The dynamics of water in nanoporous silica studied by dielectric spectroscopy

Broad-band dielectric spectroscopy is used to investigate the dynamics of hydration water on the surface of the cylindrical pores of a nanostructured silica material (MCM-41, with pore diameter of 3.2 nm) at various hydrations, in the temperature range 250-150 K. We focus our attention on orientational relaxations that shift from 0.5 MHz at 250 K to less than 1 Hz at 150 K. The measurements distinguish the relaxation of the hydroxyl groups at the surface of silica from the orientational dynamics of hydration water which strongly depends on the degree of hydration. Although it is significantly faster than the dynamics of water in ice, the orientational relaxation of non-freezing water has an activation energy comparable to that in ice when the hydration layer is complete and approximately two-molecule thick.     

Novel colloidal crystalline states on two-dimensional periodic substrates

We show, using numerical simulations, that a rich variety of novel colloidal crystalline states are realized on square and triangular two-dimensional periodic substrates which can be experimentally created using crossed-laser arrays. When there are more colloids than potential substrate minima, multiple colloids are trapped at each substrate minima and act as a single particle with a rotational degree of freedom, giving rise to a new type of orientational order. We call these states colloidal molecular crystals. A two-step melting can also occur in which individual colloidal molecules initially rotate, destroying the overall orientational order, followed by the onset of interwell colloidal hopping.     

Dielectric relaxation of chlorinated polyvinyl chloride (CPVC) stabilized with cyanoguanidine

Dielectric spectra of CPVC stabilized with cyanoguanidine were studied in the temperature range 300–450 K and frequency range 10 kHz to 1 MHz. In these conditions, only one clear dielectric relaxation band (α-type) associated with dipolar polarization was observed. Dielectric losses was found to directly proportional to the number of dipoles (N) which reflects the orientational distribution of polymer chains in the amorphous region, at which dielectric losses concerned. Calculations of the dielectric modulus M′(T) at low temperature indicate that there is a role of the electrode polarization in the relaxation process. Dielectric loss data were used to calculate the activation enthalpy by two different methods, the obtained value was 450 kJ/mol.     

Zero-THz spectroscopy of rotator phases

The molecular dynamical evolution from hertzian frequencies to the far infrared (THz) is reported for various kinds of rotator phase environment. The zero to THz frequency spectral profiles are analysed in terms of itinerant libration and developments on this theme. The systems analysed are (i) the plastic rotator phase of t-butyl chloride and solid solutions of this solute in tetramethylsilane; (ii) these results are compared with those from a spectral study in the range up to THz frequencies of t-butyl chloride in vitreous decalin solvent; (iii) single crystalline pentachloronitrobenzene and a pressed disc thereof. This solid is representative of a class of rotator phases where orientational disorder is gradually frozen in at low temperatures, unlike the plastic rotators; (iv) solid solutions of dipolar solute molecules in non-dipolar rotator phase benzene and tetramethylsilane. This is an attempt at evaluating the electrodynamics of dipole-dipole interaction.

The data over the complete range of frequency is rationalized in terms of the itinerant libration mean square torque and theoretical evaluations of the observable sub-THz Debye relaxation times. This is the first attempt at modelling the molecular dynamics of the various kinds of rotator phases over the complete range of frequency at which the kinematics and electrodynamics of the constituent molecules are observable, either as the complex dielectric permittivity or the optical power absorption coefficient. This methodology is named zero-THz spectroscopy. Used properly, it is one of the most incisive methods available for studies of molecular dynamics.     

Levy (stable) probability densities and mechanical relaxation in solid polymers

Early investigations by Weber, R. and F. Kohlrausch, Maxwell, and Boltzmann of relaxation in viscoelastic solids are reviewed. A two-state model stress-tensor describing strain coupling to internal conformations of a polymer chain is used to derive a linear response version of the Boltzmann superposition principle for shear stress relaxation. The relaxation function of Kohlrausch(t)= exp[–(t/ )] is identical to the Williams-Watts empirical dielectric relaxation function and in the model corresponds to the autocorrelation function of a segment's differential shape anisotropy tensor. By analogy with the dielectric problem, exp[–(t/ )] is interpreted as the survival probability of a frozen segment in a swarm of hopping defects with a stable waiting-time distributionAt ^– for defect motion. The exponent a is the fractal dimension of a hierarchical scaling set of defect hopping times. Integral transforms of(t) needed for data analysis are evaluated; the cosine and inverse-Laplace transforms are stable probability densities. The reciprocal kernel for short-time compliance is discussed.Reference 1 contains a thorough discussion of historical background.     

