共查询到20条相似文献,搜索用时 734 毫秒
1.
B.-Q. Ma I. Schmidt J.-J. Yang 《The European Physical Journal A - Hadrons and Nuclei》2001,12(3):353-359
We investigate the spin structure of the nucleon in an extended Jaffe-Lipkin quark model. In addition to the conventional
3q structure, different (3q)(Q
) admixtures in the nucleon wave function are also taken into account. The contributions to the nucleon spin from various
components of the nucleon wave function are discussed. The effect due to the Melosh-Wigner rotation is also studied. It is
shown that the Jaffe-Lipkin term is only important when antiquarks are negatively polarized. We arrive at a new “minimal”
quark model, which is close to the naive quark model, in order to understand the proton spin “puzzle”.
Received: 4 November 2000 / Accepted: 23 October 2001 相似文献
2.
Gorce JP Bovey D McDonald PJ Palasz P Taylor D Keddie JL 《The European physical journal. E, Soft matter》2002,8(4):421-429
We present a systematic study of the vertical uniformity of water distribution during the drying of waterborne colloidal films,
testing the predictions of a Peclet number Pe defined for this system. Pe indicates the relative contributions of water evaporation and Brownian diffusion in determining the concentration profile
in the vertical direction (i.e. normal to the substrate). When Pe < 1, the water concentration in films cast from an alkyd emulsion is found via magnetic-resonance profiling to be uniform with depth, which is consistent with expectations. When Pe > 1, a gradient in the water concentration develops, with less water near the interface with air. The water profiles reveal
that the alkyd particles do not coalesce immediately upon contact in close-packing. At later times, a concentrated surface
layer develops, but particles are not coalesced in this layer to form a continuous “skin”, but rather the structure is likely
to be that of a biliquid foam.
Received 20 March 2002 and Received in final form 12 June 2002 相似文献
3.
Ye.S. Golubeva L.A. Kondratyuk M. Büscher W. Cassing V. Hejny H. Ströher 《The European Physical Journal A - Hadrons and Nuclei》2001,11(2):237-245
The ω resonance production and its π0γ decay in pA reactions close to threshold is considered within the Intranuclear Cascade (INC) model. The π0γ invariant-mass distribution shows two components which correspond to the ω decay “inside” and “outside” the nucleus, respectively.
The “inside” component is distorted by medium effects, which introduce a mass shift as well as collisional broadening for
the ω-meson and its decaying pion. The relative contribution of the “inside” component is analyzed in detail for different
kinematical conditions and nuclear targets. It is demonstrated that a measurement of the correlation in azimuthal angle between
the π0 and γ momenta allows to separate events related to the “inside”ω decay from different sources of background when uncorrelated
π0's and γ's are produced.
Received: 2 April 2001 / Accepted: 5 June 2001 相似文献
4.
C. Signorini 《The European Physical Journal A - Hadrons and Nuclei》2002,13(1-2):129-133
The experimental data relative to the interaction for the systems 9, 10, 11Be + 209Bi at the Coulomb barrier are critically discussed and compared also with present theories. The break-up (BU) of the two loosely
bound projectiles, 9, 11Be, seems to influence the fusion process by “hindering” the fusion cross-sections; but, contrary to expectations, the 11Be halo structure has no influence, since no “enhancement” is evident from the existing data. Attempt to describe simultaneously
all the 9Be + 209Bi system data: fusion, elastic scattering and BU, within a coupled-channel (CC) approach is only partly successful. It is
important, from a theoretical viewpoint, to include in the CC formalism as well as possible the BU process both to continuum
states as well as to unbound resonances. More accurate and well-focused experiments are also necessary to pin down this problem.
Received: 1 May 2001 / Accepted: 4 December 2001 相似文献
5.
C. Devaux J.P. Chapel E. Beyou Ph. Chaumont 《The European physical journal. E, Soft matter》2002,7(4):345-352
Thin layers of polystyrene were grown from surface-grafted nitroxide initiators via controlled “living” free radical polymerization.
