抛物量子线中强耦合极化子的有效质量

采用改进的线性组合算符法、Lagrange乘子和变分法,在考虑电子与LO声子相互作用情况下,研究了抛物量子线中强耦合极化子的有效质量和光学声子平均数。通过数值计算,讨论了约束强度ω₀和拉格朗日乘子u对极化子的有效质量m^*和光学声子平均数N及极化子振动频率λ的影响。计算结果表明:有效质量m^*和光学声子平均数N及极化子振动频率λ都随着约束强度ω₀和拉格朗日乘子u的增加而增大。     

在不同应力下石墨烯中拉曼谱的G峰劈裂的变化

应用计及五阶近邻的力常数模型,研究了单轴应力下的石墨烯和芳香烃分子三明治型贴层的石墨烯中拉曼谱的G峰劈裂.计算结果表明对称性的降低解除了G峰对应的在Γ点的面内的纵波光学模声子和横波光学模声子能量简并,从而G峰劈裂为G⁺和G^- 两个峰.在单轴应力作用下,C—C键的伸长致使力常数减小,软化了面内的光学模声子,导致两个G峰都红移;芳香烃分子对石墨烯产生的沿分子长短边方向不同的应力作用,使得G峰对应的两支光学模声子的频率一支发生蓝移,而另一支发生红移.这解 关键词： 力常数模型 石墨烯 拉曼G峰劈裂     

表面极化子光学声子平均数的磁场和温度依赖性

采用变分法、幺正变换和拉格朗日乘子法,研究了有限温度下纯二维晶体中表面磁极化子的性质.讨论了表面光学声子平均数、磁极化子振动频率λ与磁场B、温度T及Lsgrange乘子u之间的关系.对KCl晶体进行了数值计算,结果表明:磁极化子振动频率、表面光学声子平均数均随磁场B的增强而增加,且随温度T升高而增加.当bgrange乘子u超出慢电子范围时磁极化子振动频率、表面光学声子平均数均随u增加而增大且变化越来越显著.     

MX络合物中电荷密度波和自旋密度波对三次谐波产生系数的影响

研究了电子-电子相互作用与电子-声子相互作用的竞争对MX络合物基态及三次谐波产生系数(THG)的影响。根据哈伯德模型,当电子-电子相互作用强度U不超过2t₀(t₀是最近邻的M原子d_z²和x原子Pz轨道间的跳跃能)时,可用Hartree-Fock(HF)近似来研究体系的状态,结果发现随着U的增大,体系会从电荷密度波(CDW)变为自旋密度波(SDW),相交点发生在U～1.2t₀,且是一级相变。在相变点     

抛物量子线中弱耦合极化子的有效质量和光学声子平均数

讨论电子与体纵光学(LO)声子弱耦合时对抛物量子线中极化子性质的影响.采用Tokuda改进的线性组合算符法、Lagrange乘子和变分法,导出了抛物量子线中弱耦合极化子的有效质量和光学声子平均数随拉格朗日乘子变化的规律及极化子振动频率随量子线约束强度的变化规律.并以ZnS量子线为例进行了数值计算,结果表明:抛物量子线中弱耦合极化子的有效质量m^*和光学声子平均数N随着拉格朗日乘子u的增加而增大;该结论与体材料中结论基本一致,但量子线中的效应比体材料更明显,表明量子线对电子约束的增强,使极化子效应更明显.同时,极化子振动频率λ随约束强度ω₀的增强而增大.     

