Statistical mechanics of cation ordering in PbSc<Subscript>1/2</Subscript>Ta<Subscript>1/2</Subscript>O<Subscript>3</Subscript> and PbSc<Subscript>1/2</Subscript>Nb<Subscript>1/2</Subscript>O<Subscript>3</Subscript> solid solutions

A model Hamiltonian for B cation ordering (Sc-Nb(Ta)) in PbSc_1/2Nb_1/2O₃ and PbSc_1/2Ta_1/2O₃ solid solutions is constructed. The parameters of the model Hamiltonian are determined from the ab initio calculation within the ionic crystal model with allowance made for the deformability and the dipole and quadrupole polarizabilities of the ions. The temperatures of the phase transition due to the ordering of the B cations are calculated by the Monte Carlo method in the mean-field and cluster approximations. The phase transition temperatures calculated by the Monte Carlo method (1920 K for PbSc_1/2Ta_1/2O₃ and 1810 K for PbSc_1/2Nb_1/2O₃) are consistent with the experimental data (1770 and 1450 K, respectively). The thermodynamic properties of the cation ordering are investigated using the Monte Carlo method.     

Coulomb correlations do not fill the e<Superscript>′</Superscript><Subscript>g</Subscript> hole pockets in Na<Subscript>0.3</Subscript>CoO<Subscript>2</Subscript>

There exists presently considerable debate over the question whether local Coulomb interactions can explain the absence of the small e^′ _g Fermi surface hole pockets in photoemission studies of Na_0.3CoO₂. By comparing dynamical mean field results for different single particle Hamiltonians and exact diagonalization as well as quantum Monte Carlo treatments, we show that, for realistic values of the Coulomb energy U and Hund exchange J, the e^′ _g pockets can be slightly enhanced or reduced compared to band structure predictions, but they do not disappear.     

Study of ferroelectric phase transitions in disordered PbSc<Subscript>1/2</Subscript>Nb<Subscript>1/2</Subscript>O<Subscript>3</Subscript> and PbSc<Subscript>1/2</Subscript>Ta<Subscript>1/2</Subscript>O<Subscript>3</Subscript> solid solutions

Ferroelectric phase transitions and thermodynamic properties of disordered PbSc_1/2Ta_1/2O₃ and PbSc_1/2Nb_1/2O₃ solid solutions have been investigated by the Monte Carlo method. The parameters of the effective Hamiltonian were calculated within the generalized Gordon-Kim model. The obtained values of the phase-transition temperatures and spontaneous polarization are in satisfactory agreement with the experimental data.     

NaBi(WO<Subscript>4</Subscript>)<Subscript>2</Subscript>: In Cherenkov crystals for electromagnetic calorimetry

NaBi(WO₄)₂:In (called NBW) colorless Cherenkov crystals were synthesized and studied. A decrease in the concentration of color centers provides a 50-nm blueshift of the optical transmission spectrum of NBW crystals. Upon exposing these crystals to gamma rays at a dose of 3 · 10⁷ rad, the optical transmission spectra in these crystals remain almost the same. The energy resolution (calculated by the Monte Carlo method) of calorimeters based on NBW crystals of different compositions is discussed.     

Quenching of the Haldane gap in LiVSi<Subscript>2</Subscript>O<Subscript>6</Subscript> and related compounds

We report results of susceptibility χ and ⁷Li NMR measurements on LiVSi₂O₆. The temperature dependence of the magnetic susceptibility χ(T) exhibits a broad maximum, typical for low-dimensional magnetic systems. Quantitatively it is in agreement with the expectation for an S=1 spin chain, represented by the structural arrangement of V ions. The NMR results indicate antiferromagnetic ordering below T_N=24 K. The intra- and interchain coupling J and J_p for LiVSi₂O₆, and also for its sister compounds LiVGe₂O₆, NaVSi₂O₆ and NaVGe₂O₆, are obtained via a modified random phase approximation which takes into account results of quantum Monte Carlo calculations. While J_p is almost constant across the series, J varies by a factor of 5, decreasing with increasing lattice constant along the chain direction. The comparison between experimental and theoretical susceptibility data suggests the presence of an easy-axis magnetic anisotropy, which explains the formation of an energy gap in the magnetic excitation spectrum below T_N, indicated by the variation of the NMR spin-lattice relaxation rate at T≪T_N.     

