Stiffness exponents for lattice spin glasses in dimensions $\mathsf{d = 3,\ldots,6}$

The stiffness exponents in the glass phase for lattice spin glasses in dimensions are determined. To this end, we consider bond-diluted lattices near the T = 0 glass transition point p^*. This transition for discrete bond distributions occurs just above the bond percolation point p_c in each dimension. Numerics suggests that both points, p_c and p^*, seem to share the same 1/d-expansion, at least for several leading orders, each starting with 1/(2d). Hence, these lattice graphs have average connectivities of near p^* and exact graph-reduction methods become very effective in eliminating recursively all spins of connectivity , allowing the treatment of lattices of lengths up to L = 30 and with up to 10⁵-10⁶ spins. Using finite-size scaling, data for the defect energy width over a range of p > p^* in each dimension can be combined to reach scaling regimes of about one decade in the scaling variable . Accordingly, unprecedented accuracy is obtained for the stiffness exponents compared to undiluted lattices (p = 1), where scaling is far more limited. Surprisingly, scaling corrections typically are more benign for diluted lattices. We find in for the stiffness exponents y₃ = 0.24(1), y₄ = 0.61(2), y₅ = 0.88(5), and y₆ = 1.1(1).Received: 29 October 2003, Published online: 20 April 2004PACS: 05.50. + q Lattice theory and statistics (Ising, Potts, etc.) - 64.60.Cn Order-disorder transformations; statistical mechanics of model systems - 75.10.Nr Spin-glass and other random models - 02.60.Pn Numerical optimization     

Bond dilution in the 3D Ising model: a Monte Carlo study

We study by Monte Carlo simulations the influence of bond dilution on the three-dimensional Ising model. This paradigmatic model in its pure version displays a second-order phase transition with a positive specific heat critical exponent . According to the Harris criterion disorder should hence lead to a new fixed point characterized by new critical exponents. We have determined the phase diagram of the diluted model, starting from the pure model limit down to the neighbourhood of the percolation threshold. For the estimation of critical exponents, we have first performed a finite-size scaling study, where we concentrated on three different dilutions to check the stability of the disorder fixed point. We emphasize in this work the great influence of the cross-over phenomena between the pure, disorder and percolation fixed points which lead to effective critical exponents dependent on the concentration. In a second set of simulations, the temperature behaviour of physical quantities has been studied in order to characterize the disorder fixed point more accurately. In particular this allowed us to estimate ratios of some critical amplitudes. In accord with previous observations for other models this provides stronger evidence for the existence of the disorder fixed point since the amplitude ratios are more sensitive to the universality class than the critical exponents. Moreover, the question of non-self-averaging at the disorder fixed point is investigated and compared with recent results for the bond-diluted q = 4 Potts model. Overall our numerical results provide evidence that, as expected on theoretical grounds, the critical behaviour of the bond-diluted model is indeed governed by the same universality class as the site-diluted model.Received: 24 February 2004, Published online: 28 May 2004PACS: 05.40.-a Fluctuation phenomena, random processes, noise, and Brownian motion - 64.60.Fr Equilibrium properties near critical points, critical exponents - 75.10.Hk Classical spin models     

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     

Full-band extended states with random phases in one-dimensional disordered system with specific impurities

We investigate transport properties of electrons in a one-dimensional (1D) disordered system consisting of a host chain attached with specific impurities. Every impurity, labelled by j and possessing site energy , is side-coupled to two adjacent sites of the host chain with hopping integral t _1j and changesthe original nearest-neighbor (NN) hopping to t _2j . We show that if and for all impurities, with t ₀ being the NN hopping of the host chain, the states in the whole band are extended, even though s and positions of impurities are random. The phases of these states, however, are spatially random, corresponding to finite free path and infinite localization length in such a 1D system.Received: 6 May 2004, Published online: 12 July 2004PACS: 72.15.Rn Localization effects (Anderson or weak localization) - 72.80.Ng Disordered solids - 73.20.Jc Delocalization processes     

The anisotropic quantum antiferromagnet on the Sierpiński gasket: Ground state and thermodynamics

