共查询到20条相似文献,搜索用时 218 毫秒
1.
A. Strigazzi 《Il Nuovo Cimento D》1988,10(11):1335-1344
Summary The presence in the distortion free energy of theK
13-term, depending on the first derivatives of the walls’ tilt angles, leads to some paradoxes. ButK
13 can be shown to vanish in the linear elastic theory. Instead, the saddle splayK
24-term is independent of the first derivatives of the anchoring angles. This fact allows us to obtain Fréedericksz’ threshold
as a function of the involved elastic constants and of the anchoring strengths in the frame of the standard continuum theory.
Work presented at the First USSR-Italy Bilateral Meeting on Liquid Crystals held in Portonovo, Ancona (Italy), September 30–October
2, 1987. 相似文献
2.
The contributions of the anisotropic exchange to the firstand second-order uniaxial anisotropy constants have been calculated
for hematite at T = 0 K and arbitrary temperatures. The results of the calculations and the most significant mechanisms are taken into account
in interpreting the temperature dependence of the anisotropy in rhombohedral antiferromagnetic crystals. The first-order anisotropy
constant for hematite is described by the dipole interaction, contributions of the “single-ion” nature, and relatively small
contributions of the anisotropic exchange. The second-order constant for hematite includes the “single-ion” contribution and
the contribution of the anisotropic exchange. For FeBO3 and MnCO3 crystals, the main contributions to the first-order anisotropy constant come from the dipole and single-ion mechanisms. 相似文献
3.
Perturbative solution to order βɛ of the Percus-Yevick equation for triangular well potential forn=2
The radial distribution function for a fluid in which the molecules interactvia a triangular well potential is considered. Expanding the radial distribution function in pwoers of βɛ, where ɛ is the depth
of the potential andβ=1/k
BT the first-order terms are calculated analytically using the Percus-Yevick theory in the Baxter’s formulation. The first-order
terms in the direct correlation functionc(r) are also calculated. The first- and second-order terms in the free energy obtained from the energy equation of state are
calculated and compared with other calculations.
An erratum to this article is available at . 相似文献
4.
S. V. Ul’yanov 《Journal of Experimental and Theoretical Physics》1998,87(6):1098-1104
The Landau-de Gennes model for the free energy of a nematic liquid crystal near the phase transition to the smectic A-phase is used to determine the frequency dependence of the fluctuation corrections to the Frank elastic constants. It is
shown that the interaction of the fluctuations of the smectic order parameter and the director results in corrections to all
the Frank elastic constants. In the low-frequency limit (ω→0), the corrections to the Frank elastic constants K
22 and K
33 are the largest, and decrease to zero in the infinite-frequency limit. The correction to K
11 is negative, and vanishes in both limits. The absolute value of the correction to K
11 is the largest at frequencies in the megahertz range. It is shown that in oriented nematics the interaction of the smectic
fluctuations and the director limits deviations of the director from the direction of preferred orientation, as a result of
which relaxation of both inhomogeneous and homogeneous distortions of the director field can be observed. It is also shown
that this gives rise to a frequency interval in the megahertz range in which shear waves begin to propagate in the nematic.
The propagation speed of these waves is roughly a hundred times smaller than that of sound and strongly depends on the direction
of propagation.
Zh. éksp. Teor. Fiz. 114, 2022–2033 (December 1998) 相似文献
5.
Litov and Anderson after various considerations suggested a four constant potential function for a-Se as well as a-As2S3. Hence we also used a four constant potential function with the sole purpose of applying this potential function to obtain
several acoustic, thermodynamic and other properties. We calculated several acoustic properties of a-Se like second order
elastic constants (SOECs), their pressure derivatives, the longitudinal and transverse Grüneisen constant by two different
methods, phonon frequencies, absorption band position through the use of Nath-Smith-Delaunay’s equation, and the thermodynamic
properties like heat capacity, bulk modulus, thermal Grüneisen constant, the pressure derivative of the bulk modulus (dK
T/dP=C
1), the pressure derivative ofC
1 which is related to Anderson-Grüneisen parameter, pressure derivative of Grüneisen constant namelyγ′
g
which is related to second Grüneisen constant, characteristics of phonon frequencies, potential energy function through the
use of fitted parameters and third order elastic constants. Finally we calculatedK
T at the reduced density ofρ/ρ
0=1.1.K
T is obtained from the potential function with the fitted parameters. In all the above cases the calculated values are found
to be in good agreement with experiment wherever available. In this connection it is important to point out that we eliminated
‘C’ a constant in the potential function using the equilibrium condition as was done by Litovet al in a-Se and Gerlichet al in the case of a-As2S3 as all amorphous substances are isotropic as mentioned by several authors. We contemplate to calculate several other properties
for a-Se and a-As2S3 and present them at a later stage. 相似文献
6.
