摘要:

An overview is presented of the theory of continuum (“Fro¨hlich‐Pekar”‐) polarons. Experimental manifestations of polarons are analysed and compared to theory. The emphasis is on the response characteristics of one‐ and many polarons: optical absorption, cyclotron resonance, photoluminescence, Raman scattering. I consider free polarons and polarons confined in space, polarons in 3D, 2D, 0D (quantum dots). © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1612391

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

The electron system with its many phases and realizations remains a fascinating subject of study. It has been extensively analysed in three, two, one and zero dimensions, as well as in confined nanoscopic and mesoscopic geometries generated with microfabrication techniques. An obvious next step in this investigation is to study the electron system in curved geometries. In particular we will focus in these lectures on the two‐dimensional spherical electron system, realized in for example multielectron bubbles (MEBs), buckyballs, and nanoshells. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1612392

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

We present a review of the auxiliary field (i.e. determinantal) Quantum Monte Carlo method applied to various problems of correlated electron systems. The ground state projector method, the finite temperature approach as well as the Hirsch‐Fye impurity algorithm are described in details. It is shown how to apply those methods to a variety of models: Hubbard Hamiltonians, periodic Anderson model, Kondo lattice and impurity problems, as well as hard core bosons and the Heisenberg model. An introduction to the world‐line method with loop upgrades as well as an appendix on the Monte Carlo method is provided. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1612393

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

We regard the Heisenberg model, the Hubbard model, the tJ‐model and the sd‐model as the basic models of the quantum theory of magnetism in solids. They can describe localized and itinerant magnets and strongly correlated electron systems. This review is devoted to analytical approaches for these models: diagrammatic techniques and the method of generating functional. The diagrammatic techniques are based on a generalization of the Wick theorem for spin andXoperators. Peculiarities of such techniques for the basic models appear because the spin andXoperators do not commute on aC‐value, but their commutator (anticommutator) is an operator itself. The method of generating functional is a generalization of the Kadanoff‐Baym approach, developed earlier for usual Fermi systems. The generating functional describes the interaction of a system with fluctuating fields, and different Green’s functions can be treated as variational derivatives with respect to these fields. Such approach allows to derive the equation of motion for the Green’s functions in each model in terms of functional derivatives. These equations help to find common features in the behavior of the basic models, particularly in finding the multiplicative structure of one‐particle Green’s functions. Iteration of the equations generates perturbation theory, which is compared with the diagrammatic techniques. Both approaches are applied to the calculation of the quasiparticle spectrum of the models and of collective excitations. A generalized random phase approximation (GRPA) is suggested for calculation of different dynamical susceptibilities. This approximation is developed in both approaches: the diagrammatic technique and the generating functional method. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1612394

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

We report about our study on unconventional density waves (UDW) (i.e. density waves with wavevector dependent gap) in quasi‐one dimensional systems. Due to the zero average of the gap over the Fermi surface, these systems are not characterized by the periodic modulation of either the charge or spin density, and are referred to as systems with hidden‐order parameter. The quantities signaling the phase transition are determined. Depending on the explicit wavevector dependence of the gap, optical absorption is possible at low frequencies. In the presence of magnetic field, the different phases exhibit distinct behaviour, and the nonlinear electric response is strongly influenced by the applied field. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1612395

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

We study the ground state properties of the 1‐D boson and fermion Hubbard models in harmonic traps with an underlying lattice. The physics of these systems is found to be fundamentally different from the homogeneous case. Global quantities lose their meaning and Mott domains appear for a continuous range of incommensurate fillings. A local order parameter is defined to characterize the Mott insulating phases that coexist with the metallic ones. Finally, the phase diagrams show richer structures than in the homogeneous models. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1612396

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

Using the Hartree‐Fock approximation we analyze the properties and stability of the filled (one doped hole per stripe site) vertical stripes (VS) and diagonal stripes (DS), found in the two‐dimensional Hubbard model at two representative doping levelsx= 1/8 andx= 1/6, by varying the ratio of the on‐site Coulomb repulsionUto the hopping elementt. In the weak coupling regime ofU∼ 3.5tthe stability of VS is best explained by the solitonic mechanism which leads to the kinetic energy gain due to the hopping perpendicular to the stripes, while DS have a lower potential energy which explains their stability in the regime of largeU. The results obtained beyond the Hubbard model are shortly reviewed as well. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1612397

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

The problem of pseudo‐gap formation in an electronic system, induced by the fluctuations of the order parameter is revisited. We make the observation that a large class of current theories are theoretically equivalent to averaging the Free energy of the pseudo‐gap system over quenched‐disordered distribution of the order parameter. To illustrate how our approach works we examine the interplay of pseudogap, superconductivity and disorder. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1612398

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

We review our recent results concerning the electronic properties of a nanoscopic system with inclusion oflong‐rangeCoulomb interactions that are obtained by combiningexact diagonalizationin the Fock space with anab initiomethod (EDABI). The system evolution is discussed as a function of lattice parameterR. A transformation from a nanometal to localized spin system is observed. With the help of afinite‐size scalingwe determine the discontinuity of the momentum distribution at the Fermi points. Our results implyFermi‐liquid‐like behaviorforR≲ 3a0(a0is the Bohr radius). The quasiparticle mass is almost divergent at the localization threshold, where the particle distribution of the Fermi‐Dirac type gets smeared out. The charge‐gap evolution is compared with that of the Slater gap. Also, the single‐particle spectral function and the corresponding density of states are presented. The analysis is performed using 1s‐like Gaussian‐type STO‐3G basis. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1612399

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

We study the entanglement near a quantum phase transition in a class of exactly solvable one‐dimensional quantum magnets, the anisotropic XY models. The critical behaviour (scaling) is reflected by the derivatives of the bipartite entanglent, quantified by the concurrence. However, we found a profound difference between classical correlation and non‐local quantum correlation: The range of the concurrence is found to behave in a non‐universal way, being extremely short‐ranged for the Ising model. This range tends to infinity towards the isotropic XY model. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1612400

出版商:AIP    

年代:1903

数据来源: AIP