We investigate an antiferromagnetic s = 1/2 quantum spin system with anisotropic spin exchange on a fractal lattice, the Sierpiski gasket. We introduce a novel approximative numerical method, the configuration selective diagonalization (CSD) and apply this method to a the Sierpiski gasket with N = 42. Using this and other methods we calculate ground state energies, spin gap, spin-spin correlations and specific heat data and conclude that the s = 1/2 quantum antiferromagnet on the Sierpinski gasket shows a disordered magnetic ground state with on the Sierpinski gasket shows a disordered magnetic ground state with a very short correlation length of and an, albeit very small, spin gap. This conclusion holds for Heisenberg as well a for XY exchange.Received: 18 February 2004, Published online: 20 April 2004PACS: 75.10.-b General theory and models of magnetic ordering - 05.45.Df Fractals - 75.40.Mg Numerical simulation studies     

$\mathsf{\sqrt{3}}$ linear structures in the Te/Ni(111) system

Surface structures in the Te/Ni(111) system are revealed by using reflection high-energy electron diffraction combined with X-ray and ultraviolet photoelectron spectroscopies. At a 0.33 mono-layer (ML)-Te/Ni(111) surface, a reversible structural phase transition is observed with a transition temperature T_c of 380 C. The diffraction pattern from the low temperature phase is accompanied by streaks. The high and low temperature phases are characterized by and rectangle, respectively. The mechanism of the phase transition is explained by the order-disorder transition with a rumpled chain model. Both 0.51 ML- and 0.44 ML-Te/Ni(111) surfaces exhibit the complex diffraction patterns accompanied by diffuse streaks. These surface structures are characterized by the rectangle and , respectively. All diffuse streaks obtained at the above surfaces are consistently interpreted in the view of the ill-ordered arrangements of the well-ordered linear chains. It is shown that the linear structure is the key in the Te/Ni(111) system.Received: 1 December 2003, Published online: 20 April 2004PACS: 61.14.Hg Low-energy electron diffraction (LEED) and reflection high-energy electron diffraction (RHEED) - 68.65.-k Low-dimensional, mesoscopic, and nanoscale systems: structure and nonelectronic properties - 64.60.Cn Order-disorder transformations; statistical mechanics of model systems     

An $\mathsf{\epsilon}$-expansion for small-world networks

I construct a well-defined expansion in for diffusion processes on small-world networks. The technique permits one to calculate the average over disorder of moments of the Greens function, and is used to calculate the average Greens function and fluctuations to first non-leading order in , giving results which agree with numerics. This technique is also applicable to other problems of diffusion in random media.Received: 28 July 2004, Published online: 14 December 2004PACS: 89.75.Hc Networks and genealogical trees 64.60.Ak Renormalization-group studies of phase transitions     

Spin-dependent transport in films composed of Co clusters and C $_{\mathsf{60}}$ fullerenes

Granular films of Co-cluster/C₆₀ mixtures have been prepared by the co-deposition of well defined Co clusters (mean diameter 4.5 nm) and C₆₀ fullerenes onto a cold ( 35 K) substrate. Films having a Co cluster volume fraction show a resistivity , typical for tunneling with a Coulomb barrier. The tunneling magnetoresistance (TMR) has a value of TMR ( % for and is decreasing by almost one order of magnitude going to . We explain this unusual decrease of the TMR with increasing as caused by electron-doping of the C₆₀ fullerenes due to the known charge transfer process occurring between transition metal surface and C₆₀. Increasing electron doping may lead to an increasing probability for spin-flip processes within the tunneling barrier, resulting in a decrease of the TMR.Received: 17 March 2004, Published online: 3 August 2004PACS: 75.47.-m Magnetotransport phenomena; materials for magnetotransport - 73.40.Gk Tunneling - 73.40.Rw Metal-insulator-metal structures     

Electrical resistivity in the ferromagnetic metallic state of La-Ca-MnO\mathsf{_{3}}: Role of electron-phonon interaction

