Low-temperature properties of the Ce(Pd1−xNix) system

The influence of substituting Pd by Ni is described in Ce(Pd_1-xNi_x)₃ alloys with x taken up to about 0.25. Thermal and magnetization measurements point out a transition from a non-magnetic state (CePd₃) to a ferromagnetic state for x > 0.05, with a Curie temperature K. The Ce-L_2,3 absorption edges and magnetic circular dichroism (XMCD) study reveals the coexistence of strong 4f hybridization and ferromagnetic order. The Ce-L_2,3 XMCD signal measured in CePd₃ demonstrates that in the Ce-based dense Kondo materials only the 4f¹ channel gives a magnetic response.Received: 2 September 2003, Published online: 15 March 2004PACS: 75.20.Hr Local moment in compounds and alloys; Kondo effect, valence fluctuations, heavy fermions - 75.30.Mb Valence fluctuation, Kondo lattice, and heavy-fermion phenomena - 71.10.Hf Non-Fermi-liquid ground states, electron phase diagrams and phase transitions in model systems     

Dilute limit of a strongly-interacting model of spinless fermions and hardcore bosons on the square lattice

In our model, spinless fermions (or hardcore bosons) on a square lattice hop to nearest neighbor sites, and also experience a hard-core repulsion at the nearest neighbor separation. This is the simplest model of correlated electrons and is more tractable for exact diagonalization than the Hubbard model. We study systematically the dilute limit of this model by a combination of analytical and several numerical approaches: the two-particle problem using lattice Green functions and the t-matrix, the few-fermion problem using a modified t-matrix (demonstrating that the interaction energy is well captured by pairwise terms), and for bosons the fitting of the energy as a function of density to Schicks analytical result for dilute hard disks. We present the first systematic study for a strongly-interacting lattice model of the t-matrix, which appears as the central object in older theories of the existence of a two-dimensional Fermi liquid for dilute fermions with strong interactions. For our model, we can (Lanczos) diagonalize the 7 x 7 system at all fillings and the 20 x 20 system with four particles, thus going far beyond previous diagonalization works on the Hubbard model.Received: 8 May 2003, Published online: 28 May 2004PACS: 71.10.Fd Lattice fermion models (Hubbard model, etc.) - 71.10.Pm Fermions in reduced dimensions (anyons, composite fermions, Luttinger liquid, etc.) - 05.30.Jp Boson systems - 74.20.Mn Nonconventional mechanisms (spin fluctuations, polarons and bipolarons, resonating valence bond model, anyon mechanism, marginal Fermi liquid, Luttinger liquid, etc.)N.G. Zhang: Present address: Dept. of Physics, George Washington University, Washington, DC 20052     

Spin-gap phase in the extended t — J chain

We study the one-dimensional deformed model in terms of the continuum field theories. We found that at low doping concentration and far away from the phase separation regime, there are two phases: the Luttinger liquid and the Luther-Emery liquid, depending on or , where . Moreover, the singlet superconducting correlations are dominant in the Luther-Emery liquid.Received: 12 October 2003, Published online: 19 February 2004PACS: 71.10.Fd Lattice fermion models - 71.10.Hf Non-Fermi-liquid ground states - 74.20.Mn Non-conventional mechanisms     

Charge density plateaux and insulating phases in the $\mathsf{t-J}$ model with ladder geometry

We discuss the occurrence and the stability of charge density plateaux in ladder-like t-J systems (at zero magnetization M = 0) for the cases of 2- and 3-leg ladders. Starting from isolated rungs at zero leg coupling, we study the behaviour of plateaux-related phase transitions by means of first order perturbation theory and compare our results with Lanczos diagonalizations for t-J ladders (N = 2 × 8) with increasing leg couplings. Furthermore we discuss the regimes of rung and leg couplings that should be favoured for the appearance of the charge density plateaux.Received: 28 July 2003, Published online: 8 December 2003PACS: 71.10.Fd Lattice fermion models (Hubbard model, etc.) - 71.27. + a Strongly correlated electron systems; heavy fermions - 75.10.-b General theory and models of magnetic ordering - 75.10.Jm Quantized spin models     

Fourth-order perturbation theory for the half-filled Hubbard model in infinite dimensions

We calculate the zero-temperature self-energy to fourth-order perturbation theory in the Hubbard interaction U for the half-filled Hubbard model in infinite dimensions. For the Bethe lattice with bare bandwidth W, we compare our perturbative results for the self-energy, the single-particle density of states, and the momentum distribution to those from approximate analytical and numerical studies of the model. Results for the density of states from perturbation theory at U/W = 0.4 agree very well with those from the Dynamical Mean-Field Theory treated with the Fixed-Energy Exact Diagonalization and with the Dynamical Density-Matrix Renormalization Group. In contrast, our results reveal the limited resolution of the Numerical Renormalization Group approach in treating the Hubbard bands. The momentum distributions from all approximate studies of the model are very similar in the regime where perturbation theory is applicable, . Iterated Perturbation Theory overestimates the quasiparticle weight above such moderate interaction strengths.Received: 9 September 2003, Published online: 30 January 2004PACS: 71.10.Fd Lattice fermion models (Hubbard model, etc.) - 71.27. + a Strongly correlated electron systems; heavy fermions - 71.30. + h Metal-insulator transitions and other electronic transitions     

