共查询到20条相似文献,搜索用时 46 毫秒
1.
J.E. Santos U.C. Täuber 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,28(4):423-440
Second-order phase transitions in a non-equilibrium liquid-gas model with reversible mode couplings, i.e., model H for binary-fluid critical dynamics, are studied using dynamic field theory and the renormalization group. The system
is driven out of equilibrium either by considering different values for the noise strengths in the Langevin equations describing
the evolution of the dynamic variables (effectively placing these at different temperatures), or more generally by allowing
for anisotropic noise strengths, i.e., by constraining the dynamics to be at different temperatures in d
|| - and d
⊥-dimensional subspaces, respectively. In the first, isotropic case, we find one infrared-stable and one unstable renormalization group fixed point. At the stable fixed point, detailed
balance is dynamically restored, with the two noise strengths becoming asymptotically equal. The ensuing critical behavior
is that of the standard equilibrium model H. At the novel unstable fixed point, the temperature ratio for the dynamic variables
is renormalized to infinity, resulting in an effective decoupling between the two modes. We compute the critical exponents
at this new fixed point to one-loop order. For model H with spatially anisotropic noise, we observe a critical softening only in the d
⊥-dimensional sector in wave vector space with lower noise temperature. The ensuing effective two-temperature model H does
not have any stable fixed point in any physical dimension, at least to one-loop order. We obtain formal expressions for the
novel critical exponents in a double expansion about the upper critical dimension d
c = 4 - d
|| and with respect to d
|| , i.e., about the equilibrium theory.
Received 4 April 2002 Published online 13 August 2002 相似文献
2.
H. Barentzen 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,24(2):197-209
An analytic study is presented of the E⊗e Jahn-Teller (JT) polaron, consisting of a mobile eg electron linearly coupled to the local eg normal vibrations of a periodic array of octahedral complexes. Due to the linear coupling, the parity operator and the angular momentum operator commute with the JT part and cause a twofold degeneracy of each JT eigenvalue. This degeneracy is lifted by the anisotropic
hopping term. The Hamiltonian is then mapped onto a new Hilbert space, which is isomorphic to an eigenspace of belonging to a fixed angular momentum eigenvalue j > 0. In this representation, the Hamiltonian depends explicitly on j and decomposes into a Holstein term and a residual JT interaction. While the ground state of the JT polaron is shown to belong
to the sector j = 1/2, the Holstein polaron is obtained for the “unphysical” value j = 0. The new Hamiltonian is then subjected to a variational treatment, yielding the dispersion relations and effective masses
for both kinds of polarons. The calculated polaron masses are in remarkably good agreement with recent quantum Monte Carlo
data. The possible relevance of our results to the magnetoresistive manganite perovskites is briefly discussed.
Received 6 July 2001 相似文献
3.
H.W. Diehl M. Smock 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,21(4):567-587
A class of continuum models with a critical end point is considered whose Hamiltonian [φ,ψ] involves two densities: a primary order-parameter field, φ, and a secondary (noncritical) one, ψ. Field-theoretic methods
(renormalization group results in conjunction with functional methods) are used to give a systematic derivation of singularities
occurring at critical end points. Specifically, the thermal singularity ∼ | t|2 - α of the first-order line on which the disordered or ordered phase coexists with the noncritical spectator phase, and the coexistence
singularity ∼ | t|1 - α or ∼ | t|β of the secondary density <ψ> are derived. It is clarified how the renormalization group (RG) scenario found in position-space
RG calculations, in which the critical end point and the critical line are mapped onto two separate fixed points
CEP
* and
λ
*, translates into field theory. The critical RG eigenexponents of
CEP
* and
λ
* are shown to match.
CEP
* is demonstrated to have a discontinuity eigenperturbation (with eigenvalue y = d), tangent to the unstable trajectory that emanates from
CEP
* and leads to
λ
*. The nature and origin of this eigenperturbation as well as the role redundant operators play are elucidated. The results
validate that the critical behavior at the end point is the same as on the critical line.
Received 18 January 2001 相似文献
4.
