Electronic phase transitions in the spinless falicov-kimball model with correlated hopping

The extrapolation of small-cluster exact-diagonalization calculations is used to examine the influence of correlated hopping on valence and metal-insulator transitions in the one-dimensional Falicov-Kimball model. It is shown that in the half-filled band case the ground-state phase diagram as well as the picture of valence and metal-insulator transitions found for the conventional Falicov-Kimball model (without correlated hopping) are strongly changed when the correlated hopping term is added. The effect of correlated hopping is so strong that it can induce the insulator-metal transition. Outside half-filling correlated hopping stabilizes the segregated phase in the ground-state, however, the nature of the ground-state remains qualitatively unchanged.     

Commensurate-incommensurate transition in two-coupled chains of nearly half-filled electrons

We investigate the physical properties of two coupled chains of electrons, with a nearly half-filled band, as a function of the interchain hopping t _⊥ and the doping. We show that upon doping, the system undergoes a metal-insulator transition well described by a commensurate-incommensurate transition. By using bosonization and renormalization we determine the full phase diagram of the system, and the physical quantities such as the charge gap. In the commensurate phase two different regions, for which the interchain hopping is relevant and irrelevant exist, leading to a confinement-deconfinement crossover in this phase. A minimum of the charge gap is observed for values of t _⊥ close to this crossover. At large t _⊥ the region of the commensurate phase is enhanced, compared to a single chain. At the metal-insulator transition the Luttinger parameter takes the universal value K _ρ ^* = 1, in agreement with previous results on special limits of this model. Received 31 July 2000     

Phase diagram of the Mott transition in a two-band Hubbard model in infinite dimensions

The Mott metal-insulator transition in the two-band Hubbard model in infinite dimensions is studied by using the linearized dynamical mean-field theory recently developed by Bulla and Potthoff. The phase boundary of the metal-insulator transition is obtained analytically as a function of the on-site Coulomb interaction at the d-orbital, the charge-transfer energy between the d- and p-orbitals and the hopping integrals between p-d, d-d and p-p orbitals. The result is in good agreement with the numerical results obtained from the exact diagonalization method. Received 5 October 2000 and Received in final form 8 December 2000     

Magnetism and phase separation in the ground state of the Hubbard model

We discuss the ground state magnetic phase diagram of the Hubbard model off half filling within the dynamical mean-field theory. The effective single-impurity Anderson model is solved by Wilson's numerical renormalization group calculations, adapted to symmetry broken phases. We find a phase separated, antiferromagnetic state up to a critical doping for small and intermediate values of U, but could not stabilize a Néel state for large U and finite doping. At very large U, the phase diagram exhibits an island with a ferromagnetic ground state. Spectral properties in the ordered phases are discussed. Received 9 January 2002 Published online 25 June 2002     

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     

A fractional-spin phase in the power-law Kondo model

We consider a Kondo impurity coupled to a fermionic host with a power-law density of states near the Fermi level, ρ(ε) ∼ |ε|^r, with exponent r < 0. Using both perturbative renormalization group (poor man's scaling) and numerical renormalization group methods, we analyze the phase diagram of this model for ferromagnetic and antiferromagnetic Kondo coupling. Both sectors display non-trivial behavior with several stable phases separated by continuous transitions. In particular, on the ferromagnetic side there is a stable intermediate-coupling fixed point with universal properties corresponding to a fractional ground-state spin. Received 18 February 2002 Published online 31 July 2002     

Phase diagram of the three-dimensional Hubbard model at half filling

We investigate the phase diagram of the three-dimensional Hubbard model at half filling using quantum Monte Carlo (QMC) simulations. The antiferromagnetic Néel temperature is determined from the specific heat maximum in combination with finite-size scaling of the magnetic structure factor. Our results interpolate smoothly between the asymptotic solutions for weak and strong coupling, respectively, in contrast to previous QMC simulations. The location of the metal-insulator transition in the paramagnetic phase above is determined using the electronic compressibility as criterion. Received 11 April 2000 and Revised in final form 29 June 2000     

X-ray scattering study of the phase transitions in crystalline fullerene-biphenyl C 60 [(C 6 H 5 ) 2 ]

The phase diagram of the newly synthesized mixed crystal C₆₀-biphenyl is investigated as a function of temperature by single-crystal X-ray scattering. Diffuse scattering investigations evidencing complex disorder and local order effects are presented. Two phase transitions leading to two different doublings of the high temperature unit cell are observed, at 212 K and 147 K. The first transition is attributed to the ordering of twisted biphenyls, which couples to the orientational ordering of the C₆₀ molecules as the temperature decreases. Full ordering of the C₆₀ molecules is achieved below 100 K only, in the low temperature phase. The rich phase diagram of C₆₀-biphenyl is due to the interplay between fullerene and biphenyl ordering phenomena. Received 31 August 2001 and Received in final form 4 December 2001     

Some exact results for the spin-1/2 Falicov-Kimball model in the strong coupling limit

The spin-1/2 Falicov-Kimball model for electronically driven valence and metal-insulator transitions is studied analytically using strong-coupling perturbation theory. It is shown that in the limit of the infinite interaction strength between localized and itinerant electrons the Falicov-Kimball model undergoes two types of discontinuous valence transitions: the insulator-metal transitions from an integer-valence ground state (n _f = 1) into an inhomogeneous intermediate-valence ground state (0 < n _f < 1) and the insulator-metal transitions from n _f = 1 to n _f = 0. In addition, we discuss the role of the electron-phonon interaction on the mechanism of valence transitions and we present possible extensions of the exact one dimensional results to higher dimensions.     

