共查询到20条相似文献,搜索用时 15 毫秒
1.
M. Jochum H.-G. Unruh 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,5(2):163-168
Dielectric and Raman spectroscopic measurements have been performed to investigate the ferroelectric phase transition in . Single crystals were grown by the zone melting method. The frequency dependence of the dielectric permittivity from 1 MHz
to 1 GHz has been studied in a temperature range between 265 and 285 K. A Debye like dielectric dispersion was found, showing
a critical slowing down around K. Polarized Raman spectra have been taken between 220 and 310 K. Two softening modes have been found, one of A- and another one of B
/
B
g-symmetry. The phase transition mechanism in can be classified as partially order-disorder and partially displacive, confirming former structural results. It resembles
strongly that of monoclinic .
Received: 7 April 1998 / Revised: 5 June 1998 / Accepted: 16 June
1998 相似文献
2.
Y.S. Lee M. Greven B.O. Wells R.J. Birgeneau G. Shirane 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,5(1):15-22
We report a neutron scattering study of the instantaneous spin correlations in the two-dimensional spin S
=5/2 square-lattice Heisenberg antiferromagnet Rb2MnF4. The measured correlation lengths are quantitatively described, with no adjustable parameters, by high-temperature series
expansion results and by a theory based on the quantum self-consistent harmonic approximation. Conversely, we find that the
data, which cover the range from about 1 to 50 lattice constants, are outside of the regime corresponding to renormalized
classical behavior of the quantum non-linear model. In addition, we observe a crossover from Heisenberg to Ising critical behavior near the Néel temperature; this crossover
is well described by a mean-field model with no adjustable parameters.
Received: 3 March 1998 / Received in final form: 4 May 1998 /
Accepted: 19 May 1998 相似文献
3.
H. Kallabis P.L. Krapivsky D.E. Wolf 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,5(3):801-804
The typical island distance in submonolayer epitaxial growth depends on the growth conditions via an exponent . This exponent is known to depend on the substrate dimensionality, the dimension of the islands, and the size i* of the critical nucleus for island formation. In this paper we study the dependence of on i* in one-dimensional epitaxial growth. We derive that for and confirm this result by computer simulations.
Received: 26 May 1998 / Accepted: 23 June 1998 相似文献
4.
P. Entel R. Meyer K. Kadau H.C. Herper E. Hoffmann 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,5(3):379-388
Results are presented of first-principles total-energy calculations and molecular-dynamics simulations of structural transformations
in magnetic transition metal alloys like Fe1-xNix. While first-principles calculations allow to identify those structures having the lower total energy, molecular-dynamics
simulations can be used to trace out the dependence of the transformation on temperature, composition, concentration of defects
etc. We have used the method of the semi-empiric embedded-atom potential in the molecular-dynamics simulations which yields remarkable
good results for the structural changes.
Received: 3 February 1998 / Revised: 4 May 1998 / Accepted: 24 June 1998 相似文献
5.
S.W.H. Eijt R. Currat J.E. Lorenzo P. Saint-Grégoire B. Hennion Y.M. Vysochanskii 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,5(2):169-178
The low frequency lattice dynamics and its relationship to the second order paraelectric-to-ferroelectric transition in Sn2P2S6 is studied. The dispersion branches of the acoustic and lowest lying optical phonons in the a*-c* plane have been obtained in the ferroelectric phase, for x-polarized phonons. Close to the phase transition a considerable softening is found for the lowest optical mode (Px), comparable to the behaviour observed in previous Raman investigations. As found previously in Sn2P2Se6, a strong coupling between the TO(Px) and TA(uxz) phonons is observed, although, apparently, not strong enough to lead to an incommensurate phase. The soft TO(Px) mode at the zone center is observed. The temperature dependence of its frequency and damping shows that the transition is
not entirely displacive. At low temperatures an unusual apparent negative LO-TO splitting is observed which is shown to arise
from the coupling of the x-polarized soft mode to the nearby z-polarized optical phonon. For comparison, the soft TO(Px) dispersion in the a*-b* plane is measured in both the paraelectric and ferroelectric phases. Consistent frequency changes and LO-TO splitting are
observed, revealing a significant interaction between the TA(uyx) and LA(uxx) acoustics branches and the TO and LO soft optic branches, respectively. In contrast, the nearby y-polarized optic branch shows almost no temperature dependence. Finally, the influence of piezoelectric effects on the limiting
acoustic slopes in the ferroelectric phase is discussed.
Received: 11 May 1998 / Revised and Accepted: 15 June 1998 相似文献
6.
