Monte Carlo simulations of the classical two-dimensional discrete frustrated model

The classical two-dimensional discrete frustrated φ ⁴ model is studied by Monte Carlo simulations. The correlation function is obtained for two values of a parameter d that determines the frustration in the model. The ground state is a ferro-phase for d = - 0.35 and a commensurate phase with period N = 6 for d = - 0.45. Mean field predicts that at higher temperature the system enters a para-phase via an incommensurate state, in both cases. Monte Carlo data for d = - 0.45 show two phase transitions with a floating-incommensurate phase between them. The phase transition at higher temperature is of the Kosterlitz-Thouless type. Analysis of the data for d = - 0.35 shows only a single phase transition between the floating-fluid phase and the ferro-phase within the numerical error. Received 16 December 2002 / Received in final form 17 January 2003 Published online 6 March 2003 RID="a" ID="a"e-mail: vladimir@shg.ru     

CEF nature of the magnetic excitations in ordered HoNi B C

The magnetic excitations in the antiferromagnetic phase of HoNi₂B₂C are studied by inelastic neutron scattering on single crystals for the first time. Spectra measured at constant T = 2 K along symmetry directions of the reciprocal space are well explained in terms of crystal electric field (CEF) magnetic excitons within the J = 8 ground state multiplet of Ho³⁺. Very modest bandwidth with planar energy dispersion describes the magnetic exciton dynamics. A perturbative model approach consisting of the CEF states in the effective exchange mean-field provides a simple but applicable characterization of the experimental observations. The microscopic determination of the relevant exchange parameters is discussed in connection with previous works on the subject. Received 25 February 2002 / Received in final form 13 May 2002 Published online 14 October 2002 RID="a" ID="a"e-mail: nordal.cavadini@psi.ch     

Electron-phonon interaction in carbon clathrate hex- $\mathsf{C_{40}}$

We investigate an extended spin ladder with diagonal frustrated exchanges in a wide parameter regime. By representing the model as a sum of semidefinite positive projection operators, we prove that this model has exactly a dimer ground state. Smoothly changing parameters may lead the model cover several exactly known models. Starting from this ladder model, we proposed two two-dimensional net models with exact ground states. The quantum phase transition of the ground state, due to the change of exchange strengths along perpendicular rungs, is also discussed. Received 13 October 2002 Published online 27 January 2003 RID="a" ID="a"e-mail: schen@thphy.uni.duesseldorf.de     

Reply to the Comment by A. Rabhi on “Energies of the ground state and first excited 0<Superscript> + </Superscript> state in an exactly solvable pairing model”

It is pointed out that the exact solutions given in the recent Comment by A. Rabhi on the paper by N. Dinh Dang Energies of the ground state and first excited 0⁺ state in an exactly solvable pairing model (Eur. Phys. J. A 16, 181 (2003)) do not correspond to the standard exact solution of the well-known two-level pairing model. Other issues raised in the Comment reiterate the discussions already published in the original paper by N. Dinh Dang.Received: 21 July 2003, Published online: 17 February 2004PACS: 21.60.Jz Hartree-Fock and random-phase approximations - 21.60.-n Nuclear-structure models and methods     

Polar Kerr rotation of the ferromagnet

Magneto-optical data on EuB ₆ , a ferromagnet with a Curie temperature T _C ∼ 15 K, are presented and discussed in detail. We have measured the polar Kerr rotation, covering a spectral range from the infrared up to the ultraviolet, as a function of temperature between 1.5 and 20 K and in external magnetic fields between 0 and 10 T. The Kerr rotation in high fields and at low temperatures is enormous. Our observations, which implicitly reflect the large magnetoresistive effects, are shown to discriminate between the spectroscopic response of localized and itinerant electronic states. Our data analysis is based on the phenomenological Lorentz-Drude model, following from the classical dispersion theory and appropriately extended to magneto-optical experiments. Received 30 January 2003 / Received in final form 21 March 2003 Published online 23 May 2003 RID="a" ID="a"e-mail: degiorgi@solid.phys.ethz.ch     