R. Takayama N.H. Kwong I. Rumyantsev M. Kuwata-Gonokami R. Binder 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,25(4):445-462
A detailed numerical analysis of exciton-exciton interactions in semiconductor quantum wells is presented. The theory is based
on the dynamics-controlled truncation formalism and evaluated for the case of resonant excitation of 1s-heavy-hole excitons. It is formulated in terms of standard concepts of scattering theory, such as the forward-scattering
amplitude (or T-matrix). The numerical diagonalization of the exciton-exciton interaction matrix in the 1s-approximation yields the excitonic T-matrix. We discuss the role of the direct and exchange interaction in the effective two-exciton Hamiltonian, which determines
the T-matrix, evaluated within the 1s-subspace, and also analyze the effects of the excitonic wave function overlap matrix. Inclusion of the latter is shown to
effectively prevent the 1s-approximation from making the Hamiltonian non-hermitian, but a critical discussion shows that other artefacts may be avoided
by not including the overlap matrix. We also present a detailed analysis of the correspondence between the excitonic T-matrix in the 1s-approximation and the well-known T-matrix governing two-particle interactions in two dimensional systems via short-range potentials.
Received 3 August 2001 and Received in final form 26 December 2001 相似文献
5.
F. Cinti M. Affronte A.G.M. Jansen 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,30(4):461-468
The level crossing mechanism between the ground and the first excited state of Na:Fe6 antiferromagnetically coupled iron rings is studied by torque magnetometry down to 40 mK and in magnetic fields up to 28
T. The step width at the crossing field Bc assumes a finite value at the lowest temperatures. This fact is ascribed to the presence of level anticrossing, not expected
for a ring with axial, i.e. S6 point group, symmetry. Assuming a reduced symmetry, we revised the model Hamiltonian of such a spin system by introducing
a Dzyaloshinsky-Moriya (DM) term and we show, by exact diagonalization, that DM term can account for the mixing of states with different parity.
In particular, analytical as well numerical analysis show that the introduction of the DM term may contribute to the broadening
of the torque step as well as for the finite energy gap at Bc observed by heat capacity in a similar ring Li:Fe6 as previously reported [#!aclbg!#].
Received 3 September 2002 Published online 31 December 2002 相似文献
6.
Haibo Xu Guangrui Wang Shigang Chen 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,22(1):65-69
By adjusting external control signal, rather than some available parameters of the system, we modify the straight-line stabilization
method for stabilizing an unstable periodic orbit in a neighborhood of an unstable fixed point formulated by Ling Yang et al., and derive a more simple analytical expression of the external control signal adjustment. Our technique solves the problem
that the unstable fixed point is independent of the system parameters, for which the original straight-line stabilization
method is not suitable. The method is valid for controlling dissipative chaos, Hamiltonian chaos and hyperchaos, and may be
most useful for the systems in which it may be difficult to find an accessible system parameter in some cases. The method
is robust under the presence of weak external noise.
Received 10 January 2001 相似文献
7.
The double differential dilepton spectrum d/(d
2
d
⊥
2) at fixed transverse mass M
⊥ allows a direct access to the vector meson spectral functions. Within a fireball model the sensitivity of d/(d
2
d
⊥
2) against variations of both the in-medium properties of mesons and the dynamics of the fireball is investigated. In contrast
to the integrated invariant-mass spectrum d/d
2, in the spectrum d/(d
2
d
⊥
2) with fixed M
⊥ the ω signal is clearly seen as bump riding on the ρ background even in case of strong in-medium modifications.[3mm]
Received: 16 November 2000 / Accepted: 16 January 2001 相似文献
8.
I.G. Alekseev N.A. Bazhanov Yu.A. Beloglazov P.E. Budkovsky E.I. Bunyatova V.P. Kanavets A.I. Kovalev L.I. Koroleva S.P. Kruglov B.V. Morozov V.M. Nesterov D.V. Novinsky V.V. Ryltsov V.A. Shchedrov A.D. Sulimov V.V. Sumachev D.N. Svirida V.Yu. Trautman V.V. Zhurkin 《The European Physical Journal A - Hadrons and Nuclei》2001,12(1):117-120
The spin rotation parameters A and R were measured for the elastic pion-proton scattering by the PNPI-ITEP collaboration in the D13(1700) resonance region. The main goal of the experimental program is to resolve the current partial-wave analyses (PWA) uncertainties.