The “reactive” Langmuir-Blodgett deposition method allowed an effective control of the initiator layer density leading to
PS brushes with different and high grafting density and stretching. The influence of the grafting density on the layer structure
was studied. Comparison with theoretical predictions for monodispersed brushes in bad solvent was discussed. The thickness
was found to vary linearly with molecular weight and the density dependence was shown using wetting measurements. Special
features of controlled radical nitroxide polymerization from a surface were discussed. A direct comparison of the molecular
weight and polydispersity between surface and bulk polymers was made by de-grafting the brushes into a toluene/HF solution.
Finally, some evidence of a “surface Fischer” effect was shown from re-initiated layers.
Received 20 December 2001 相似文献
6.
J. Gidden M.T. Bowers 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2002,20(3):409-419
Ion mobility experiments and molecular modeling calculations were used to investigate the gas-phase conformations and folding
energetics of 16 deprotonated dinucleotides. [M-H]- ions were formed by MALDI and their collision cross-sections measured in helium using ion mobility based techniques. Cross-sections
of theoretical structures, generated by molecular mechanics/dynamics calculations, were compared to the experimental values
for conformational identification of the dinucleotides. Temperature dependent measurements and kinetic theory were also used
to obtain energetic and dynamic data concerning the folding properties of the dinucleotides. Three distinct families of conformations,
with significantly different collision cross-sections, were identified: a “stacked” family in which the two nucleobases stack;
an “H-bonded” family in which the two nucleobases stay in the same plane and are hydrogen-bonded to each other; and an “open”
family in which the two nucleobases are separated from each other. At temperatures ≥ 300 K these conformers rapidly interconvert
in most systems, but they can be separated and individually observed in the lower temperature (80-200 K) experiments. The
types and relative amounts of each conformer observed, and the temperature at which they can be separated, are base and sequence
dependent. Theoretical modeling of the temperature-dependent data was used to determine isomerization barrier heights between
the various conformers and yielded values between 0.8-12.9 kcal/mol, depending on the dinucleotide.
Received 17 May 2002
Published online 13 September 2002 相似文献
7.
We investigate, using numerical simulations and analytical arguments, a simple one-dimensional model for the swelling or the
collapse of a closed polymer chain of size N, representing the dynamical evolution of a polymer in a Θ-solvent that is rapidly changed into a good solvent (swelling)
or a bad solvent (collapse). In the case of swelling, the density profile for intermediate times is parabolic and expands
in space as t
1/3, as predicted by a Flory-like continuum theory. The dynamics slows down after a time ∝N
2 when the chain becomes stretched, and the polymer gets stuck in metastable “zig-zag” configurations, from which it escapes
through thermal activation. The size of the polymer in the final stages is found to grow as . In the case of collapse, the chain very quickly (after a time of order unity) breaks up into clusters of monomers (“pearls”).
The evolution of the chain then proceeds through a slow growth of the size of these metastable clusters, again evolving as
the logarithm of time. We enumerate the total number of metastable states as a function of the extension of the chain, and
deduce from this computation that the radius of the chain should decrease as 1/ln(ln t). We compute the total number of metastable states with a given value of the energy, and find that the complexity is non-zero
for arbitrary low energies. We also obtain the distribution of cluster sizes, that we compare to simple “cut-in-two” coalescence
models. Finally, we determine the aging properties of the dynamical structure. The subaging behaviour that we find is attributed
to the tail of the distribution at small cluster sizes, corresponding to anomalously “fast” clusters (as compared to the average).
We argue that this mechanism for subaging might hold in other slowly coarsening systems.
Received 23 October 2000 相似文献
8.