Metal-insulator transition in the Hubbard model: a simple description including the Kondo effect

The electron spectrum structure in the half-filled Hubbard model is considered in terms of the one-particle Greens functions within many-electron representation. A simple analytical generalization of the single-site Hubbard-III approximation is obtained, which takes into account the Fermi excitations (Kondo terms). The problem of the metal-insulator transition in the paramagnetic phase is investigated. The occurrence of a three-peak density-of-states structure including the Kondo peak at the Fermi level is discussed. A comparison with large-d calculations is performed.Received: 5 January 2004, Published online: 8 June 2004PACS:   71.10.Fd Lattice fermion models (Hubbard model, etc.) - 71.27. + a Strongly correlated electron systems; heavy fermions - 71.30. + h Metal-insulator transitions and other electronic transitions     

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     

The 2-site Hubbard and $\mathsf{t}$-$\mathsf{J}$ models

The fermionic and bosonic sectors of the 2-site Hubbard model have been exactly solved by means of the equation of motion and Greens function formalism. The exact solution of the t-J model has been also reported to investigate the low-energy dynamics. We have successfully searched for the exact eigenoperators, and the corresponding eigenenergies, having in mind the possibility to use them as an operatorial basis on the lattice. Many local, single-particle, thermodynamical and response properties have been studied as functions of the external parameters and compared between the two models and with some numerical and exact results. It has been shown that the 2-site Hubbard model already contains the most relevant energy scales of the Hubbard model: the local Coulomb interaction U and the spin-exchange one . As a consequence of this, for some relevant properties (kinetic energy, double occupancy, energy, specific heat and entropy) and as regards the metal-insulator transition issue, it has resulted possible to almost exactly mime the behavior of larger systems, sometimes using a higher temperature to get a comparable level spacing. The 2-site models have been also used as toy models to test the efficiency of the Greens function formalism for composite operators. The capability to reproduce the exact solutions, obtained by the exact diagonalization technique, gives a firm ground to the approximate treatments based on this formalism.Received: 16 July 2003, Published online: 30 January 2004PACS: 71.10.-w Theories and models of many-electron systems - 71.10.Fd Lattice fermion models (Hubbard model, etc.)     

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     

Analytical and numerical treatment of the Mott-Hubbard insulator in infinite dimensions

We calculate the density of states in the half-filled Hubbard model on a Bethe lattice with infinite connectivity. Based on our analytical results to second order in t/U, we propose a new Fixed-Energy Exact Diagonalization scheme for the numerical study of the Dynamical Mean-Field Theory. Corroborated by results from the Random Dispersion Approximation, we find that the gap opens at . Moreover, the density of states near the gap increases algebraically as a function of frequency with an exponent in the insulating phase. We critically examine other analytical and numerical approaches and specify their merits and limitations when applied to the Mott-Hubbard insulator.Received: 14 July 2003, Published online: 2 October 2003PACS: 71.10.Fd Lattice fermion models (Hubbard model, etc.) - 71.27.+a Strongly correlated electron systems; heavy fermions - 71.30.+h Metal-insulator transitions and other electronic transitions     

Phase diagram of the three-dimensional NJL model

With the exception of confinement the three-dimensional Nambu-Jona-Lasinio (NJL₃) model incorporates many of the essential properties of QCD. We discuss the critical properties of the model at non-zero temperature T and/or non-zero chemical potential . We show that the universality class of the thermal transition is that of the d = 2 classical spin model with the same symmetry. We provide evidence for the existence of a tricritical point in the -plane. We also discuss numerical results by Handset al. which showed that the system is critical for and the diquark condensate is zero.Received: 30 September 2002, Published online: 22 October 2003PACS: 71.10.Fd Lattice fermion models (Hubbard model, etc.) - 05.70.Fh Phase transitions: general studies     

Finite temperature properties and frustrated ferromagnetism in a square lattice Heisenberg model

The spin 1/2 Heisenberg model on a square lattice with antiferromagnetic nearest- and next-nearest neighbour interactions (the J ₁-J ₂ model) has long been studied as a paradigm of a two-dimensional frustrated quantum magnet. Only very recently, however, have the first experimental realisations of such systems been synthesized. The newest material, Pb₂VO(PO₄)₂ seems to have mixed ferro- and antiferromagnetic exchange couplings. In the light of this, we extend the semiclassical treatment of the J ₁-J ₂ model to include ferromagnetic interactions, and present an analysis of the finite temperature properties of the model based on the exact diagonalization of 8, 16 and 20 site clusters. We propose that diffuse neutron scattering can be used to resolve the ambiguity inherent in determining the ratio and sign of J ₁ and J ₂ from thermodynamic properties alone, and use a finite temperature Lanczos algorithm to make predictions for the relevant high temperature spin-spin correlation functions. The possibility of a spin-liquid phase occurring for ferromagnetic J ₁ is also briefly discussed.Received: 19 March 2004, Published online: 8 June 2004PACS: 71.27. + a Strongly correlated electron systems; heavy fermions - 71.10.-w Theory and models of many-electron systems - 75.40.Cx Static properties (order parameter, static susceptibility, heat capacities, critical exponents, etc.)     

