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991.
O. Golinelli Th. Jolicœur E.S. Sørensen 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,11(2):199-206
We study the magnetic excitation spectrum of the spin-1 chain with Hamiltonian .We focus on the range where the spin chain is in the gapped Haldane phase. The excitation spectrum and static structure factor is studied using
direct Lanczos diagonalization of small systems and density-matrix renormalization group techniques combined with the single-mode
approximation. The magnon dispersion has a minimum at until a critical value is reached at which the curvature (velocity) vanishes. Beyond this point, which is distinct from the VBS point and the Lifshitz
point, the minimum lies at an incommensurate value that goes smoothly to when approaches , the Lai-Sutherland point. The mode remains isolated from the other states: there is no evidence of spinon deconfinement
before the point .These findings explain recent observation of the behavior of the magnetization curve for .
Received 16 December 1998 相似文献
992.
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
993.
V. Žigman 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》1999,7(1):11-16
The results of the evaluation of the viscosity cross-section for elastic electron-xenon collisions, taking into account the
spin-orbit interaction of the continuum electron, in the energy interval from 0.1 eV to 50 eV are presented and discussed.
The calculations are performed on the basis of three theoretically derived sets of phase shift data obtained by different
authors and on the deduced relativistic expression for the viscosity cross-section in terms of phase shifts discerning the
spin-up and spin-down states of the scattered electrons. Comparison with viscosity cross-sections, as evaluated from non-relativistic
phase shifts extracted from experiments, strongly favours the relativistic results. The assumption of isotropic scattering
is critically examined and the error induced by its use is shown to persist to the same extent as in non-relativistic calculations,
at least in the energy region considered.
Received: 22 April 1998 / Received in final form: 16 December 1998 相似文献
994.
M. Sassetti F. Napoli B. Kramer 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,11(4):643-654
Using the bosonization technique, a theory for the collective excitations of the interacting electrons in quantum wires with
two subbands occupied is developed. The dispersion relations for the inter-subband charge and spin density excitations are
determined. The results are used to interpret the features observed in recent measurements of the Raman spectra of AlGaAs/GaAs
quantum wires, particularly for photon energies near band gap resonance. It is shown that peaks previously identified as “single
particle excitations” are signatures of higher order collective spin density excitations. Predictions about the observability
of the interband modes are made.
Received 8 February 1999 相似文献
995.
C. Djurberg K. Jonason P. Nordblad 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,10(1):15-21
Experiments on the temperature and time dependence of the response function and the field cooled magnetisation of a Cu(Mn)
spin glass at temperatures below the zero field spin glass temperature are used to explore the non-equilibrium nature of the
underlying spin configuration. The results imply that a certain spin configuration is imprinted on the system as the temperature
is decreased at a constant cooling rate. The cooling rate governs the magnitude of the FC magnetisation ((H,T)). Any intermittent halt at a constant temperature, , imprints an extended spin configuration, a process that is reflected e.g. in a downward relaxation of . On continued cooling at the same rate, the magnitude of (T) remains at a lower level than that of a continuous cooling curve. These results are put into the context of the corresponding
behaviour of the response function as observed in measurements of the relaxation of the zero field cooled magnetisation.
Received 27 October 1998 and Received in final form 30 November 1998 相似文献
996.
A. B. D. Brown C. Ferrero T. Narayanan A. R. Rennie 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,11(3):481-489
The structure and phase behaviour of a colloidal dispersion of plate-like particles are described. The plates are nickel (II)
hydroxide and have short-range, repulsive interactions and a low polydispersity. As the concentration of the plates is increased,
an equilibrium phase separation between a columnar phase and a less ordered phase is observed. Complementary measurements
using small-angle neutron and small-angle X-ray scattering have been used to distinguish the columnar phase from other possible
ordered structures. Previously isotropic-nematic phase transitions have been observed [#!ref1!#], however this dispersion
forms the more highly ordered columnar phase, due to the aspect ratio and the low polydispersity of the plate-like particles.
The concentration at which phase separation occurs, increases as the range of the particle interactions is reduced. This system
provides an interesting model for comparison with theory and calculations of structures in liquid crystal and mesophase in
which the particle interactions can be altered.
Received 24 February 1999 相似文献
997.