J. Campos-Martínez E. Carmona-Novillo J. Echave M.I. Hernández R. Hernández-Lamoneda J. Palma 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》1998,4(2):159-168
A reduced dimensionality model is used to study the reaction OO
O
3
(
X1A1
) + O(
3
P
) by means of time-dependent and time-independent quantum-mechanical methods. State-selected probabilities and rate constants
are obtained for the reactive process as well as for the inelastic collision in which the vibrationally excited oxygen loses
one or more quanta. It is found that the experimentally observed jump in depletion rates above a critical value of v could be partially explained by the vibrational relaxation rather than reaction. Reaction only becomes important for relatively
high translational energies and therefore the calculated rates are too small at the temperatures of interest. It is concluded,
however, that the reaction saddle point region in the potential energy surface plays a crucial role in the enhancement of
vibrational relaxation.
Received: 3 February 1998 / Revised: 27 March 1998 / Accepted: 15 May
1998 相似文献
7.
K. Mølmer 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》1999,5(2):301-305
We discuss the possibility of preparing an atomic sample of atoms with minimum fluctuations in the difference between populations
of two levels. A first scheme involves absorption of twin beams of light, and it presents a variant of a recent proposal for
atomic spin squeezing within an excited state manifold [Kuzmich et al., Phys. Rev. Lett. 79, 4782 (1997)]. A second scheme involves atoms with two stable states, and we suggest that by use of quantum non-demolition
detection and feed-back optical pumping, we may ensure a perfect agreement between the number of atoms in these two states.
Received: 14 May 1998 / Revised: 10 August 1998 / Accepted: 8 October 1998 相似文献
8.
L. Piraux S. Dubois A. Fert L. Belliard 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,4(4):413-420
A theory, based on earlier work by Valet and Fert, is first presented to describe the influence of temperature on the perpendicular
giant magnetoresistance (GMR) in multilayers. Then we present GMR measurements performed at T=77 K and at room temperature on Co/Cu multilayered nanowires with layer thicknesses ranging from a few nm to 1 μm. We use
our model to obtain a good quantitative fit to the experimental results in both the short spin diffusion length limit and
out of this limit. We discuss the temperature dependence of the bulk parameters, the scattering spin asymmetry coefficient
and spin diffusion length in the Co layers.
Received: 25 January 1998 / Accepted: 6 May 1998 相似文献
9.
R. Enjalbert J. Galy Y. Vysochanskii A. Ouédraogo P. Saint-Grégoire 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,8(2):169-177
The structural change occurring in between the paraelectric and the ferroelectric phases is investigated by means of X-ray diffraction. Details of the structure
in both phases are obtained and the role of the lone pair is discussed in the light of structural data. In agreement with the 2/
m to m symmetry lowering, polar displacements are found within the m plane away from a particular crystallographic direction but antiparallel displacements occur also. These results are discussed
in the frame of the phenomenological theory which predicts a particular temperature dependence of the dielectric polarization
in this crystal.
Received: 10 July 1998 相似文献
10.
11.
Numerical data for the distribution of citations are examined for: (i) papers published in 1981 in journals which are catalogued
by the Institute for Scientific Information (783,339 papers) and (ii) 20 years of publications in Physical Review D, vols.
11-50 (24,296 papers). A Zipf plot of the number of citations to a given paper versus its citation rank appears to be consistent with a power-law dependence for leading rank papers, with exponent close to -1/2.
This, in turn, suggests that the number of papers with x citations, N(x), has a large-x power law decay , with .
Received: 12 May 1998 / Accepted: 12 May 1998 相似文献
12.
T. Ohm C. Sangregorio C. Paulsen 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,6(2):195-199
We observed non-exponential relaxation for a quantum tunneling molecular magnetic system at very low temperatures and argue
that it results from evolving intermolecular dipole fields. At the very beginning of the relaxation, the magnetization follows
a square-root time dependence. A simple model is developed for the intermediate time range that is in good agreement with
the data over 4 decades in time. Detailed numerical calculations as well as measurements are presented which indicate unusual
correlation effects in these systems.
Received: 15 May 1998 / Revised: 10 July 1998 / Accepted: 11 July 1998 相似文献
13.
J.P. Santos F. Parente P. Indelicato 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》1998,3(1):43-52
A theoretical study of the one- and two-photon spontaneous emission rates from the 2
s1/2 state of one-electron ions is presented. High-precision values of the relativistic emission rates for ions with nuclear charge
Z up to 100 are obtained through the use of finite basis sets for the Dirac equation constructed from B-splines. Furthermore,
we analyze the influence of the inclusion of quantum electrodynamics corrections in the initial and final state energies.
