Uniaxial stress dependence of the dielectric properties of barium titanate single crystals

BaTiO₃ single crystals were grown by the melt-grown method. The effect of uniaxial pressure (0–1700 bar) on the dielectric properties of these crystals has been systematically studied. The external stress showed obvious effects on these properties. An increase in the difference between the Curie T_c and Curie–Weiss T₀ temperatures induced by the applied pressure is observed. This could be ascribed to the inducing of non-ferroelectric cubic islands in the tetragonal phase by the applied compressive stress. On the other hand, the pressure behavior of thermal hysteresis and the ??/?T vs. T plot strongly suggests that the phase transition changes to second-order type with increasing pressure. The Curie–Weiss constant obtained from a modified Curie–Weiss law strongly decreases with increasing pressure, suggesting that the mechanism of phase transition is going to order–disorder type. An increase in the difference between the Curie T_c and Burn's T_B temperatures with increasing pressure is observed. This could be ascribed to the narrowing of the temperature range on which the Curie–Weiss law is valid. In general, the obtained results are in good agreement with hydrostatic pressure data. Some kind of relaxation near T_c, which is strongly coupled with strain caused by applied compressive stress, is postulated.     

Originalarbeiten / Travaux originaux / Original Papers

An account is given of a variational calculation to estimate the amplitudes of resonance factorsɛ in a recent theory to describe the enhancement of crystalline field potentials by conduction electrons in heavy rare earth metals. It is demonstrated that the values ofɛ obtained by minimising the energy of interaction between conduction electrons and rare earth ions are consistent with those previously used to form a comparison with experiment. These latter values were obtained by maximising theA ₂ ⁰ crystal field coefficient with respect toɛ. Consistency is exhibited in both the sign and order of magnitude of the resonance amplitudes and renders the theory parameterless. The values ofɛ show an approximate linear dependence with the number of electrons in the incompletef shell of the rare earth ions.     

On the area of square lattice polygons

We consider the generating function of self-avoiding square lattice polygons grouped by both area and perimeter. The generating function for polygons of arean is found to diverge atx _c =0.251834, with an exponent of zero. The mean perimeter of polygons with arean is found to be proportional ton, while the mean area of polygons with perimetern is found to be proportional ton ^1.5.     

Magnetic tunneling told to ignorants by two ignorants

Magnetic tunneling of a large spin subject to the Hamiltonian is investigated by elementary methods for weak fields H. In zero field (H=0) the tunnel frequency in the ground state is found to be equal to [0pt] multiplied by a quantity whose variation with s is slower than exponential. This result coincides with that of path integral methods [#!schi!#]. For the values of the longitudinal field which allow tunneling, the tunnel frequency is shown to vanish when H _y =0 for certain “diabolic" values of [0pt] , in qualitative agreement with experiments by Wernsdorfer and Sessoli. The particular case H _z =0 was already obtained by Garg by means of path integrals. The diabolic values of are in agreement with numerical results but, as already noticed by Wernsdorfer and Sessoli, they disagree with the experimental ones. This may be an effect of higher order anisotropy terms, which is briefly addressed in the conclusion. Received 17 May 2000     

A quasi analytical method for the calculation of virial coefficients for realistic intermolecular potentials

The difficulties of accurately representing the Ursell function, f, for molecules interacting with a Lennard-Jones potential by a hard sphere core and a truncated exponential series are discussed. The range of R for which a hard sphere approximates f with negligible error is established and the range of arguments in f is determined. The accuracy with which the exponential series, S_k , truncated after k terms, represents the function is established for several ranges of arguments. It is shown that it is possible to represent f accurately by a hard sphere core and S ₄ or S ₆ for all values of R except over a short range of R.

Values of B and C, accurate to within a few per cent, can be obtained analytically through approximating f by S ₄ or S ₆ and a hard core whose diameter is determined by the range of arguments for which the series is valid. More accurate values of B and C required additional approximations for f. Two of these, the repulsive square well and the Barker-Henderson radius, give accurate values of B and C at all temperatures that were examined.     

