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1.
J. Riera D. Poilblanc E. Dagotto 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,7(1):53-66
Several experiments in the context of ladder materials have recently shown that the study of simple models of anisotropic
ladders (i.e. with different couplings along legs and rungs) is important for the understanding of these compounds. In this paper Exact
Diagonalization studies of the one-band Hubbard and t-J models are reported for a variety of densities, couplings, and anisotropy ratios. The emphasis is given to the one-particle
spectral function which presents a flat quasiparticle dispersion at the chemical potential in some region of parameter space. This is correlated
with the existence of strong pairing fluctuations, which themselves are correlated with an enhancement of the bulk-extrapolated
value for the two-hole binding energy as well as with the strength of the spin-gap in the hole-doped system. Part of the results
for the spectral function are explained using a simple analytical picture valid when the hopping along the legs is small.
In particular, this picture predicts an insulating state at quarter filling in agreement with the metal-insulator transition
observed at this special filling for increasing rung couplings. The results are compared against previous literature, and
in addition pair-pair correlations using extended operators are also here reported.
Received: 22 April 1998 / Revised: 23 July 1998 / Accepted: 30 July 1998 相似文献
2.
Sonic stop-bands for cubic arrays of rigid inclusions in air 总被引:2,自引:0,他引:2
M.S. Kushwaha B. Djafari-Rouhani L. Dobrzynski J.O. Vasseur 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,3(2):155-161
Extensive band structures have been computed for cubic arrays of rigid spheres and cubes in air. Complete stop bands are obtained
for the face-centered-cubic (fcc) structure; however, there is no gap for the body-centered-cubic (bcc) and simple-cubic (sc)
structures. These gaps start opening up for a filling fraction of (27%) for spherical (cubic) inclusions and tend to increase with the filling fraction, exhibiting a maximum at the close-packing.
We also propose a tandem structure that allows the achievement of an ultrawideband filter for environmental or industrial
noise in the desired frequency range. This work is motivated by the recent experimental measurement of sound attenuation on
the sculpture, by Eusebio Sempere, exhibited at the Juan March Foundation in Madrid (Nature 378, 241 (1995)) and complements the corresponding theoretical work (Appl. Phys. Lett. 70, 3218 (1997)).
Received: 24 October 1997 / Revised: 1 December 1997 and 26 January 1998 / Accepted: 6 March 1998 相似文献
3.
C. Schuster U. Eckern 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,5(3):395-402
In order to study the effect of interaction and lattice distortion on quantum coherence in one-dimensional Fermi systems,
we calculate the ground state energy and the phase sensitivity of a ring of interacting spinless fermions on a dimerized lattice.
Our numerical DMRG studies, in which we keep up to 1000 states for systems of about 100 sites, are supplemented by analytical
considerations using bosonization techniques. We find a delocalized phase for an attractive interaction, which differs from
that obtained for random lattice distortions. The extension of this delocalized phase depends strongly on the dimerization
induced modification of the interaction. Taking into account the harmonic lattice energy, we find a dimerized ground state
for a repulsive interaction only. The dimerization is suppressed at half filling, when the correlation gap becomes large.
Received: 11 February 1998 / Revised: 1st April 1998 / Accepted: 30 April 1998 相似文献
4.
T. Boutreux 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,6(3):419-424
We present the generalization of a theoretical model for segregation of granular mixtures due to surface flows, published
in J. Phys. I France 6, 1295 (1996). Our generalized model is valid for grains differing by their size and/or their surface properties; in the present
paper, we describe the case of two species with the same surface properties but two different sizes. The rolling stream is
assumed to be homogeneous. Exchanges between the grains at rest and the rolling stream are modelized via binary collisions.
The model predicts that during the filling of a two-dimensional silo, continuous segregation appears inside the static phase: small (respectively large) grains tend to stop uphill (respectively downhill), although
both species remain present everywhere. This fits the observations when the size difference between the species is small.
When the size difference is large, a different regime is observed. We argue that in this case, segregation occurs directly inside the rolling stream.
Received: 25 February 1998 / Received in final form and Accepted: 6 July 1998 相似文献
5.
E. Muñoz-Sandoval J. Dorantes-Dávila G.M. Pastor 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》1999,5(1):89-96
The electronic and magnetic properties of clusters are investigated in the framework of the Hubbard model by treating electron
correlations effects in a saddle-point slave-boson approximation. The size dependent single-particle spectrum is calculated
using a third moment real-space expansion of the local density of states. Results for the magnetic moments, magnetic order,
average number of double occupations and hopping renormalizations are given as a function of the local coordination number
z, for different representative values of the Coulomb interaction strength U/t and band filling n. Several transitions between paramagnetic, ferromagnetic and antiferromagnetic behaviors are obtained as a function of z. The environment dependence of the magnetic behavior and of the degree of electron delocalization is analyzed. Advantages
and limitations of the present approach are discussed.