Electronic structure and metal-insulator transition in SrTi 1 - x Ru xO 3

We studied the changes in the electronic structure of SrTi_1-xRu_xO₃ across the metal-insulator transition. The parent compound, SrTiO₃, is a well known diamagnetic insulator; whereas the doped compound, SrTi_1-xRu_xO₃, becomes a ferromagnetic metal above x _C = 0.35. The techniques used in the study were photoemission (PES) and O 1 s X-ray absorption (XAS) spectroscopy. The experimental spectra were analyzed in terms of band structure and Hubbard model calculations. The PES and XAS spectra of SrTi_1-xRu_xO₃ show the Ru 4 d bands growing in the band gap of SrTiO₃ . The analysis in terms of the Hubbard model indicates that the Ti 3 d and Ru 4 d bands are mostly decoupled. This suggests that the metal-insulator transition is a percolation transition like that of metals embedded in a rare gas matrix. Electron correlation effects are present in this system, but they do not seem to play a major role in the transition. Received 10 September 2001     

Finite-size scaling of the level compressibility at the Anderson transition

We compute the number level variance Σ ₂ and the level compressibility χ from high precision data for the Anderson model of localization and show that they can be used in order to estimate the critical properties at the metal-insulator transition by means of finite-size scaling. With N, W, and L denoting, respectively, linear system size, disorder strength, and the average number of levels in units of the mean level spacing, we find that both χ(N, W) and the integrated Σ ₂ obey finite-size scaling. The high precision data was obtained for an anisotropic three-dimensional Anderson model with disorder given by a box distribution of width W/2. We compute the critical exponent as ν≈ 1.45±0.12 and the critical disorder as W _c≈ 8.59±0.05 in agreement with previous transfer-matrix studies in the anisotropic model. Furthermore, we find χ≈ 0.28±0.06 at the metal-insulator transition in very close agreement with previous results. Received 1st November 2001 and Received in final form 8 March 2002 Published online 6 June 2002     

Critical behaviour near the metal-insulator transition of a doped Mott insulator

We have studied the critical behaviour of a doped Mott insulator near the metal-insulator transition for the infinite-dimensional Hubbard model using a linearized form of dynamical mean-field theory. The discontinuity in the chemical potential in the change from hole to electron doping, for U larger than a critical value U _c, has been calculated analytically and is found to be in good agreement with the results of numerical methods. We have also derived analytic expressions for the compressibility, the quasiparticle weight, the double occupancy and the local spin susceptibility near half-filling as functions of the on-site Coulomb interaction and the doping. Received 15 March 2001 and Received in final form 22 May 2001     

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     

Spin glass and ferromagnetism in Kondo lattice compounds

The Kondo lattice model has been analyzed in the presence of a random inter-site interaction among localized spins with non zero mean J₀ and standard deviation J. Following the same framework previously introduced by us, the problem is formulated in the path integral formalism where the spin operators are expressed as bilinear combinations of Grassmann fields. The static approximation and the replica symmetry ansatz have allowed us to solve the problem at a mean field level. The resulting phase diagram displays several phase transitions among a ferromagnetically ordered region,a spin glass one, a mixed phase and a Kondo state depending on J₀, J and its relation with the Kondo interaction coupling J_K. These results could be used to address part of the experimental data for the CeNi _{1 - x} Cu _x compound, when x ⩽ 0.8. Received 24 June 2002 Published online 31 December 2002     

Investigation of α-nucleus interaction in the 27Al (α,α)27Al scattering and 27Al (α, d )29Si reaction

Full finite-range macroscopic calculations in the distorted-wave Born approximation have been performed using the molecular and Michel α-nucleus potentials to analyze the angular distributions of cross-sections of the ²⁷Al(α, d)²⁹Si reaction, at 26.5 and 27.2 MeV incident energies, leading to seven transitions up to the excitation energy E _X = 4.08 MeV of the final nucleus. The parameters of the two types of the α-nucleus potentials are determined from the elastic-scattering data. Both the molecular and Michel potentials, without any adjustment to the parameters needed to fit the elastic-scattering data, are able in most cases to reproduce, simultaneously, the absolute cross-sections particularly at large angles, where the previous calculations failed to reproduce by orders of magnitude, and the gross pattern of angular distributions of the reaction. The deuteron-cluster spectroscopic factors for most of the seven transitions, deduced using the two α-²⁷Al potentials, differ from those obtained in earlier works. The spectroscopic factor for the ground-state transition, deduced in the present work for the 25.8 MeV data, agrees well with the shell model prediction. Received: 15 July 2002 / Accepted: 8 August 2002 / Published online: 10 December 2002 RID="a" ID="a"e-mail: akbasak2001@Yahoo.com Communicated by G. Orlandini     