Exciton transfer between localized states in ZnO quantum well structures

The dynamics of photocreated excitons in a CdZnO/MgZnO quantum well (QW) was studied by comparing the experimental photoluminescence (PL) data with the results of Monte Carlo simulations of the exciton hopping. The temperature-dependent PL linewidth was found to be in reasonable agreement with the model of exciton hopping, with an additional inhomogeneous broadening (Γ) accounted for. The simulation analysis revealed fluctuations of the band potential to be 20 meV with an additional inhomogeneous broadening of , and a crossover from a non-thermalized to thermalized exciton energy distribution at about 100 K. In addition, a Bose–Einstein distribution like temperature dependence of the exciton energy in the wells was extracted using the data on the PL peak position.     

Study of dielectric and ac impedance properties of Ti doped Mn ferrites

We have reported dielectric and ac impedance properties of Ti doped Mn_1+xFe_2−2xO₄ (0x0.5) ferrites prepared by solid-state reaction method, using dielectric and impedance spectroscopy in the frequency range of 42 Hz–5 MHz, between the temperatures (300K–473K). The dielectric constant and dielectric loss (tan δ) decreases with increasing frequency but these parameters increase with increasing temperature. The dielectric loss tangent curves exhibit dielectric relaxation peaks at high frequencies (3.6 kHz–5 MHz), which are attributed to the coincidence of the frequency of charge hopping between the localized charge states and the external field. The dielectric properties have been explained on the basis of space charge polarization according to Maxwell–Wagner’s two-layer model and the hopping of charge between Fe²⁺ and Fe³⁺ as well as between Mn³⁺ and Mn²⁺ ions at B-sites. The complex impedance analysis has been used to separate grain and grain boundary in studied samples. Two semicircles corresponding to grain and grain boundary have been observed at low temperature, while only one semicircle has been seen at high temperatures. The resistance of grain and grain boundary both increase with Ti⁴⁺ doping.     

TheA 4 glass transition singularity

Formulae for the -relaxation process as obtained within the mode coupling theory for the dynamics of supercooled liquids are derived for states near anA ₄ glass transition singularity. A discussion of the expected anomalies of susceptibility spectra is presented. In particular the parameter regions for (1/f)-noise, for spectra exhibiting two minima and regions of linear variations in ln(1/f) are identified. The results are used to interprete quantitatively dielectric data for the following polymeric systems: polychlorotrifluoroethylene (PCTFE), polyhexamethylene sebacamide (nylon 610), polyoxymethylene (delrin) and polyparaethylene oxybenzoate (PEOB).     

Topological defect motifs in two-dimensional Coulomb clusters

We study the distribution of topological defects in two-dimensional Coulomb clusters with parabolic lateral confinement. The minima hopping algorithm based on molecular dynamics is used to efficiently locate the ground-?and low-energy metastable states, and their structure is analysed by means of the Delaunay triangulation. The size, structure and distribution of geometry-induced lattice imperfections strongly depends on the system size and the energetic state. Besides isolated disclinations and dislocations, classification of defect motifs includes defect compounds-grain boundaries, rosette defects, vacancies and interstitial particles. Proliferation of defects in metastable configurations destroys the orientational order of the Wigner lattice.     

Conductivity of a generalized hopping system with internal dynamics

We derive explicit exact expressions for the frequency dependent conductivity of a system whose dynamics is described by a Master Equation in which the transition rates _ab can be functions of other dynamical variables. We apply this method to study the hopping of particles in a lattice with the hopping rate modulated by an additional harmonic variable. The results of this model are related to the observed microwave conductivity of superionic conductors. In particular we show that the modulation of the hopping rate can produce a structure similar to the one observed in AgI type systems.     