The normal state scattering rate in high- cuprates

A detailed phenomenological model is developed for the normal state transport properties of optimal and overdoped high-T_c cuprates. In particular, an explicit form of scattering rate is identified that may account, qualitatively and quantitatively, for the normal state (magneto)-transport properties of Tl₂Ba₂CuO _{6 + δ} and Bi₂Sr₂CaCu₂O _{8 + δ} across the overdoped side of the phase diagram. The proscribed form of the scattering rate is consistent also with features seen in the photoemission spectroscopy of Bi₂Sr₂CaCu₂O _{8 + δ} and offers a new and intuitive way to understand the evolution of the temperature dependence of the inverse Hall angle in Bi-based cuprates with carrier concentration. Received 5 June 2002 / Received in final form 17 December 2002 Published online 6 March 2003 RID="a" ID="a"e-mail: n.e.hussey@bristol.ac.uk     

Vertex cover problem studied by cavity method: Analytics and population dynamics

We study the vertex cover problem on finite connectivity random graphs by zero-temperature cavity method. The minimum vertex cover corresponds to the ground state(s) of a proposed Ising spin model. When the connectivity c > e = 2.718282, there is no state for this system as the reweighting parameter y, which takes a similar role as the inverse temperature β in conventional statistical physics, approaches infinity; consequently the ground state energy is obtained at a finite value of y when the free energy function attains its maximum value. The minimum vertex cover size at given c is estimated using population dynamics and compared with known rigorous bounds and numerical results. The backbone size is also calculated. Received 11 November 2002 Published online 1st April 2003 RID="a" ID="a"e-mail: zhou@mpikg-golm.mpg.de     

Electron-electron bound states in Maxwell-Chern-Simons-Proca QED <Emphasis Type="Bold">3</Emphasis>

We start from a parity-breaking MCS QED₃ model with spontaneous breaking of the gauge symmetry as a framework for evaluation of the electron-electron interaction potential and for attainment of numerical values for the e ^- e ^- - bound state. Three expressions ( V _eff , V _eff , V _eff ) are obtained according to the polarization state of the scattered electrons. In an energy scale compatible with condensed matter electronic excitations, these three potentials become degenerated. The resulting potential is implemented in the Schr?dinger equation and the variational method is applied to carry out the electronic binding energy. The resulting binding energies in the scale of 10-100 meV and a correlation length in the scale of 10-30 ? are possible indications that the MCS-QED₃ model adopted may be suitable to address an eventual case of e ^- e ^- pairing in the presence of parity-symmetry breakdown. The data analyzed here suggest an energy scale of 10-100 meV to fix the breaking of the U (1)-symmetry. Received 24 September 2002 / Received in final form 15 January 2003 Published online 1st April 2003 RID="a" ID="a"e-mail: belich@cbpf.br RID="b" ID="b"e-mail: delcima@cbpf.br RID="c" ID="c"e-mail: manojr@cbpf.br RID="d" ID="d"e-mail: helayel@cbpf.br     

Comparative investigation of <Emphasis Type="Bold">39</Emphasis>K and <Emphasis Type="Bold">40</Emphasis>K trap loss rates: alternative loss channel at low light intensities

We report a comparative investigation of trap loss rates in a magneto-optical trap for two potassium isotopes, ³⁹K and ⁴⁰K, as a function of trap light intensity. The isotopes present a quite similar behavior for the loss rates at high intensities, and a sudden increase of the loss rates at low intensities is present in both cases. While for ³⁹K such increase can be explained assuming that the major contribution to the losses comes from hyperfine changing collisions, a different loss mechanism must be considered for ⁴⁰K, which has an inverted ground state hyperfine structure. The experimental results of both isotopes are well reproduced by an alternative model based on radiative escape as the dominant loss mechanism. Received 1st May 2002 / Received in final form 19 October 2002 Published online 4 March 2003 RID="a" ID="a"Alternative address: Dipartimento di Fisica, Universitá di Trento, 38050 Povo (Tn), Italy. RID="b" ID="b"e-mail: marcassa@if.sc.usp.br     

An Effective Field Theory Model to Describe Nuclear Matter in Heavy-Ion Collisions

Relativistic mean field theory with mesons , , and mediating interactions and nucleons as basic fermions has been very successful in describing nuclear matter and finite nuclei. However, in heavy-ion collisions, where the c. m. energy of two colliding nucleons will be in the hundreds of GeV region, nucleons are not expected to behave as point-like particles. Analyses of elastic pp and ¯pp scattering data in the relevant c. m. energy range show that the nucleon is a composite object—a topological soliton or Skyrmion embedded in a condensed quark-antiquark ground state. Against this backdrop, we formulate an effective field theory model of nuclear matter based on the gauged linear -model where quarks are the basic fermions, but the mesons still mediate the interactions. The model describes the nucleon as a Skyrmion and produces a q¯q ground state analogous to a superconducting ground state. Quarks are quasi-particles in this ground state. When the temperature exceeds a critical value, the scalar field in the ground state vanishes, quarks become massless, and a chiral phase transition occurs leading to chiral symmetry restoration. We explore the possibility of a first order phase transition in this model by introducing suitable self-interactions of the scalar field. Internal structures of the Skyrmions are ignored, and they are treated as point-like fermions.     

