共查询到20条相似文献,搜索用时 968 毫秒
1.
R. Guirado-López D. Spanjaard M.-C. Desjonquères A.M. Oleś 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,3(4):437-446
We analyze the stability of magnetic states obtained within the tight-binding model for cubooctahedral (Oh) and icosahedral (Ih) clusters of early 4d (Y, Zr, Nb, Mo, and Tc) transition metals. Several metastable magnetic clusters are identified which suggests the existence
of multiple magnetic solutions in realistic systems. A bulk-like parabolic behavior is observed for the binding energy of
Oh and Ih clusters as a function of the atomic number along the 4
d-series. The charge transfer on the central atom changes sign, while the average magnetic moments present an oscillatory behavior
as a function of the number of d electrons in the cluster. Our results are in agreement with other theoretical calculations.
Received: 20 November 1997 / Received in final form: 9 March 1998 / Accepted: 30 March 1998 相似文献
2.
B. Reusch R. Blümel 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》1998,3(2):123-127
Numerical simulations of the equations of motion of 300 charged particles confined to a plane with an additional magnetic
field orthogonal to the plane reproduce recently observed self-organization of non-neutral plasmas into a small number of
interacting vortices. In the presence of damping we observe crystallized vortices, i.e. vortices with regular internal structure. We also observe crystallized vortex crystals, i.e. geometric patterns of crystallized vortices. Fractal vortex arrangements are investigated and found to be stable. Our results
are relevant for quantum dots and artificial atoms.
Received: 24 February 1998 / Revised: 4 March 1998 / Accepted: 4 May
1998 相似文献
3.
H. Razafimandimby M. Taguchi J. C. Parlebas 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,11(1):21-26
We investigate the competition between magnetic order and local Kondo effect in a Kondo lattice model (i.e. the Coqblin-Schrieffer Hamiltonian extended to a lattice) in a mean-field approximation, taking account of the spin-orbit
degeneracy of each localized f level. This leads to the definition of a dependent Kondo temperature. We study the Kondo phase and compare its energy with the energies of magnetic phases, when the
number of the conduction band electron per site is near one. We present a phase diagram which shows the occurrence of three
phases: Kondo, antiferromagnetic and paramagnetic phases. Our model in the mean-field approximation also shows a somewhat
flat Kondo temperature, for large values of , as a function of the exchange coupling J between conduction and localized f electrons. Finally we show some scaling effects between and J and we define a corresponding Kondo temperature.
Received 21 September 1998 and Received in final form 8 February 1999 相似文献
4.
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
5.
P. Schieffer C. Krembel M.-C. Hanf M.-H. Tuilier P. Wetzel G. Gewinner K. Hricovini 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,8(2):165-168
We studied the magnetic properties of ultra-thin Mn films deposited on Ag (001) held at 80 K with soft X-ray absorption and
magnetic circular dichroism. The observed shape and branching ratio of the Mn 2p absorption edge as a function of Mn coverage demonstrate that, up to , the Mn adopts a stable high spin state similar to the Mn atom Hund's rule 6
S
5/2
ground state. Above this coverage a rapid transition from localized high spin to itinerant low spin behavior of the Mn 3d electrons is evidenced. Magnetic circular dichroism shows no sign of long range ferromagnetic order in these films at 80
K. The data, first confirm the large atomic-like local magnetic moment, and second are in line with the in-plane antiferromagnetic order, reported recently (Phys. Rev. B 57, 1141 (1998)), for Mn in the nearly ideal on-top Mn monolayer formed by 0.9 ML deposited at 80 K.
Received: 4 May 1998 相似文献
6.