Received: 8 January 1998 / Revised: 22 June 1998 / Accepted: 6 August 1998 相似文献
6.
N. Demoncy O. Stéphan N. Brun C. Colliex A. Loiseau H. Pascard 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,4(2):147-157
Various filled carbon nanotubes have recently been successfully produced by the arc-discharge method by doping a 99.4% graphite
anode with a transition metal like Cr, Ni, a rare earth like Yb, Dy, or a covalent element like S, Ge. In this work, the structural
characteristics of these encapsulated nanowires were studied by High Resolution Transmission Electron Microscopy and their
chemical composition was investigated using Electron Energy-Loss Spectroscopy with high spatial resolution: this analysis
mode provides elemental concentration profiles across or along the filled nanotubes. Except in the case of Ge for which only
pure Ge fillings were identified, surprising amounts of sulfur, which was present as an impurity ( 0.25%) in the graphite rods, were found within numerous filling materials. When using high purity carbon rods, no filled
nanotube was obtained. We chose the case of Cr to clearly evidence that the addition of sulfur in catalytic quantity is responsible
for the formation of filled nanotubes, including sulfur free encapsulated nanowires. A growth mechanism based on a catalytic
process involving three elements, i.e. carbon, a metal and sulfur, and taking into account the experimental results is proposed.
Received: 20 January 1998 / Received in final form and accepted: 9 April 1998 相似文献
7.
T. Boutreux H.A. Makse P.-G. de Gennes 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,9(1):105-115
We present the generalization of the minimal model for surface flows of granular mixtures, proposed by Boutreux and de Gennes
[J. Phys. I France 6, 1295 (1996)]. The minimal model was valid for grains differing only in their surface properties. The present model also
takes into account differences in the size of the grains. We apply the model to study segregation in two-dimensional silos
of mixtures of grains differing in size and/or surface properties. When the difference in size is small, the model predicts
that a continuous segregation appears in the static phase during the filling of a silo. When the difference in size is wide,
we take into account the segregation of the grains in the rolling phase, and the model predicts complete segregation and stratification
in agreement with experimental observations.
Received 9 September 1998 and Received in final form 4 November 1998 相似文献
8.
S.F. Özeren U. Tirnakli F. Büyükkiliç D. Demirhan 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,2(1):101-106
In this study, using the base of coherent states, Landau diamagnetism has been generalized within Tsallis Thermostatistics.
As far as we know, this is the first attempt to introduce coherent states in this formalism. The magnetization and the susceptibility
of the system have been obtained and compared with the standard result to illustrate the effect of nonextensivity. Then, adding
a perturbation term to the Hamiltonian of the system, nonextensive effects on diamagnetic susceptibility have been investigated.
In addition to this, making use of the -deformed partition function of the -oscillator system, the magnetization for -deformed Landau diamagnetism has been derived, with the aim of comparing the results obtained within both formalisms.
Received: 21 July 1997 / Revised: 12 December 1997 / Accepted: 23 January 1998 相似文献
9.
R. Lacaze A. Morel B. Petersson J. Schröper 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,2(4):509-523
We present an investigation of the 2D attractive Hubbard model, considered as an effective model relevant to superconductivity
in strongly interacting electron systems. We use both hybrid Monte-Carlo simulations and existing hopping parameter expansions
to explore the low temperature domain. The increase of the static S-wave pair correlation with decreasing temperature, which depends weakly on the band filling in the explored temperature range,
is analyzed in terms of an expected Kosterlitz-Thouless superconducting transition. Using both our data and previously published
results, we show that the evidence for this transition is weak: If it exists, its temperature is very low. The number of unpaired
electrons remains nearly constant with temperature at fixed attractive potential strength. In contrast, the static magnetic
susceptibility decreases fast with temperature, and cannot be related only to pair formation. We introduce a method by which
the Padé approximants of the existing series for the susceptibility give sensible results down to rather low temperature region,
as shown by comparison with our numerical data.
Received: 30 October 1996 / Revised: 23 October 1997 / Accepted: 29 January 1998 相似文献
10.
