共查询到20条相似文献,搜索用时 190 毫秒
1.
A two-field model provides a unifying framework for elasticity, lattice dynamics and electromechanical coupling in graphene and carbon nanotubes, describes optical phonons, nontrivial acoustic branches, strain-induced gap opening, gap-induced phonon softening, doping-induced deformations, and even the hexagonal graphenic Brillouin zone, and thus explains and extends a previously disparate accumulation of analytical and computational results. 相似文献
2.
Fn the present paper, the linear Eliashberg equation is converted into a new integral equation and from it, the following TC formula is derived analytically in the weak coupling limit:Tc=28/πωlog·(πlog/πc)μ*/λ*-μ* exp{-1+λ/λ-μ*}, where In γ=C=0.5772 is the Eater constant. 相似文献
3.
The dispersion of acoustic and optical phonons of rubidium cyanide has been measured at 300 K. The acoustic branches are affected by rotation-translation coupling in a way similar to the findings in KCN. The data are explained by a model which combines a shell model and rotation-translation coupling. 相似文献
4.
K.R.A. Ziebeck 《Journal of magnetism and magnetic materials》1977,5(4):332-338
A model is presented for the magnetic excitations and magnon-phonon coupling in a localised moment ferromagnet in which spins can reorientate by application of a magnetic field. The model is suitable for those materials which possess a spin wave gap at zero wave vector and therefore the magnon and acoustic phonon branches can intersect. A magnon-phonon coupling linear in both spin and phonon operators is employed which has proved successful for the ferrous salts. The main effect of the applied field is to modify the spin wave gap, and to introduce a critical value for the coupling constant which enables the system to remain stable as the gap goes to zero. Furthermore the wave vector of the anticrossing point decreases as the spin wave gap increases and therefore the value of the sound velocity determined by high resolution inelastic neutron experiments is dependent on the gap. 相似文献
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ZHU Shan-Hua HUANG Guo-Xiang 《理论物理通讯》2001,(10)
The dynamics of coupled band gap solitons in one-dimensional Heisenberg ferromagnetic chains with bond alternation is considered analytically. Using the method of multiple scales the nonlinear coupled-mode equations (i.e.Manakov equations) for the upper cutoff mode of acoustic band and the lower cutoff mode of optical band are derived under the quasi-discreteness approximation. Due to the cross-phase modulation the type of soliton excitations may be changed and the vibrating frequencies of these soliton excitations may locate within or outside the gap of magnon frequency bands.`` 相似文献
9.
The dynamics of coupled band gap solitons in one-dimensional
Heisenberg ferromagnetic chains with bond alternation is considered analytically. Using the method of multiple scales the nonlinear coupled-mode equations (i.e. Manakov equations) for the upper cutoff mode of acoustic band and the lower cutoff mode of optical band are derived under the quasi-discreteness approximation. Due to the cross-phase modulation the type of soliton excitations may be changed and the vibrating frequencies of these soliton excitations may locate within or outside the gap of magnon frequency bands. 相似文献
10.
We use recurrence relations method to study a classical harmonic diatomic chain. The momentum autocorrelation function results from contributions of acoustic and optical branches. By use of convolution theorem, analytical expressions for the acoustic and optical branches are derived as even-order Bessel function expansions. The expansion coefficients are given in terms of integrals of real and complex elliptic functions for the acoustic and optical branches, respectively. Double convolution results respectively in integrals of trigonometric and hyperbolic functions for expansion coefficients of acoustic and optical branches. 相似文献
11.
利用自洽重整化方法,研究了不同电-声藕合作用下MX络合物中晶格零点振动对电荷密度波(CDW)的影响。对聚乙炔,无论电-声耦合作用强弱,键序波(BOW)总存在。而对MX络合物,计算表明,量子起伏对CDW的削弱作用依赖于电-声耦合作用的强弱。当电-声耦合作用强时,CDW的削弱小;相反,当电-声耦合作用弱时,CDW削弱大,甚至几乎被破坏。这一差异来源于以下事实,即在弱电-声耦合λ情况下,聚乙炔中光学模声子频率随λ1/2减小,而在MX络合物中光学模声子频率与其裸值ω0差
关键词: 相似文献
12.
The reduction of charge density wave (CDW) amplitude of MX solids caused by the lattice quantum fluctuation is investigated in a wide range of electron-phonon coupling by using the self-consistent renormalization method. In contrast to polyacetylene, where the bond order wave (BOW) exists well in arbitrary coupling, the effect of the quantum Auctuatipn on CDW of MX solids distinctly depends on the coupling. When the coupling is strol; the reduction of CDW is small; but, if the coupling is weak, CDW is seriously suppressed and even nearly destroyed. This contrast is related to the fact that for weak coupling, the renormalized frequencies of both the acoustic and optical branches are greatly reduced in polyacetylene, but these frequencies in MX solids are not well different from the bare value ω0. 相似文献
13.