On the basis of a fractal model the macroscopic elastic properties of an inhomogeneous medium with random structure have been
determined. It is shown that if the ratio of the bulk moduli of the phases K
2/K
1→0, then the percolation threshold p
c the Poisson coefficient is equal to 0.2. A study of the behavior of a two-phase medium with negative Poisson coefficient
is carried out.
Fiz. Tverd. Tela (St. Petersburg) 41, 2147–2153 (December 1999) 相似文献
7.
G. Colangelo J. Gasser A. Rusetsky 《The European Physical Journal C - Particles and Fields》2009,59(4):777-793
Data on K
e4 decays allow one to extract experimental information on the elastic π
π scattering amplitude near threshold, and to confront the outcome of the analysis with predictions made in the framework of
QCD. These predictions concern an isospin symmetric world, while experiments are carried out in the real world, where isospin-breaking
effects—generated by electromagnetic interactions and by the mass difference of the up and down quarks—are always present.
We discuss the corrections required to account for these, so that a meaningful comparison with the predictions becomes possible.
In particular, we note that there is a spectacular isospin-breaking effect in K
e4 decays. Once it is taken into account, the previous discrepancy between NA48/2 data on K
e4 decays and the prediction of π
π scattering lengths disappears. 相似文献
8.
Ab initio calculations were performed to investigate electronic and elastic properties of the newly discovered 7.5 K superconductor:
layered Nb2InC. As a result, electronic bands, total and site-projected l—decomposed density of states at the Fermi level, shape of the Fermi surface for Nb2InC were obtained for the first time. Besides, independent elastic constants, bulk modulus, compressibility, shear modulus,
Young’s modulus, Poisson’s ratio together with the elastic anisotropy parameters and indicator of brittle/ductile behavior
of Nb2InC were evaluated and analyzed in comparison with the available data. 相似文献
9.
The hydrostatic pressure derivatives of the single crystal elastic moduli of Gd, Dy and Er have been measured at 298°K, to pressures near 5 Kbar. The very small pressure derivatives of the adiabatic bulk moduli indicate that a small ion core model should be appropriate for interpreting the data. The long-range electrostatic contributions to the shear moduli have a dominant influence on the pressure derivatives of the shear moduli of Er, whereas the Gd and Dy data evidently reflect band structure contributions. The values of the longitudinal stiffnesses correspond remarkably well with the Bohm-Staver model for velocity of waves in an ion plasma dispersed in a sea of electrons, where the ionic interaction is perely Coulombic. This model is extended to provide an interpretation of the volume derivatives of the longitudinal moduli in terms of the volume derivative of the density of electron states at the Fermi energy.The Grüneisen parameters calculated from averages of the acoustic model gammas are in relatively poor agreement with those determined from thermal expansion data. An explanation based on the changes in c/a ratio with volume change is tested quantitatively and found to be reasonably successful. The values of dKT/dP, where KT is the isothermal bulk modulus, are applied to the Murnaghan equation of state and give excelent agreement with Bridgman's direct compression data for Dy and Er to 40 Kbar. For Gd, Bridgman's data indicate either that (dKT/dP)p=0 should be considerably larger than deduced from the adiabatic dKs/dP measurements or that a phase change occurs near 20 Kbar. The occurrence of a phase change in Er at ~90 Kbar is definitely indicated when comparing the Murnaghan equation with X-ray diffraction data. 相似文献
10.
V. A. Stephanovich 《JETP Letters》1997,65(5):446-451
A theory of self-localized states of free carriers near polarization fluctuations (fluctuons) in disordered ferroelectrics
is developed. Calculations are carried out for the model disordered ferroelectric K1− x
LixTaO3 (x≪0.05). The basic characteristics of the fluctuon — the energy and radius of the fluctuon state — are calculated as functions
of the impurity dipole concentration and temperature. The theory predicts the appearence of stable fluctuon states in both
the mixed ferroelectric-dipole-glass phase (a dipole glass is the electric analog of a spin glass) and the dipole-glass state
of disordered ferroelectrics. The possible role of fluctuons in kinetic phenomena such as conductivity in these substances
is discussed.
Pis’ma Zh. éksp. Teor. Fiz. 65, No. 5, 425–429 (10 March 1997)
Published in English in the original Russian journal. Edited by Steve Torstveit. 相似文献
11.