Controlled delocalization of electronic states in a multi-strand quasiperiodic lattice

Finite strips, composed of a periodic stacking of infinite quasiperiodic Fibonacci chains, have been investigated in terms of their electronic properties. The system is described by a tight binding Hamiltonian. The eigenvalue spectrum of such a multi-strand quasiperiodic network is found to be sensitive on the mutual values of the intra-strand and inter-strand tunnel hoppings, whose distribution displays a unique three-subband self-similar pattern in a parameter subspace. In addition, it is observed that special numerical correlations between the nearest and the next-nearest neighbor hopping integrals can render a substantial part of the energy spectrum absolutely continuous. Extended, Bloch like functions populate the above continuous zones, signalling a complete delocalization of single particle states even in such a non-translationally invariant system, and more importantly, a phenomenon that can be engineered by tuning the relative strengths of the hopping parameters. A commutation relation between the potential and the hopping matrices enables us to work out the precise correlation which helps to engineer the extended eigenfunctions and determine the band positions at will.     

Dielectric relaxation and AC conduction of an AgTlSe2 semiconductor in the solid and liquid states

Measurements of the dielectric properties of AgTlSe₂ in the solid and liquid states were carried out in a wide range of frequencies and temperatures. The material displayed dielectric dispersion, and a loss peak was observed. Cole-Cole diagrams have been used to determine the distribution parameter (a) and the molecular relaxation time (). The process of dielectric relaxation (loss) and ac conduction was attributed to the correlated barrier hopping model suggested by Elliott for amorphous solids, where two carriers simultaneously hop over a barrier between charged defectD ⁺ andD ^– states.     

Magnetic interactions and electronic structure of $$\hbox {Pt}_{2}\hbox {Mn}_{1-x}\hbox {Y}_{x} \hbox {Ga (Y = Cr and Fe)}$$ system: An ab-initio calculation

Structural, optical, electrical conductivity and dielectric relaxation properties of bulk 4-amino-3-mercapto-6-(2-(2-thienyl)vinyl)-1,2,4-triazin-5(4H)-one donor (AMT) are studied. The structure of AMT in its powder form was analysed by X-ray diffraction (XRD), infrared spectroscopy (FT-IR) and atomic force microscopy (AFM). AC measurements (impedance, capacitance and phase angle) are done over the temperature range 303–373 K and in the frequency range from 42 Hz to 5 MHz. Analytical approaches for the experimental results of the σ _AC(ω, T) and the temperature behaviour of the frequency exponent show that the correlated barrier hopping (CBH) model is a good model to explain the AC electrical conductivity of bulk AMT organic semiconductor material. Application of the dielectric modulus formulism gives a simple method for evaluating the activation energy of the dielectric relaxation. The activation energy from the DC conductivity and the relaxation time are quite similar suggesting a hopping mechanism for AMT. The optical band gap of AMT is investigated using spectrophotometric measurement of transmittance at normal incidence of light in the wavelength range 300–1100 nm.     

Mean field kinetic theory of a classical electron gas in a periodic potential. III. The high-temperature limit in two dimensions

The Vlasov-like mean field kinetic equation for a classical electron plasma in the periodic field of an ionic lattice is solved in the high-temperature limit in two dimensions. The predictions for the one-electron density, the static structure factor, and the long-wavelength charge fluctuation spectrum are compared to the results of extensive molecular dynamics simulations. The predicted shift and damping of the plasma oscillation mode are in reasonable agreement with the simulation data at intermediate couplings (=e ²/k _B T1), but the agreement deteriorates as the temperature or the density is lowered, because mean field theory does not lead to the expected Kosterlitz-Thouless transition to the dielectric phase where electrons are localized.     

