Antiferromagnetism of almost localized fermions: Evolution from Slater-type to Mott-Hubbard gap

We supplement (and critically overview) the existing extensive analysis of antiferromagnetic solution for the Hubbard model with a detailed discussion of two specific features, namely (i) the evolution of the magnetic (Slater) gap (here renormalized by the electronic correlations) into the Mott-Hubbard or atomic gap, and (ii) a rather weak renormalization of the effective mass by the correlations in the half-filled-band case, which contrasts with that for the paramagnetic case. The mass remains strongly enhanced in the non-half-filled-band case. We also stress the difference between magnetic and non-magnetic contributions to the gap. These results are discussed within the slave boson approach in the saddle-point approximation, in which there appears a non-linear staggered molecular field due to the electronic correlations that leads to the appearance of the magnetic gap. They reproduce correctly the ground-state energy in the limit of strong correlations. A brief comparison with the solution in the limit of infinite dimensions and the corresponding situation in the doubly-degenerate-band case with one electron per atom is also made. Received 16 December 2002 / Received in final form 5 February 2003 Published online 11 April 2003 RID="a" ID="a"e-mail: ufspalek@if.uj.edu.pl     

Influence of the spin-orbit couplingon nuclear superfluidity along the N = Z line

We show that the spin-orbit potential of the nuclear mean field destroys isoscalar superfluid correlations in self-conjugate nuclei. Using group theory and boson mapping techniques on a Hamiltonian including single particle splittings and a SO _ST(8) pairing interaction, we give analytical expressions for the spin-orbit dependence of some N = Z properties such as the relative position of T = 0 and T = 1 states in odd-odd systems or double binding-energy differences of even-even nuclei. Received: 12 April 2000 / Accepted: 25 May 2000     

Noise correlations in shear flows

We consider the effects of a shear on velocity fluctuations in a flow. The shear gives rise to a transient amplification that not only influences the amplitude of perturbations but also their time correlations. We show that, in the presence of white noise, time correlations of transversal velocity components are exponential and that correlations of the longitudinal components are exponential with an algebraic prefactor. Cross correlations between transversal and downstream components are strongly asymmetric and provide a clear indication of non-normal amplification. We suggest experimental tests of our predictions. Received 26 February 2002 / Received in final form 11 March 2003 Published online 20 June 2003 RID="a" ID="a"e-mail: bruno.eckhardt@physik.uni-marburg.de RID="b" ID="b"Also at: Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore, India     

Beta-decay of the N = Z nucleus 72Kr

The beta-decay of the N = Z, even-even nucleus ⁷²Kr has been studied at the ISOLDE PSB facility at CERN. Measurements of βγ and βγγ coincidences have enriched the decay scheme of the daughter nucleus ⁷²Br with 27 new low spin levels. A more precise half-life of T _1/2 = 17.1(2) s has been determined. Strong feeding to the ⁷²Br ground state is established yielding an unambiguous J ^π = 1⁺ assignment for this state. Candidates for the ⁷²Br g.s. wave function are discussed in the framework of a self-consistent deformed mean-field calculation with SG2 Skyrme force and pairing correlations. A search for beta-delayed particle emission was made and an upper limit of 10^-6 for this decay branch obtained. The cumulated experimental level density of 1⁺ states has been fitted with the constant temperature formula. The comparison indicates that most likely all 1⁺ levels up to 1.2 MeV have been observed in this investigation. The corresponding nearest-neighbour level spacing does not follow a Poisson distribution. The Gamow-Teller strength distribution is compared, in terms of nuclear deformation, with different calculations made in the framework of the quasiparticle random phase approximation. Received: 7 February 2002 / Accepted: 31 October 2002 / Published online: 6 March 2003 RID="a" ID="a"e-mail: borge@pinar2.csic.es RID="b" ID="b"Present address: Centre d' Etudes Nucléaires de Bordeaux-Gradignan, Le Haut Vigneau, F-33175 Gradignan Cedex, France. RID="c" ID="c"Present address: University Mentouri, 25000 Constantine, Algeria. Communicated by J. ?yst?     

