共查询到20条相似文献,搜索用时 15 毫秒
1.
F.A. Tamarit S.A. Cannas C. Tsallis 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,1(4):545-548
We consider biological evolution as described within the Bak and Sneppen 1993 model. We exhibit, at the self-organized critical
state, a power-law sensitivity to the initial conditions, calculate the associated exponent, and relate it to the recently
introduced nonextensive thermostatistics. The scenario which here emerges without tuning strongly reminds of that of the tuned onset of chaos in say logistic-like one-dimensional maps. We also calculate the dynamical exponent z.
Received: 5 November 1997 / Received in final form: 11 November 1997 / Accepted: 19 November 1997 相似文献
2.
N. Andrenacci P. Pieri G.C. Strinati 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,13(4):637-642
We consider a system of fermions in the continuum case at zero temperature, in the strong-coupling limit of a short-range
attraction when composite bosons form as bound-fermion pairs. We examine the density dependence of the size of the composite
bosons at leading order in the density (“dilute limit”), and show on general physical grounds that this size should decrease with increasing density, both in three and two dimensions. We then compare with the analytic zero-temperature mean-field
solution, which indeed exhibits the size shrinking of the composite bosons both in three and two dimensions. We argue, nonetheless,
that the two-dimensional mean-field solution is not consistent with our general result in the “dilute limit”, to the extent
that mean field treats the scattering between composite bosons in the Born approximation which is known to break down at low
energy in two dimensions.
Received 3 June 1999 and Received in final form 29 July 1999 相似文献
3.
S. Galam A. Mauger 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,1(2):255-258
An universal invariant for site and bond percolation thresholds ( and respectively) is proposed. The invariant writes where and are positive constants, and d the space dimension. It is independent of the coordination number, thus exhibiting a topology invariance at any d. The formula is checked against a large class of percolation problems, including percolation in non-Bravais lattices and
in aperiodic lattices as well as rigid percolation. The invariant is satisfied within a relative error of for all the twenty lattices of our sample at d=2, d=3, plus all hypercubes up to d=6.
Received: 7 July 1997 / Accepted: 5 November 1997 相似文献
4.
R. Zeyher A. Greco 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,6(4):473-485
The feasibility of a perturbation expansion for Green's functions of the t-J model directly in terms of X-operators is demonstrated using the Baym-Kadanoff functional method. As an application we derive explicit expressions for
the kernel of the linearized equation for the superconducting order parameter in leading order of a 1/N expansion. The linearized equation is solved numerically on a square lattice taking instantaneous and retarded contributions
into account. Classifying the order parameter according to irreducible representations of the point group C4v of the square lattice and according to even or odd parity in frequency we find that a reasonably strong instability occurs
only for even frequency pairing with d-wavelike symmetry. The corresponding transition temperature Tc is where t is the nearest-neighbor hopping integral. The underlying effective interaction consists of an attractive, instantaneous term
and a retarded term due to charge and spin fluctuations. The latter is weakly attractive at low frequencies below ,strongly repulsive up to and attractive towards even higher energies. Tc increases with decreasing doping until a d-wavelike bond-order wave instability is encountered near optimal doping at for J=0.3. Tc is essentially linear in J and rather insensitive to an additional second-nearest neighbor hopping integral t'. A rather striking property of Tc is that it is hardly affected by the soft mode associated with the bond-order wave instability or by the Van Hove singularity
in the case with second-nearest neighbor hopping. This unique feature reflects the fact that the solution of the gap equation
involves momenta far away from the Fermi surface (due to the instantaneous term) and many frequencies (due to the retarded
term) so that singular properties in momentum or frequency are averaged out very effectively.
Received: 16 June 1998 / Accepted: 14 July 1998 相似文献
5.
P. Politi M.G. Pini 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,2(4):475-481
We study the effect of dipolar interactions on a magnetic striped monolayer with a microscopic unit cell of square symmetry,
and of size spins. Even if the aspect ratio r=N
x
/N
y
is very large, an in-plane shape anisotropy is always negligible, except if Ny is fairly small (N
y
<40). In-plane domains are not possible, except for values of the dipolar coupling larger than the domain wall energy.
Received: 11 July 1997 / Revised: 24 September 1997 / Accepted: 24 October 1997 相似文献
6.