V. Yamakov A. Milchev G.M. Foo R.B. Pandey D. Stauffer 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,9(4):659-667
A coarse-grained off-lattice bead-spring model is used to reveal the complex dynamics of a polymer chain in a quenched porous
medium in the presence of an external field B. The behavior of the mean square displacement (MSD) of the center chain bead and that of the center of mass of the chain
as a function of time is studied at different values of the barrier concentration C, the field strength B and the chain length N. In a field, important information on the way in which chains move between obstacles and overcome them is gained from the
MSD vs. time analysis in the directions parallel and perpendicular to the flow. Instead of a steady approach to uniform drift-like
motion at low C, for sufficiently strong field B we observe logarithmic oscillations in the effective exponents describing the time dependence of the MSD along and perpendicular to field. A common nature of
this phenomenon with oscillatory behavior, observed earlier for biased diffusion of tracers on random lattices, is suggested.
Received 7 August 1998 相似文献
998.
J.C. Gimel T. Nicolai D. Durand J.M. Teuler 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,12(1):91-97
Dynamic properties of Brownian particles immersed in a periodic potential with two barriers V1 and V2 (symmetric bistable potential) are studied by using the Fokker-Planck equation which we solve numerically by the matrix continued
fraction method. This study will therefore serve to demonstrate the influence of this form of potential, which is of great
interest for superionic conductors and for many other solid systems, on the diffusion process. Thus, we have calculated the
full width at half maximum (FWHM) ) of the quasi-elastic line of the dynamic structure factor, for a large range of values of the wave-vectors q. Our results show clearly that, by varying the ratio of the barriers strictly between and 1, the Fokker-Planck equation describes a diffusive process which has some characteristic of jump and
liquid-like regimes. While in the limit cases, i.e. when tends to or 1, the diffusion process can be described only by a simple jump motion. However, the jump-lengths corresponding
to each limit case are not equal. In general the change of the ratio is found to have a significant effect on the character
of the diffusive motion. We have also performed Fokker-Planck dynamics calculations of the diffusion coefficient in a bistable
potential. We have found a good agreement between numerical calculations and analytical approximation results obtained in
the high friction limit.
Received 25 May 1998 and Received in final form 15 November 1998 相似文献
999.
S.G. Magalhães A. Theumann 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,9(1):5-13
We have examined the role of the BCS pairing mechanism in the formation of the magnetic moment and henceforth a spin glass
(SG) phase by studying a fermionic Sherrington-Kirkpatrick model with a local BCS coupling between the fermions. This model
is obtained by using perturbation theory to trace out the conduction electrons degrees of freedom in conventional superconducting
alloys. The model is formulated in the path integral formalism where the spin operators are represented by bilinear combinations
of Grassmann fields and it reduces to a single site problem that can be solved within the static approximation with a replica
symmetric ansatz. We argue that this is a valid procedure for values of temperature above the de Almeida-Thouless instability
line. The phase diagram in the T-g plane, where g is the strength of the pairing interaction, for fixed variance J
2
/N of the random couplings Jij, exhibits three regions: a normal paramagnetic (NP) phase, a spin glass (SG) phase and a pairing (PAIR) phase where there
is formation of local pairs.The NP and PAIR phases are separated by a second order transition line g=g
c
(T) that ends at a tricritical point T
3
=0.9807J, g
3
=5,8843J, from where it becomes a first order transition line that meets the line of second order transitions at T
c
=0.9570J that separates the NP and the SG phases. For T<T
c
the SG phase is separated from the PAIR phase by a line of first order transitions. These results agree qualitatively with
experimental data in .
Received 14 May 1998 相似文献
1000.
B. Lobe W. Janke K. Binder 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,7(2):283-291
We analyze recently extended high-temperature series expansions for the “Edwards-Anderson” spin-glass susceptibility of the
p-state Potts glass model on d-dimensional hypercubic lattices for the case of a symmetric bimodal distribution of ferro- and antiferromagnetic nearest-neighbor
couplings . In these star-graph expansions up to order 22 in the inverse temperature , the number of Potts states p and the dimension d are kept as free parameters which can take any value. By applying several series analysis techniques to the new series expansions,
this enabled us to determine the critical coupling Kc and the critical exponent of the spin-glass susceptibility in a large region of the two-dimensional (p,d)-parameter space. We discuss the thus obtained information with emphasis on the lower and upper critical dimensions of the
model and present a careful comparison with previous estimates for special values of p and d.
Received: 25 May 1998 / Revised and Accepted: 11 August 1998 相似文献