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     

Nature of the Peierls- to Mott-insulator transition in 1D

In order to clarify the physics of the crossover from a Peierls band insulator to a correlated Mott-Hubbard insulator, we analyze ground-state and spectral properties of the one-dimensional half-filled Holstein-Hubbard model using quasi-exact numerical techniques. In the adiabatic limit the transition is connected to the band to Mott insulator transition of the ionic Hubbard model. Depending on the strengths of the electron-phonon coupling and the Hubbard interaction the transition is either first order or evolves continuously across a narrow intermediate phase with finite spin, charge, and optical excitation gaps. Received 7 July 2002 / Received in final form 21 October 2002 Published online 27 January 2003 RID="a" ID="a"e-mail: holger.fehske@physik.uni-greifswald.de     

Electrodynamics of the vortex lattice in untwinned YBaCuO by complex impedance measurements

We report complex impedance measurements in an untwinned YBaCuO crystal. Our broad frequency range covers both the quasi static response and the resistive response of the vortex lattice. It allow us to characterize the irreversibility line without the need of any frequency dependent pinning parameters. We confirm the validity of the two modes model of vortex dynamic, and extract both the surface critical current and the flux flow resistivity around the first order transition T_m. This latter is identified by the abrupt loss of pinning and by an unexpected step of (T) at T_m. Received 22 November 2002 / Received in final form 17 February 2003 Published online 20 June 2003 RID="a" ID="a"e-mail: alain.pautrat@ismra.fr RID="b" ID="b"UMR 6508 associée au CNRS     

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     

Effect of interfacial alloying on the surface plasmon resonance of nanocrystalline Au-Ag multilayer thin films

Nanocrystalline Au and Ag in multilayer thin film form with Au/Ag/Au structure were prepared by high pressure (∼40 Pa) d.c. sputtering techniques. The Ag concentrations in Ag_xAu_1-x films were changed from x = 0 to 1. These multilayer films with varying Ag concentration showed significant changes in microstructures obtained from TEM and XRD analyses. The optical absorption spectra of these multilayer films showed a single plasmon band confirming the formation of Au-Ag alloy. We ascribe this alloying to the interfacial reactions in nanophase limited at the Au-Ag interface. The red-shift and broadening of the plasmon bands with the increase in silver concentration could be associated to the increase in size of the nanoparticles and its distribution. The observed red shift in the plasmon band may be associated with the change in electronic structure at the Au-Ag interface due to configuration mixing of the atomic energy levels of Au and Ag. Received 17 October 2002 / Received in final form 26 February 2003 Published online 23 May 2003 RID="a" ID="a"e-mail: msakp@mahendra.iacs.res.in     

Statics and dynamics of phase segregation in multicomponent fermion gas

We investigate the statics and dynamics of spatial phase segregation process of a mixture of fermion atoms in a harmonic trap using the density functional theory. The kinetic energy of the fermion gas is written in terms of the density and its gradients. Several cases have been studied by neglecting the gradient terms (the Thomas-Fermi limit) which are then compared with the Monte-Carlo results using the full gradient corrected kinetic energy. A linear instability analysis has been performed using the random-phase approximation. Near the onset of instability, the fastest unstable mode for spinodal decomposition is found to occur at q = 0. However, in the strong coupling limit, many more modes with q ≈ K _F decay with comparable time scales. Received 12 September 2001 Published online 24 September 2002 RID="a" ID="a"e-mail: k1@sharif.edu RID="b" ID="b"Permanent address: Dr. Vijay Kumar Foundation, 45 Bazaar Street, K.K. Nagar (West), Chennai 600 078, India.     

Giant entropy change at the co-occurrence of structural and magnetic transitions in the Ni Mn Ga Heusler alloy

We have studied the isothermal entropy change around a first-order structural transformation and in correspondence to the second-order Curie transition in the ferromagnetic Heusler alloy Ni_2.15Mn_0.85Ga. The results have been compared with those obtained for the composition Ni_2.19Mn_0.81Ga, in which the martensitic structural transformation and the magnetic transition occur simultaneously. With a magnetic field span from 0 to 1.6 T, the magnetic entropy change reaches the value of 20 J/kg K when transitions are co-occurring, while 5 J/kg K is found when the only structural transition occurs. Received 27 September 2002 / Received in final form 17 February 2003 Published online 11 April 2003 RID="a" ID="a"e-mail: solzi@fis.unipr.it     

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     

Metastable states of the Ising chain with Kawasaki dynamics

We consider a ferromagnetic Ising chain evolving under Kawasaki dynamics at zero temperature. We investigate the statistics of the blocking time, as well as various characteristics of the metastable configurations reached by the system, including the statistics of the final energy, the spin correlations, and the distribution of domain sizes. Results of extensive numerical simulations are compared with analytical predictions made for the a priori ensemble of all blocked configurations with equal weights. Qualitative differences are found, e.g. in the domain sizes, which are found to be neither statistically independent nor exponentially distributed. Received 24 October 2002 / Received in final form 13 January 2003 Published online 1st April 2003 RID="a" ID="a"e-mail: luck@spht.saclay.cea.fr RID="b" ID="b"URA 2306 of CNRS     

