Thermal conductivity of doped $\mathsf{La_2CuO_4}$ as an example for heat transport by optical phonons in complex materials

We investigate the phonon thermal conductivity of doped based on out-of-plane thermal conductivity measurements. When room temperature is approached the temperature dependence of strongly deviates from the T^-1-decrease which is usually expected for heat transport by acoustic phonons. Instead, decreases much weaker or even increases with rising temperature. Simple arguments suggest that such unusual temperature dependencies of are caused by heat transport via dispersive optical phonons.Received: 20 October 2003, Published online: 20 April 2004PACS: 66.70. + f Nonelectronic thermal conduction and heat-pulse propagation in solids; thermal waves - 74.72.Dn La-based cuprates - 44.10. + i Heat conduction     

Mo6S6 nanowires: structural, mechanical and electronic properties

The properties of nanowires were investigated with ab initio calculations based on the density-functional theory. The molecules build weakly coupled one-dimensional chains, like and Mo₆S_9-xI_x, and the crystals are strongly uniaxial in their mechanical and electronic properties. The calculated moduli of elasticity and resilience along the chain axis are c₁₁ = 320 GPa and E_R = 0.53 GPa, respectively. The electronic band structure and optical conductivity indicate that the crystals are good quasi-one-dimensional conductors. The frequency-dependent complex dielectric tensor ε, calculated in the random-phase approximation, shows a strong Drude peak in ε_∥, i.e., for the electric field polarised parallel to the chain axis, and several peaks related to interband transitions. The electron energy loss spectrum is weakly anisotropic and has a strong peak at the plasma frequency ħω_p ≈20 eV. The stability analysis shows that is metastable against the formation of the layered .     

On the Meissner effect in a superconductor with 4-fermion attraction

A presence of a Meissner-Ochsenfeld effect in a gas of spin 1/2 fermions with an interaction , where is a volume of a region in real space which is taken by thesystem and with , satisfying Fermi anticommutation relations, is investigated. The effect proves to be weaker than in BCS by a factor 3/4 at T = 0, implying a greater penetration depth of external magnetic field. V ₄ is nonzero only within a thin layer of 1-fermion energies around the chemical potential .Received: 14 June 2004, Published online: 12 October 2004PACS: 74.20.-z Theories and models of superconducting state - 74.20.Fg BCS theory and its development     

Scaling behavior of jamming fluctuations upon random sequential adsorption

It is shown that the fluctuations of the jamming coverage upon Random Sequential Adsorption (RSA) ( ), decay with the lattice size according to the power-law , with ,where D is the dimension of the substrate and is the fractal dimension of the set of sites belonging to the substrate where the RSA process actually takes place. This result is in excellent agreement with the figure recently reported by Vandewalle et al. [Eur. Phys. J. B 14, 407 (2000)], namely for the RSA of needles with D = 2 and , that gives . Furthermore, our prediction is in excellent agreement with different previous numerical results. The derived relationships are also confirmed by means of extensive numerical simulations applied to the RSA of dimers on both stochastic and deterministic fractal substrates.Received: 17 October 2003, Published online: 8 December 2003PACS: 05.40.-a Fluctuation phenomena, random processes, noise, and Brownian motion - 02.50.-r Probability theory, stochastic processes, and statistics     

Structural and magnetic properties of \lbrackEr|Tb\rbrackmultilayers

We have investigated the structural and magnetic properties of Er|Tb multilayers by different scattering methods. Diffuse X-ray scattering under grazing incidence reveals the interface structure in Er|Tb bilayers and trilayers, indicating vertically correlated roughness between the Er and Tb interfaces. The magnetic properties of Er_nEr|Tb_nTb superlattices have been studied as a function of the superlattice composition (indices denote the number of atomic layers). Coupled ferromagnetic structures exist in all investigated samples. The phase transition temperature varies with the Tb layer thickness. Modulated magnetic order is short range for all samples beside the Er₂₀|Tb₅ superlattice, the sample with the smallest Tb layer thickness. We observe dipolar antiferromagnetic coupling between single ferromagnetic Tb layers in all samples, with the onset of this ordering depending on the Tb layer thickness. Due to competing interactions, exchange coupling is limited to the interface near region. Therefore long range modulated magnetic order is observed in the Er₂₀|Tb₅ superlattice only, where the interface regions overlap. The distinct differences to the magnetic structure of an Er_0.8Tb_0.2 alloy film are explained by a highly anisotropic arrangement of neighbouring atoms due to the correlated roughness.     

