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1.
A so-called boson peak in the reduced density g(ω)ω 2 of vibrational states is one of the most universal properties of amorphous solids (glasses). It quantifies the excess density of states above the Debye value at low frequencies ω. Its nature is not fully understood and, at a first sight, is nonuniversal. It is shown in this work that, under rather general assumptions, the boson peak emerges in a natural way in very dissimilar models of stable random dynamic matrices possessing translational symmetry. This peak can be shifted toward both higher and lower frequencies (down to zero frequency) by varying the parameters of the distribution and the degree of disorder in the system. The frequency ω b of the boson peak appears to be proportional to the elastic modulus E of the system in all cases under investigation. 相似文献
2.
Within the model of stable random matrices possessing translational invariance, a two-dimensional (on a square lattice) disordered oscillatory system with random strongly fluctuating bonds is considered. By a numerical analysis of the dynamic structure factor S(q, ω), it is shown that vibrations with frequencies below the Ioffe-Regel frequency ωIR are ordinary phonons with a linear dispersion law ω(q) ∝ q and a reciprocal lifetime б ~ q3. Vibrations with frequencies above ωIR, although being delocalized, cannot be described by plane waves with a definite dispersion law ω(q). They are characterized by a diffusion structure factor with a reciprocal lifetime б ~ q2, which is typical of a diffusion process. In the literature, they are often referred to as diffusons. It is shown that, as in the three-dimensional model, the boson peak at the frequency ωb in the reduced density of vibrational states g(ω)/ω is on the order of the frequency ωIR. It is located in the transition region between phonons and diffusons and is proportional to the Young’s modulus of the lattice, ω
b
≃E. 相似文献
3.
We present explicit predictions of the statistical bootstrap model for inclusive single particle spectra in e +e ? annihilation. The distribution ( ω/ σtot· E) ( dσ/ p2 d p) is found to become, for E ? 3 GeV, a function only of the secondary energy ω, independent of the incident e +d ? energy E. 相似文献
4.
The meaning of a high frequency cut-off for the Coulomb interaction at ±iω max with ω max ≈ E f is investigated. It is not difficult to justify such a cut-off for nearly-free-electron metals. The real frequency Coulomb pseudopotential corresponding to the usual imaginary axis (μ 1, iω c) pseudopotential is rigorously derived. It is markedly different from any assumed in previous work. 相似文献
5.
Experimentally observed Huang-Rhys numbers for Co 2+ impurities in several zincblende type hosts correlate well with our theoretically predicted ω -3Da-12 dependence (ω D is the Debye frequency and a the lattice constant of the host). 相似文献
6.
The first observation of interference effects between two sharp resonant contributions to the third-order nonlinear susceptibility χ (3) (ω 3=ω 0+ω 1?ω 2) is reported. Using two tunable near ir frequencies, ω 0=ω 1 and ω 2, the was seen in crystalline CuCl when 2ω 1 and ω 1?ω 2 were close to the sharp Z3 exciton at 3.21 eV and the 210 cm -1 Raman line, respectively. Three different tunable visible frequencies were used to observe similar interference effects in a benzene-cyclohexane mixture when ω 0?ω 2 and ω 1?ω 2 were close to the 992 cm -1 Raman line of benzene and the 801 cm -1 Raman line of cyclohexane, respectively. 相似文献
7.
Results of theoretical studies of laser and Kerr nonlinear susceptibilities of vapors and solutions of organic dyes using a series of polycyclic arenes as an example are presented. Nonlinear susceptibilities of the third χ (3) (?3ω; ω, ω, ω) and the fifth χ (5) (?3ω; ω, ω, ω, ω, ?ω) orders of a series of organic dyes responsible for third harmonic generation of Nd:YAG laser radiation are calculated within the context of the free electron model. Results of calculations of their Kerr third-order nonlinear susceptibilities χ (3) (?ω; ω, ?ω, ω) and non-linear refractive indices n 2 are presented. The calculation results are compared with experimental data on third harmonic generation in naphthalene vapors and with χ (3) (?ω; ω, ?ω, ω) as well as n 2 of paraterphenyl and naphthalene solutions. 相似文献
8.