Andreev-Lifshitz supersolid revisited for a few electrons on a square lattice II

In this second paper, using N = 3 polarized electrons (spinless fermions) interacting via a U/r Coulomb repulsion on a two dimensional L×L square lattice with periodic boundary conditions and nearest neighbor hopping t, we show that a single unpaired fermion can co-exist with a correlated two particle Wigner molecule for intermediate values of the Coulomb energy to kinetic energy ratio r _s = UL/(2t ). This supports in an ultimate mesoscopic limit a possibility proposed by Andreev and Lifshitz for the thermodynamic limit: a quantum crystal may have delocalized defects without melting, the number of sites of the crystalline array being smaller than the total number of particles. When L = 6, the ground state exhibits four regimes as r_s increases: a Hartree-Fock regime, a first supersolid regime where a correlated pair co-exists with a third fully delocalized particle, a second supersolid regime where the third particle is partly delocalized, and eventually a correlated lattice regime. Received 22 October 2002 Published online 23 May 2003 RID="a" ID="a"e-mail: jpichard@cea.fr     

Reliability of the pseudospin symmetry in atomic nuclei

The reliability of the pseudospin symmetry (PSS) in atomic nuclei is analyzed in the framework of the relativistic Hartree approach. We find that the nuclear surface strongly increases the effect of the pseudospin-orbit potential (PSOP), spoiling the possibility of the exact realization of the PSS even in the limit of a vanishing PSOP. It is also shown that the PSS cannot be explained by the fact that Σ_S ≃ - Σ. New arguments to explain the PSS in finite nuclei are given. The important role the spin-orbit interaction plays in the achievement of the PSS is also discussed. Received: 22 July 2002 / Accepted: 18 February 2003 / Published online: 20 May 2003     

Metal-insulator transition in two-dimensional systems with long-range correlated disorder

We study the localization properties of electrons in a two-dimensional model with on-site energies exhibiting long-range correlated disorder. The localization length and conductance of the system are calculated by using the finite size scaling method combined with transfer matrix technique. In the presence of long-range correlations, we find that there is a continuous line of fixed points indicating that the system undergoes a disorder driven Kosterlitz-Thouless-type metal-insulator transition. Received 6 March 2003 Published online 20 June 2003 RID="a" ID="a"e-mail: wsliu@sjtu.edu.cn     

SU(3) coupling schemes for odd-odd nuclei in the interacting boson – fermion – fermion model with both odd proton and odd neutron in natural parity orbits

The SU^BFF(3) dynamical symmetry limits of interacting boson – fermion – fermion model are identified and they are appropriate for heavy deformed odd – odd nuclei for configurations with both the odd proton and odd neutron occupying all the natural parity orbits in the corresponding valence shells. There are three symmetry limits and their correspondence with two quasi-particle (proton-neutron) Nilsson configurations is established; one of the limits mixes both Nilsson n_z's and Λ's and other two limits mix only Nilsson Λ's. The ¹⁹¹Ir (d,t) ¹⁹⁰Ir single nucleon transfer spectroscopic strengths are well described by one of the symmetry limits that mixes only Nilsson Λ's. Received: 22 June 1998     

A simple theory for high Δ T c ratio in d-wave superconductors

We investigate a simple explanation for the high maximum gap to T _c ratio found experimentally in high T _c compounds. We ascribe this observation to the lowering of T _c by boson scattering of electrons between parts of the Fermi surface with opposite sign for the order parameter. We study the simplest possible model within this picture. Our quantitative results show that we can account for experiment for a rather small value of the coupling constant, all the other ingredients of our model being already known to exist in these compounds. A striking implication of this theory is the fairly high value of the critical temperature in the absence of boson scattering. Received 12 March 2001 and Received in final form 25 May 2001     

Application of the Dyson boson expansion method to the treatment of the pairing force

The treatment of the separable pairing interaction in the context of the BRST formalism and in the Dyson boson expansion method is discussed. The approach is based on the use of the vacuum expectation value of the boson number operator to define a suitable mean field. Received: 10 April 2000 / Accepted: 6 July 2001     

Coexistence of color superconductivity and chiral symmetry breaking within the NJL model

The phase diagram for quark matter is investigated within a simple Nambu-Jona-Lasinio model without vector correlations. It is found that the phase structure in the temperature-density plane depends sensitively on the parametrization of the model. We present two schemes of parametrization of the model where, within the first one, a first-order phase transition from a phase with broken chiral symmetry to a color superconducting phase for temperatures below the triple point at T _t = 55 MeV occurs, whereas for the second one a second-order phase transition for temperatures below T _t = 7 MeV is found. In the latter case, there is also a coexistence phase of broken chiral symmetry with color superconductivity, which is a new finding within this class of models. Possible consequences for the phenomenology of the QCD phase transition at high baryon densities are discussed. Received: 3 January 2003 / Accepted: 21 February 2003 / Published online: 24 April 2003