On the scaling of probability density functions with apparent power-law exponents less than unity

We derive general properties of the finite-size scaling of probability density functions and show that when the apparent exponent of a probability density is less than 1, the associated finite-size scaling ansatz has a scaling exponent τ equal to 1, provided that the fraction of events in the universal scaling part of the probability density function is non-vanishing in the thermodynamic limit. We find the general result that τ≥1 and . Moreover, we show that if the scaling function approaches a non-zero constant for small arguments, , then . However, if the scaling function vanishes for small arguments, , then τ= 1, again assuming a non-vanishing fraction of universal events. Finally, we apply the formalism developed to examples from the literature, including some where misunderstandings of the theory of scaling have led to erroneous conclusions.     

Omnidirectional zero-\bar {\mathsf n} gap in symmetrical Fibonacci sequences composed of positive and negative refractive index materials

The band structures of symmetrical Fibonacci sequences (SFS) composed of positive and negative refractive index materials are studied with a transfer matrix method. A new type of omnidirectional zero- gaps is found in the SFS. In contrast to the Bragg gaps, such an omnidirectional zero- gap is insensitive to the incident angles and polarization, and is invariant upon the change of the ratio of the thicknesses of two media. It is found that omnidirectional zero- gap exists in all the SFS, and it is rather stable and independence of the structure sequence.     

Aging of the field-induced asymmetry in a disordered ferroelectric

The isothermal aging of the asymmetry induced in the disordered dielectric crystal (x=0.027) submitted to the biasing electric field E, is investigated. To this end, the response of the complex dielectric constant to infinitesimal field changes , applied to the sample after a variable aging delay, has been measured for different magnitudes of E and after different aging delays. Two different experimental procedures have been used: in both cases the response strongly depends on the time spent under field. For short aging delays, the response has a strong contribution proportional to δE and a weak quadratic contribution proportional to . As time elapses, the linear and the quadratic contributions age in opposite ways: the former decreases whereas the latter increases. This paradoxical behaviour is analyzed in the framework of a model which attributes aging and the related effects (rejuvenation, memory) to the evolution of polarization domain walls: the decrease of the linear contribution is related to the decrease of the total wall area, while the increase of the quadratic term is attributed to wall reconformations.     

Structural,electronic, and optical properties of beryllium monochalcogenides

The results of first-principles theoretical study of the structural, electronic and optical properties of beryllium monochalcogenides BeTe, BeSe and BeS, performed using the full potential linearized augmented plane wave (FP-LAPW) method are presented. The calculated structural parameters and band gaps compare very well with previous theoretical results. The trends of the band gap pressure coefficients and volume deformation potentials for these II-VI compounds are investigated. The linear pressure coefficients for the X and band gaps increase with decrease in anion atomic weight. The dependence of the direct and indirect band gaps on the relative change of lattice constant are found to follow almost the same type of trends in each of these compounds. The volume deformation potential ( ) for the direct ( ) and indirect ( ) gaps are positive, but negative for the indirect ( ) gap. Furthermore, , for transitions decreases with increase in anion atomic number whereas , increases. The optical properties have also been calculated. From the reflectivity spectra, the compounds will be useful for optical applications. The variation of the band gaps with respect to the application of pressure and the origin of some of the peaks in the optical spectra are discussed in terms the calculated electronic structure.Received: 26 September 2003, Published online: 18 June 2004PACS: 71.15.Ap Basis sets (LCAO, plane-wave, APW, etc.) and related methodology (scattering methods, ASA, linearized methods, etc.) - 71.15.Mb Density functional theory, local density approximation, gradient and other corrections - 71.20.Nr Semiconductor compounds     

Surface critical behavior of fluids: Lennard-Jones fluid near a weakly attractive substrate