Higher Spin Conserved Currents in Sp(2M) Symmetric Spacetime

An infinite set of higher spin conserved charges is found for the sp(2M) symmetric dynamical systems in M(M+ 1)/2-dimensional generalized spacetime _M. Since the dynamics in _M is equivalent to the conformal dynamics of infinite towers of fields in d-dimensional Minkowski spacetime with d = 3, 4, 6, 10, ... for M = 2, 4, 8, 16, ..., respectively, the constructed currents in _M generate infinite towers of (mostly new) higher spin conformal currents in Minkowski spacetime. The charges have a form of integrals of M-forms which are bilinear in the field variables and are closed as a consequence of the field equations. Conservation implies independence of a value of charge of a local variation of a M-dimensional integration surface _M analogous to Cauchy surface in the usual spacetime. The scalar conserved charge provides an invariant bilinear form on the space of solutions of the field equations that gives rise to a positive-definite norm on the space of quantum states.     

Messung der komplexen Dielektrizitätskonstanten von Seignette-Salz bei 10 GHz in Abhängigkeit von der Temperatur

We determined the complex dielectric constant of Rochelle Salt at 9.61·10⁹ Hz and its temperature dependence using a microwave interferometer. In thex-direction of the crystal which becomes its ferroelectric axis between ?18°C and +24°C, the real part of the dielectric constant shows distinct minima at the transition points according to our measurements. We find agreement with results ofJäckle but discrepancies to older measurements. Neither the humidity of the surrounding atmosphere nor the position of the sample in the growing crystal proved to have an effect. In they- andz-directions, we observed no anomaly of the dielectric constant.     

Simulation calculation of dielectric constants: Comparison of methods on an exactly solvable model

A mean spherical model of classical dipoles on a simple cubic lattice of sideM=2N+1 sites is considered. Exact results are obtained for finite systems using periodic boundary conditions with an external dielectric constant and using reaction field boundary conditions with a cutoff radiusR _c N and an external dielectric constant. The dielectric constant in the disordered phase is calculated using a variety of fluctuation formulas commonly implemented in Monte Carlo and molecular dynamics simulations of dipolar systems. The coupling in the system is measured by the parametery=4 ²/9kT, where ² is the fixed mean square value of the dipole moments on the lattice. The system undergoes a phase transition aty2.8, so that very high dielectric constants cannot be obtained in the disordered phase. The results show clearly the effects of system size, cutoff radius, external dielectric constant, and different measuring techniques on a dielectric constant estimate. It is concluded that with periodic boundary conditions, the rate of approach of the dielectric constant estimate to its thermodynamic limit is asN ^–2/3 and depends only weakly on. Methods of implementing reaction field boundary conditions to give rapid convergence to the thermodynamic limit are discussed.     

Distribution for fermionic discrete lattice gas within the canonical ensemble

The distinct deviations from the Fermi-Dirac statistics ascertained recently at low temperatures for a one-dimensional, spinless fermionic discrete lattice gas with conserved number of noninteracting particles hopping on the non-degenerated, well-separated single-particle energy levels are studied in numerical and theoretical terms. The generalized distribution is derived in the formn(h)={Y _h exp [(_h–)]+1{su–1 valid even in the thermodynamic limit, when the discreteness of the energy levels is kept. This distribution demonstrates good agreement with the data obtained numerically both by the canonical partition function technique and by Monte Carlo simulation.     

Hubbard-like models in high spatial dimensions

The present paper studies the properties of Hubbard-like models in high spatial dimensionsD. In a first par the limit of infinite dimension and its main features-i.e.i) the mapping onto a generalized atomic model with an additional auxiliary field andii) the validity of the local approximation for the self-energy-are worked out in a systematic (1/D)-expansion. Since the hopping matrix elements have to be properly scaled with the dimensionD, the (1/D)-expansion is also an expansion in the hopping amplitude. Thus for small hopping theD-limit may serve as a proper approximation for finite-dimensional systems. The second part of the paper adopts the hybridisation-perturbation theory of the single impurity Anderson model in order to construct a perturbation theory for the auxiliary field of the generalized atom which can also be interpreted as an expansion in the hopping amplitude. The non-crossing approximation (NCA) is used to study the antiferromagnetic phase transtion of theD-Hubbard model in the case of half filling: the critical temperature, the antiferromagnetic order parameter and the free energy of the lattice system are calculated. The NCA-results are in quite good agreement with recent results from the imaginary-time discretisation method.