Simulation of the magnetic properties of manganese oxide Pb<Subscript>3</Subscript>Mn<Subscript>7</Subscript>O<Subscript>15</Subscript>

The magnetic susceptibility, heat capacity, and spin-spin correlation functions of manganese oxide Pb₃Mn₇O₁₅ are calculated by the Monte Carlo method. Two critical temperatures are determined: T ₁ ≈ 20 K, above which a modulated structure along the hexagonal axis is formed, and T ₂ ≈ 70 K, at which the long-range magnetic order disappears. The antiferromagnetic exchange interaction constant in a hexagonal plane is estimated to be J ₁ ～ 7 K, and the antiferromagnetic and ferromagnetic exchange interaction constants between hexagonal planes are calculated to be J ₂ ～ 3 K and K ～ 50 K, respectively.     

Migration of Mn<Superscript>2+</Superscript> excitations in Cd<Subscript>0.5</Subscript>Mn<Subscript>0.5</Subscript>Te crystal

The experimentally obtained intensity decay curves for the 2-eV intracenter luminescence band of Mn²⁺ ions in Cd_0.5Mn_0.5Te semiconductor solid solution at a temperature of 77 K have been simulated by the Monte Carlo method. The calculations show that the initial nonexponential behavior of the intensity decay curves at the band wings, as well as the time dependence of the band peak energy, are determined by the fast migration of excitations through the Mn²⁺ ion subsystem. There are more than 200 jumps per each emitted photon, and the migration rate increases by almost two orders of magnitude in comparison with the rate at 4 K. The analysis of the simulation results and the calculation based on the experimental data show the interaction between ions to be resonant. The estimate derived from the Anderson criterion suggests that the excited state is not delocalized. An increase in the migration rate with an increase in temperature significantly reduces the inhomogeneous broadening dispersion.     

Constrained path calculations of the <Superscript>4</Superscript><Emphasis Type="Italic">He</Emphasis> and <Superscript>16</Superscript><Emphasis Type="Italic">O</Emphasis> nuclei

We give results for the energy of the ⁴He and ¹⁶O nuclei using the auxiliary field diffusion Monte Carlo and a path constraint. We compare the results with previous FHNC and cluster Monte Carlo calculations.Received: 1 November 2002, Published online: 15 July 2003PACS: 21.10.Dr Binding energies and masses - 21.60.Ka Monte Carlo models     

Effect of small admixtures of N<Subscript>2</Subscript>, H<Subscript>2</Subscript> or O<Subscript>2</Subscript> on the electron drift velocity in argon: experimental measurements and calculations

The electron drift velocity in argon with admixtures of up to 2% of nitrogen, hydrogen or oxygen is measured in a pulsed Townsend system for reduced electric fields ranging from 0.1 Td to 2.5 Td. The results are compared with those obtained by Monte Carlo simulations and from the solution of the electron Boltzmann equation using two different solution techniques: a multiterm method based on Legendre polynomial expansion of the angular dependence of the velocity distribution function and the S _n method applied to a density gradient expansion representation of the distribution function. An almost perfect agreement between the results of the three numerical methods and, in general, very good agreement between the experimental and the calculated results is obtained. Measurements in Ar-O₂ mixtures were limited by electron attachment to oxygen molecules, which contributes to the measured drift velocity. As a result of this attachment contribution, the bulk drift velocity becomes larger than the flux drift velocity if attachment is more probable for electrons with energy below the mean value and smaller in the opposite case. Attachment also contributes to the negative differential conductivity observed in Ar-O₂ mixtures.     

Mobility and diffusion of oxygen isotopes in YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>6+<Emphasis Type="Italic">x</Emphasis></Subscript>: Monte carlo simulation

The self-diffusion (D₀) and isotope diffusion (D*) coefficients of oxygen in YBa₂Cu₃O_6+x are calculated as functions of the temperature (600–1200 K) and the oxygen content (0<x<1). The Monte Carlo simulation is performed with due regard for both the interaction of oxygen ions at lattice sites in the basal planes of YBa₂Cu₃O_6+x and the interaction between a jumping ion at a saddle point and the environment. Equilibrium thermodynamic characteristics (including the phase diagram and the heat capacity) are calculated in terms of the Hamiltonian of interaction between oxygen ions at the lattice sites. It is found that an increase in the oxygen content leads to a decrease in the diffusion coefficients D₀ and D*, an increase in the effective activation energies for diffusion by 0.3–0.5 eV, and a decrease in the Haven ratio from 1 to ～0.5.     