The phase behavior of fluids near weakly attractive substrates is studied by computer simulations of the coexistence curve of a Lennard-Jones (LJ) fluid confined in a slitlike pore. The temperature dependence of the density profiles of the LJ fluid was used to study the surface critical behavior. A universal critical behavior of the local order parameter, defined as the difference between the local densities of the coexisting liquid and vapor phases at some distance from the pore walls, , is observed in a wide temperature range and found to be consistent with the surface critical behavior of the Ising model. Near the surface the dependence of the order parameter on the reduced temperature obeys a scaling law ~1 with a critical exponent 1 of about 0.8, corresponding to the surface transition. A crossover from bulk-like to surface-like critical behavior occurs, when the distance to the surface is about twice the correlation length at the given temperature. Relations between the and transitions in Ising systems and the surface critical behavior of fluids are discussed.     

Charge orderings in the atomic limit of the extended Hubbard model

The extended Hubbard model in the atomic limit (AL-EHM) on a square lattice with periodic boundary conditions is studied with use of the Monte Carlo (MC) method. Within the grand canonical ensemble the phase and order-order boundaries for charge orderings are obtained. The phase diagrams include three types of charge ordered phases and the nonordered phase. The system exhibits very rich structure and shows unusual multicritical behavior. In the limiting case of t_ij=0, the EHM is equivalent to the pseudospin model with single-ion anisotropy , exchange interaction W in an effective magnetic field . This classical spin model is analyzed using the MC method for the canonical ensemble. The phase diagram is compared with the known results for the Blume-Capel model.     

Scale dependent partitioning of one-dimensional aperiodic set diffraction

We give a multiresolution partition of pure point parts of diffraction patterns of one-dimensional aperiodic sets. When an aperiodic set is related to the Golden Ratio, denoted by , it is well known that the pure point part of its diffractive measure is supported by the extension ring of , denoted by . The partition we give is based on the formalism of the so called -integers, denoted by . The set of -integers is a selfsimilar set obeying , . The pure point spectrum is then partitioned with respect to this Russian doll like sequence of subsets . Thus we deduce the partition of the pure point part of the diffractive measure of aperiodic sets.Received: 24 March 2004, Published online: 12 July 2004PACS: 61.44.Br Quasicrystals - 61.10.Dp Theories of diffraction and scattering     

The optimal velocity traffic flow models with open boundaries

The effects of the open boundaries on the dynamical behavior of the optimal velocity traffic flow models with a delay time allowing the car to reach its optimal velocity is studied using numerical simulations. The particles could enter the chain with a given injecting rate probability , and could leave the system with a given extracting rate probability . In the absence of the variation of the delay time , it is found that the transition from unstable to metastable and from metastable to stable state occur under the effect of the probabilities rates and . However, for a fixed value of , there exist a critical value of the extraction rate above which the wave density disappears and the metastable state appears and a critical value above which the metastable state disappears while the stable state appears. and depend on the values of and the variation of the delay time . Indeed and increase when increasing and/or decreasing . The flow of vehicles is calculated as a function of , and for a fixed value of . Phase diagrams in the ( ) plane exhibits four different phases namely, unstable, metastable, stable. The transition line between stable phase and the unstable one is curved and it is of first order type. While the transition between stable (unstable) phase and the metastable phase are of second order type. The region of the metastable phase shrinks with increasing the variation of the delay time and disappears completely above a critical value .Received: 23 July 2003, Published online: 8 December 2003PACS: 05.50. + q Lattice theory and statistics (Ising, Potts, etc.) - 64.60.Cn Order-disorder transformations; statistical mechanics of model systems - 75.30.Kz Magnetic phase boundaries (including magnetic transitions, metamagnetism, etc.) - 82.20.Wt Computational modeling; simulation     

Hyperfine quenching of the 4s4p <Superscript>3</Superscript>P<Subscript>0</Subscript> level in Zn-like ions

In this paper, we used the multiconfiguration Dirac-Fock method to compute with high precision the influence of the hyperfine interaction on the [Ar] P₀ level lifetime in Zn-like ions for stable and some quasi-stable isotopes of nonzero nuclear spin between Z=30 and Z=92. The influence of this interaction on the [Ar] P P₀ separation energy is also calculated for the same ions.     