A universal mechanism of the Boson peak formation in glasses is proposed. The mechanism is based on the concept of interacting quasilocal oscillators. Even in the case of weak interaction, the low-frequency spectrum becomes unstable. Due to anharmonicity, the system undergoes a transition into a new stable configuration. As a result, below some characteristic frequency ω c, proportional to the typical strength of interaction, the renormalized density of states becomes a universal function of ω with a Boson peak feature; i.e., the reduced density of states g(ω)/ω 2 has a maximum at a frequency ω b?ω c. We derive an analytic form of this function. 相似文献
9.
In the frame work of TREDER 's gravitational theory we consider two classes of field equations which are derivable from two classes of LAGRANGE ian densities Ω (1)(ω 1, ω 2), Ω (2)(s? 1, s? 2). ω 1, ω 2; s? 1, s? 2 are parameters. Ω (2)(ω 1, ω 2) gives us field equations which are up to the post-NEWTON ian approximation in the sense of NORDTVEDT , THORNE and WILL equivalent to the field equations given by BRANS and DICKE . For ω 2 = ?1 ?2ω 1 field equations follow from Ω (1)(ω 1, ?1 ?2ω 1) which are in the above mentioned sense of post-NEWTON ian approximation equivalent to EINSTEIN 's equations. The field equations following from Ω (1)(ω 1, ω 2) have a cosmological model with the well known cosmological singularities for T → ± ∞ in case that ω 1/(1 +3ω 1 +ω 2) ? γ > 0. For ω 1/(1 +3ω 1 +ω 2) ≤ 0 cosmological models with no cosmological singularities exist. From Ω (2)(s? 1, s? 2) we obtain field equations which at the best give us perihelion rotation 7% above EINSTEIN 's value and light deflection 7% below the corresponding EINSTEIN 's value. But in that case we are able to show the existence of a cosmological model without any cosmological singularity. 相似文献
10.
Non-degenerate four-wave mixing using two non-collinear laser beams with frequencies (wavevectors) ω p, ω t ( kp, kt) respectively is studied in CuCl. Two emission lines at frequencies ω (1)=2ω t-ω p, and ω (2)=2ω p-ω t are observed. Their excitation spectrum is sharply peaked if the phase-match condition k(1)=2 kt- kp is fulfilled. This is the case, if ω p coincides with the hyper-Raman lines ( R+T, R-T) of the laser labelled (t) in a well-defined geometrical configuration. 相似文献
11.
We consider the influence of an ω-dependent ionic dielectric constant ?(ω) on the properties of a superconductor. Assuming that the pairing interaction is proportional to ? 2 we have solved the Eliashberg equations for this case, both for imaginary and real frequencies. The interaction potential depends on a coupling constant λ and on a longitudinal phonon frequency Ω. The dielectric constant is assumed to be independent of wavevector q, and to depend on frequency through the expression: ?(ω) = (ω 2 - ω 2long)/(ω 2 - ω 2trans), where ω long, ω trans are the frequencies of optical phonons of the dielectric. We find that along the imaginary frequency axis (but not for real frequencies) the weighted phonon propagator can be modeled by an appropriate choice of a cutoff frequency and an effective coupling constant. The influence of ?(ω) on Tc, the gap δ(ω), and the renormalization function Z(ω) are studied and it is found that these quantities increase significantly with the dielectric constant. 相似文献
12.
Abstract In this paper, the authors have derived the expression for the combination frequency components ω 3 = 2ω 1-ω 2 in the current density by solving the appropriate Boltzmann transfer equation for electrons, when two laser beams of frequencies of ω 1 and ω 2 are incident on a degenerate nonparabolic semiconductor viz. InSb; the nonlinearity due to collision mechanism as well as the nonparabolicity of conduction band has been taken into account. The ionized impurity scattering has been considered to be the sole mechanism of electron scattering. The expression for the current density is further substituted in the wave equation to obtain the expression for the amplitude of combination frequency wave in the transmitted component. The calculated value of output power of frequency ω 2 is found to be 1/10th of the experimental value (Patel, Slusher and Fleury, 1966), (7) and thus is in better agreement as compared to earlier investigators (Wolff and Pearson, 1966; Kaw, 1968). (5, 10) 相似文献
13.
Based on the dielectric function of an excitonic system, a simple model dielectric function is constructed. The high energy zero of this function yields the plasma frequency, and is given by: (h ωx) 2 = (h ωf) 2 + E x2 where ω f is the free electron plasma frequency and E x the lowest exciton energy. It is argued that this formula is valid for both, Frenkel and Wannier excitons, and comparison is made with experiments on a variety of crystals, ranging from InSb to Ar. In all cases, surprisingly good agreement is found. 相似文献
14.