Prospects to search for E<Subscript>6</Subscript> isosinglet quarks in ATLAS

We consider pair production of new down-type isosinglet quarks originating from E₆, which is the favorite gauge symmetry group in superstring inspired GUT models. The study concentrates on the possibility of observing the pair production of the lightest of the new quarks, D, in the ATLAS detector at the forthcoming LHC accelerator, in the channel DD̄→ZjZj . Both signal and background events are studied using tree level event generators based on Monte Carlo techniques. The detector effects are taken into account using the ATLAS fast simulation tool. It is shown that ATLAS can observe the D quark within the first year of low luminosity LHC operation if its mass is less than 650 GeV. For the case of two years of full luminosity running, 1 TeV can be reached with about three sigma significance.     

Transforming Fixed-Length Self-avoiding Walks into Radial SLE<Subscript>8/3</Subscript>

We conjecture a relationship between the scaling limit of the fixed-length ensemble of self-avoiding walks in the upper half plane and radial SLE_8/3 in this half plane from 0 to i. The relationship is that if we take a curve from the fixed-length scaling limit of the SAW, weight it by a suitable power of the distance to the endpoint of the curve and apply the conformal map of the half plane that takes the endpoint to i, then we get the same probability measure on curves as radial SLE_8/3. In addition to a non-rigorous derivation of this conjecture, we support it with Monte Carlo simulations of the SAW. Using the conjectured relationship between the SAW and radial SLE_8/3, our simulations give estimates for both the interior and boundary scaling exponents. The values we obtain are within a few hundredths of a percent of the conjectured values.     

The use of quantum-correlated D<Superscript>0</Superscript> decays for φ<Subscript>3</Subscript> measurement

We report the results of the Monte Carlo study of the method to determine the CKM angle φ₃ using Dalitz plot analysis of D⁰ produced in B^±→DK^± decay. Our main goal is to find the optimal strategy for a model-independent φ₃ extraction. We find that the analysis using decays of CP-tagged D mesons only cannot provide a completely model-independent measurement in the case of a limited data sample. The procedure involving binned analysis of B^±→DK^± and ψ(3770)→(K⁰ _Sπ⁺π^-)_D(K⁰ _Sπ⁺π^-)_D decays is proposed which, in contrast, allows not only to reach the φ₃ precision comparable to an unbinned model-dependent fit, but also provides an unbiased measurement with currently available data. PACS 11.30.Er; 12.15.Hh     

<Emphasis Type="Italic">W</Emphasis><Superscript>+</Superscript><Emphasis Type="Italic">W</Emphasis><Superscript>+</Superscript> plus dijet production in the <Emphasis FontCategory="NonProportional">POWHEG BOX</Emphasis>

We present an implementation of the calculation of the production of W ⁺ W ⁺ plus two jets at hadron colliders, at next-to-leading order (NLO) in QCD, in the POWHEG framework, which is a method that allows the interfacing of NLO calculations to shower Monte Carlo programs. This is the first 2→4 process to be described to NLO accuracy within a shower Monte Carlo framework. The implementation was built within the POWHEG BOX package. We discuss a few technical improvements that were needed in the POWHEG BOX to deal with the computer intensive nature of the NLO calculation, and argue that further improvements are possible, so that the method can match the complexity that is reached today in NLO calculations. We have interfaced our POWHEG implementation with PYTHIA and HERWIG, and present some phenomenological results, discussing similarities and differences between the pure NLO and the POWHEG+PYTHIA calculation both for inclusive and more exclusive distributions. We have made the relevant code available at the POWHEG BOX web site.     

The dependence of critical temperature on oxygen concentration in YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>6+<Emphasis Type="Italic">x</Emphasis></Subscript> in terms of the fragmented chain model

An extended ASYNNNI model, that beside nearest-neighbour and next-nearest neighbour O-O interactions in the basal plane also includes interactions between the three nearest oxygen atoms, is used to describe the statistics of CuO chain fragmentation and to calculate doping and T _c in YBa₂Cu₃O_6+x . Calculations were made by the Monte Carlo method employing the recently proposed charge transfer model that assumes only chains whose length is equal to, or exceeds, a characteristic (critical) length, l _cr , can provide holes to the layers and contribute to doping p. The obtained p(x) is then combined with a universal T _c versus p relation to yield T _c (x) characteristics that correlate remarkably with those reported in recent experiments. The best coordination between theoretical and experimental T _c (x) characteristics has been achieved for l _cr = 2, implying that only isolated basal plane oxygen atoms (trivial chains) do not contribute holes to CuO₂ layers.     