A. Banerjee M.K. Harbola 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》1999,5(2):201-206
Employing a near exact Hylleraas wavefunction we calculate various third-order nonlinear optical properties for the helium
atom within the time-dependent Kohn-Sham theory. In our calculations we employ the adiabatic local-density approximation (ALDA)
for the exchange and correlation kernels fxc and gxc, and compare the numbers obtained by us with the available accurate theoretical as well as experimental results. Our results
demonstrate the accuracy of ALDA for the calculation of nonlinear optical properties of many electron systems.
Received: 22 June 1998 / Accepted: 15 October 1998 相似文献
11.
M.C. Larsen P. Hvelplund M.O. Larsson H. Shen 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》1999,5(2):283-289
The fragmentation of anions and cations resulting from 50 keV collisions with rare gas targets is studied. Positive ion fragment
patterns are recorded, and dramatic changes in these patterns are observed as a function of target atom number. The fragment
pattern dependence on the target atom size is investigated within a simple model, normally used for stopping power calculations.
Fair agreement is obtained between calculated and experimental spectra. From these comparisons we conclude that the range
of the screened atomic potentials, as e.g., the Thomas-Fermi potential, is an essential parameter in the collisional induced fragmentation process.
Received: 13 February 1998 / Revised: 27 October 1998 / Accepted: 29 October 1998 相似文献
12.
W. Kleinig V.O. Nesterenko P.-G. Reinhard Ll. Serra 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》1998,4(3):343-352
The systematics of the plasmon response in spherical K, Na and Li clusters in a wide size region is studied. Two simplifying approximations whose validity has been established previously are considered: (a) a separable
approach to the random-phase-approximation, involving an expansion of the residual interaction into a sum of separable terms,
(b) the electron-ion interaction is modeled within the pseudo-Hamiltonian jellium model (PHJM) including nonlocal effects
by means of realistic atomic pseudoHamiltonians. In cases where nonlocal effects turn out to be negligible, the Structure
Averaged Jellium Model (SAJM) has been used. The leading role of Landau damping in forming the plasmon width in medium and
large clusters is demonstrated. Good agreement with available experimental data is achieved for K, Na (using the SAJM) and
small Li clusters (invoking the PHJM). The trends for peak position and width are generally well reproduced, even up to details
of the Landau fragmentation in several clusters. Less good agreement, however, is found for large Li clusters. The possible
reasons of the discrepancy are discussed.
Received: 22 April 1998 / Accepted: 24 July 1998 相似文献
13.
S. Karshenboim U. Jentschura V. Ivanov G. Soff 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》1998,2(3):209-215
We present an analytical evaluation of radiative corrections in exotic atoms induced by the one-loop electronic vacuum polarization.
We evaluate corrections to the energy levels, to the wave function (at the origin) and to the hyperfine structure. We treat
all corrections analytically within a non-relativistic approximation. Agreement is found with a few available numerical results.
The analytical treatment allows to determine the asymptotic forms of the corrections in the limit of a small atomic radius,
which for the atomic systems considered corresponds to a large mass of the constituent particle as compared to the electron
mass. The asymptotics can be verified using the effective charge approach.
Received: 28 January 1998 / Accepted: 13 March 1998 相似文献
14.
A. Halperin 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,3(3):359-364
The compression of brushes of terminally anchored chain within the de Gennes n-cluster model is analysed. This model was developed for Poly(ethylene oxide) in water but may apply to other systems. Brushes
described by this model exhibit discontinuous concentration profile associated with the coexistence of an inner dense “phase”
and an outer, dilute, one. The compression induces growth of the dense, weakly compressible region. This, in turn, gives rise
to distinctive force profiles associated with changes of slope. When the dilute region disappears, the compression of two
brushes can give rise to a transient attraction.
Received: 4 November 1997 / Revised: 29 January 1998 / Accepted: 24 March 1998 相似文献
15.
L. Elcoro J.M. Perez-Mato 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,7(1):85-89
The diffraction properties of a quenched Al-Mg alloy which has been recently termed as a “cubic quasicrystal” are quantitatively
reanalyzed. It is shown that the phase can be interpreted within the superspace formalism as an ordinary incommensurately modulated structure. The cubic six-dimensional superspace group that describes its symmetry properties has
been determined. The additional inflation symmetry features exhibited by the diffraction diagram can be summed up by its invariance
for the inflation factor , but this property has its origin in the specific value of the modulus of the modulation wave vectors, which is composition
dependent. Other particular values of this modulus can give rise to similar scaling properties. Further experiments are required
to ellucidate if the mentioned inflation symmetry is a fortuitous situation in a composition dependent wave vector, or has
indeed the physical significance which would allow to describe the system as a “cubic quasicrystal”.
Received: 25 June 1998 / Received in final form and Accepted: 23 September 1998 相似文献
16.