On crystalline silicon specimens with a nonuniform carrier concentration distribution produced by an optical method, a dispersion of the effective transverse conductivity σ ⊥ eff (ω) is observed near the frequency ω≈ω c=τ ⊥ ?1 ≡ε/4πσ ⊥ eff . At ω<ω c, an anomalous transverse effective conductivity is observed: σ ⊥ eff (ω) is greater than the transverse conductivity of a homogeneous specimen σ ⊥ h (ω) (in the frequency range studied in the experiment σ ⊥ h (ω) = const). Near ω≈ω c, the conductivity σ ⊥ eff decreases, and, at ω>ω c it coincides with σ ⊥ h . 相似文献
15.
半整数四极核的中心跃迁对梳状脉冲序列的响应随四极常数ω q而变化。在梳伏脉冲的平均射频汤强度ω e(以弧度·秒 -1为单位)满足ω q/ω e<1/5时,测量中心线所得的谱,位于频率ω e/2π处;而当ω q/ω e>2时,所得的谱线在2ω e/2π处,理论计算与实验结果是一致的。本文得到的结论表明,梳状脉冲序列将可能成为研究这
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16.
我们用缀饰模型研究强近共振激光脉冲驱动下二能级系统与弱试探脉冲的相互作用。当试探脉冲的频率ω 2调到共振频率ω 2=ω 1±<Ω 21D>时,试探脉冲呈现吸收或放大效应。还得到非线性混频效应的瞬态过程。共振增强存在于此非线性混频过程中。
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17.
讨论了垂直入射的、频率为ω g的平面高频引力波对环形波导管中频率为ω e的电磁波的调制作用.一般情况下,在波导管中与引力波传播方向垂直的对称平面附近,电磁波的能流密度会出现三种新的频率成分(2ω e±ω g)和ω g.在ω g》ω e时,能流密度振幅的相对调制量与引力波的无量纲振幅h 0数量级相同.特别是当ω g,ω e和电磁波绕波导传播的绕行频率ω 0满足关系ω g=2ω e》ω 0时,由于类似于共振的机制,能流密度会出现比h 0大几个数量级的振幅相对调制量.这个结果不会由于参数的微小改变而消失,这对于探测极微弱的引力辐射信号将是十分重要的
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18.
A lattice dynamical calculation for the photoinduced infrared vibrational modes is reported. By using the perturbed Green function method we have been able to compute the new frequencies and the integrated intensities of the photoinduced infrared modes at ω 1H⋍500 cm −1, ω 2H⋍1260 cm −1 and ω 3H⋍1370 cm −1 in trans(CH) x and at ω 1D⋍400 cm −1 and ω 2D⋍1045 cm −1 in trans(CD) x. Most of the features of the photoinduced infrared spectra can be explained in terms of long conjugation length segment properties. Only the higher frequency tail of the band shape peaked at ω 1H and ω 1D can be related to the pertubbation determined by a photogenerated charge trapped in short segments. The calculated frequencies and integrated intensities are in good agreement with the experimental results. 相似文献
19.
本文运用无序层状系统模型模拟无序超晶格的性质,根据β-S方程,在弱局域限,给出了动力学密度响应函数x(q,ω),并由x(q,ω)求得了平行和垂直于平面的电导率和局域化长度的表达式,导出了依赖于频率ω的扩散系数D k(ω)的自洽方程,并进行了讨论。
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20.
Experimental data on the spin susceptibility of HTSC cuprates are reproduced on the basis of a spherically symmetric approach in the frustrated Heisenberg model. The inclusion of real and imaginary renormalizations in spin Green’s functions makes it possible to explain the evolution of spin excitation spectrum ω( q) and susceptibility spectrum χ( q, ω) in the range from insulator to optimal doping. In the low-frustration limit corresponding to the weakly doped mode, the saddle singularity of ω( q) and scaling of χ 2D(ω) =∫ d q Im χ( q, ω) are reproduced and an analytic expression is derived for the scaling function. In the strong frustration (optimal doping) mode, the stripe scenario is demonstrated; this leads to a peak of χ 2D (ω) in the region of ω~60 meV. 相似文献
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