Y.Y. Goldschmidt Y. Shiferaw 《The European Physical Journal B - Condensed Matter and Complex Systems》2003,32(1):87-95
In this paper we investigate the problem of a long self-avoiding polymer chain immersed in a random medium. We find that in
the limit of a very long chain and when the self-avoiding interaction is weak, the conformation of the chain consists of many
“blobs” with connecting segments. The blobs are sections of the molecule curled up in regions of low potential in the case
of a Gaussian distributed random potential or in regions of relatively low density of obstacles in the case of randomly distributed
hard obstacles. We find that as the strength of the self-avoiding interaction is increased the chain undergoes a delocalization
transition in the sense that the appropriate free energy per monomer is no longer negative. The chain is then no longer bound
to a particular location in the medium but can easily wander around under the influence of a small perturbation. For a localized
chain we estimate quantitatively the expected number of monomers in the “blobs” and in the connecting segments.
Received 13 November 2002 Published online 14 March 2003 相似文献
9.
Ph. Tordjeman C. Robert G. Marin P. Gerard 《The European physical journal. E, Soft matter》2001,4(4):459-465
In order to study the effect of the α,β crystalline structure of polypropylene (PP) on its mechanical properties, it is necessary
to prepare samples with variable α/β-phase content but with constant crystallinity and constant spherulite size. With this
objective, heat treatment was first defined to be applied to an isotactic PP containing a β nucleating agent in order to achieve
these conditions. Then study of the effect of the β-phase content on the tensile properties and fracture behaviour has been
done at room temperature. The mechanical properties at fracture were assessed by three-point bending tests and were analysed
on the basis of the “Essential Work of Fracture” (EWF). The results show that the elongation at fracture under tensile stress
and the “near” Plane-Strain Essential Work of Fracture, w
Ie, increase substantially with the β-phase content. Besides, Young's modulus and the yield stress in tensile tests decrease
slowly with the β-phase content. Finally, these results are analysed taking account the differences in structure of the α
and β spherulites.
Received 18 September 2000 and Received in final form 19 December 2000 相似文献
10.
X.H. Yan J.W. Ding Q.B. Yang 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,20(2):157-163
Developing a renormalization group approach, we study the hopping conductivity of nanocrystalline chains with different site
energies. Exact calculations show that many parameters including nano-sizes, randomness of grain distributions, lattice distortions,
site energies, transition rates, Fermi energy, and temperature influence the conductivity. Some new singular features, for
example the frequency shift, the amplitude fluctuations, and the interchange between “peak” and “valley” behavior of the imaginary
part of the conductivity can be caused by certain parameters mentioned above, while the interface distortions modulate mainly
the overall amplitudes of the conductivity at the whole frequency region.
Received 13 January 2000 and Received in final form 12 September 2000 相似文献
11.
B. Lamine M.-T. Jaekel S. Reynaud 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2002,20(2):165-176
We study the decoherence of atomic interferometers due to the scattering of stochastic gravitational waves. We evaluate the
“direct” gravitational effect registered by the phase of the matter waves as well as the “indirect” effect registered by the
light waves used as beam-splitters and mirrors for the matter waves. Considering as an example the space project HYPER, we
show that both effects are negligible for the presently studied interferometers.
Received 15 February 2002 / Received in final form 12 April 2002 Published online 19 July 2002 相似文献
12.
13.
H. Sakurai 《The European Physical Journal A - Hadrons and Nuclei》2002,13(1-2):49-53
Recent studies on nuclear structure by using radioactive isotope beams available at the RIKEN projectile-fragment separator
(RIPS) are introduced. Special emphasis is given to two selected experiments from recent programs that highlight studies on
the magicity loss observed for very neutron-rich nuclei beyond N = 20 in the “island-of-inversion” region; the particle stability of 31F, and the low-lying excited states of 34Mg.
Received: 1 May 2001 / Accepted: 4 December 2001 相似文献
14.