Simultaneously with A and R the polarization parameter P was measured with the purpose to improve the experimental database and estimate systematic errors. The constraint which demands
a smooth energy dependence of all π-
p transverse amplitude zeros in the complex plane together with the new experimental data on A parameter can lead to the conclusion that the Barrelet branch of “zero trajectories" is chosen improperly in PWA of the Carnegie-Mellon-Lawrence-Berkely-Laboratory
groups at the range of the pion beam momentum near 1.0 GeV/c. The setup included a longitudinally polarized proton target with superconductive magnet, multiwire spark chambers and carbon
polarimeter with thick filter. The experiment was performed at the ITEP proton synchrotron, Moscow.
Received: 17 August 2001 / Accepted: 13 September 2001 相似文献
9.
V.S. Gerdjikov A. Kyuldjiev G. Marmo G. Vilasi 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,29(2):177-181
We consider generalizations of the standard Hamiltonian dynamics to complex dynamical variables and introduce the notions
of real Hamiltonian form in analogy with the notion of real forms for a simple Lie algebra. Thus to each real Hamiltonian
system we are able to relate several nonequivalent ones. On the example of the complex Toda chain we demonstrate how starting
from a known integrable Hamiltonian system (e.g. the Toda chain) one can complexify it and then project onto different real forms.
Received 18 October 2001 / Received in final form 24 May 2002 Published online 2 October 2002
RID="a"
ID="a"e-mail: gerjikov@inrne.bas.bg 相似文献
10.
V. Bernard N. Kaiser Ulf-G. Meißner 《The European Physical Journal A - Hadrons and Nuclei》1999,4(3):259-275
We evaluate the threshold matrix–element for the reaction pp→ppπ0 in a fully relativistic Feynman diagrammatic approach. We employ a simple effective range approximation to take care of the
S–wave pp final–state interaction. The experimental value for the threshold amplitude A = (2.7 −i0.3) fm4 can be reproduced by contributions from tree level chiral (long–range) pion exchange and short–range effects related to vector
meson exchanges, with ω-exchange giving the largest individual contribution. Pion loop effects appear to be small. We stress
that the commonly used heavy baryon formalism is not applicable in the NN–system above the pion production threshold due to
the large external momentum, |p|≃ (Mm
π)−1/2, with M and m
π the nucleon and the pion mass, respectively. We furthermore investigate the reaction pp→pnπ+ near threshold within the same approach. We extract from the data the triplet threshold amplitude as B = (2.8 −i1.5) fm4. Its real part can be well understood from (relativistic) tree level meson–exchange diagrams. In addition, we investigate
the process pp→ppη near threshold. We use a simple factorization ansatz for the ppη final–state interaction and extract from the data the modulus of the threshold amplitude, |C|= 1.32 fm4. With g
ηN= 5.3, this value can be reproduced by (relativistic) tree level meson–exchange diagrams and η–rescattering, whose strength
is fixed by the ηN scattering length. We also comment on the recent near threshold data for η′–production.
Received: 27 November 1998 相似文献
11.
A. Torcini A. Vulpiani A. Rocco 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,25(3):333-343
We study the front propagation in reaction-diffusion systems whose reaction dynamics exhibits an unstable fixed point and
chaotic or noisy behaviour. We have examined the influence of chaos and noise on the front propagation speed and on the wandering
of the front around its average position. Assuming that the reaction term acts periodically in an impulsive way, the dynamical
evolution of the system can be written as the convolution between a spatial propagator and a discrete-time map acting locally.
This approach allows us to perform accurate numerical analysis. They reveal that in the pulled regime the front speed is basically
determined by the shape of the map around the unstable fixed point, while its chaotic or noisy features play a marginal role.
In contrast, in the pushed regime the presence of chaos or noise is more relevant. In particular the front speed decreases
when the degree of chaoticity is increased, but it is not straightforward to derive a direct connection between the chaotic
properties (e.g. the Lyapunov exponent) and the behaviour of the front. As for the fluctuations of the front position, we observe for the
noisy maps that the associated mean square displacement grows in time as t
1/2 in the pushed case and as t
1/4 in the pulled one, in agreement with recent findings obtained for continuous models with multiplicative noise. Moreover we
show that the same quantity saturates when a chaotic deterministic dynamics is considered for both pushed and pulled regimes.
