directly related to the interlayer phase coherence. In this paper we report about developments and achievments in spectroscopy of JPR. The study of the vortex state of Bi₂Sr₂CaCu₂O_8+δ and c-axis electrodynamics of Bi₂Sr₂CaCu₂O_8+δ and Bi₂Sr₂CuO_6+δw in zero magnetic field by JPR.

Perovskite structured ferroelectric (Na_1/2 Bi_1/2)_0.945Ba_0.055TiO₃ (BNBT-5.5) material has been synthesized by the conventional sintering technique. X-ray analysis on the material showed a single phase compound with rhombohedral structure with lattice parameters a = 3.89 Åand α = 89.893 Å. Frequency and temperature dependence of dielectric permittivity, impedance, modulus and conductivity have been performed in the frequency and temperature range 45 Hz–5 MHz and 35–595^°C, respectively. The observed low frequency dielectric dispersion (LFDD) in the material could be explained by Jonschers power law and evaluated activation energies at different temperature regions. Impedance spectroscopy study showed the presence of both bulk and grain boundary effects in the materials. The ac conductivity spectrum obeyed the Jonscher's power law. Modulus analysis indicated the possibility of hopping mechanism for electrical process in the system.

The gapped graphene is technologically more important as electronic devices made up of conventional semiconductors are based on the presence of a gap between the valence and conduction bands. In this paper, we report our analytical and numerical calculation of conductivity of gapped graphene on SiO₂ substrate and its variation with the governing parameters of carrier density, band gap, long range charge impurity scatterers, short range neutral defect scatterers, impurity distance from the graphene interface and potential strength of short range scattering centers. We find that the conductivity for unscreened potential decreases with increasing band gap but when screening is incorporated the trend is not only reversed but there is a sharp enhancement in its magnitude also. The sub linear behavior seen in conductivity in the case of gapless graphene at high carrier concentration due to enhanced scattering from the short range neutral defects is also observed for gapped graphene, and it is further observed that increasing the potential strength of the defect scatterers brings in the same behavior at relatively smaller carrier densities.

Transparent films of PMMA (poly methyl methacrylate), PVAc (polyvinyl acetate) and their blends, have been prepared by using a solution-casting technique. The dielectric properties and the electrical conductivity are reported. The frequency and temperature dependence of the dielectric constant, ε′ and tan δ, have been investigated for the studied samples in the frequency range from 1 kHz to 5 MHz and over a range of temperature from 303–413 K. In addition, AC conductivity values were calculated from the dielectric data and the conduction mechanism is discussed. The frequency-dependent conductivity behavior at different temperatures provides a qualitative description of the conduction mechanism. Also, differential scanning calorimetry (DSC) scans have been measured for the studied samples.

Fullerene grafted poly (methylmethacrylate) polymers have been studied using X-ray diffraction and raman spectroscopy to analyze their polymeric structure. The electrical properties of prepared polymers have also been studied. The X-ray diffraction and raman spectroscopy pattern of pure buckminster fullerene (C₆₀) has been compared with prepared polymeric samples. Raman spectroscopy indicates that a structural change occurs due to grafting. The vibrations in the relative intensity of peaks and shifts in energy are observed. The lower scattering efficiency of pure fullerene (C₆₀) is observed at 1468 cm^-1.The values for poly (methylmethacrylate) are observed at 3000 cm^-1. The result show a large shift in the intense raman spectral region of fullerene-based poly (methylmethacrylate) which lies between 2500 cm^-1 and 3000 cm^-1. X-ray diffraction shows an unusual variation in crystallite size of polymeric samples. Generally it has been observed that polymers are insulators, but noticeably fullerene-based poly (methylmethacrylate) show conducting behavior.

In the present study, samarium doped ceria (SDC) and SDC-based composite with the addition of K₂CO₃ were prepared by co-precipitation route and effects of pH of the solution and calcination temperature on microstructure of SDC and SDC-K₂CO₃, respectively, were investigated. Furthermore, experimentation was performed to investigate into the ionic conductivity of pure SDC by co-doping with yttrium i.e., YSDC, XRD and SEM studies show that the crystallite size and particle size of SDC increases with the increase in pH. The SEM images of all the samples of SDC synthesized at different pH values showed the irregular shaped and dispersed particles. SDC-K₂CO₃ was calcined at 600${}^{\circ }$C, 700${}^{\circ }$C and 800${}^{\circ }$C for 4 h and XRD results showed that crystallite size increases while lattice strain, decreases with the increase in calcination temperature and no peaks were detected for K₂CO₃ as it is present in an amorphous form. The ionic conductivity of the electrolytes increases with the increase in temperature and SDC-K₂CO₃ shows the highest value of ionic conductivity as compared to SDC and YSDC. Chemical compatibility tests were performed between the co-doped electrolyte and lithiated NiO cathode at high temperature. It revealed that the couple could be used up to the temperature of 700${}^{\circ }$C.

Based on density functional theory (DFT) and local density approximation (LDA), the band structure and the density of states of all models of pure and V-doped anatase TiO₂ systems were calculated using plane-wave ultra-soft pseudo potential method after geometry optimization. The calculation results show that, with the increase of the doping concentration of V in anatase TiO₂, the free electron concentration and electron effective mass increase; the electronic mobility and electronic conductivity decrease, which is consistent with the change trend of the experimental results.