Achard MF Bedel JP Marcerou JP Nguyen HT Rouillon JC 《The European physical journal. E, Soft matter》2003,10(2):129-134
Taking advantage of the great number of bent-core or “banana" compounds synthesized and studied in the laboratory, we describe
their behaviour under the application of an external electric field. If the field were a static one, we would work within
the frame of an equilibrium phase diagram in a (field E, temperature T) space where some phases would be simple dielectrics and others ferroelectric ones with a macroscopic polarization, either
spontaneous or induced by the field. In this paper, we deal with the basic responses of “banana” liquid crystals under the
application of a low frequency (1 to 100 Hz) AC field. Firstly square-wave voltages allow us to locate the phase boundary
between dielectric (at lower field) and ferroelectric phases (higher field) at a given temperature and field threshold. Then
we apply slowly varying AC voltages with shapes like triangle or “triple-plateau” to check out the stability of the induced
ferroelectric phase versus field removal. Three behaviours are encountered, the unstable one (short lifetime of the high-field ferroelectric phase)
where the macroscopic polarization is destroyed and then rebuilt in the opposite direction during each half period and usually
called “antiferroelectric”; the stable one (long lifetime) with a polarization that rotates at constant modulus which is labeled
as “ferroelectric” and a new one where the macroscopic polarization is proportional to the applied fied, we named this behaviour
as “superparaelectric”. Let us stress that these observations apply to the ferroelectric phases of the (E, T) phase diagram not to the zero field (0,T) phases observed in the usual phase characterization experiments except for an
eventual spontaneous ferroelectric phase.
Received 18 April 2002 and Received in final form 17 January 2003 Published online: 16 April 2003
RID="a"
ID="a"e-mail: marcerou@crpp.u-bordeaux.fr
RID="b"
ID="b"URL: http://www.crpp-bordeaux.cnrs.fr 相似文献
15.
We have examined a number of candidates for the minimum-surface-energy arrangement of two-dimensional clusters composed of
N bubbles of area 1 and N bubbles of area λ ( λ≤1). These include hexagonal bubbles sorted into two monodisperse honeycomb tilings, and various mixed
periodic tilings with at most four bubbles per unit cell. We identify, as a function of λ, the minimal configuration for N → ∞. For finite N, the energy of the external (i.e., cluster-gas) boundary and that of the interface between honeycombs in “phase-separated” clusters have to be taken into account.
We estimate these contributions and find the lowest total energy configuration for each pair (N,λ). As λ is varied, this alternates between a circular cluster of one of the mixed tilings, and “partial wetting” of the
monodisperse honeycomb of bubble area 1 by the monodisperse honeycomb of bubble area λ.
Received 1 August 2002
RID="a"
ID="a"e-mail: paulo@ist.utl.pt 相似文献
16.
The present paper develops a Statistical Mechanics approach to the inherent states of glassy systems and granular materials
by following the original ideas proposed by Edwards for granular media. We consider three lattice models (a diluted spin glass,
a system of hard spheres under gravity and a hard-spheres binary mixture under gravity) introduced to describe glassy and
granular systems. They are evolved using a “tap dynamics” analogous to that of experiments on granular media. We show that
the asymptotic states reached in such a dynamics are not dependent on the particular sample history and are characterized
by a few thermodynamical parameters. We assume that under stationarity these systems are distributed in their inherent states
satisfying the principle of maximum entropy. This leads to a generalized Gibbs distribution characterized by new “thermodynamical”
parameters, called “configurational temperatures” (related to Edwards compactivity for granular materials). Finally, we show
by Monte Carlo calculations that the average of macroscopic quantities over the tap dynamics and over such distribution indeed
coincide. In particular, in the diluted spin glass and in the system of hard spheres under gravity, the asymptotic states
reached by the system are found to be described by a single “configurational temperature”. Whereas in the hard-spheres binary
mixture under gravity the asymptotic states reached by the system are found to be described by two thermodynamic parameters,
coinciding with the two configurational temperatures which characterize the distribution among the inherent states when the
principle of maximum entropy is satisfied under the constraint that the energies of the two species are independently fixed.