Received 17 July 2001 相似文献
12.
I.P. Johnstone L.D. Skouras 《The European Physical Journal A - Hadrons and Nuclei》2001,11(2):125-136
The matrix elements of the effective Hamiltonian in the 0g
9/2-1p
1/2 space are determined by a least-square fit to the energies of 477 levels of nuclei with 38≤Z≤50 and 47≤N≤50. The results of the calculation are found to be in better agreement with experiment than those obtained with previously
determined interactions.
Received: 31 May 2001 / Accepted: 14 June 2001 相似文献
13.
G. Puddu 《The European Physical Journal A - Hadrons and Nuclei》2000,9(2):171-181
We extend the recently presented formalism for Monte Carlo calculations of the partition function, for both even and odd particle
number systems (Phys. Rev. C 59, 2500 (1999)), to the calculation of many-body matrix elements of the type <ψ| e
- βℋ|ψ> where |ψ> is a many-body state with a definite angular momentum, parity, neutron and proton numbers. For large β such
matrix elements are dominated by the lowest eigenstate of the many-body Hamiltonian ℋ, corresponding with a given angular
momentum parity and particle number. Emphasis is placed on odd-mass nuclei. Negligible sign fluctuations in the Monte Carlo
calculation are found provided the neutron and proton chemical potentials are properly adjusted. The formalism is applied
to the J
π = 0+ state in 166
Er and to the J
π = 9/2-, 13/2+, 5/2- states in 165
Er using the pairing-plus-quadrupole model.
Received: 28 April 2000 / Accepted: 20 September 2000 相似文献
14.
R.-J. Tarento P. Joyes J. van de Walle M. Nakamura 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2001,16(1):111-114
We investigate the electron capture occurring in the collision between an ion A+ and a cluster An (n = 5). The process has been modelled within the Hubbard Hamiltonian,which takes into account the intrasite U electron correlation. An exact procedure has been numerically applied which involves all the excited states to examine the
time evolution of the system during the collision. We have applied the model to the sodium case. We have investigated the
time evolution of the electron population during the collision on the projectile versus the kinetic energy of the projectile. It displays some oscillations which means that the electron exchanges between the ion
and the cluster occurs alternatively in one direction and the other. We also vary U and examine its influence on the dynamics of the oscillation of the average population. Finally the cross section is derived
versus the energy and U.
Received 29 November 2000 相似文献
15.
V.Yu. Grishina L.A. Kondratyuk W. Cassing A.B. Kaidalov E. De Sanctis P. Rossi 《The European Physical Journal A - Hadrons and Nuclei》2001,10(3):355-364
We investigate deuteron two-body photodisintegration within the framework of the Quark-Gluon Strings Model with nonlinear
baryon Regge trajectories. Special attention is paid to the use of QCD motivated Regge trajectories of the logarithmic and
square-root form. We find that the recent experimental data from TJNAF in the few-GeV region can be reasonably described by
the model. Angular distributions at different γ-energies are presented and the effect of a forward-backward asymmetry is discussed.
Predictions for the energy dependence of dσ/dt at higher energies and different Θc.m. are presented, too.
Received: 12 March 2001 / Accepted: 19 March 2001 相似文献
16.
E. Arrigoni G.C. Strinati 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,19(3):433-448
We study the zero-temperature spin fluctuations of a two-dimensional itinerant-electron system with an incommensurate magnetic
ground state described by a single-band Hubbard Hamiltonian. We introduce the (broken-symmetry) magnetic phase at the mean-field
(Hartree-Fock) level through a spiral spin configuration with characteristic wave vector Q different in general from the antiferromagnetic wave vector Q
AF, and consider spin fluctuations over and above it within the electronic random-phase (RPA) approximation. We obtain a closed system of equations for the generalized wave vector and frequency dependent susceptibilities, which are equivalent to the
ones reported recently by Brenig. We obtain, in addition, analytic results for the spin-wave dispersion relation in the strong-coupling
limit of the Hubbard Hamiltonian and find that at finite doping the spin-wave dispersion relation has a hybrid form between that associated with the (localized) Heisenberg model and that associated with the (long-range) RKKY exchange interaction.