C.P. Beulshausen L. Fritsche 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,2(2):237-247
A characteristic feature of the Full Potential Linearized Augmented Plane Wave (FLAPW)-method consists in the spatial subdivision of the charge density analogous to that of the one-particle wavefunctions,
i.e. into a portion that is expanded in terms of spherical harmonics Ylm inside the muffin-tin spheres and into a plane wave expansion of the interstitial charge density. To obtain the Hartree potential
inside the spheres one is hence forced to solve a boundary value problem at the sphere surface. In addition, in all non-equivalent
spheres each (l,m)-component of the charge density is mapped onto 300-400 radial grid points. To ensure an accelerated convergence of the calculation,
the pertinent schemes require this rather large data set to be stored and mixed within 3-6 iteration steps. We show and illustrate
for the example of a spin-polarized Ni-film with and without an oxygen overlayer and for bulk Si that this data set can be
compressed by at least two orders of magnitude if one partitions the charge density in a different way so that the relevant
portion determining the interatomic bonding can be Fourier expanded throughout the lattice cell. One thereby arrives at a
modified FLAPW-scheme that combines favorable features of the original method with virtues of the pseudopotential method which
consist in the simple construction of the Hartree potential and the efficient way of achieving self-consistency. These advantages
can be exploited to the fullest by using Fast Fourier Transform. Moreover, forces that atoms experience in off-equilibrium
positions attain a particularly simple form in terms of the charge density expansion coefficients.
Received: 31 May 1997 / Revised: 12 December 1997 / Accepted: 18 December 1997 相似文献
7.
C.M. Marques D. Izzo T. Charitat E. Mendes 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,3(3):353-358
We study the scattering intensity of dilute and semi-dilute solutions of star polymers. The star conformation is described
by a model introduced by Daoud and Cotton. In this model, a single star is regarded as a spherical region of a semi-dilute
polymer solution with a local, position dependent screening length. For high enough concentrations, the outer sections of
the arms overlap and build a semi-dilute solution (a sea of blobs) where the inner parts of the actual stars are embedded.
The scattering function is evaluated following a method introduced by Auvray and de Gennes. In the dilute regime there are
three regions in the scattering function: the Guinier region (low wave vectors, ) from where the radius of the star can be extracted; the intermediate region () that carries the signature of the form factor of a star with f arms: ; and a high wavevector zone () where the local swollen structure of the polymers gives rise to the usual q
-5/3
decay. In the semi-dilute regime the different stars interact strongly, and the scattered intensity acquires two new features:
a liquid peak that develops at a reciprocal position corresponding to the star-star distances; and a new large wavevector
contribution of the form q
-5/3
originating from the sea of blobs.
Received: 3 September 1997 / Revised: 13 January 1988 / Accepted: 31 March 1998 相似文献
8.
L.-H. Tang P. Smilauer D.D. Vvedensky 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,2(3):409-412
Two types of mechanisms are proposed for mound coarsening during unstable epitaxial growth: stochastic, due to deposition
noise, and deterministic, due to mass currents driven by surface energy differences. Both yield the relation H=(RWL)2 between the typical mound height W, mound size L, and the film thickness H. An analysis of simulations and experimental data shows that the parameter R saturates to a value which discriminates sharply between stochastic () and deterministic () coarsening. We derive a scaling relation between the coarsening exponent 1/z and the mound-height exponent which, for a saturated mound slope, yields .
Received: 11 November 1997 / Revised in final form: 28 November 1997 /
Accepted: 28 November 1997 相似文献
9.
G. Bergmann 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,2(2):233-235
The n-electron ground state of the Friedel resonance model can be written as a single Slater determinant of n s-electrons plus d-electron-s-hole companion. This new formula is derived geometrically in the Hilbert space. The derivation uses the fact
that a n-electron Slater determinant, built from N band states, corresponds to a n-dimensional subspace in the N-dimensional Hilbert space.
Received: 4 November 1997 / Accepted: 19 November 1997 相似文献
10.
11.
12.
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
13.
S. Chakravarty 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,1(3):337-343
The interlayer tunneling mechanism of the cuprate high temperature superconductors involves a conversion of the confinement
kinetic energy of the electrons perpendicular to the CuO-planes (c-axis) in the normal state to the pair binding energy in the superconducting state. This mechanism is discussed and the arguments
are presented from the point of view of general principles. It is shown that recent measurements of the c-axis properties support the idea that the electrons substantially lower their c-axis kinetic energy upon entering the superconducting state, a change that is nearly impossible in any conventional mechanism.
The proper use of a c-axis conductivity sum rule is shown to resolve puzzles involving the penetration depth and the optical measurements.
Received: 5 January 1998 / Accepted: 17 March 1998 相似文献
14.
G. Grégoire M. Mons C. Dedonder-Lardeux C. Jouvet 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》1998,1(1):5-7
clusters (solvents being , or ) have been studied by resonance enhanced two photons ionization, leading to the detection of clusters. When water is the solvent, large clusters up to n>50 can be observed, whereas for and no clusters larger than 10 could be evidenced. Because the first step in the ionization process is the excitation from the
ground solvated () ion pair state to a covalent excited state, the differences in the cluster size distribution for different solvent may be
interpreted as a difference in cluster structures leading to a difference in the charge separation in the ground state.