利用经典力学方法研究了二维反式聚乙炔链的光学模和声学模的维度效应.链的二聚化引起系统的对称性降低,从而导致光频支格波和声频支格波的宽度(ω2+(k=0)-ω2-(k=0))明显变窄,而对BZ边界面上两支格波的间隙(ω2+(k=π/a)-ω2-(k=π/a))几乎没有影响. 相似文献
14.
We combine well-controlled analytical and numerical methods to determine the optical conductivity of the one-dimensional Mott-Hubbard insulator at zero temperature. A dynamical density-matrix renormalization group method provides the entire absorption spectrum for all but very small coupling strengths. In this limit we calculate the conductivity analytically using exact field-theoretical methods. Above the Lieb-Wu gap the conductivity exhibits a characteristic square-root increase. For small to moderate interactions, a sharp maximum occurs just above the gap. For larger interactions, another weak feature becomes visible around the middle of the absorption band. 相似文献
15.
Two-Dimensional Discrete Gap Breathers in a Two-Dimensional Diatomic β Fermi--Pasta--Ulam Lattice 下载免费PDF全文
We study the existence of two-dimensional discrete breathers in a two-dimensional face-centred square lattice consisting of alternating light and heavy atoms, with nearest-neighbour coupling containing quartic soft or hardnonlinearity. This study is focused on two-dimensional breathers with frequency in the gap that separates the acoustic and optical bands of the phonon spectrum. We demonstrate the possibility of existence of two-dimensional gap breathers by using the numerical method, the local anharmonicity approximation and the rotating wave approximation. We obtain six types of two-dimensional gap breathers, i.e., symmetric, mirror-symmetric and asymmetric, no matter whether the centre of the breather is on a light or a heavy atom. 相似文献
16.
In this paper we study the existence and stability of two-dimensional discrete gap
breathers in a two-dimensional diatomic face-centered square lattice
consisting of alternating light and heavy atoms, with on-site
potential and coupling potential. This study is focused on
two-dimensional breathers with their frequency in the gap that
separates the acoustic and optical bands of the phonon spectrum. We
demonstrate the possibility of the existence of two-dimensional gap
breathers by using a numerical method. Six types of two-dimensional
gap breathers are obtained, i.e., symmetric, mirror-symmetric and
asymmetric, whether the center of the breather is on a light or a
heavy atom. The difference between one-dimensional discrete gap
breathers and two-dimensional discrete gap breathers is also
discussed. We use Aubry's theory to analyze the stability of
discrete gap breathers in the two-dimensional diatomic face-centered
square lattice. 相似文献
17.
Bing-Hong WANG 《理论物理通讯》1994,21(3):289-298
The universal crossover behavior of Lyapunov exponents in transition from conservative limit to dissipative limit of dynamical system is studied. We discover numerically and prove analytically that for homogeneous dissipative two-dimensional maps, along the equal dissipation line in parameter space, two Lyapunov exponents λ1 and λ2 of periodic orbits possess a plateau structure, and around this exponent plateau value, there is a strict symmetrical relation between λ1 and λ2 no matter whether the orbit is periodic, quasiperiodic, or chaotic.The method calculating stable window and Lyapunov exponent plateau widths is given. For Hénon map and 2-dimensional circle map, the analytical and numerical results of plateau structure of Lyapunov exponents for period-1,2 and 3 orbits are presented. 相似文献
18.
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
19.
The vibrational spectra of uranium dioxide UO2 and plutonium dioxide PuO2, as well as the one-phonon densities of states and thermal occupation number weighted two-phonon densities of states, have been calculated within the framework of the phenomenological rigid ion model. It has been shown that the acoustic and optical branches of the spectra are predominantly determined by vibrations of the metal and oxygen atoms, respectively, because the atomic masses of the metal and oxygen differ from each other by an order of magnitude. On this basis, the vibrational spectra can be represented in two Brillouin zones, i.e., in the Brillouin zone of the crystal and the Brillouin zone of the oxygen sublattice. In this case, the number of optical branches decreases by a factor of two. The two-phonon densities of states consist of two broad structured peaks. The temperature dependences of the upper peak exhibit a thermal broadening of the phonon lines L01 and L02 in the upper part of the optical branches. The lower peak is responsible for the thermal broadening of the lowest two optical (T02, T01) and acoustic (LA, TA) branches. 相似文献
20.
EUAN JING-HUI CHENG ZHI-SHIUH CHEN GUI-YING LI ZHU-CHI LU TING WU SHAN-LING 《中国物理C(英文版)》1978,2(5):441-449
Inelastic scattered neutron spectra of metal hydrides ZrH1.66 and TiH1.92 have been measured by beryllium filter detector spectrometer and their phonon spectra have been deduced from the neutron spectra according to the A. Sjölander formula by iterative method. The optical branches of the two samples thus obtained are approximately Gaussian distributions and their positions are in agreement with other works, while their PWHM are smaller than the value uncorrected for multiphonons effect reported elsewhere. Their acoustic branches obtained are of neither Debye nor Gaussian type but show a double-numped configurations. Some new configurations have been observed between the optical and acoustic branches which may be attributed to localized modes as a result of disorder. 相似文献