R. Thissen P. Lablanquie R.I. Hall M. Ukai K. Ito 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》1998,4(3):335-342
Photoionization of rare gas clusters in the innervalence shell region has been investigated using threshold photoelectron
and photoion spectrometers and synchrotron radiation. Two classes of states are found to play an important role: (A) valence
states, correlated to dissociation limits involving an ion with a hole in its innervalence ns shell, (B) Rydberg states correlated to dissociation limits involving an ion with a hole in its outervalence np shell plus an excited neutral atom. In dimers, class A states are “bright”, that is, accessible by photoionization, and serve
as an entrance step to form the class B “dark” states; this character fades as the size of the cluster increases. In the dimer,
the “Mulliken” valence state is found to present a shallow potential well housing a few vibrational levels; it is predissociated by the
class B Rydberg states. During the predissociation a remarkable energy transfer process is observed from the excited ion that
loses its innershell electron to its neutral partner.
Received: 10 February 1998 / Revised: 17 July 1998 / Accepted: 31 July 1998 相似文献
7.
R. Guirado-López D. Spanjaard M.-C. Desjonquères 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,6(4):459-466
The magnetic properties of small YN clusters are studied by using a tight-binding Hubbard Hamiltonian in the unrestricted Hartree-Fock approximation. Several
types of cluster geometries are considered in order to see the effects of the size and symmetry of the structures on the magnetic
properties. The average magnetic moments are found to be constant over large domains of variations in the interatomic distance, a fact that can be explained by the
existing closed shell electronic configurations at least for one spin direction in all our magnetic solutions. Small energy
gains upon the onset of magnetization are obtained, which reveals the low stability of the magnetic solutions. Our results
contradict the prediction of a magnetic-nonmagnetic transition at a large cluster size (about 90 atoms) for these kinds of
systems.
Received: 27 April 1998 / Received in final form: 23 June 1998 / Accepted: 17 July 1998 相似文献
8.
R. López-Sandoval G. M. Pastor 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2006,38(3):507-514
The ground-state properties of C20 fullerene clusters
are determined in the framework of the Hubbard model by using
lattice density-functional theory (LDFT) and
scaling approximations to the interaction-energy functional.
Results are given for the ground-state energy, kinetic and Coulomb
energies, local magnetic moments, and charge-excitation
gap, as a function of the Coulomb repulsion U/t and for
electron or hole doping δ close to half-band filling
(|δ| ≤1). The role of electron correlations is
analyzed by comparing the LDFT results with fully unrestricted
Hartree-Fock (UHF) calculations which take into account
possible noncollinear arrangements of the local spin-polarizations.
The consequences of the spin-density-wave symmetry breaking,
often found in UHF, and the implications of this
study for more complex fullerene structures are discussed. 相似文献
9.
C. Kohl S.M. El-Gammal F. Calvayrac E. Suraud P.-G. Reinhard 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》1999,5(2):271-274
We investigate spin modes in the ground state and the polarized first isomer of the Na12 cluster describing the valence electrons in time-dependent local-spin-density approximation (TDLSDA) and the detailed ionic
background using local pseudopotentials. The spin modes show a collective redshift compared to the unperturbed particle-hole
excitations. They are strongly fragmented and the average energy of the modes along the principal axes are related to the
underlying geometry (triaxial or axially symmetric). For the polarized isomer, we find significant cross talk between the
spin modes and the dipole plasmon, which hints at a possible spectroscopic identification.
Received: 22 June 1998 / Accepted: 29 July 1998 相似文献
10.
S. Cueille C. Sire 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,7(1):111-127
We define a block persistence probability p
l
(t) as the probability that the order parameter integrated on a block of linear size l has never changed sign since the initial time in a phase-ordering process at finite temperature T<T
c
. We argue that in the scaling limit of large blocks, where z is the growth exponent (), is the global (magnetization) persistence exponent and f(x) decays with the local (single spin) exponent for large x. This scaling is demonstrated at zero temperature for the diffusion equation and the large-n model, and generically it can be used to determine easily from simulations of coarsening models. We also argue that and the scaling function do not depend on temperature, leading to a definition of at finite temperature, whereas the local persistence probability decays exponentially due to thermal fluctuations. These
ideas are applied to the study of persistence for conserved models. We illustrate our discussions by extensive numerical results.
We also comment on the relation between this method and an alternative definition of at finite temperature recently introduced by Derrida [Phys. Rev. E 55, 3705 (1997)].