周期力调制噪声驱动下单模激光系统的多重随机共振

在周期力调制噪声驱动下单模激光系统的光强方程中加入调幅波, 用线性化近似方法计算了系统的光强关联函数和输出信噪比, 并对信噪比进行数值计算和分析, 发现低频调制频率Ω、高频载波频率ω和周期力频率Ω_λ对系统的输出信噪比有很大的影响. 具体表现为信噪比R 随低频调制频率Ω 的变化过程中出现了多重随机共振和极强的单峰共振, 当Ω << ω 时, 系统出现的是多峰共振, 且随着Ω_λ 增加, 共振峰间的距离增大, 峰值位置不变; 当Ω → ω 时, 输出信噪比R迅速增大, 而Ω_λ 的影响被削弱甚至可以忽略, 多峰共振消失; 当Ω = ω 时, 系统出现了极强的单峰共振. 此外, 信噪比随周期力频率的变化呈现振幅减小的多重随机共振, 而随载流频率的变化出现单峰随机共振.     

基态非简并的聚合物中的晶格振动频谱

反式聚乙炔[trans-(CH)_x]和顺式聚乙炔[cis-(CH)_x]的振动谱在弱耦合情况下,光谱声子ω₀(k=0)随电子-晶格耦合系数λ的变化关系反映出完全不同的规律。这是由于在顺式聚乙炔中存在禁闭势垒的原因,使基态呈非简并态。 关键词：     

Ta对C—15相V2Hf和V2(Zr0.5Hf0.5)系列声子性能的影响

本文用中子飞行时间方法对C-15相的超导材料V₂Hf,V₂Ta和V₂Hf_0.8Ta_0.2以及V₂Zr_0.5。Hf_0.5和V₂Zr_0.5Hf_0.33Ta_0.17的热中子非弹性散射谱作了测量,并计算出相对的广义声子态密度。结果与早先发表的Nb对C-15相V₂Zr和V₂(Hf_0.5Zr_0.5)系列声子性能的影响一致:声子频率随超导转变温度T_c增加而软化,随T_c减小而硬化。这表明,对于此类材料弹性软化在一定程度上对提高T_c起了作用。结果还进一步表明V₂Zr或V₂Hf与V₂(Zr_0.5Hf_0.5)之间有着质的差别,V₂Hf加Ta后,T_c增加,声子频率软化,而V₂(Zr_0.5Hf_0.5)加Ta后,T_c减小,声子频率则略有硬化。这与V₂Zr和V₂(Hf_0.5Zr_0.5)加Nb的结果是一致的。此结果可以用角动量分波表象的能带论方法分析电-声耦合相互作用得出的杂化理论来定性解释。 关键词：     

抛物量子线中束缚磁极化子的性质

研究抛物量子线中束缚磁极化子的性质,采用线性组合算符和幺正变换方法导出了强、弱耦合两种情况下的基态能量、振动频率和光学声子平均数.结果表明,无论是强耦合还是弱耦合情况,抛物量子线中束缚磁极化子的振动频率λ、基态能量E₀和光学声子平均数N都随约束强度ω₀的增大而迅速增大.     

电子关联对顺式聚乙炔中双极化子附近局域振动模的影响

用自洽迭代的方法研究了顺式聚乙炔中双极化子的能谱及其附近的局域振动模.结果发现:1)在顺式聚乙炔中,双极化子的电子束缚态数目和位置仅取决于耦台参数λ和电子相互作用U,这类似于反式聚乙炔;参数t_l(消除基态二度简并)虽不能改变电子局域态的个数,但影响了双极化子附近局域模的数目.2)电子相互作用不仅使局域模的频率发生了漂移,而且增加了局域模的个数,这不同于反式,3)局域模的局域性随耦合参数λ以及电子相互作用U的增加而增强,随参数(?)的增大而减弱.     

Effect of a Gaussian white noise on the charge density wave dynamics in a one dimensional compound

The effect of a white Gaussian noise (WGN) on the charge density wave (CDW) dynamics properties of a one-dimensional conductor is studied numerically in the weak pinning limit. In agreement with a recent experimental work on the charge density wave conductor K_0.3MoO₃, the addition of a Gaussian noise affects the CDW dynamics of the system. The results are discussed in the context of CDW dislocations.     