The temperature-dependent resistivity of the perovskite manganites La_1-x Ca _x MnO₃, with x = 0.33, is theoretically analysed within the framework of the classical electron-phonon model of resistivity, i.e., the Bloch-Gruneisen model. Due to inherent acoustic (low-frequency) phonons ( as well as high-frequency optical phonons ( , the contributions to the resistivity have first been estimated. The acoustic phonons of the oxygen-breathing mode yield a relatively larger contribution to the resistivity compared to the contribution of optical phonons. Furthermore, the nature of phonons changes around T = 167 K exhibiting a crossover from an acoustic to optical phonon regime with elevated temperature. The contribution to resistivity estimated by considering both phonons, i.e. and , when subtracted from thin film data, infers a power temperature dependence over most of the temperature range. The quadratic temperature dependence of is understood in terms of electron-electron scattering. Moreover, in the higher temperature limit, the difference can be varies linearly with T ^4.5 in accordance with the electron-magnon scattering in the double exchange process. Within the proposed scheme, the present numerical analysis of temperature dependent resistivity shows similar results as those revealed by experiment.Received: 8 April 2004, Published online: 12 August 2004PACS: 5.47.Gk Colossal magnetoresistance - 72.15.-v Electronic conduction in metals and alloys - 74.25.Kc Phonons - 75.30.Ds Spin wavesD. Varshney: dvboson.sop@dauniv.ac.in     

Temperature and field dependence of the magnetic properties of Nd \mathsf{_{2-x}}Ce\mathsf{_{x}}CuO\mathsf{_4}

We have investigated the magnetic ordering of Nd_2-x Ce _x CuO₄ for x = 0, 0.09, 0.13, 0.15 and 0.18 by neutron diffraction at low temperatures down to 33 mK and under magnetic field up to 5 tesla applied along [1,-1,0] crystallographic direction. At zero applied magnetic field Cu magnetic sublattice orders at , 210, 130 and 105 K for x = 0, 0.09, 0.13, 0.15, respectively. No long range magnetic order of the Cu could be detected for x = 0.18. The magnetic order of Nd was found in all samples, with a gradual increase of the polarized magnetic moment with decreasing temperature, saturating around 1 K. Hyperfine induced nuclear polarization of the Nd nuclear spin has been observed below about 400 mK for samples with x = 0, 0.13, 0.15 and 0.18. Field variation of the intensities of the principal and superstructure reflections of Nd_2-x Ce _x CuO₄ at millikelvin temperatures shows a field-induced second-order double-k to single-k phase transition at H _c = 0.75 and 0.56 tesla for samples with x = 0 and 0.15, respectively at T = 50 mK. We have also investigated the polarization of the Nd electronic sublattice due to the field of the Cu sublattice by the element specific X-ray resonant magnetic scattering investigation with synchrotron radiation.Received: 9 April 2004, Published online: 14 December 2004PACS: 75.25. + z Spin arrangements in magnetically ordered materials (including neutron and spin-polarized electron studies, synchrotron-source X-ray scattering, etc.)T. Chatterji: Has changed his surname from Chattopadhyay to Chatterji.     

High resolution multiphoton ionization and dissociation of acetone via $\mathsf{3s\leftarrow n}$ Rydberg transitions

Measurements of multiphoton ionization and dissociation of acetone are reported in the wavelength range 582.60-585.80 nm at photon energy resolution of 0.3 cm^-1. To our knowledge there are not available results of (3 + 2) REMPI on acetone at 0.01 nm in this region. The experiments were performed using an Nd: YAG-OPO (optical parametric oscillator) laser system coupled to time-of-flight mass spectrometer. The ion yield and the single ion channel are presented. The high-resolution three-photon resonance multiphoton spectrum of the acetone Rydberg transition is also reported. The experimental results show three dissociation channels of the acetone ion, leading to the products: (CH₃CO ⁺ ), (CH₃ ⁺ ) and (COH ⁺ ); the channel CH₃COCH ₃CO ⁺ + CH₃ being the most favored. The acetone and acetyl ions are observed in all wavelength range investigated. In addition, we have measured the origin of the and transitions, and vibrational bands of the 3s state.Received: 21 March 2004, Published online: 11 May 2004PACS: 33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states) - 33.15.Ta Mass spectra - 33.80.Gj Diffuse spectra; predissociation, photodissociation     

Dependence of the average to-node distance on the node degree for random graphs and growing networks

In a connected graph, nodes can be characterised locally (with their degree k) or globally (e.g. with their average length path to other nodes). Here we investigate how depends on k. The numerical algorithm based on the construction of the distance matrix is applied to random graphs and the growing networks: the scale-free ones and the exponential ones. The results are relevant for search strategies in different networks.Received: 15 June 2004, Published online: 21 October 2004PACS: 02.10.Ox Combinatorics; graph theory - 05.10.-a Computational methods in statistical physics and nonlinear dynamics     