Signals of <Emphasis Type="Italic">Z</Emphasis>′ boson in the Bhabha process within the LEP2 data set

The LEP2 data set on the Bhabha process is analyzed with the aim to detect the signals of the heavy virtual Z′ gauge boson. The state interacting with the left-handed Standard Model doublets and called the chiral Z′ is investigated. This particle was introduced already as the low-energy state allowed by the renormalizability. The contribution of the chiral-Z′ state to the Bhabha process is described by two parameters: the coupling to electrons and the Z–Z′ mixing angle. The sign-definite one-parameter observable is proposed to measure the Z′ coupling to the electron current. The one-parameter fit of the data shows no signals of the particle. The alternative two-parameter fit of the differential cross sections is also performed. It also shows no chiral-Z′ signals. The comparisons with other fits are discussed. The text was submitted by the authors in English.     

Non-Abelian magnetic monopoles

It is shown that a general, irreducible, SU(n), Sp(n), SO(2n) monopole with maximal symmetry breaking is determined by its spectral data. For SU(n) with minimal symmetry breaking the spectral data is defined and also shown to determine the monopole.Research supported in part by NSF Grant 8120790     

On ergodic properties for harmonic crystals

We consider the dynamics of a harmonic crystal in n dimensions with d components, where d and n are arbitrary, d, n ⩾ 1. The initial data are given by a random function with finite mean energy density which also satisfies a Rosenblatt-or Ibragimov-type mixing condition. The random function is close to diverse space-homogeneous processes as x _n → ±∞, with the distributions μ±. We prove that the phase flow is mixing with respect to the limit measure of statistical solutions. Partially supported by RFBR under grant no. 06-01-00096.     

Asymmetry in the nuclear magnetic shielding tensor

The applicability of Green's function (GF) and Feynman path-integral quantum Monte Carlo (QMC) methods for the simulation of cyclic networks with (4n + 2) and 4n (n = 1, 2, 3, …) electrons is analysed. Both QMC techniques are employed in simulations on the basis of the simple Hückel Hamiltonian which is exclusively defined by nearest-neighbour hopping elements. In addition we have used the Pariser-Parr-Pople (PPP) Hamiltonian to perform GF QMC simulations. The electronic energies E derived by the QMC methods are compared either with Hückel molecular orbital (HMO) results or exact configuration interaction data where (π) electronic correlations are fully taken into account. A sign problem occurs in QMC simulations of 4n annulenes. This leads to an error in the total energy in the standard formulations of the employed QMC techniques, which is enhanced with decreasing ring size. A simple modification in the QMC formalisms is suggested to avoid the numerical uncertainties caused by the sign problem in 4n annulenes. Renormalization of the kinetic hopping integrals t by t cos (π/M) with M abbreviating the number of atomic sites leads to ground state energies as well as any other quantity close to the values derived by conventional diagonalization techniques. Substitution of t against t cos (π/M) conserves a common sign of all matrix elements containing the hopping. The occurrence of negative probabilities, which lead to numerical problems in the QMC simulations, is thereby prevented. The transformation suggested in 4n rings has a formal connection to so-called Möbius rings.     

Non-polar solute-water pair correlation functions

The microscopic theory of the solvation of non-polar solutes in water proposed by Pratt and Chandler has been generalized in order to obtain separate solute-oxygen and solute-hydrogen radial distribution functions, g(r). The g(r)s predicted by this method for a spherical solute have been tested by comparison with the corresponding functions from two computer simulation studies. The water-water interactions were described by the configuration intereaction potential (CI) in both cases. The agreement between theoretical and simulation results is good for the solute-oxygen g(r), less so for the solute-hydrogen function. Moreover, the influence of the model of water on the calculated solute-solvent g(r)s has been examined by comparing results obtained with partial structure functions derived from the CI model and from recent neutron diffraction measurements. It is found that CI model and real water yield remarkably different solute-water radial distribution functions. Finally, the solvation of a model two-site solute has been studied for various bond lengths and the results confirm that when the space between the sites is sufficient to host a water molecule, the solvation of each site is the same as that of an isolated site, with respect to oxygen as well as hydrogen.     