We also find an instability of the spin-wave spectrum in a finite region about the center of the Brillouin zone, which signals
a physical instability toward a different spin- or, possibly, charge-ordered phase, as, for example, the stripe structures
observed in the high-T
c materials. We expect, however, on physical grounds that for wave vectors external to this region the spin-wave spectrum that
we have determined should survive consideration of more sophisticated mean-field solutions.
Received 15 September 2000 相似文献
17.
We show that the spin-orbit potential of the nuclear mean field destroys isoscalar superfluid correlations in self-conjugate
nuclei. Using group theory and boson mapping techniques on a Hamiltonian including single particle splittings and a SO
ST(8) pairing interaction, we give analytical expressions for the spin-orbit dependence of some N = Z properties such as the relative position of T = 0 and T = 1 states in odd-odd systems or double binding-energy differences of even-even nuclei.
Received: 12 April 2000 / Accepted: 25 May 2000 相似文献
18.
A. Avella F. Mancini D. Villani H. Matsumoto 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,20(3):303-311
The addition to the Hubbard Hamiltonian of a t' diagonal hopping term, which is considered to be material dependent for high-T
c cuprate superconductors, is generally suggested to obtain a model capable to describe the physics of high-T
c cuprate materials. In this line of thinking, the two-dimensional t-t'-U model has been studied by means of the Composite Operator Method, which allows to determine the dynamics in a fully self-consistent
way by use of symmetry requirements, as the ones coming from the Pauli principle. At first, some local quantities have been
calculated to be compared with quantum Monte Carlo data. Then, the structure of the energy bands, the shape of the Fermi surface
and the position of the van Hove singularity have been computed as functions of the model parameters and studied by the light
of the available experimental data. The results of our study show that there exists two sets of parameters that allows the
model to describe the relevant features of the 1-layer compounds Nd2-xCexCuO4 and La2-xSrxCuO4. On the other hand, for the 2-layer compound YBa2Cu3O 7 - δ is not possible to find a reasonable set of parameters which could reproduce the position of the van Hove singularity as
predicted by ARPES experiments. Hence, it results questionable the existence of an unique model that could properly describe
the variety of cuprate superconductors, as the two-dimensional t-t'-U model was thought to be.
Received 29 March 2000 and Received in final form 10 August 2000 相似文献
19.
We present a quantum mechanical framework for defining the statistics of measurements of
, A(t) being a quantum mechanical variable. This is a generalization of the so-called full counting statistics proposed earlier for DC electric currents. We develop an influence functional formalism that allows us to study the quantum system along with the measuring device while fully accounting for the back action of the detector on the system to be measured. We define the full counting statistics of an arbitrary variable by means of an evolution operator that relates the initial and final density matrices of the measuring device. In this way we are able to resolve inconsistencies that occur in earlier definitions. We suggest two schemes to observe the so defined statistics experimentally.Received: 30 June 2003, Published online: 15 October 2003PACS:
73.50.Td Noise processes and phenomena - 73.23.-b Electronic transport in mesoscopic systems - 74.40.+k Fluctuations (noise, chaos, nonequilibrium superconductivity, localization, etc.) 相似文献
20.
J.P. Salas M. I narrea A.I. Pascual 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2002,20(1):45-54
In the frame work of classical mechanics, we study the nonlinear dynamics of a single ion trapped in a Penning trap perturbed
by an electrostatic sextupolar perturbation. The perturbation is caused by a deformation in the configuration of the electrodes.
By using a Hamiltonian formulation, we obtain that the system is governed by three parameters: the z-component of the canonical angular momentum P
φ - which is a constant of the motion because the perturbation we assume is axial-symmetric -, the parameter δ that determines
the ratio between the axial and the cyclotron frequencies, and the parameter a which indicates how far from the ideal design the electrodes are. We study the case P
φ = 0. By means of surfaces of section, we show that the phase space structure is made of three fundamental families of orbits:
arch, loop and box orbits. The coexistence of these kinds of orbits depends on the parameter δ. The escape is also explained on the basis of
the shape of the potential energy surface as well as of the phase space structure.
Received 6 September 2001 / Received in final form 19 March 2002 Published online 28 June 2002 相似文献