Anisotropy and interplane interactions in the dielectric response of graphite

We determined the anisotropic dielectric response of graphite by means of time-dependent density-functional theory and high-resolution valence electron energy-loss spectroscopy. The calculated loss function was in very good agreement with the experiment for a wide range of momentum-transfer orientations with respect to the graphitic basal planes, provided that local-field effects were included in the response. The calculations also showed strong effects of the interlayer Coulomb interaction on the total pi+sigma plasmon. This finding must be taken into account for the explanation of recent loss spectra of carbon nanotube materials.     

Study on ionic conductivity and dielectric properties of PEO-based solid nanocomposite polymer electrolytes

The ionic conductivity and dielectric properties of the solid nanocomposite polymer electrolytes formed by dispersing a low particle-sized TiO₂ ceramic filler in a poly (ethylene oxide) (PEO)-AgNO₃ matrix are presented and discussed. The solid nanocomposite polymer electrolytes are prepared by hot press method. The optimum conducting solid polymer electrolyte of polymer PEO and salt AgNO₃ is used as host matrix and TiO₂ as filler. From the filler concentration-dependent conductivity study, the maximum ionic conductivity at room temperature is obtained for 10 wt% of TiO₂. The real part of impedance (Z′) and imaginary part of impedance (Z″) are analyzed using an LCR meter. The dielectric properties of the highest conducting solid polymer electrolyte are analyzed using dielectric permittivity (ε′), dielectric loss (ε″), loss tangent (tan δ), real part of the electric modulus (M′), and imaginary part of the electric modulus (M″). It is observed that the dielectric constant (ε′) increases sharply towards the lower frequencies due to the electrode polarization effect. The maxima of the loss tangent (tan δ) shift towards higher frequencies with increasing temperature. The peaks observed in the imaginary part of the electric modulus (M″) due to conductivity relaxation shows that the material is ionic conductor. The enhancement in ionic conductivity is observed when nanosized TiO₂ is added into the solid polymer electrolyte.     

Some dielectric properties of monoclinic lysozyme crystals

The frequency dependences of the dielectric loss tangent and the magnitude of the impedance for monoclinic lysozyme single crystals and the solution used for preparing the crystals were investigated. The measurements were performed in the frequency range 1–10⁷ Hz under exposure of the crystals at a temperature of 25°C, cooling to ?20°C, and subsequent heating. The analysis of the experimental dielectric properties of the crystals demonstrated that drying of the crystals at room temperature in air initially led to the removal of “free” water with the content approximately equal to 65% of the total content. Further drying resulted in the removal of “bound” water. The solvent contained in the crystals (～26 wt %) was frozen at a constant temperature of approximately ?6.5°C. The permittivity of the dehydrated crystal at high frequencies was considerably higher than that of ice.     

Anisotropy of graphite optical conductivity

The graphite conductivity is evaluated for frequencies between 0.1 eV, the energy of the order of the electronhole overlap, and 1.5 eV, the electron nearest hopping energy. The in-plane conductivity per single atomic sheet is close to the universal graphene conductivity e ²/4ħ and, however, contains a singularity conditioned by peculiarities of the electron dispersion. The conductivity is less in the c direction by the factor of the order of 0.01 governed by electron hopping in this direction.     

Band structure and dielectric properties of orthorhombic SrZrO3

Ab initio density functional theory has been used to study the electronic band structure, $k=0$ (zone center) vibrational modes, and dielectric properties of the crystalline high-$\kappa$ perovskite oxide SrZrO₃. The frequencies of the $k=0$ normal modes are computed by the linear response technique and then grouped according to their symmetry. The dielectric intensity of each IR-active mode has been determined. It is found that the calculated orientationally averaged static dielectric constant ${\epsilon }^0=21.49$ is in good agreement with experiment.     

Entropy and ionic conductivity

It is known that the ionic conductivity can be obtained by using the diffusion constant and the Einstein relation. We derive it here by extracting it from the steady electric current which we calculate in three ways, using statistics analysis, an entropy method, and an entropy production approach.     

Anisotropy of the dielectric function for wurtzite InN

A -plane InN film grown by molecular beam epitaxy on -plane sapphire substrate with an AlN nucleation layer and a GaN buffer was studied by spectroscopic ellipsometry. The data analysis yields both the ordinary and the extraordinary dielectric tensor components perpendicular and parallel to the optical axis, respectively. Strong optical anisotropy is demonstrated over the whole energy range from 0.72 up to 9.5 eV. The line shapes of the tensor components and the polarisation behaviour are in very good agreement with the results of recently published band structure and dielectric function calculations. Above the band gap, five van Hove singularities are evidenced from the ordinary component, while three are resolved from the extraordinary part. The polarisation dependence below 1 eV can be interpreted in terms of optical selection rules for three energetically split valence bands around the Γ-point of the Brillouin zone, similar to the well known behaviour of wurtzite GaN. This emphasises a band gap of hexagonal InN of about 0.7 eV.     

Volume dependence of the effective ionic charge of alkaline-earth fluorides

The importance of the use of low temperature data with the second Szigeti relation is shown for the fluorite structure; a comparison with the NaCl and CsCl structures is given. The volume dependence of the generalized first Szigeti relation is used to calculate the Grüneisen parameter γ₁ with low temperature data; γ₁ is calculated with different models. Values of the volume dependence of the effective ionic charge are obtained independently using Hardy's model.     