Mixed spin transverse Ising model with longitudinal random crystal field interactions 总被引:1,自引:0,他引:1
N. Benayad R. Zerhouni A. Klümper 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,5(3):687-695
The effect of a longitudinal random crystal field interaction on the phase diagrams of the mixed spin transverse Ising model
consisting of spin-1/2 and spin-1 is investigated within the finite cluster approximation based on a single-site cluster theory.
In order to expand a cluster identity of spin-1, we transform the spin-1 to spin-1/2 representation containing Pauli operators.
We derive the state equations applicable to structures with arbitrary coordination number N. The phase diagrams obtained in the case of a honeycomb lattice (N=3) and a simple-cubic lattice (N=6), are qualitatively different and examined in detail. We find that both systems exhibit a variety of interesting features
resulting from the fluctuation of the crystal field interactions.
Received: 13 February 1998 / Accepted: 17 March 1998 相似文献
17.
W. Michalke S. Kreitmeier 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,4(4):469-473
The diffusion Quantum-Monte-Carlo method of solving the Schr?dinger equation is applied to the vibrational ground state of
a polyethylene molecule. The results for the ground state energy show good agreement with normal mode analysis. In addition
to stretching, bending and torsional interaction van-der-Waals interaction is applied to a single chain showing a decrease
of the energy of 5%. The decrease for a polyethylene system of 5 chains with 10 atoms per molecule at the positions of a unit
cell is determined to be 4.8% per molecule. Finally first steps towards simulating excited states were performed.
Received: 9 February 1998 / Revised: 2 April 1998 / Accepted: 23 April 1998 相似文献
18.
M. Hamm M.M. Kozlov 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,6(4):519-528
We present an alternative model of structure and energetics of the inverted amphiphilic mesophases. The previous studies of
the inverted hexagonal, HII, and inverted micellar cubic, QII, phases considered the amphiphilic monolayers to be homogeneously bent. In contrast, we assume a unit cell of an inverted
mesophase to consist of flat fragments of monolayer. Hence, the unit cells of the HII and QII phases are represented by a hexagonal rod and a polyhedron, respectively. Our model is motivated by Turner and Gruner's X-ray
diffraction reconstruction of structure of the HII phase. The only deformation of the amphiphilic monolayers we consider is tilt of the hydrocarbon chains with respect to the
monolayer surface, determined by the packing constraints imposed in the mesophases. Applying our recent model for the elastic
energy of tilt in liquid membranes [#!ref23!#], we show that: i) tilt accounts in a natural way for the frustration energy
of mesophases resulting from filling by the hydrocarbon chains the corners of the unit cells, ii) the energy of tilt variation
along the membrane surface is analogous to the bending energy. We compute the energetics of the HII, QIIsc and QIIfcc phases and obtain a hypothetical phase diagram in terms of the elastic constants of monolayers. Moreover, we calculate the
structural dimensions of the mesophases. We verify the model showing that the obtained phase diagram describes the recent
data for the glycolipids/water systems; the predicted dimensions of the QII phase are in accord with the measured values; the model treats quantitatively the structural features observed for the HII phase.
Received: 9 February 1998 / Revised: 4 June 1998 / Accepted: 3 July 1998 相似文献
19.
K. Mølmer 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》1999,5(2):301-305
We discuss the possibility of preparing an atomic sample of atoms with minimum fluctuations in the difference between populations
of two levels. A first scheme involves absorption of twin beams of light, and it presents a variant of a recent proposal for
atomic spin squeezing within an excited state manifold [Kuzmich et al., Phys. Rev. Lett. 79, 4782 (1997)]. A second scheme involves atoms with two stable states, and we suggest that by use of quantum non-demolition
detection and feed-back optical pumping, we may ensure a perfect agreement between the number of atoms in these two states.
Received: 14 May 1998 / Revised: 10 August 1998 / Accepted: 8 October 1998 相似文献
20.
M. Henkel D. Karevski 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,5(3):787-792
The effect of point defects on persistent currents in mesoscopic rings is studied in a simple tight-binding model. Using an
analogy with the treatment of the critical quantum Ising chain with defects, conformal invariance techniques are employed
to relate the persistent current amplitude to the Hamiltonian spectrum just above the Fermi energy. From this, the dependence
of the current on the magnetic flux is found exactly for a ring with one or two point defects. The effect of an aperiodic
modulation of the ring, generated through a binary substitution sequence, on the persistent current is also studied. The flux-dependence
of the current is found to vary remarkably between the Fibonacci and the Thue-Morse sequences.
Received: 4 March 1998 / Revised: 20 April 1998 / Accepted: 30 April 1998 相似文献