Third harmonics of the AC magnetic susceptibility: a method for the study of flux dynamics in high temperature superconductors

The temperature dependence of the 1st and 3rd harmonics ( ) of the AC magnetic susceptibility has been measured on melt grown YBCO samples for different frequencies and amplitudes of the AC magnetic field and intensity of a contemporaneously applied DC field. With the help of critical state models and of numerical simulations [22], we have devised a novel method, based on the combined analysis of the 1st and the 3rd harmonics (specifically on the comparison between and ), that allows to distinguish different temperature ranges dominated by the different dissipative magnetic flux regimes. In particular, we identified three principal zones in the temperature dependence of the real part of the 3rd harmonic: the zone 1, in the temperature range below the peak of the imaginary part of the 1st harmonic, , and the zone 2, characterized by negative values in a temperature region just above , both dominated by the creep regime; the zone 3, just below T _c , in which we revealed the presence of Thermally Assisted Flux Flow (TAFF). By the identification of these zones, an estimation of the value of the pinning potential can be obtained.Received: 23 September 2003, Published online: 19 November 2003PACS: 74.25.Ha Magnetic properties - 74.60.Ge Flux pinning, flux creep, and flux-line lattice dynamics     

Singular behavior of the Emery model with O-O hopping for high-T$\mathsf{_c}$ superconductors

Mean field slave boson (MFSB) solution is found analytically for the Emery model of the high-T_c cuprates with infinite interaction U_d on the Cu-site and finite O-O hopping t', in addition to the Cu-O hopping t₀. The solution is found for arbitrary Cu-O charge transfer energy and small doping , assuming only t' < 0 for the insulating phase, appropriate for the superconducting cuprates. It is shown analytically that the Brinkmann-Rice metal-insulator transition is preserved, though shifted to higher values of . To the leading order in t', the transition conserves its t'=0 triple point nature with complete electron-hole symmetry with respect to doping. This symmetry is broken by logarithmically small terms related to the t'-induced shift of the chemical potential of the half-filled band from the van Hove singularity. Qualitative change in the behavior of the effective charge transfer energy is found at finite t' for large . The prerequisite to MFSB solution is the discussion of the singularities in the 3-band dispersion of the Emery model. The range of parameters leading to the touching of two bands is determined analytically, showing that it is wider than known before. The anticrossing (rather than touching) is however consistent with the observed Fermi surfaces, leading to the MFSB theory with t' < 0. The detailed comparison of the theory with the angle-resolved photoemission spectroscopy data in La Sr CuO₄, Bi₂Sr₂CaCuO and YBa₂Cu₃O is finally given, using the properties of the resonant band of the physical fermions, discussed in the companion paper. Fast evolution of the band-structure with doping is explained for LSCO. The three-band fits for Bi2212 and Y123 are equally good, although the Luttinger sum rule is not obeyed there.Received: 30 May 2002, Published online: 23 July 2003PACS:   71.18.+y Fermi surface: calculations and measurements; effective mass, g factor - 74.72.Dn La-based cuprates - 74.25.Jb Electronic structure - 79.60.-i Photoemission and photoelectron spectra     

The effect of mixture lengths of vehicles on the traffic flow behaviour in one-dimensional cellular automaton

The effect of mixture lengths of vehicles on the asymmetric exclusion model is studied using numerical simulations for both open and periodic boundaries in deterministic parallel dynamics. The vehicles are filed according to their length, the small cars type 1 occupy one cell whereas the big ones type 2 takes two. In the case of open boundaries two cases are presented. The first case corresponds to a chain with two entries where densities are calculated as a function of the injecting rates and of vehicles type 1 and type 2 respectively, and the phase diagram ( ) is presented for a fixed value of the extracting rate . In this situation the first order transition from low to high density phases occurs at and disappears for . The second case corresponds to a chain with one entry, where is the injecting rate of vehicles independent of their nature. Type 2 are injected with the conditional probability , where and n is the concentration of type 2. Densities are calculated as a function of the injecting rates , and the phase diagrams ( , ) are established for different values of n. In this situation the gap which is a characteristic of the first order transition vanishes with increasing for . However, the first order transition between high and low densities exhibit an end point above which the global density undergoes a continuous passage. The end point coordinate depends strongly on the value of n. In the periodic boundaries case, the presence of vehicles type 2 in the chain leads to a modification in the fundamental diagram (current, density). Indeed, the maximal current value decreases with increasing the concentration of vehicles type 2, and occurs at higher values of the global density.Received: 13 May 2004, Published online: 3 August 2004PACS: 05.50. + q Lattice theory and statistics (Ising, Potts, etc.) - 64.60.Cn Order-disorder transformations; statistical mechanics of model systems - 75.30.Kz Magnetic phase boundaries (including magnetic transitions, metamagnetism, etc.) - 82.20.Wt Computational modeling; simulation     