Received: 25 February 1998 / Revised: 24 July 1998 / Accepted: 27 July 1998 相似文献
11.
12.
J. des Cloizeaux 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,8(3):439-443
Dirac equation is reformulated in terms of real local observables, which are mean values of the wave function . The quadrivector current is shown to be a function of the potential vector and of other local observables. The equations
describe the evolution of a four dimensional system T, X, Y, Z, and of two scalars, in the coordinate system ct, x, y, z. The current is proportional to the T vector. The Z vector is associated with the spin of the electron. Energy and gauge transformations correspond to rotations in the plane
(X, Y). In the presence of a static field, the (real) solutions of the equations appear as eigenfunctions associated with energy
eigenvalues.
Received 7 September 1998 相似文献
13.
C. Coudray G. Blaise M.J. Malliavin 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2000,11(1):127-136
With the help of ab initio methods the clusters [(MgO)13Mg]
Q+
are simulated for Q = 0, 1, 2. Then, vacancy clusters [(MgO)12Mg2]
Q+
obtained by removing one oxygen atom are computed for Q running from 0 to 4. These clusters exhibit a slight sphericity and generally shorter interatomic distances than in the crystal.
The electronic densities variations are studied in function of Q. In particular, it is observed that the electronic density in the oxygen vacancy goes to a maximum when Q = 2. The ionisation potentials vary from approximately 4 to 14 eV when Q varies from 0 to 3, with a more rapid increase from Q = 1 to Q = 2. The stability study of vacancy clusters show that they experience a phase transition when their charge becomes equal
to 2, in accordance with the features mentioned above.
Received 14 September 1999 and Received in final form 2 December 1999 相似文献
14.
We study irreversible A-B reaction kinetics at a fixed interface separating two immiscible bulk phases, A and B. Coupled equations
are derived for the hierarchy of many-body correlation functions. Postulating physically motivated bounds, closed equations
result without the need for ad hoc decoupling approximations. We consider general dynamical exponent z, where is the rms diffusion distance after time t. At short times the number of reactions per unit area, , is 2nd order in the far-field reactant densities . For spatial dimensions dabove a critical value , simple mean field (MF) kinetics pertain, where Qb is the local reactivity. For low dimensions , this MF regime is followed by 2nd order diffusion controlled (DC) kinetics, , provided . Logarithmic corrections arise in marginal cases. At long times, a cross-over to 1st order DC kinetics occurs: . A density depletion hole grows on the more dilute A side. In the symmetric case (), when the long time decay of the interfacial reactant density, , is determined by fluctuations in the initial reactant distribution, giving . Correspondingly, A-rich and B-rich regions develop at the interface analogously to the segregation effects established by
other authors for the bulk reaction . For fluctuations are unimportant: local mean field theory applies at the interface (joint density distribution approximating
the product of A and B densities) and . We apply our results to simple molecules (Fickian diffusion, z=2) and to several models of short-time polymer diffusion (z>2).
Received 8 June 1998 and Received in final form 10 September 1999 相似文献
15.
C. Bréchignac Ph. Cahuzac B. Concina J. Leygnier I. Tignères 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2000,12(1):185-192
The cross-sections for collisional charge transfer between singly charged free clusters M
n
+ (M = Li, Na; n=1...50) and atomic targets A (cesium, potassium) have been measured as a function of collisional relative velocity in laboratory
energy range 1–10 keV. For each cluster size, the experimental values of the charge transfer cross-section are fitted with an universal parametric curve with two independent parameters and vm, the maximum cross-section and the corresponding velocity. For small size clusters (), the characteristic parameters show strong variations with the number of atoms in the cluster. Abrupt dips observed for n=10 and n=22 are attributed to electronic properties. Charge transfer patterns observed for various collisional systems present similarities,
which appear more sensitive to cluster quantum size effects than to collision energy defects. In their whole, the and vm parameters show differences in both their size evolution and their absolute values discussed in term of projectile and target
electronic structures.