Universal properties of shortest paths in isotropically correlated random potentials

We consider the optimal paths in a d-dimensional lattice, where the bonds have isotropically correlated random weights. These paths can be interpreted as the ground state configuration of a simplified polymer model in a random potential. We study how the universal scaling exponents, the roughness and the energy fluctuation exponent, depend on the strength of the disorder correlations. Our numerical results using Dijkstra's algorithm to determine the optimal path in directed as well as undirected lattices indicate that the correlations become relevant if they decay with distance slower than 1/r in d = 2 and 3. We show that the exponent relation 2ν - ω = 1 holds at least in d = 2 even in case of correlations. Both in two and three dimensions, overhangs turn out to be irrelevant even in the presence of strong disorder correlations. Received 20 December 2002 / Received in final form 10 April 2003 Published online 20 June 2003 RID="a" ID="a"e-mail: schorr@lusi.uni-sb.de     

On pionic hydrogen. Quantum field theoretic,relativistic covariant and model-independent approach

We consider pionic hydrogen , the bound state. Within the quantum field theoretic and relativistic covariant approach we calculate the shift and width of the energy level of the ground state of pionic hydrogen caused by strong low-energy interactions treated perturbatively. The generalization of the Deser-Goldberger-Baumann-Thirring (DGBT) formulas (S. Deser, M.L. Goldberger, K. Baumann, W. Thirring, Phys. Rev. 96, 774 (1954)) is given. The generalized DGBT formulas for the energy level displacement of the ground state of pionic hydrogen contain the non-perturbative and model-independent correction of about , caused by the relativistic covariant smearing of the wave function of the ground state around the origin. This non-perturbative correction is very important for the precise extraction of the S-wave scattering lengths of the scattering from the experimental data on the energy level displacements in pionic hydrogen by the PSI Collaboration. In addition, the shift of the energy level of the ground state of pionic hydrogen is improved by the second-order correction of strong low-energy interactions which is about 0.1%. This testifies the applicability of the perturbative treatment of strong low-energy interactions to the analysis of pionic hydrogen. We show that the width of the energy level of the ground state of pionic hydrogen is valid to all orders of the perturbation theory in strong low-energy interactions.Received: 28 April 2003, Revised: 26 June 2003, Published online: 2 December 2003PACS: 11.10.Ef Lagrangian and Hamiltonian approach - 11.55.Ds Exact S matrices - 13.75.Gx Pion-baryon interactions - 21.10.-k Properties of nuclei; nuclear energy levels - 36.10.-k Exotic atoms and molecules (containing mesons, muons, and other unusual particles)A.N. Ivanov: Permanent address: State Polytechnical University, Department of Nuclear Physics, 195251 St. Petersburg, Russian Federation;N.I. Troitskaya: Permanent address: State Polytechnical University, Department of Nuclear Physics, 195251 St. Petersburg, Russian Federation.     

Extended RPA with ground-state correlations

We propose a time-independent method for finding a correlated ground state of an extended time-dependent Hartree-Fock theory, known as the time-dependent density matrix theory (TDDM). The correlated ground state is used to formulate the small amplitude limit of TDDM (STDDM) which is a version of extended RPA theories with ground-state correlations. To demonstrate the feasibility of the method, we calculate the ground state of ²² O and study the first 2 ⁺ state and its two-phonon states using STDDM.Received: 7 November 2003, Published online: 10 August 2004PACS: 21.10.Re Collective levels - 21.60.Jz Hartree-Fock and random-phase approximations - 27.30. + t      

Atomic coherent states studied by virtue of the EPR entangled state and their Wigner functions

Based on the newly constructed Einstein, Podolsky and Rosen (EPR) entangled state representation we introduce macroscopic classical functions associated with atomic coherent state τ with angular momentum value j. These functions are proportional to the ordinary one-variable Hermite polynomials of order 2j. The corresponding Wigner quasiprobability function for τ in phase space is also derived which turns out to be a two-variable Hermite polynomial H _{2j, 2j}. In so doing, a new classical-quantum correspondence scheme for angular momentum system is established. Received 7 August 2002 / Received in final form 14 December 2002 Published online 24 April 2003 RID="a" ID="a"Work supported by the National Natural Science Foundation of China under grant 10175057. RID="b" ID="b"e-mail: fhym@sjtu.edu.en     