The knowledge of elasticity of the minerals is useful for interpreting the structure and composition of the lower mantle and
also in seismic studies. The purpose of the present study is to discuss a simple and straightforward method for evaluating
thermoelastic properties of minerals at high temperatures. We have extended the Kumar’s formulation by taking into the account
the concept of anharmonicity in minerals above the Debye temperature (θ
D). In our present study, we have investigated the thermophysical properties of two minerals (pyrope-rich garnet and MgAl2O4) under high temperatures and calculated the second-order elastic constant (C
ij
) and bulk modulus (K
T) of the above minerals, in two cases first by taking Anderson–Gruneisen parameter (δ
T) as temperature-independent and then by treating δ
T as temperature-dependent parameter. The results obtained when δ
T is temperature-dependent are in close agreement with experimental data. 相似文献
12.
This paper is mainly concerned with elastic and acoustic properties of vitrous silica besides the computation of phonon frequencies.
Thus the phonon frequencies of vitrous silica have been calculated assuming the electronic bulk modulus,K
e, as equal to zero. New equations have been derived to relate the pressure derivatives of second order elastic constants to
the acoustic Gruneisen’s parameters using both Bhatia-Singh’s parameters and Schofield’s equations. The calculated longitudinal
and transverse Gruneisen’s parameters and the predicted absorption band spectra from Nagendranath’s equation and Bhatia Singh’s
parameters are in good agreement with experiment. The calculated mean acoustic mode Gruneisen’s parameter evaluated from the
pressure derivative of Nagendranath’s equation is also in good agreement with experiment.
An erratum to this article is available at . 相似文献
13.
A statistical mechanical theory of the Frank elastic constants is formulated. The free energy functional is constructed for the deformed sample and the free energy density is defined for the case of small spatial gradients. The Frank constants are expressed in terms of the direct correlation function c(1, 2) and the orientational single particle distribution function. For the example of Onsager spherocylinders three constants K 1, K 2 and K 3 are calculated. The results of these calculations are similar to those given by Priest and by Straley. 相似文献
14.
A method for determination of magnetocrystalline anisotropy constants of arbitrary order is proposed. The method is based on a least squares fitting of a phenomenological anisotropy energy for a given symmetry truncated at an arbitrary order term to a theoretical anisotropy energy computed exactly for a given energy level model. Several applications of the method to cubic systems are considered. The study reveals that the widely used expressions in the literature for the cubic constants K1 and K2 in terms of free energy for the three symmetry direction are of rather limitedvalidity only. The higher-order cubic constants K3, K4 and K5 are determined besides the usual K1 and K2 in temperature range 0 to 300 K. The importance of the higher- order terms with respect to the first term in the cubic anisotropy energy is discussed. The results show that the cubic constants K3 and K4 cannot be neglected for most of the energy level models studied at certain temperatures. 相似文献
15.
The concentration dependences of the elastic constants of the two-dimensional Si
x
C1 − x
system have been investigated with the use of the Harrison bonding-orbital method and the Keating model. The central and
non-central force constants and the Grüneisen parameter have been considered by means of the bonding-orbital method. All quantities
under consideration have been shown to exhibit a nonlinear behavior during the transition from graphene to silicene. A nontrivial
role of the short-range repulsion has been discussed. The second-order and third-order elastic constants, the pressure dependences
of the second-order elastic constants, as well as the Poisson’s ratio and Young’s modulus have been investigated in the Keating
model. It has been found that the elastic constants and Young’s modulus change almost linearly upon the transition from graphene
to silicene, whereas the other quantities under consideration exhibit nonlinearity. 相似文献
16.
V.M. MathewC.S. Menon K.P. Jayachandran 《Journal of Physics and Chemistry of Solids》2002,63(10):1835-1838
The second- and third-order elastic constants and pressure derivatives of second-order elastic constants of tetragonal β-tin have been obtained using the deformation theory. The strain energy density derived using the deformation theory is compared with the strain dependent lattice energy obtained from the elastic continuum model approximation to get the expressions for the second- and third-order elastic constants. Higher order elastic constants are a measure of the anharmonicity of a crystal lattice. The 12 non-vanishing third-order elastic constants and the six pressure derivatives of the second-order elastic constants in tetragonal β-tin are obtained in the present work and are compared with the available experimental values. The second-order elastic constant C33 obtained in the present study is in reasonable agreement with the experimental values. The third-order elastic constants are generally one order of magnitude greater than the second-order elastic constants as expected of a crystalline solid. The third-order elastic constant C333 is higher in magnitude than all other values. This shows a greater anharmonicity of β-tin along the c-axis direction of the crystal. 相似文献
17.
