588 research outputs found
Double fluctuations on the attractive Hubbard model: ladder approximation
We explore, for the first time, the effect of double fluctuations on both the
diagonal and off-diagonal self-energy. We use the T-Matrix equations below
, developed recently by the Z\"urich group (M.H. Pedersen et al) for the
local pair attraction Hamiltonian. Here, we include as well the effect of
fluctuations on the order parameter (beyond the BCS solution) up to second
order in . This is equivalent to approximating the effective interaction
by in the off-diagonal self-energy. For , , and , we find four peaks both for the diagonal,
, and off-diagonal, ,
spectral functions. These peaks are not symmetric in pairs as previously found.
In addition: (a) in , the far left peak has a
vanishing small weight; (b) in the far left and
far right peaks have very small weights. The physical picture is, then, that
the pair physics in the normal phase () is still valid below .
However, the condensation of the e-h pairs produces an additional gap around
the chemical potential as in BCS, in other words, superconductivity opens a gap
in the lower branch of a Hubbard-type-I solution.Comment: LaTeX, 7 pages. 8 figures available on request. To appear in Z.
Physik
Self-consistent calculation of particle-hole diagrams on the Matsubara frequency: FLEX approximation
We implement the numerical method of summing Green function diagrams on the
Matsubara frequency axis for the fluctuation exchange (FLEX) approximation. Our
method has previously been applied to the attractive Hubbard model for low
density. Here we apply our numerical algorithm to the Hubbard model close to
half filling (), and for , in order to study the
dynamics of one- and two-particle Green functions. For the values of the chosen
parameters we see the formation of three branches which we associate with the a
two-peak structure in the imaginary part of the self-energy. From the imaginary
part of the self-energy we conclude that our system is a Fermi liquid (for the
temperature investigated here), since Im
around the chemical potential. We have compared our fully self-consistent FLEX
solutions with a lower order approximation where the internal Green functions
are approximated by free Green functions. These two approches, i.e., the fully
selfconsistent and the non-selfconsistent ones give different results for the
parameters considered here. However, they have similar global results for small
densities.Comment: seven pages, nine figures as ps files. Accepted in Int. J. Modern
Phys. C (1997
Pre-formed Cooper pairs and Bose-Einstein condensation in cuprate superconductors
A two-dimensional (2D) assembly of noninteracting, temperature-dependent,
pre-formed Cooper pairs in chemical/thermal equilibrium with unpaired fermions
is examined in a binary boson-fermion statistical model as the Bose-Einstein
condensation (BEC) singularity temperature is approached from above.
Compared with BCS theory (which is {\it not} a BEC theory) substantially higher
's are obtained without any adjustable parameters, that fall roughly
within the range of empirical 's for quasi-2D cuprate superconductors.Comment: 4 page
Nonlinear interference in a mean-field quantum model
Using similar nonlinear stationary mean-field models for Bose-Einstein
Condensation of cold atoms and interacting electrons in a Quantum Dot, we
propose to describe the original many-particle ground state as a one-particle
statistical mixed state of the nonlinear eigenstates whose weights are provided
by the eigenstate non-orthogonality. We search for physical grounds in the
interpretation of our two main results, namely, quantum-classical nonlinear
transition and interference between nonlinear eigenstates.Comment: RevTeX (pdfLaTeX), 7 pages with 5 png-figures include
Bose-Einstein condensation of the magnetized ideal Bose gas
We study the charged non-relativistic Bose gas interacting with a constant
magnetic field but which is otherwise free. The notion of Bose-Einstein
condensation for the three dimensional case is clarified, and we show that
although there is no condensation in the sense of a phase transition, there is
still a maximum in the specific heat which can be used to define a critical
temperature. Although the absence of a phase transition persists for all values
of the magnetic field, we show how as the magnetic field is reduced the curves
for the specific heat approach the free field curve. For large values of the
magnetic field we show that the gas undergoes a "dimensional reduction" and
behaves effectively as a one-dimensional gas except at very high temperatures.
These general features persist for other spatial dimensions D and we show
results for D=5. Finally we examine the magnetization and the
Meissner-Ochsenfeld effect.Comment: 4 pages RevTex 2 column format with 4 eps figures, uses epsf.
Replaced version has missing acknowledgements and a discussion of two
references is corrected thanks to discussions with J. Daicic and N. Franke
From the Test Benches to the First Prototype of the muFly Micro Helicopter
The goal of the European project muFly is to build a fully autonomous micro helicopter, which is comparable to a small bird in size and mass. The rigorous size and mass constraints infer various problems related to energy efficiency, flight stability and overall system design. In this research, aerodynamics and flight dynamics are investigated experimentally to gather information for the design of the helicopter's propulsion group and steering system. Several test benches are designed and built for these investigations. A coaxial rotor test bench is used to measure the thrust and drag torque of different rotor blade designs. The effects of cyclic pitching of the swash plate and the passive stabilizer bar are studied on a test bench measuring rotor forces and moments with a 6-axis force sensor. The gathered knowledge is used to design a first prototype of the muFly helicopter. The prototype is described in terms of rotor configuration, structure, actuator and sensor selection according to the project demands, and a first version of the helicopter is shown. As a safety measure for the flight tests and to analyze the helicopter dynamics, a 6DoF vehicle test bench for tethered helicopter flight is use
Modeling and System Identification of the muFly Micro Helicopter
An accurate mathematical model is indispensable for simulation and control of a micro helicopter. The nonlinear model in this work is based on the rigid body motion where all external forces and moments as well as the dynamics of the different hardware elements are discussed and derived in detail. The important model parameters are estimated, measured or identified in an identification process. While most parameters are identified from test bench measurements, the remaining ones are identified on subsystems using the linear prediction error method on real flight data. The good results allow to use the systems for the attitude and altitude controller desig
CARATTERISTICHE FONDAMENTALI DI UN LEARNERâS DICTIONARY ITALIANO
Il presente studio si dedica a una categoria di dizionari ancora assente nella lessicografia italiana: i dizionari per apprendenti L2. Mentre il mercato italiano Ăš dotato di una serie di dizionari pedagogici ideati per alunni e studenti italiani, unâopera concepita secondo il modello dei leaner's dictionaries inglesi e tedeschi, esclusivamente indirizzati a utenti stranieri (o di italiano L2 in generale) non esiste. Lâarticolo descrive le caratteristiche fondamentali di un tale dizionario, comprendenti le definizioni, i rapporti semantico-lessicali e morfosemantici tra i lemmi, i contesti sintagmatici tipici (collocazioni e valenze) e la dimensione pragmatica nel trattamento lessicografico dei segnali discorsivi e dei fraseologismi. Ne risulta chiaramente come dovrebbe essere un learnerâs dictionary italiano.   Fundamental characteristics of an italian learnerâs dictionary  This study discusses a category of dictionaries which are still absent from Italian lexicography: dictionaries for L2 learners. While the Italian market offers a series of pedagogical dictionaries designed for Italian pupils ad students, a work along the lines of an English or German learnerâs dictionary model, exclusively for foreign learners (or Italian L2 learners in general) does not exist. The article describes the basic characteristics for such a dictionary, including definitions, the semantic-lexical and morphosyntatical relationships between the entries, typical syntagmatic contexts (collocations and valences) and the pragmatic dimension in the lexographical treatment of discourse signals and phraseologisms. It is clear how an Italian learnerâs dictionary should be designed
- âŠ