Received 19 March 2002 and Received in final form 14 June 2002 相似文献
17.
E.G. Mishchenko M. Patra C.W.J. Beenakker 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2001,13(2):289-297
A theory is presented for the frequency dependence of the power spectrum of photon current fluctuations originating from a
disordered medium. Both the cases of an absorbing medium (“grey body”) and of an amplifying medium (“random laser”) are considered
in a waveguide geometry. The semiclassical approach (based on a Boltzmann-Langevin equation) is shown to be in complete agreement
with a fully quantum mechanical theory, provided that the effects of wave localization can be neglected. The width of the
peak in the power spectrum around zero frequency is much smaller than the inverse coherence time, characteristic for black-body
radiation. Simple expressions for the shape of this peak are obtained, in the absorbing case, for waveguide lengths large
compared to the absorption length, and, in the amplifying case, close to the laser threshold.
Received 8 August 2000 相似文献
18.
M. Bellour A. Knaebel J.P. Munch S.J. Candau 《The European physical journal. E, Soft matter》2000,3(2):111-121
Static and dynamic light scattering and conductivity experiments were carried out on salt-free aqueous micellar solutions
of cetyltrimethylammonium n-hexane sulfonate (CTAC_6SO_3) and cetyltrimethylammonium n-heptane sulfonate (CTAC_7SO_3) as a function of surfactant concentration. This study confirms the analogy between the behavior
in the semi-dilute regime of elongated micellar systems and “classical” polyelectrolyte solutions. Time-resolved scattering
experiments performed after a variation of concentration from about twice the overlap volume fraction to less than half of it revealed the existence of a structural relaxation with a characteristic time of several hours.
Received 21 December 1999 相似文献
19.
The lattice QCD studies indicate that the critical temperature T
c ≃ 260-280 MeV of the deconfinement phase transition in quenched QCD is considerably smaller than the lowest-lying glueball
mass m
G ≃ 1500-1700 MeV, i.e., T
c ≪ m
G. As a consequence of this large difference, the thermal excitation of the glueball in the confinement phase is strongly suppressed
by the statistical factor e
-mG/Tc ≃ 0.00207 even near T ≃ T
c. We consider its physical implication, and argue the abnormal feature of the deconfinement phase transition in quenched QCD
from the statistical viewpoint. To appreciate this, we demonstrate a statistical argument of the QCD phase transition using
the recent lattice QCD data. From the phenomenological relation between T
c and the glueball mass, the deconfinement transition is found to take place in quenched QCD before a reasonable amount of
glueballs is thermally excited. In this way, quenched QCD reveals a question “what is the trigger of the deconfinement phase
transition ?”
Received: 18 November 2002 / Accepted: 4 February 2003 / Published online: 29 April 2003 相似文献
20.
U. Schröter 《The European Physical Journal B - Condensed Matter and Complex Systems》2003,33(3):297-310
Green's dyadic technique represents a powerful tool for calculations in electrodynamics, especially in modelling optical properties
of nanoscopic objects. The method does not only provide field distributions, but also maps of susceptibilities and densities
of states. Whereas the formalism is well established for dielectrics and electric fields, I present here a straight forward
extension to tensors of both electric and magnetic type as well as mixed ones and furthermore to the situation where objects
with dielectric and magnetic permeabilities are present together. As examples, characteristic field patterns are compared
for elementary dielectric and magnetic perturbations. Green's tensors calculated for a coral structure reveal that mixed susceptibilities
can exhibit other symmetries than pure electric or magnetic ones. Maps of all tensor components can thus give essential clues
to the interpretation of near-field images.
Received 15 December 2002 Published online 20 June 2003
RID="a"
ID="a"Files “maths.ps” and “tensors.ps” are only available in electronic form at http://www.edpsciences.org
RID="b"
ID="b"e-mail: Ursula.Schroeter@uni-konstanz.de 相似文献