摘要:

We analyze the problem of the interplay of superconductivity and ferromagnetism, by focusing on their coexistence. The interest in this field has been renewed after the recent discovery of coexistence of superconductivity and ferromagnetism in UGe2, URhGe, ZrZn2, and in rutheno‐cuprate RuSr2RECu2O8compounds, with RE= Eu or Gd. In this paper we concentrate on the competition between itinerant ferromagnetism and singlet superconductivity looking at the case in which the same electrons participate in both the ordered phases. By using a mean field approximation, we have studied the stability of the coexistence phase when the ferromagnetic instability is described through the Stoner model, and when it can be ascribed to a kinetic exchange mechanism. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1639591

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

Superconductivity in the two component model of coexisting local electron pairs (charged bosons) and itinerant fermions coupled via a charge exchange mechanism is discussed. The cases of isotropics‐wave and anisotropic pairing of extendeds‐wave anddx2−y2symmetries are analyzed for a 2D square lattice within the BCS‐mean field approximation and the Kosterlitz‐Thouless theory. The phase diagrams and superconducting characteristics of the model as a function of the position of the local pair (LP) level and the total carrier concentration, are determined. The model exhibits interesting crossovers from the BCS like behavior to that of LP’s. Finally, we analyze the pairing fluctuation effects (in 3D) within a generalized T‐matrix approach and determine the transition temperatures from the pseudogap state. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1639592

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

A formalism for the study of highly interacting electronic systems is presented. The proposed scheme is based on two key concepts: composite operators and algebra constraints. Composite field operators, that naturally appear as a consequence of interaction, are promoted to the rank of basic fields in terms of which a perturbation formulation is set up. The formalism is based on the use of Green’s function and equation of motion method. The use of composite operators requires a revisitation of the Green’s function formulation, where the representation is determined by means of algebra constraints which are a manifestation at macroscopic level of the algebra rules and symmetry properties obeyed at microscopic level. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1639593

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

The possibility to match the projection technique at the basis of the Composite Operator Method and the determination of the residual self‐energy through the mode‐coupling approximation is explored. The natural irreducibility given by the projection procedure is fully preserved and exploited. The expressions of the higher order propagators, within the mode‐coupling approximation, in terms of the basic fermionic and bosonic propagators is given. The self‐energy for the spin and charge propagators is computed in terms of convolutions of electronic propagators. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1639594

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

In a large variety of systems, like the high‐temperature superconductors, the dominant parameter of the system is the on‐site Coulomb repulsion that becomes comparable with the kinetic energy in the vicinity of the metal‐insulator transition. This implies that perturbation expansions must include this interaction in the zeroth order Hamiltonian and treat the hopping term as a perturbation. Thus, one gets a situation radically different from the usual one in which the particles are initially free and needs to develop a nonstandard perturbation theory based on a cumulant expansion. In this paper, we revisit the general formalism of the cumulant expansion approach for the single band and two‐band Hubbard model, being interested in introducing a systematic way to perform approximations which take into account cooperative phenomena. As an example we analyze the effect of the charge fluctuations on the single‐particle spectral properties. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1639595

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

The subject of consideration is the pseudospin‐electron model (PEM) which appears in the description of locally anharmonic crystalline and molecular systems with strong electron correlations. A review of the main results concerning thermodynamics, energy spectrum and dynamics of the model is given. Thermodynamically stable states and phase transitions in the system are investigated in the regimes of constant chemical potential or the fixed electron concentration both within the framework of the generalized random phase approximation and the dynamical mean field scheme. A comparison is performed with thermodynamics of the Falicov‐Kimball model. An attention is called to the possibility of applying of the PEM to the description of high‐Tcsuperconductors as well as to the H‐bonded systems with the transition metal ions. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1639596

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

We briefly summarize the fundamental properties of correlated nanoscopic systems obtained within the EDABI method combining anexact diagonalizationin the Fock space with anab initioreadjustment of the single‐particle wave functions in the resultant ground state. Explicitly, we address the following questions: evolution of various systems from a nanometal to a nanoinsulator of the Mott‐Hubbard type, the appearance of the Mott‐Hubbard gap in molecular and cluster (Hn) systems, as well as the stability of those clusters. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1639597

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

We study the spin ordering of localized quantum spins interacting with itinerant electrons via antiferromagnetic (Kondo model) or ferromagnetic (Hund coupling) exchange interactions in the one‐dimensional lattice models. In the case of ferromagnetic coupling we also include a super‐exchange interaction between the localized spins. Using the Density Matrix Renormalization Group (DMRG) we show, for both models, the existence of new magnetic phases with spin ordering consisting offerromagnetic islandscoupled antiferromagnetically. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1639598

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

We outline a general approach to the computation of transport properties of interacting systems at low temperetures and frequencies. We show that if the fixed point and the irrelevant operators around it are known, then by studying the structure of the softly violated conserved currents chracterizing the fixed point one may set up an effective calculation in terms of a memory matrix formalism. We apply this approach to the computation of thermal conductivity of spin chains embedded in a matter matrix and interacting with its phonons. The results are found to be in very good agreement with experiment. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1639599

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

The quantum properties of a particle interacting with a dissipative environment are studied. We show the way in which different types of environments reduce quantum effects. The influence of certain environments, like that described by a Caldeira‐Leggett bath of oscillators suppress very efficiently interference processes, while the coupling to other (ohmic) environments does not modify qualitatively the main features of a free particle. Examples of physical realizations of the different environments are also given. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1639600

出版商:AIP    

年代:1903

数据来源: AIP