Non-uniformities in polymer/liquid crystal mixtures

We theoretically model the nucleation of nematic droplets during phase ordering in mixtures of a flexible polymer and a low-molecular-weight liquid crystal. By appealing to classical nucleation theory (CNT), we calculate the energy barrier to nucleation and the size of a critical nucleus. We study the influence of a metastable intermediate phase on the nucleation of the nematic. Below a triple point in the phase diagram, there are two distinct mechanisms for the formation of a nematic nucleus: 1) direct nucleation from the isotropic phase and 2) nucleation via a precursor metastable isotropic phase. We calculate the crossover concentration as a function of temperature, delineating the regions of the phase diagram in which each mechanism prevails. In the latter case, the presence of a hidden metastable isotropic-isotropic binodal may either promote or delay the nucleation of a nematic phase. Received 9 August 2002 RID="a" ID="a"e-mail: matuyama@chem.mie-u.ac.jp     

Nuclear levels in 228Th populated in the decay of 228Pa (II)

The electron-capture decay of ²²⁸Pa to levels in ²²⁸Th was studied using mass-separated sources and a γ-ray detection system consisting of five Compton-suppressed Ge detectors. A total of 87 levels were observed up to an excitation energy of 2 MeV which are connected by approximately 500 γ-ray transitions. The complete octupole quadruplet, three excited K ^π=0⁺ bands and two K ^π=2⁺ bands were identified below 1.4 MeV. The observed level structure is compared to calculations within the quasiparticle-phonon nuclear model. The surprisingly good agreement indicates that ²²⁸Th has less transitional character than hitherto assumed. Received: 30 March 1998     

The glassy phase of Gallager codes

Gallager codes are the best error-correcting codes to date. In this paper we study them by using the tools of statistical mechanics. The corresponding statistical mechanics model is a spin model on a sparse random graph. The model can be solved by elementary methods (i.e. without replicas) in a large connectivity limit. For low enough temperatures it presents a completely frozen glassy phase (q _EA = 1). The same scenario is shown to hold for finite connectivities. In this case we adopt the replica approach and exhibit a one-step replica symmetry breaking order parameter. We argue that our ansatz yields the exact solution of the model. This allows us to determine the whole phase diagram and to understand the performances of Gallager codes. Received 9 April 2001     

Collapse of sodium polyacrylate chains in calcium salt solutions

The sodium salt of polyacrylic acid (NaPA) precipitates in the presence of Ca²⁺-ions. This phase behaviour can be represented by a phase diagram where the critical NaPA concentration is plotted versus the critical Ca²⁺ concentration resulting in a straight line as a phase boundary. The location of this phase boundary is influenced by the presence of an inert monovalent salt like NaCl. The present contribution focuses on the coil dimensions of NaPA chains in dilute aqueous solution corresponding to the one phase region of such a phase diagram. A variety of parameters with which the size and shape of the polyelectrolyte chains can be modulated are revealed. Approaching the phase boundary by decreasing the NaPA concentration at a constant Ca²⁺ content leads to a collapse of the NaPA chains. Combined static and dynamic light scattering suggests a compact spherical shape as the final state of this transition, both in 0.1 M NaCl and in 0.01 M NaCl. In the lower NaCl concentration, indication is presented for the existence of a cigar or pearl necklace like intermediate. Most strikingly, the collapsed chains can be reexpanded by increasing the concentration of inert NaCl at constant content of NaPA and Ca²⁺. Clearly, excessive Na⁺-ions displace the Ca²⁺-ions from the NaPA chains. Received 18 July 2000 and Received in final form 24 August 2000     

Magnetic, electronic, dielectric and optical properties of Pr(Ca:Sr)MnO 3

The charge-ordered perovskite Pr_0.65Ca_0.28Sr_0.07MnO₃ was investigated by means of magnetic susceptibility, specific heat, dielectric and optical spectroscopy and electron-spin resonance techniques. Under moderate magnetic fields, the charge order melts yielding colossal magnetoresistance effects with changes of the resistivity over eleven orders of magnitude. The optical conductivity is studied from audio frequencies far into the visible spectral regime. Below the phonon modes hopping conductivity is detected. Beyond the phonon modes the optical conductivity is explained by polaronic excitations out of a bound state. ESR techniques yield detailed informations on the (H,T ) phase diagram and reveal a broadening of the linewidth which can be modeled in terms of activated polaron hopping. Received 9 August 2000