摘要:

This paper addresses the question of “how large is large enough for a crystallite to be a three dimensional Bravais lattice?” Based on the premise that the ratio of the bulk volume to that of the volume of the unit cell for a given material should determine the transition to a large molecule, it is proposed that for values of the ratio < 106crystallites can be considered as large molecules. The universality of this criterion has been established by examples from magnetism, optics, ferroelectrics and superconductivity. It is expected, due to the wide variety of materials considered, that other classes of inorganic materials would also obey this rule leading to a quantitative definition for a nanocrystal or nanostructure. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1639581

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

In the development of Thermo Field Dynamics (the quantum field theory at finite temperature and density), Prof. Mancini acted as one of the originators and played the important role for its birth through the work of superconductivity. Recent development of the Bose‐Einstein condensation in the dilute alkaline atomic gas enables us to investigate deeply the quantum coherence in bose and fermi particle systems, and gives us a plenty of experimental data even for the problem in nonequilibrium formation of the condensate. There the relaxation time is reasonably slow to make the laboratory observation possible. In this talk, we will show that Thermo Field Dynamics gives us a reasonable approximation scheme to describe time‐dependent ordered states with spatial inhomogeniety by use of a single‐time quasi‐particle field equation, which contains the information of the spatial and temporal dependent order parameter, particle energy and particle distribution. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1639582

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

We propose to employ the phenomenon of modulational instability in order to create regularly arranged localized excitations in arrays of Bose‐Einstein condensates. These excitations are narrow tubes in 2D and small hollows in 3D arrays filled in with the condensed atoms of much greater density compared to surrounding array sites. As the regions with high atomic concentration develop due to the modulational instability, they can be preserved by increasing the strength of the optical lattice. Theoretical model, based on the multiple scale expansion, describes the main features of the phenomenon. Analytical predictions are confirmed by numerical simulations of the Gross‐Pitaevskii equation. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1639583

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

By means of the multifractal analysis (MFA), the expressions of the probability density functions (PDFs) are unified in a compact analytical formula which is valid for various quantities in turbulence. It is shown that the formula can explain precisely the experimentally observed PDFs both on log and linear scales. The PDF consists of two parts, i.e., thetailpart and thecenterpart. The structure of the tail part of the PDFs, determined mostly by the intermittency exponent, represents the intermittent large deviations that is a manifestation of the multifractal distribution of singularities in physical space due to the scale invariance of the Navier‐Stokes equation for large Reynolds number. On the other hand, the structure of the center part represents small deviations violating the scale invariance due to thermal fluctuations and/or observation error. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1639584

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

Combinatorial aspects of all statistics based on the permutation group are analyzed by imposing the requirements of indistinguishability in the permutation group sense on the Hilbert space describingNidentical particles. Compact expressions for the grand canonical partition functions are given wherever possible. The theory of symmetric functions is found to play a significant role in this development. An analysis of the semion statistics of Haldane is also presented from this perspective together with some recent developments in the field of exclusion statistics. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1639585

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

Our recent studies for the intrinsic Josephson effect are reviewed. We propose a simple phenomenological model based on the time‐dependent Ginzburg‐Landau model atT= 0Kfor describing the phase dynamics of a stack of intrinsic Josephson junctions. In this model the capacitive and the inductive couplings between junctions dominate the dynamics of the gauge‐invariant phase differences. It is shown that our model well explains the intrinsic Josephson effects observed in strongly anisotropic layered high‐Tcsuperconductors. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1639586

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

We review the self‐consistent mean‐field theory for charge‐frustrated Josephson junction arrays. Using ⟨cos&Jgr;⟩ (&Jgr; is the phase of the superconducting wavefunction) as order parameter and imposing the self‐consistency condition, we compute the phase boundary line between the superconducting region (⟨cos&Jgr;⟩ ≠ 0) and the insulating one (⟨cos&Jgr;⟩ = 0). For a uniform offset chargeq = ethe superconducting phase increases with respect to the situation in whichq= 0. We generalize the self‐consistent mean‐field theory to include the effects induced by a random distribution of offset charges and/or of diagonal self‐capacitances. We find results in agreement with the ones obtained in studies using the path‐integral approach. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1639587

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

We review the frustrated magnetic interactions in spin‐orbital models which describe superexchange in transition metal oxides with orbital degeneracy, and analyze the reasons for the symmetry breaking in cubic perovskites. The superexchange inegsystems is dominated by orbital interactions responsible for the orbital ordering, and theA‐type antiferromagnetic ordering follows at lower temperatures. Instead, a generic tendency towards dimerization, found already in the degenerate Hubbard model, occurs int2gsystems. In this case the quantum orbital fluctuations may stabilize orbital liquid states along one directions even in some undopedt2gsystems, leading to theC‐type antiferromagnetic order. The orbital liquid in manganites is triggered by doping. The present understanding of the spectroscopic parameters provides reliable information on the magnetic interactions, as shown on the example of magnons in ferromagnetic cubic and bilayer manganites. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1639588

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

To elucidate the similarities and differences between the physics displayed by orbital and spin degrees of freedom, we analyze an orbital‐Hubbard model with two orbital flavors, corresponding to pseudospin 1/2, and contrast its behavior with that of the familiar (spin‐1/2) Hubbard model. The orbital‐Hubbard model describes a partly filled spin‐polarizedegband on a cubic lattice, as occurs in ferromagnetic manganites. We demonstrate that the absence of SU(2) invariance in orbital space has important implications — superexchange contributes in all orbital ordered states, the Nagaoka theorem does not apply, and the kinetic energy is enhanced as compared with the spin case. As a result orbital‐ordered states are destabilized by doping, and instead a strongly correlatedorbital liquidwith disordered orbitals is realized. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1639589

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

We describe local‐moment systems by the (multiband) s‐f model (ferromagnetic Kondo‐lattice model) which traces back the characteristic properties of such materials to an interband exchange coupling between itinerant conduction electrons and localized magnetic moments. We first present a many‐body approach to the electronic and magnetic properties of the single‐band model. The exchange coupling leads, on the one hand, to a distinct temperature‐dependence of the electronic quasiparticle spectrum and, on the other hand, to magnetic properties, as e. g. the Curie temperatureTCor the magnon dispersion, which are strongly influenced by the band electron selfenergy and therewith in particular by the carrier density. Results for the electronic part are given in terms of quasiparticle densities of states and quasiparticle band structures and for the magnetic part in terms of the selfconsistently derived Curie temperature and spin wave spectra. The transition from weak‐coupling (RKKY) to strong‐coupling (double exchange) behaviour is worked out. The multiband model is combined with an ab‐initio bandstructure calculation to describe real magnetic materials. The proposed method avoids the double counting of relevant interactions and takes into account the correct symmetry of atomic orbitals. For the ferromagnetic metal Gd we get a selfconsistently derived Curie temperature of 301.5 K and aT= 0‐moment of 7.81 &mgr;B, very close to the experimental values. Furthermore a striking induced temperature‐dependence of the 5d conduction bands explains respective photoemission data. For the ferromagnetic semiconductors EuO and EuS we present results for electronic and magnetic bulk properties as well as for thin films. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1639590

出版商:AIP    

年代:1903

数据来源: AIP