Strongest gravitational waves from neutrino oscillationsat supernova core bounce

Resonant active-to-active ( ), as well as active-to-sterile ( ) neutrino ( ) oscillations can take place during the core bounce of a supernova collapse. Besides, over this phase, weak magnetism increases the antineutrino ( ) mean free path, and thus its luminosity. Because the oscillation feeds mass-energy into the target species, the large mass-squared difference between the species ( ) implies a huge amount of energy to be given off as gravitational waves ( erg s^-1), due to anisotropic but coherent flow over the oscillation length. This asymmetric -flux is driven by both the spin-magnetic and the universal spin-rotation coupling. The novel contribution of this paper stems from (1) the new computation of the anisotropy parameter -0.01, and (2) the use of the tight constraints from neutrino experiments as SNO and KamLAND, and the cosmic probe WMAP, to compute the gravitational-wave emission during neutrino oscillations in supernovae core collapse and bounce. We show that the mass of the sterile neutrino that can be resonantly produced during the flavor conversions makes it a good candidate for dark matter as suggested by Fuller et al. , Phys. Rev. D 68, 103002 (2003). The new spacetime strain thus estimated is still several orders of magnitude larger than those from diffusion (convection and cooling) or quadrupole moments of neutron star matter. This new feature turns these bursts into the more promising supernova gravitational-wave signals that may be detected by observatories as LIGO, VIRGO, etc., for distances far out to the VIRGO cluster of galaxies.Received: 26 November 2003, Revised: 26 February 2004, Published online: 3 June 2004     

Search for chargino and neutralino productionat $\sqrt{s} = 192$-209 GeV at LEP

Approximately 438 pb^-1 of e ⁺ e^- data from the OPAL detector, taken with the LEP collider running at centre-of-mass energies of 192-209 GeV, are analyzed to search for evidence of chargino pair production, , or neutralino associated production, . Limits are set at the 95% confidence level on the product of the cross-section for the process and its branching ratios to topologies containing jets and missing energy, or jets with a lepton and missing energy, and on the product of the cross-section for and its branching ratio to jets. R-parity conservation is assumed throughout this paper. When these results are interpreted in the context of the Constrained Minimal Supersymmetric Standard Model, limits are also set on the masses of the and , and regions of the parameter space of the model are ruled out. Nearly model-independent limits are also set at the 95% confidence level on with the assumption that each chargino decays via a W boson, and on with the assumed to decay via a Z⁰.Received: 15 December 2003, Revised: 16 February 2004, Published online: 23 April 2004     

Di-jet production in photon-photon collisionsat $\sqrt{s_{\protect\bf\mathrm{ee}}}$ from 189 to 209 GeV

Di-jet production is studied in collisions of quasi-real photons at e ⁺ e^- centre-of-mass energies from 189 to 209 GeV at LEP. The data were collected with the OPAL detector. Jets are reconstructed using an inclusive -clustering algorithm for all cross-section measurements presented. A cone jet algorithm is used in addition to study the different structure of the jets resulting from either of the algorithms. The inclusive di-jet cross-section is measured as a function of the mean transverse energy of the two leading jets, and as a function of the estimated fraction of the photon momentum carried by the parton entering the hard sub-process, , for different regions of . Angular distributions in di-jet events are measured and used to demonstrate the dominance of quark and gluon initiated processes in different regions of phase space. Furthermore the inclusive di-jet cross-section as a function of and is presented, where is the jet pseudo-rapidity. Different regions of the - -space are explored to study and control the influence of an underlying event. The results are compared to next-to-leading order perturbative QCD calculations and to the predictions of the leading order Monte Carlo generator PYTHIA.Received: 20 December 2002, Published online: 17 October 2003