Effect of CDW and magnetic interactions on the electrons of the manganite systems

We address a model study which includes the co-existence of the charge density wave (CDW) and ferromagnetic interactions in order to explain the colossal magnetoresistance (CMR) in manganites. The Hamiltonian consists of the ferromagnetic Hund's rule exchange interaction between e_g and t_2g spins, Heisenberg core spin interactions and the CDW interaction present in the e_g band electrons. The core electron magnetization, induced e_g electron magnetization and the CDW gap are calculated using Zubarev's Green's function technique and determined self-consistently. The effect of core electron magnetization and the CDW interaction on the induced magnetization as well as on the occupation number in the different spin states of the e_g band electrons are investigated by varying the model parameters of the system like the CDW coupling, the exchange coupling, the Heisenberg coupling and the external field. It is observed that the induced magnetization exhibits re-entrant behaviour and exists within a narrow temperature range just below the Curie temperature. This unusual behaviour of the e_g band electrons will throw some new insights on the physical properties of the manganite systems.     

Stochastic resonance and charge density wave dynamics in quasi-one-dimensional conductors

Stochastic resonance (SR) in nonlinear systems is a counterintuitive concept in which a weak periodic signal and noise cooperate and give rise to a maximum in the signal-to-noise ratio at the output of the system when the noise is tuned to a certain value. The spatial coupling of a large number of oscillators showing SR may have a constructive effect and leads to the so-called array enhanced stochastic resonance (AESR). We discuss the possible application of SR and AESR concepts to charge density wave (CDW) dynamics in quasi-one-dimensional conductors. We show in a preliminary experiment that the addition of noise can modify the behavior of the CDW in the quasi-one-dimensional conductor K_0.30MoO₃.     

Microscopic theory of longitudinal sound velocity in CDW and SDW ordered cuprate systems

We address here the self-consistent calculation of the spin density wave and the charge density wave gap parameters for high-T_c cuprates on the basis of the Hubbard model. In order to describe the experimental observations for the velocity of sound, we consider the phonon coupling to the conduction band in the harmonic approximation and then the expression for the temperature dependent velocity of sound is calculated from the real part of the phonon Green’s function. The effects of the electron–phonon coupling, the frequency of the sound wave, the hole doping concentration, the CDW coupling and the SDW coupling parameters on the sound velocity are investigated in the pure CDW phase as well as in the co-existence phase of the CDW and SDW states. The results are discussed to explain the experimental observations.     

Scanning tunnelling microscopy of charge-density waves in transition metal chalcogenides

We have used scanning tunnelling microscopes (STMs) operating at liquid helium and liquid nitrogen temperatures to image the charge-density waves (CDWs) in transition metal chalcogenides. The layer structure dichalcogenides TaSe₂, TaS₂, NbSe₂, VSe₂, TiSe₂ and TiS₂ have been studied including representative polytype phases such as 1T, 2H and 4Hb. Experimental results are presented for the complete range of CDW amplitudes and structures observed in these materials. In most cases both the CDW and the surface atomic structure have been simultaneously imaged. Results on the trichalcogenide NbSe₃ are also included.

The formation of the CDW along with the associated periodic lattice distortion gaps the Fermi surface (FS) and modifies the local density-of-states (LDOS) detected by the tunnelling process. The tunnelling microscopes have been operated mostly in the constant current mode which maps the LDOS at the position of the tunnelling tip. The relative amplitudes and profiles of the CDW superlattice and the atomic lattice have been measured and confirm on an atomic scale the CDW structures predicted by X-ray, electron and neutron diffraction. The absolute STM deflections are larger than expected for the CDW induced modifications of the LDOS above the surface and possible enhancement mechanisms are reviewed.

In the 2H trigonal prismatic coordination phases the CDWs involve a relatively small charge transfer and the atomic structure dominates the STM images. In the 1T octahedral coordination phases the charge transfer is large and the CDW structure dominates the STM image with an anomalously large enhancement of the STM profile. Systematic comparison of the STM profiles with band structure and FS information is included.