Isomerization and dissociation of small (CH <Emphasis Type="Bold">3</Emphasis>CN) <Emphasis Type="Bold">n</Emphasis> molecular clusters: a computational study

The isomerization and evaporation processes in the neutral homogeneous (CH₃CN)_n molecular clusters (n = 2-7) have been investigated using classical molecular dynamics simulations. The evaporation rate constants and the kinetic energy release in the dissociation have been analysed as a function of the cluster size and as a function of the internal energy in the parent cluster. The competition between monomer and dimer ejections has been also carefully studied. All the dynamical properties in these dissociative processes have been discussed in relation to the static properties of the clusters involved in the dissociation and also in relation to the solid-liquid like transition which appears in these homogeneous molecular clusters. Received 19 November 2002 / Received in final form 5 February 2003 Published online 29 April 2003 RID="a" ID="a"e-mail: pascal.parneix@ppm.u-psud.fr RID="b" ID="b"Laboratoire associé à l'université Paris-Sud.     

Instabilities in foam clusters related to the pressures in the bubbles

We reinterpret an instability in a two-dimensional free foam cluster previously discussed by Weaire et al. (Eur. Phys. J. E 7, 123 (2002)) in terms of the excess pressure in the bubbles. We conclude that in a free foam cluster no bubble can have a pressure below that of the surrounding gas. Received: 20 September 2002 / Accepted: 5 May 2003 / Published online: 27 May 2003 RID="a" ID="a"e-mail: fatima.vaz@ist.utl.pt     

Band structure of tetragonal BaTiO <Emphasis Type="Bold">3</Emphasis>

The electronic structure, total density of states DOS and electronic density in ferroelectric tetragonal crystal BaTiO₃ are studied using WIEN2k package. This employs the full potential-linearized augmented plan wave FP-LAPW method in the framework of the density functional theory DFT with the generalized gradient approximation (GGA). The results show an indirect band gap of 2.30 eV at the Γ point in the Brillouin zone. The calculated band structure and density of states of BaTiO₃ agree with the previous experimental and theoretical results, as do the charge distribution and the prediction of the nature of the chemical bonding. Received 11 December 2002 / Received in final form 3 February 2003 Published online 1st April 2003 RID="a" ID="a"e-mail: salehihamid@yahoo.com     

On the nature of the low-temperature phase in discontinuous mean-field spin glasses

The low-temperature phase of discontinuous mean-field spin glasses is generally described by a one-step replica symmetry breaking (1RSB) ansatz. The Gardner transition, i.e. a very-low-temperature phase transition to a full replica symmetry breaking (FRSB) phase, is often regarded as an inessential, and somehow exotic phenomenon. In this paper we show that the metastable states which are relevant for the out-of-equilibrium dynamics of such systems are always in a FRSB phase. The only exceptions are (to the best of our knowledge) the p-spin spherical model and the random energy model (REM). We also discuss the consequences of our results for aging dynamics and for local search algorithms in hard combinatorial problems. Received 10 February 2003 Published online 20 June 2003 RID="a" ID="a"e-mail: Federico.Ricci@roma1.infn.it RID="b" ID="b"UMR 8549, Unité Mixte de Recherche du Centre National de la Recherche Scientifique et de l' école Normale Supérieure     

Persistent currents in a lattice correlated electron ring with a magnetic impurity

The behavior of charge and spin persistent currents in an integrable lattice ring of strongly correlated electrons with a magnetic impurity is exactly studied. Our results manifest that the oscillations of charge and spin persistent currents are similar to the ones, earlier obtained for integrable continuum models with a magnetic impurity. The difference is due to two (instead of one) Fermi velocities of low-lying excitations. The form of oscillations in the ground state is “saw-tooth”-like, generic for any multi-particle coherent one-dimensional models. The integrable magnetic impurity introduces net charge and spin chiralities in the generic integrable lattice system, which determine the initial phase shifts of charge and spin persistent currents. We show that the magnitude of the charge persistent current in the generic Kondo situation does not depend on the parameters of the magnetic impurity, unlike the (magneto)resistivity of transport currents. Received 30 January 2003 / Received in final form 12 March 2003 Published online 11 April 2003 RID="a" ID="a"e-mail: zvyagin@fy.chalmers.se