Canonical local algorithms for spin systems: heat bath and Hasting’s methods

We introduce new fast canonical local algorithms for discrete and continuous spin systems. We show that for a broad selection of spin systems they compare favorably to the known ones except for the Ising 1 spins. The new procedures use discretization scheme and the necessary information have to be stored in computer memory before the simulation. The models for testing discrete spins are the Ising 1, the general Ising S or Blume-Capel model, the Potts and the clock models. The continuous spins we examine are the O(N) models, including the continuous Ising model (N = 1), the Ising model (N = 1), the XY model (N = 2), the Heisenberg model (N = 3), the Heisenberg model (N = 3), the O(4) model with applications to the SU(2) lattice gauge theory, and the general O(N) vector spins with .Received: 16 August 2004, Published online: 21 October 2004PACS: 05.70.Fh Phase transitions: general studies - 64.60.Cn Order-disorder transformations; statistical mechanics of model systems - 75.10.Hk Classical spin models - 75.10.Nr Spin-glass and other random models     

Test of universality in the Ising spin glass using high temperature graph expansion

We calculate high-temperature graph expansions for the Ising spin glass model with 4 symmetric random distribution functions for its nearest neighbor interaction constants J _ij . Series for the Edwards-Anderson susceptibility are obtained to order 13 in the expansion variable (J/(k _B T))² for the general d-dimensional hyper-cubic lattice, where the parameter J determines the width of the distributions. We explain in detail how the expansions are calculated. The analysis, using the Dlog-Padé approximation and the techniques known as M1 and M2, leads to estimates for the critical threshold (J/(k _B T _c ))² and for the critical exponent in dimensions 4, 5, 7 and 8 for all the distribution functions. In each dimension the values for agree, within their uncertainty margins, with a common value for the different distributions, thus confirming universality.Received: 22 July 2004, Published online: 12 October 2004PACS: 05.70.Jk Critical point phenomena - 75.10.Nr Spin-glass and other random models     

Spin-Peierls lattice fluctuations and disorders in CuGeO$_\mathsf{3}$ and its solid solutions

The inorganic quasi-one dimensional (1D) S = 1/2 antiferromagnetic (AF) system CuGeO₃ undergoes a 2nd order spin-Peierls (SP) phase transition at T _SP = 14.2 K. In this study we present an X-ray synchrotron radiation investigation which confirms that the SP instability is announced by an important regime of pretransitional structural fluctuations which have been detected until 36 K. Furthermore we show that these fluctuations are 1D above 24 K, a feature expected for a structural instability triggered by the Cu^{2 +} chains of spin 1/2. By extrapolating the thermal dependence of the correlation length in the chain direction, we show that formation of singlet dimers begins at about 50 K, a temperature that we identify as the mean field temperature of the SP chain. The critical nature of the pretransitional fluctuations does not change when low amounts (<1%) of non-magnetic dopants substitute either the Cu site (case of Zn and Mg) or the Ge site (case of Si and Al) of CuGeO₃. However, the spatial extension of the fluctuations is considerably reduced when the magnetic dopant Ni substitutes the Cu site. In the SP ground state of doped materials we have been able to detect, in addition to the superlattice SP reflections previously observed, a very weak anisotropic diffuse scattering. We give evidences that this scattering originates from dopant-induced quasi-1D domains in which the dimerisation is perturbed. If we assume that each domain is limited by a soliton-antisoliton pair, pinned either on the substituent of the Cu site or by the deformation field induced by the substituent of the Ge site, we deduce that the soliton and antisoliton are separated by a distance of about Å, and that the soliton half width amounts to about Å. With these numbers we are able to account for the rate of decrease of T _SP as a function of the dopant concentration, and to deduce the critical concentration above which the long-range SP order vanishes. The overall size of the perturbed domains thus obtained, Å, is comparable with the size of the magnetic inhomogeneities determined by muon spin spectroscopy in the AF phase of doped CuGeO₃.Received: 22 December 2003, Published online: 8 June 2004PACS: 71.27. + a Strongly correlated electron systems; heavy fermions - 61.72.Dd Experimental determination of defects by diffraction and scattering - 75.45. + j Macroscopic quantum phenomena in magnetic systems     

Prospects for production of ultracold $\mathsf{X^{1}\Sigma^{ + }}$ RbCs molecules