A. I. Frenkel D. M. Pease J. I. Budnick P. Shanthakumar T. Huang 《Journal of synchrotron radiation》2007,14(3):272-275
X‐ray absorption fine‐structure (XAFS) data were obtained for the V K‐edge for a series of anisotropic single crystals of (CrxV1–x)2O3. The data and the results were compared for the as‐prepared bulk single crystals (measured in fluorescence in two different orientations) and those ground to powder (measured in transmission). For the bulk single crystals, the glancing‐emergent‐angle (GEA) method was used to minimize fluorescence distortion. The reliability of the GEA technique was tested by comparing the polarization‐weighted single‐crystal XAFS data with the experimental powder data. These data were found to be in excellent agreement throughout the entire energy range. Thus, it was possible to reliably measure individual V–V contributions parallel and perpendicular to the c axis of the single crystals, i.e. those unavailable by powder data XAFS analysis. These experiments demonstrate that GEA is a premiere method for non‐destructive high‐photon‐count in situ studies of local structure in bulk single crystals. 相似文献
18.
M. Bak 《International Journal of Theoretical Physics》2012,51(4):1173-1186
The paper shows a way to find two-particle lattice Green functions (LGF) on any site of a two-dimensional, rectangular lattice
with hopping to nearest- and next-nearest neighbors. Exact, analytical formulas using elliptic integrals for the on-site—G
0,0, nearest neighbor—G
0,1 and G
1,0 and next-nearest neighbor—G
1,1 LGF’s are shown. Difference equation for general G
m,n
is given together with five kind of recurrence relations among LGF’s and their derivatives. A way of assembling recurrence
relations into closed sets of equations, enabling to find G
n,m
on any lattice site (requiring the knowledge of five parameters) is described. The differences between one- and two-particle
LGF’s are shown. 相似文献
19.
A simple X-ray fluorescence method using nearly parallel and monochromatic beams is analyzed and discussed in connection with
the determination of the alloy fraction in pseudobinary A1−x
B
x
C solid solutions. Applications to optoelectronic materials are considered. The reliability of the method has been analyzed
and discussed for both bulk and thin film samples. Errors due to small angular divergence in the accepted beam are seen to
be neglibile. Deviations from a symmetrical alignment of the sample can strongly influence the measured fluorescence intensity.
The experimental analysis of this effect can be used for a perfect symmetrical orientation of thick samples. The measurement
of the fluorescence intensity as a function of the glancing angle can give both thickness and composition in solid-solution
thin-film samples. The contributions of the fluorescence radiation excited by the characteristic X-ray lines of atoms within
the substrate is shown to be important in thin epitaxial layers. An approximate formula for calculating these contributions
is given and discussed in the light of experimental results. Bulk crystals and thin-film samples, vapourphase grown Cd1−x
Zn
x
S solid solutions, have been used for experiments in which the ZnK
α radiation excited by a MoK
α primary beam was measured. 相似文献
20.
Z. Rudy W. Cassing L. Jarczyk B. Kamys P. Kulessa 《The European Physical Journal A - Hadrons and Nuclei》2002,15(3):303-314
The production of K+-mesons in proton-nucleus collisions from 1.0 to 2.5GeV is analyzed with respect to one-step nucleon-nucleon ( NN → NYK
+) and two-step Δ-nucleon ( ΔN → K
+
YN) or pion-nucleon ( πN → K
+
Y) production channels on the basis of a coupled-channel transport approach (CBUU) including the kaon final-state interactions.
The influence of momentum-dependent potentials for the nucleon, hyperon and kaon in the final state are studied as well as
the importance of K+ elastic rescattering in the target nucleus. The transport calculations are compared to the experimental K+ spectra taken at LBL Berkeley, SATURNE, CELSIUS, GSI and COSY-Jülich. It is found that the momentum-dependent baryon potentials
affect the excitation function of the K+ cross-section; at low bombarding energies of ∼ 1.0GeV the attractive baryon potentials in the final state lead to a relative
enhancement of the kaon yield, whereas the net repulsive potential at bombarding energies ∼ 2GeV causes a decrease of the
K+ cross-section. Furthermore, it is pointed out that especially the K+ spectra at low momenta (or kinetic energy T
K) allow to determine the in-medium K+ potential almost model independently due to a relative shift of the K+ spectra in kinetic energy that arises from the acceleration of the kaons when propagating out of the nuclear medium to free
space, i.e. converting the potential energy to the kinetic energy of the free kaon.
Received: 28 January 2002 / Accepted: 3 June 2002 / Published online: 19 November 2002
RID="a"
ID="a"e-mail: Wolfgang.Cassing@theo.physik.uni-giessen.de
Communicated by P. Schuck 相似文献