O<Subscript>2</Subscript> photodesorption from a Ag<Subscript>8</Subscript>O<Stack><Subscript>2</Subscript><Superscript>-</Superscript></Stack> cluster

The decay path of an Ag₈(O₂)^- cluster photoexcited by a 3.1 eV photon is elucidated using time-resolved photoelectron spectroscopy. Photoabsorption results in the formation of an excited state giving rise to a peak in the photoelectron spectra with well-resolved vibrational finestructure. With a lifetime of about 100 fs this bound state decays into an anti-bonding state which dissociates into O₂ and Ag₈^- on a timescale of 10 ps. In the photoelectron spectra, this corresponds to a broad maximum shifting gradually towards higher binding energy while the O₂ and Ag₈^- separate. Finally, the spectrum of bare Ag₈^- appears. This process is unique to small clusters, because on metal surfaces excited state lifetimes are too short to allow for direct dissociation.     

Electrical switching studies on As<Subscript>40</Subscript>Te<Subscript>60-x</Subscript>Se<Subscript>x</Subscript> and As<Subscript>35</Subscript>Te<Subscript>65-x</Subscript>Se<Subscript>x</Subscript> glasses

The I–V characteristics of bulk As₄₀Te_60-xSe_x and As₃₅Te_65-xSe_x glasses have been studied with a current sweep of 0–18 mA-0, over a wide range of compositions (4≤x≤22). All the glasses studied showed a threshold electrical switching behaviour. The number of switching cycles withstood by the samples has been found to depend on the ON-state current. It is seen that the switching voltages increase with increase in selenium content. Further, the switching voltages are found to be almost independent of the thickness of the sample (d), in the range 0.18–0.3 mm. Also, the switching voltages and the number of switching cycles withstood by the samples are found to decrease with temperature. Received: 6 November 2002 / Accepted: 8 November 2002 / Published online: 29 January 2003 RID="*" ID="*"Corresponding author. Fax: +91-80/360-0135, E-mail: sasokan@isu.iisc.ernet.in     

Electron momentum density in Cu<Subscript>0.9</Subscript>Al<Subscript>0.1</Subscript>

A reconstruction technique based on the solution of the Radon transform in terms of Jacobi polynomials is used to obtain the 3D electron momentum density, ϱ(p), from nine high-resolution Compton profiles (CPs) for a Cu_0.9Al_0.1 disordered alloy single crystal. The method was also applied to theoretical CPs computed within the Korringa–Kohn–Rostoker coherent potential approximation (KKR-CPA) first-principles scheme for the same nine orientations of the crystal. The experimental ϱ(p) is in satisfactory agreement with the theoretical ϱ(p), shows most details of the Fermi surface (FS) and exhibits electron correlation effects. We comment on the map of the FS obtained by folding the reconstructed ϱ(p) into the first Brillouin zone, which yields the occupation number density, ϱ(k). A test of the validity of data via a consistency condition (within our reconstruction algorithm) as well as the propagation of experimental noise in the reconstruction of both ϱ(p) and ϱ(k) are investigated. Received: 24 October 2001 / Accepted: 20 January 2002 / Published online: 3 June 2002 RID="*" ID="*"Corresponding author. Fax: +48-71/344-10-29, E-mail: samsel@int.pan.wroc.pl     

Investigation of Critical Properties of a MnF<Subscript>2</Subscript> Antiferromagnet Model by the Monte Carlo Methods

The Wolf highly efficient single-cluster algorithm of the Monte Carlo method is used to investigate the critical static properties of a MnF ₂ antiferromagnet model. The Wolf single-cluster algorithm is modified to investigate complex magnet models in which the uniaxial anisotropy is taken into account together with the exchange interaction. All main critical static parameters of the system are calculated, including the critical exponents of heat capacity α, magnetization β, and susceptibility γ, the Fisher index η, and the correlation length ν.__________Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 53–58, February, 2005.     

Microstructures and impedance studies of Bi<Subscript>3.15</Subscript>Nd<Subscript>0.85</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript> thin films

Microstructures and impedance characteristics of chemical-solution-derived Bi_3.15Nd_0.85Ti₃O₁₂ thin films were studied as functions of temperature. A dielectric anomaly was found at around 450°C, corresponding to the paraelectric to ferroelectric transition. Via complex impedance studies, grain and grain boundary contributions to the impedance were separated. The resistance of grain and grain boundaries is found governed by the same kind of space charge with an activation energy around 1.1 eV, close to that of oxygen vacancies in perovskite ferroelectrics. The low temperature ac conductance of BNdT thin films shows a frequency dispersion, which can also be ascribed to space charges mainly due to oxygen vacancies. The results were compared with SrBi₂Ta₂O₉ in terms of oxygen vacancy conductivity.