Critical exponents of a multicomponent anisotropict−J model in one dimension

A recently presented anisotropic generalization of the multicomponent supersymmetrict–J model in one dimension is investigated. This model of fermions with general spin-S is solved by Bethe ansatz for the ground state and the low-lying excitations. Due to the anisotropy of the interaction the model possesses 2S massive modes and one single gapless excitation. The physical properties indicate the existence of Cooper-type multiplets of 2S+1 fermions with finite binding energy. The critical behaviour is described by ac=1 conformal field theory with continuously varying exponents depending on the particle density. There are two distinct regimes of the phase diagram with dominating density-density and multiplet-multiplet correlations, respectively. The effective mass of the charge carriers is calculated. In comparison to the limit of isotropic interactions the mass is strongly enhanced in general.Work performed within the research program of the Sonderforschungsbereich 341, Köln-Aachen-Jülich     

A Novel Reconfiguration CPW Leaky-Wave Antenna for Millimeter-Wave Application

A novel reconfigurable millimeter-wave leaky-wave antenna is proposed according to the radiation characteristics of periodically loaded coplanar waveguide and simulated by the finite-difference time-domain method. The simulated results demonstrate that the single main beam of this antenna operating at 35.0GHz can scan with small steps in the E-plane from –90° to 90° except the angle around 0°. The main beam scanning is obtained by the antenna structure reconfiguration with nine discrete states controlled by embedded switches and two feed ports. The antenna is useful for millimeter-wave applications.     

Dynamical and structural symmetries for the highest Landau levels on the <Emphasis Type="Italic">AdS</Emphasis><Subscript>2</Subscript>

The aim of the paper is to use the recurrence relations with respect to both indices of the associated Legendre functions for the extraction of the Dirac quantization condition and dynamical symmetry group U(1, 1) corresponding to the highest Landau levels on the hyperbolic plane with uniform magnetic field B. Irreducible representations of the su(2) algebra are obtained by the ladder differential operators which change B by 1/2 unit and mode number by one unit. Two different classes of the irreducible representations of SU(1, 1) with the even and odd boson numbers 2B − 1/2 are extracted for the Bargmann indices 1/4 and 3/4, respectively. Finally, we show that shape invariance symmetry is realized by the ladder operators which shift only the magnetic field B by 1/2 unit.     

Effect of temperature-dependent surface heat transfer coefficient on the maximum surface stress in ceramics during quenching

We study the difference in the maximum stress on a cylinder surface σ_max using the measured surface heat transfer coefficient h_m instead of its average value h_a during quenching. In the quenching temperatures of 200, 300, 400, 500, 600 and 800°C, the maximum surface stress σ_mmax calculated by h_m is always smaller than σ_amax calculated by h_a, except in the case of 800°C; while the time to reach σ_max calculated by h_m (f_mmax) is always earlier than that by h_a (f_amax). It is inconsistent with the traditional view that σ_max increases with increasing Biot number and the time to reach σ_max decreases with increasing Biot number. Other temperature-dependent properties also have a small effect on the trend of their mutual ratios with quenching temperatures. Such a difference between the two maximum surface stresses is caused by the dramatic variation of h_m with temperature, which needs to be considered in engineering analysis.     