Received 13 April 2000 and Received in final form 29 June 2000 相似文献
16.
K. Hashimoto M. Okamoto K. Takayanagi 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》1998,2(1):75-78
The stability of neutral, singly and multiply ionized silicon clusters, (N
= 2-7, M
= 0, , , ), has been investigated using an ab initio density functional method. We show that the fragmentation effect significantly affects the structure of mass-spectra of multiply
ionized silicon clusters. For clusters, the clusters with a large fragmentation energy are found to correspond to the high peaks at N = 4 and 6 in mass-spectra. For clusters, a peak at N = 5 in mass-spectra has been predicted to be especially high.
Received: 9 June 1997 / Revised: 8 January 1998 / Accepted: 25 February 1998 相似文献
17.
A. Herlert L. Schweikhard M. Vogel 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2001,16(1):65-68
Singly charged silver-cluster anions are produced in a laser vaporization source and transferred into a Penning trap. After
size selection the clusters are subjected to an electron bath in the trap, which results in the attachment of further electrons.
The relative abundance of dianions or trianions as a function of the clusters' size is analyzed by time-of-flight mass spectrometry.
Silver-cluster dianions are observed for sizes n≥ 24 and trianions for n > 100. In addition, a detailed study of the cluster sizes 24 ?n? 60 shows a pronounced resistance to electron attachment for singly charged anions Agn
- with a closed electronic shell, in particular Ag29
-, Ag33
-, and Ag39
-. Both the threshold size for the observation of dianionic silver clusters and the shell effects in the production yield correlate
favorably with previous theoretical investigations of the respective electron affinities.
Received 24 November 2000 相似文献
18.
V. Boutou A.R. Allouche F. Spiegelmann J. Chevaleyre M. Aubert Frécon 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》1998,2(1):63-73
The geometrical structure of ground state Ban clusters (n
=2-14) has been predicted from various types of calculations including two ab initio approaches used for the smaller sizes namely HF+MP2(
n
=2-6), DFT (LSDA)(
n
=2-6, 9) and one model approach HF+pairwise dispersion used for all sizes investigated here. The lowest energy configurations
as well as some isomers have been investigated. The sizes n
=4, 7 and 13 are predicted to be the relatively more stable ones and they correspond to the three compact structures: the tetrahedron,
the pentagonal bipyramid and the icosahedron. The growth behavior from Ba7 to Ba13 appears to be characterized by the addition of atoms around a pentagonal bipyramid leading to the icosahedral structure of
Ba13 which is consistent with the observed size-distribution of barium clusters. Values for vertical ionization potentials calculated
for n
=2-5 at the CI level are seen to be in quite good agreement with recent measures.
Received: 14 May 1997 / Received in final form: 2 February 1998 / Accepted: 27 February 1998 相似文献
19.
A.-L. Barra G. Chouteau A. Stepanov A. Rougier C. Delmas 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,7(4):551-562
compounds have a layered structure made of alternating Ni-O and Li-O slabs. An amount z of extra divalent Ni ions is always present in the Li-O layers. We show, using high field magnetisation, static and dynamic
susceptibility and high frequency ESR, that the magnetic properties are driven by the z parameter. The compounds can be described as ferromagnetic Ni-O layers, bridged by clusters possessing a net ferromagnetic
moment.
Received: 24 July 1998 / Revised and Accepted: 23 September 1998 相似文献
20.
We present an analytical approach of the dynamics of a polymer when it is quenched from a solvent into a good or bad solvent. The dynamics is studied by means of a Langevin equation, first in the absence of hydrodynamic
effect, then taking into account the hydrodynamic interactions with the solvent. The variation of the radius of gyration is
studied as a function of time. In both cases, for the first stage of collapse or swelling, the evolution is described by a
power law with a characteristic time proportional to N
4/3
(N), where N is the number of monomers, without (with) hydrodynamic interactions. At larger times, scaling laws are derived for the diffusive
relaxation time.
Received: 10 March 1998 / Received in final form: 15 September 1998 / Accepted: 25 September 1998 相似文献