Non-linear conductivity of monoclinic TaS3

Strongly non-ohmic conduction is observed in the semiconducting charge-density-wave (CDW) state of monoclinic TaS₃ above a very sharp threshold field. These features could be interpreted in terms of depinning or tunneling of pinned CDW under applied electric field. We also found that large noise appears with the onset of the non-linear conductivity and fades away as the current through the samples is increased, which behaves as the decreasing function of frequency.     

Anisotropy of the hopping conductivity in TlGaSe2 single crystals

The temperature dependences of the conductivities parallel and perpendicular to the layers in layered TlGaSe₂ single crystals are investigated in the temperature range from 10 K to 293 K. It is shown that hopping conduction with a variable hopping length among localized states near the Fermi level takes place in TlGaSe₂ single crystals in the low-temperature range, both along and across the layers. Hopping conduction along the layers begins to prevail over conduction in an allowed band only at very low temperatures (10–30 K), whereas hopping conduction across the layers is observed at fairly high temperatures (T?210 K) and spans a broader temperature range. The density of states near the Fermi level is determined, N _F=1.3×10¹⁹eV·cm³)^?1, along with the energy scatter of these states J=0.011 eV and the hopping lengths at various temperatures. The hopping length R along the layers of TlGaSe₂ single crystals increases from 130 Å to 170 Å as the temperature is lowered from 30 K to 10 K. The temperature dependence of the degree of anisotropy of the conductivity of TlGaSe₂ single crystals is investigated.     

Anisotropy of the electrical conductivity of lithium heptagermanate crystals

The electrical conductivity σ of single crystals of lithium heptagermanate Li₂Ge₇O₁₅ is studied in an electric field in the frequency range 0.5–100 kHz at temperatures ranging from 300 to 700 K. Heating the crystal above 500 K gives rise to a pronounced anisotropy in the electrical conductivity, which differs in magnitude by one to two orders of magnitude for different directions of the measurement field along the crystallographic axes. It is shown that an increase in the electrical conductivity σ with increasing temperature originates from charge transfer with an activation energy U = 1.04 eV. It is assumed that the thermally activated contribution to the electrical conductivity is governed by transport of lithium interstitial ions along channels in the structure of the Li₂Ge₇O₁₅ compound.     

Luminescence and ionic conductivity of bariumfluorochloride

The luminescence and ionic conductivity of pure and doped BaFCl crystals are reported. The ionic conductivity occurs mainly via chloride ion vacancies, whereas the efficient, yellow luminescence is ascribed to oxygen impurities (O′_F).     

Epitaxial growth and in-plane dielectric properties of orthorhombic HoMnO3 films

Orthorhombic HoMnO3(HMO) thin films were grown epitaxially on LaAlO3(001) substrates by using pulsed laser deposition technique. The films showed perfect orthorhombic crystallization and were well-aligned with the substrates. The in-plane dielectric constant and loss of HMO films were measured as functions of temperature(80–300 K) and frequency(120 Hz–100 kHz) by using coplanar interdigital electrodes. Two thermally activated dielectric relaxations were found, and the respective peaks shifted to higher temperatures as the measuring frequency increased. The in-plane dielectric properties of epitaxial orthorhombic HMO films were considered as universal dielectric response behavior, and the dipolar effects and the hopping conductivity induced by the charge carriers were used to explain the results.     

Electronic and ionic conductivity in CaTiO3

The present work describes the electrical conductivity of undoped CaTiO₃ in terms of the electrical conductivity components corresponding to electrons, electron holes and ionic charge carriers in the temperature range 973 K — 1323 K and under controlled oxygen partial pressure (10 Pa — 72 kPa). These data are considered in terms of the transference numbers of the respective charge carriers. It appears that the ionic conductivity component assumes maximum at the n-p transition when the ionic transfer number reaches 50% of the total conductivity value at 1323 K. The present study also includes the determination of the activation energy of the conductivity component related to ions (162.1 kJ/mol), electrons (134.2 kJ/mol) and electron holes (86.2 kJ/mol). The data obtained in this work indicate that undoped CaTiO₃ exhibits a substantial level of ionic conduction that cannot be ignored in a quantitative analysis of electrical conductivity data.     

Anisotropy of magnetothermal conductivity in Sr2RuO4

The dependence of in-plane and interplane thermal conductivities of Sr2RuO4 on temperature, as well as magnetic field strength and orientation, is reported. We found no notable anisotropy in the thermal conductivity for the magnetic field rotation parallel to the conducting plane in the whole range of experimental temperatures and fields, except in the vicinity of the upper critical field H(c2), where the anisotropy of the H(c2) itself plays a dominant role. This finding imposes strong constraints on the possible models of superconductivity in Sr2RuO4 and supports the existence of a superconducting gap with a line of nodes running orthogonal to the Fermi surface cylinder.     

Anisotropy of lattice heat conductivity of complex chalcogenides

The present paper gives the results of experimental heat conductivity studies carried out on a series of complex semiconductive AX and ABX₂-type compounds. Measurements were performed over a wide temperature range at different crystallographic directions of the materials studied. In doing so, regularities have been revealed in the variation of the heat conductivity coefficient, depending on crystal orientation and temperature as well as on the substitutions in respective sublattices.     

Anisotropy of impurity scattering and electrical conductivity in quasi-two-dimensional electronic systems

Scattering of charge carriers by impurity ions in quasi-two-dimensional electronic systems is investigated. The dependences of the longitudinal and transverse components of the relaxation time on the wave vector components and the screening length are analyzed. The anisotropy of electrical conductivity of a degenerate quasi-two-dimensional electron gas is studied as a function of the ratio between the energy at the Fermi level and the miniband width, the impurity concentration, and the lattice constant.