Raman and SERS study on cimetidine–metal complexes with biomedical interest

Cimetidine (cim) is one of the most potent histamine H₂‐receptor antagonists for inhibiting excessive acid secretion caused by histamine; it has been hypothesized that the therapeutic effects can be related to its interactions with metal ions. Raman spectra of the solid cim with Co(II), Cu(II), Ni(II) and Zn(II) metal complexes show that they can adopt two different structures: one is octahedral and the other, with Zn(II), is probably tetrahedral. The octahedral structure appears to be distorted both by the different metal ions as well as by the different anion present. The study was extended to very dilute solutions (ppm range) by using the surface‐enhanced Raman scattering (SERS) technique, mimicking the physiological concentrations of cim and its metal complexes. SERS spectra suggest that, upon the binding of cim to silver colloids, the formation of stable 1:2 cim–metal complexes is excluded, the formation of 1:1 adduct appearing more probable; in this product the metal reaches its total coordination shell by complexion with water molecules. To better explain the binding mechanism of cim to a metal (Ag) surface, we performed theoretical B3LYP calculations on cim alone as well as on cim bonded to an Ag₂ metal cluster in presence of water, observing a sufficiently good agreement between experimental and theoretical wavenumbers. Copyright © 2010 John Wiley & Sons, Ltd.     

Measuring the optical parameters of thin films by p-polarized laser beams

The refractive index n_f, extinction coefficient k_f and thickness d_f of a dip coated film are measured by using p-polarized laser beams. A sample is oblique illuminated with a p-polarized laser beam, and then two reflected beams, from the front and back surfaces, are received with a detector. After measuring their intensity ratio versus the angle of incidence, it is convenient to obtain the parameters of the film by means of data fitting. The films of polymethyltriethoxy silane (PMTES), which were made on a BK-7 glass substrate by dip coating, were measured. The method is non-contact, non-destructive and has the advantages of simplicity of both equipment and understanding. It is also shown that the values measured by this method are coincident with those measured by ellipsometry.     

Counting operator analysis of the discrete spectrum of some model Hamiltonians

The first step in the counting operator analysis of the spectrum of any model Hamiltonian H is the choice of a Hermitean operator M in such a way that the third commutator with H is proportional to the first commutator. Next one calculates operators R and R^† which share some of the properties of creation and annihilation operators, and are such that M becomes a counting operator. The spectrum of H is then decomposed into multiplets, not determined by symmetries of H, but by those of a reference Hamiltonian H_ref, which is defined by H_ref=H−R−R^†, and which commutes with M. Finally, we introduce the notion of stable eigenstates. It is shown that under rather weak conditions one stable eigenstate can be used to construct another one.     

Measurement of object height in emission electron microscopy

The possibility of measuring the height of an object in emission electron microscopy (EEM) is investigated. If the specimen is characterized by an equipotential surface with the relief h(x,y), the image is equivalent to a specimen with an ideal flat surface and a corresponding distribution of the electric potential φ(x,y)=−E _ext h(x,y). As a consequence of the interaction with the microfields grad φ(x,y), the trajectories of electrons forming the image become deformed, which leads to characteristic image distortion. From EEM, images obtained at different voltages of the extractor V _ext,φ(x,y) can be derived and thereby h(x,y) is reconstructed. If the surface of the specimen is characterized both by a distribution of the potential and in addition by a relief h(x,y), then for the reconstruction one needs an additional EEM image taken at a different voltage of the extractor. The maximal sensitivity to microfields/relief is exploited when using the electron microscope in the mirror operation mode. We illustrate the performance of the method by means of a test pattern of Au on Si. For quantitative comparison, the same structure was investigated by atomic force microscope.     

Transverse energy measurement in Au+Au collisions by the STAR experiment

Transverse energy (E _T ) has been measured with both of its components, namely hadronic (E _T ^had ) and electromagnetic (E _T ^em ) in a common phase space at mid-rapidity for 62.4 GeV Au+Au collisions by the STAR experiment. E _T production with centrality and ?{s_NN }\sqrt {s_{NN} } is studied with similar measurements from SPS to RHIC and is compared with a final state gluon saturation model (EKRT). The most striking feature is the observation of a nearly constant value of E _T /N _ch ∼ 0.8 GeV from AGS, SPS to RHIC. The initial energy density estimated by the boost-invariant Bjorken hydrodynamic model, is well above the critical density for a deconfined matter of quarks and gluons predicted by lattice QCD calculations.