In the case of the 4Hb mixed coordination phases at the lowest temperatures two nearly independent CDWs form in alternate sandwiches. STM studies on 4Hb crystals with both octahedral and trigonal prismatic surface sandwiches have been carried out. The STM scans detect the relative strengths of the two CDWs as well as the interactions between the two types of CDW structure.

The STM scans are also able to detect defects and domain structure in the CDW image. Several examples will be given demonstrating the potential of the STM to detect these local variations in LDOS on an atomic scale. In contrast to the layer structure crystals the linear chain compound NbSe₃ shows a complex surface atomic structure as well as the formation of two CDWs. The surface atomic structure is resolved in the STM scans and profiles have detected the presence of the CDW modulation at 77K and 4.2K. These results demonstrate the feasibility of detecting CDW structure in the presence of complex atomic structure and using materials where dynamical CDW effects can also be studied by STM.

The range of STM results presented here show that the STM scans are extremely sensitive to the detail of the CDW structure and its effect on the LDOS. Although much of this structure has been deduced from diffraction studies, the ability to examine the CDW structure on an atomic scale with the STM is new. The sensitivity of the STM method suggests potential applications to a wide range of electronic structures in materials.     

Competition of disorder and interchain hopping in a two-chain Hubbard model

We study the interplay of Anderson localization and interaction in a two chain Hubbard ladder allowing for arbitrary ratio of disorder strength to interchain coupling. We obtain three different types of spin gapped localized phases depending on the strength of disorder: a pinned 4k _F Charge Density Wave (CDW) for weak disorder, a pinned 2k _F CDW^π for intermediate disorder and two independently pinned single chain 2k _F CDW for strong disorder. Confinement of electrons can be obtained as a result of strong disorder or strong attraction. We give the full phase diagram as a function of disorder, interaction strength and interchain hopping. We also study the influence of interchain hopping on localization length and show that localization is enhanced by a small interchain hopping but suppressed by a large interchain hopping. Received 6 April 2001     

Soliton analysis of the electro-optical response of blue bronze

In recent measurements on the charge-density-wave (CDW) conductor blue bronze (K_0.3MoO₃), the electro-transmittance and electro-reflectance spectra were searched for intragap states that could be associated with solitons created by injection of electrons into the CDW at the current contacts [Eur. Phys. J. B16 (2000), 295; Eur. Phys. J. B35 (2003) 233]. In this work, we adapt the model of soliton absorption in dimerized polyacetylene to the blue bronze results, to obtain the (order of magnitude) estimate that current induced solitons occur on less than ∼10% of the conducting chains. We discuss the implications of these results on models of soliton lifetimes and motion of CDW phase dislocations.     

Quasiperiodic noise voltage and metastability in the charge density wave conductivity of K0.30MoO3

Slow relaxation phenomena as well as quasiperiodic noise have been studied in the non linear regime of conductivity which takes place above a well defined threshold electric field in the semiconducting incommensurate phase of the quasi one-dimensional blue bronze K_0.30MoO₃. The noise frequencies are found to be proportional to the excess current attributed to the charge density wave (CDW). In some temperature range, the CDW current is found to decrease logarithmically vs time. These results indicate the presence of metastable states related to domains and domain walls.     

Frequency pulling and heterodyning observed in voltage spectrum of the Fröhlich conductor NbSe3

The voltage power spectrum of NbSe₃ has been studied in the upper charge-density wave (CDW) state where only one CDW exists. In the non-Ohmic regime sharp spikes appear in the spectrum. The frequency of these spikes increases monotonically with current as in the lower CDW state. We study the “fine structure” of the spectrum and find evidence for “frequency pulling” between the modes. A linear coupled-mode treatment is used to model this effect. There is also evidence of heterodyning of an intrinsic (current independent) 2MHz mode. We briefly discuss these phenomena in terms of Fröhlich sliding conductivity. Difficulties with the simple two-fluid model as well as models based on solid state turbulence are pointed out.