Received: 30 September 1997 / Revised in final form: 30 October 1997 / Accepted: 30 October 1997 相似文献
15.
Shi-Biao Zheng Guang-Can Guo 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》1998,1(1):105-107
We propose a method for preparing superpositions of coherent states of the motion of an ion in an anisotropic two-dimensional
trap, in which the ion is tightly bound in the y direction. In the scheme the ion is excited by two resonant laser beams with equal amplitude, propagating along the x and y directions, respectively. In the Dicke-Lamb limit, an initial coherent state of the ion motion can be converted into a superposition
of several coherent states on a circle through the laser-ion interactions and state-selective measurements on the ion.
Received: 30 May 1997 / Revised: 29 July 1997 / Accepted: 22 October 1997 相似文献
16.
R. Kishore S. Lamba 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,8(2):161-164
An exact analytical expression for the specific heat jump at the critical temperature Tc has been obtained directly from the BCS gap equation for any shape of the energy dependent electronic density of states (DOS).
We consider a model which takes into consideration electron-electron repulsion, formulated in the Hubbard model along with
the electron-electron attraction due to electron-phonon interaction in the BCS formalism. We have analyzed this expression
for constant as well as for the Lorentzian forms of DOS. It is shown that the constant DOS in the simple BCS theory cannot
explain the large values of , found in some superconductors. The specific heat versus temperature curve has been found to have a peak, similar to that of Eliashberg theory of superconductivity. The influence
of repulsive interaction is very small and occurs mainly at higher temperatures.
Received: 26 January 1998 相似文献
17.
E. Rastelli A. Tassi 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,3(3):341-347
Ferromagnetic spin chains of a hexagonal lattice coupled by a weak antiferromagnetic interaction J1 develop a helix arrangement if the intrachain antiferromagnetic NNN exchange J2 is sufficiently large. We show that the classical minimum energy spin configuration is an umbrella when an external magnetic field is applied. The scenario is dramatically changed by quantum fluctuations. Indeed we find
that the zero point motion forces the spins in a plane containing the magnetic field so that classical expectation is deceptive
for our model. Our result is obtained by controlled expansion in the low field-long wavelength modulation limit.
Received: 9 September 1997 / Revised: 15 October 1997 / Accepted: 17 November 1997 相似文献
18.
A. Ghosh S.K. Adhikari 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,2(1):31-36
The solutions of a renormalized BCS equation are studied in three space dimensions in s, p and d waves for finite-range separable
potentials in the weak to medium coupling region. In the weak-coupling limit, the present BCS model yields a small coherence
length and a large critical temperature, , appropriate for some high- materials. The BCS gap, , and specific heat as a function of zero-temperature condensation energy are found to exhibit potential-independent universal scalings. The
entropy, specific heat, spin susceptibility and penetration depth as a function of temperature exhibit universal scaling below
in p and d waves.
Received: 18 July 1997 / Revised: 8 September 1997 / Accepted: 29 September 1997 相似文献
19.
T. Charitat B. Fourcade 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,1(3):333-336
An o-ring takes spontaneously the shape of a chair when strong enough torsion is applied in its tangent plane. This state
is metastable, since work has to be done on the o-ring to return to the circular shape. We show that this metastable state
exists in a Hamiltonian where curvature and torsion are coupled via an intrinsic curvature term. If the o-ring is constrained to be planar (2d case), this metastable state displays a kink-anti-kink
pair. This state is metastable if the ratio is less than , where C and A are the torsion and the bending elastic constants [#!landau!#]. In three dimensions, our variational approach shows that
. This model can be generalized to the case where the bend is induced by a concentration field which follows the variations
of the curvature.
Received: 27 August 1997 / Revised: 23 October 1997 / Accepted: 12 November 1997 相似文献
20.
P. Kopietz A. Völker 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,3(3):397-405
A time-dependent electric field gives rise to a stationary non-equilibrium current I
(2)
around a mesoscopic metal ring threaded by a magnetic flux. We show that this current, which is proportional to the intensity
of the field, is closely related to the exchange part of the interaction contribution to the equilibrium persistent current, and that the corresponding non-linear conductivity directly measures the weak localization correction
to the polarization. We explicitly calculate the disorder average of I
(2)
in the diffusive regime as function of the frequency of the electric field and the static flux piercing the ring, and suggest
an experiment to test our theory.
Received: 5 September 1997 / Accepted: 4 November 1997 相似文献