Recently, we have reported photoassociation of laser-cooled Rb and Cs atoms, decay of the RbCs photoassociation resonances to high levels of the state, and reexcitation to vibronic levels of the state [Kerman et al. , Phys. Rev. Lett. 92, 033004; 153001 (2004)]. Considering the reexcitation spectrum, we report here a preliminary analysis of perturbations in the c state by high levels of the and low levels of the state. Mixing with the B state provides the singlet character needed to stimulate decay to v = 0 of the ground state. We conclude that an experimental procedure that involves photoassociation of laser-cooled atoms, radiative decay, and stimulated Raman transfer to the ground electronic state is a feasible method for producing translationally, rotationally, vibrationally and electronically cold RbCs molecules. Electronic supplementary material to this article is available at and is accessible for authorized users. Received: 1 September 2004, Published online: 23 November 2004 PACS: 33.80.Ps Optical cooling of molecules; trapping - 34.50.Gb Electronic excitation and ionization of molecules; intermediate molecular states (including lifetimes, state mixing, etc.) - 33.20.Kf Visible spectra - 34.20.-b Interatomic and intermolecular potentials and forces, potential energy surfaces for collisions - 33.15.Pw Fine and hyperfine structure A supplementary table (Tab. I) is only available in electronic form at http: //www.eurphysj.org     

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     

Thermodynamic properties of an alternating-spin (1/2,1) two-leg ladder

With the aid of the Schwinger-boson mean-field method, we study the low-lying excitations and thermodynamic properties of a ferrimagnetic Heisenberg two-leg ladder (i.e., a ferrimagnetic double-chain with an antiferromagnetic interaction). The interaction between the two chains plays an important role in producing a low-lying excitation energy gap, affecting the low-lying excited spectrum, and increasing the disorder of the ferrimagnetic double-chain. The excitation spectrum, energy gap, and spin reduction in the ground state are calculated. Thermodynamic quantities such as the short-range spin correlation and short-range order are also obtained at low temperatures. In this gapful system, we observed the exponential behaviors in both the specific heat (C_V) and the product of magnetic susceptibility and temperature ( ) at low temperatures. The exponential behavior of the versus temperature agrees qualitatively with the experimental results in at low temperatures.Received: 24 October 2003, Published online: 15 March 2004PACS: 75.10.Jm Quantized spin models - 75.40.Cx Static properties (order parameter, static susceptibility, heat capacities, critical exponents, etc.) - 75.50.Ee Antiferromagnetics - 75.50.Gg Ferrimagnetics     

A new ab initio method of calculating $\mathsf{Z_{eff}}$ and positron annihilation rates using coupled-channel T-matrix amplitudes

We present a new ab initio theoretical formulation to calculate and hence the positron annihilation rates directly from the onshell and offshell (half) scattering amplitudes. The method does not require any explicit use of the scattering wave function and is formally exact within the framework of the well established Lippmann-Schwinger equation. It could serve as an effective tool as all the T-, K-, and S-matrix formulations, yield directly the scattering amplitudes; not the wave function. Numerical test of the method is presented considering sample static calculations in positron-hydrogen and positron-helium systems.Received: 15 January 2004, Published online: 15 April 2004PACS: 03.65.Nk Scattering theory - 34.85. + x Positron scattering - 71.60. + z Positron states - 78.70.Bj Positron annihilation     

Leptoproduction of $\varrho$ mesons within Regge approach

A model describing the leptoproduction of mesons is considered. For the amplitude of the photon dissociation into a pair, the light-cone wave function is used. Scattering of the colorless pair off the nucleon is computed within reggeon exchange phenomenology. The transition of the scattered quark and antiquark into the final meson is treated with the aid of the parton-hadron duality concept. Numerical calculations of and r⁰⁴₀₀ describe the world data at Q² < 4 GeV² rather reasonably. The calculations of the meson spin density matrix show that the computed matrix elements except r⁰⁴₀₀ are in good agreement with the available experimental data at Q² up to 8 GeV² and the total mass of the system greater than 4 GeV. The Regge phenomenology predictions at the highest experimentally available energies agree with both the HERA data and calculations performed within the perturbative QCD approach even for very high Q² ( -20 GeV²). The predicted scale of the S-channel helicity non-conservation is in reasonable agreement with the experimental data.Received: 18 September 2003, Revised: 17 December 2003, Published online: 13 February 2004