Received: 6 January 1998 / Accepted: 31 March 1998 相似文献
14.
E.F. Wassermann B. Rellinghaus T. Roessel J. Kästner W. Pepperhoff 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,5(3):361-365
We measured the thermal expansion and the specific heat of TixFe100-x alloys with x
= 30.5, 32.5 and 35, all with hexagonal C14 laves phase structure (MgZn2) like TiFe2, and determine the temperature dependence of the magnetic contributions to the thermal expansion and the specific heat cmag. For fixed composition and c
mag
(
T
) show the same type of behavior, demonstrating that both anomalies are of the same microscopic nature. They originate from
moment-volume fluctuations (antiferromagnetic Invar-effect) as a comparison with total energy calculations as a function of
atomic volume and moment for TiFe2 reveals.
Received: 26 January 1998 / Accepted: 17 April 1998 相似文献
15.
A general expression for the cooperative diffusion constant of weakly charged gels is derived as a function of the thermodynamic
parameters such as polyelectrolyte concentration, salt concentration, ionic strength, and the degree of crosslinking. In the
low concentration range it decreases with the monomer concentration.
Received: 30 January 1998 / Revised: 4 May 1998 / Accepted: 6 May 1998 相似文献
16.
D. Rönnow L.F. Lastras-Martínez M. Cardona 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,5(1):29-35
Within the past years the optical excitations of electrons have been measured for semiconductor samples of different isotope
compositions. The isotope shift observed have been compared with calculations of the effects of electron-phonon interaction
on the electronic band structure. While qualitative agreement has been obtained, some discrepancies remain especially concerning
the E1 and transitions. We have remeasured the effect of isotope mass on the E1 and transitions of germanium with several isotopic compositions. The results, obtained by means of spectroscopic ellipsometry,
confirm that the real part of the gap self-energies induced by electron-phonon interaction is larger than found from band
structure calculations, while the imaginary part agrees with those calculations, which are based on a pseudopotential band
structure and a bond charge model for the lattice dynamics. Our results agree with predictions based on the measured temperature
dependence of the gaps. We compare our data for E1 and with results for the lowest direct (E0) and indirect (Eg) gaps. The measured values of and increase noticeably with increasing isotope mass. Similar effects have been observed in the temperature dependence of in and . A microscopic explanation for this effect is not available.
Received: 6 March 1998 / Revised: 27 April 1998 /
Accepted: 15 May 1998 相似文献
17.
D. Görlitz J. Kötzler 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,5(1):37-43
Between 4.2 K and the Curie temperatures of the cubic Heisenberg ferromagnets EuS and EuO, their homogeneous dynamic susceptibilities
have been investigated by means of a broad-band reflectometer operating from 0.1 GHz to 40 GHz. For internal magnetic fields
larger than the anisotropy fields H
A
(
T
) of both materials, their static susceptibilities exhibit a -divergence, which reveals quantitatively the dominance of dipolar-anisotropic spin-wave fluctuations. displays a Lorentzian shape the damping frequency of which obeys scaling in terms of .The scaling function agrees quantitatively with work by Frey and Schwabl [#!FS88!#] for dipolar Heisenberg ferromagnets at
temperatures above Tc. Building upon their approach, the resonance frequency of the Lorentzian can be related to a memory effect in the damping
determined by the large value of the relaxation rate of the longitudinal magnetization fluctuations . For EuS, this relation is substantiated directly by inelastic neutron scattering. All these features reveal the hitherto
uncovered importance of the dipolar anisotropic fluctuations below Tc of ferromagnets.
Received: 4 March 1998 / Accepted: 12 May 1998 相似文献
18.
19.
J.-G. Wang T.-Q. Chang C.-Z. Dong T. Kato 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》1999,5(2):167-171
A simplified relativistic configuration interaction method is used to study the dielectronic satellite transition processes.
In this method, the infinite resonant doubly excited states can be calculated, and furthermore, the whole high-n dielectronic satellite transition processes can be treated conveniently by interpolation (rather than extrapolation) in the
frame of Quantum Defect Theory. As an example, we calculate the contributions from high-n dielectronic satellites to the resonance line in helium-like iron, and the results are in good agreement with the experimental measurements.
Received: 16 April 1998 / Revised: 8 September 1998 / Accepted: 14 September
1998 相似文献
20.
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