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     

Equation of motion method for composite field operators

The Greens function formalism in Condensed Matter Physics is reviewed within the equation of motion approach. Composite operators and their Greens functions naturally appear as building blocks of generalized perturbative approaches and require fully self-consistent treatments in order to be properly handled. It is shown how to unambiguously set the representation of the Hilbert space by fixing both the unknown parameters, which appear in the linearized equations of motion and in the spectral weights of non-canonical operators, and the zero-frequency components of Greens functions in a way that algebra and symmetries are preserved. To illustrate this procedure some examples are given: the complete solution of the two-site Hubbard model, the evaluation of spin and charge correlators for a narrow-band Bloch system, the complete solution of the three-site Heisenberg model, and a study of the spin dynamics in the Double-Exchange model.Received: 9 June 2003, Published online: 19 November 2003PACS: 71.10.-w Theories and models of many-electron systems - 71.27. + a Strongly correlated electron systems; heavy fermions - 71.10.Fd Lattice fermion models (Hubbard model, etc.)     

Nucleation of superconductivity in finite metallic multilayers: Effect of the symmetry

The influence of the finite dimensions of superconducting metallic multilayers on H-T phase diagram is studied. It is established that the geometrical symmetry of the samples determines crucially the dependencies. For samples where the symmetry plane is in the middle of the superconducting layer the values are, for temperatures close to T _c , larger than the values of the samples for which the symmetry plane lies in the middle of the normal layer. The results are analysed on the basis of the Ginzburg-Landau approach and are explained by considering the different symmetry of the wave function describing the system. The experimental results are in good agreement with the elaborated model, which takes into account the actual symmetry of the samples.Received: 23 July 2004, Published online: 5 November 2004PACS: 74.78.Fk Multilayers, superlattices, heterostructures - 74.20.De Phenomenological theories (two-fluid, Ginzburg-Landau, etc.) - 74.45. + c Proximity effects; Andreev effect; SN and SNS junctions     

Ordered phases of XXZ-symmetric spin-1/2 zigzag ladder

Using bosonization approach, we derive an effective low-energy theory for XXZ-symmetric spin-1/2 zigzag ladders and discuss its phase diagram by a variational approach. A spin nematic phase emerges in a wide part of the phase diagram, either critical or massive. Possible crossovers between the spontaneously dimerized and spin nematic phases are discussed, and the topological excitations in all phases identified.Received: 14 August 2004, Published online: 29 June 2004PACS: 75.10.Pq Spin chain models - 75.40.-s Critical-point effects, specific heats, short-range order - 75.30.Gw Magnetic anisotropy - 71.10.Pm Fermions in reduced dimensions (anyons, composite fermions, Luttinger liquid, etc.)     

$\mathsf{\sqrt{3}}$ 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     

Static magnetic susceptibility of the RbC $\mathsf{_{60}}$ polymerized fulleride

A two component model of negative U centers coupled with the Fermi sea of itinerant fermions is discussed in connection with high-temperature superconductivity of cuprates, and superfluidity of atomic fermions. We examine the phase transition and the condensed state of this boson-fermion model (BFM) beyond the ordinary mean-field approximation in two and three dimensions. No pairing of fermions and no condensation are found in two-dimensions for any symmetry of the order parameter. The expansion in the strength of the order parameter near the transition yields no linear homogeneous term in the Ginzburg-Landau-Gorkov equation and a zero upper critical field in any-dimensional BFM, which indicates that previous mean-field discussions of the model are flawed. Normal and anomalous Greens functions are obtained diagrammatically and analytically in the condensed state of a simplest version of 3D BFM. A pairing of bosons analogous to the Cooper pairing of fermions is found. There are three coupled condensates in the model, described by the off-diagonal single-particle boson, pair-fermion and pair-boson fields. These results negate the common wisdom that the boson-fermion model is adequately described by the BCS theory at weak coupling.Received: 26 February 2004, Published online: 18 June 2004PACS: 74.20.-z Theories and models of superconducting state - 74.20.Mn Nonconventional mechanisms (spin fluctuations, polarons and bipolarons, resonating valence bond model, anyon mechanism, marginal Fermi liquid, Luttinger liquid, etc.) - 74.20.Rp Pairing symmetries (other than s-wave) - 74.25.Dw Superconductivity phase diagrams     

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