铁基非晶态因瓦合金的声子谱研究

用非弹性中子散射实验方法研究了非晶态Ｆｅ_９０－ｘＣｏ_ｘＺｒ_１０（ｘ＝１０，４０）和Ｆｅ_８０－ｙＣｒ_ｙＰ_１３Ｃ_７（ｙ＝４，８）合金的广义声子谱，在低能区域（?ω≤２０ｍｅＶ）观察到了与因瓦效应相关的声子谱软化现象。初步讨论了这种动力学方面的反常行为。结果表明，声子谱软化可能与在因瓦合金中存在增强的电子－声子相互作用有关。 关键词：     

量子阱中极化子的声子平均数

采用有效质量近似下的变分法,考虑到电子同时与表面光学声子和体纵光学声子相互作用,研究了无限深量子阱中极化子的表面光学声子平均数,体纵光学声子平均数和光学声子平均数。讨论了电子与体纵光学声子耦合强度α,阱宽L和势垒材料Al_xGa_1-xAs中Al的含量x对上述光学声子平均数的影响。以GaAs/Al_xGa_1-xAs量子阱为例进行了数值计算。结果表明:量子阱中表面光学声子平均数随耦合强度α,阱宽L和Al含量x增大而增大。量子阱中体纵光学声子平均数随耦合强度α,阱宽L的增大而增大。光学声子平均数随耦合强度α,阱宽L和Al含量x的增大而增大。     

有限温度下二维Heisenberg铁磁系统的横向声频支声子激发

在二维正方Heisenberg铁磁系统的基础上建立了磁振子-声子相互作用模型.利用Matsubara格林函数理论研究了系统的横向声频支声子激发，计算了布里渊区的主要对称点线上的横向声频支声子色散曲线，发现在第一布里渊区的Σ线上，小波矢区(Γ附近k_xa＜0.2)横向声频支声子有硬化现象，但是在软化区(0.25＜k_xa＜0.7)有横向声频支声子有软化现象，在M附近(0.75＜k_xa)横向声频支声子有硬化现象.在Δ线上没有发生横向声频支声子有软化与硬化现象.在Z线上小波失区(X附近k_xa＜0.65)横向声频支声子有软化现象，但是在M附近横向声频支声子有硬化现象.并且又讨论了各项参数对横向声频支声子激发的影响，发现磁振子-声子耦合与自旋波劲度常数对横向声频支声子软化起很大的作用. 关键词： 磁振子-声子相互作用 横向声频支声子软化 Matsubara格林函数理论 铁磁系统     

拓扑声子晶体

拓扑物态是当前凝聚态及材料物理领域的关注焦点.声子晶体是具有周期性结构的人工材料,其中的声子态或声波态也可具有拓扑性质.从声子晶体的背景知识出发,介绍了2类拓扑声子晶体的研究进展,即能谷声子晶体和外尔声子晶体,它们具有良好鲁棒性及超导传输特性的拓扑界/表面波,这种无障碍的传输特性具有广阔的应用前景.     

声子之间的相互作用对量子线中磁极化子性质的影响

研究了量子线中弱耦合磁极化子的性质.采用线性组合算符和微扰法导出量子线中弱耦合磁极化子的基态能量.在计及电子在反冲效应中发射和吸收不同波矢的声子之间的相互作用时,讨论了量子线的受限强度、电子-LO声子耦合强度和声子之间相互作用对量子线中弱耦合磁极化子的基态能量的影响.数值计算结果表明:量子线中弱耦合磁极化子的基态能量随量子线的受限强度ω₀的增大而迅速增大.当受限强度ω₀取相同值时,电子-声子耦合强度α越大基态能量E₀越小,磁场的回旋频率ω_e越大基态能量E₀越大.在弱磁场情况下,当ω₀<0.5时,随着量子线的受限强度ω₀的减少p值迅速增大,即对于弱磁场声子之间相互作用的影响不能忽略.     

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：     

微波声学和微声器件的发展和应用

微波声学是一门新兴学科,十多年来发展迅速,并且已根据其原理发展出一系列新型的微声器件．本文简单地介绍了微声体波与表面波目前达到的水平,微波声子与电子、热声子、磁子、电子自旋量子以及核自旋量子等的相互作用以及它在研究固体方面的应用;表面声波的传播规律和表面波微声器件的现状及其发展前景;微声材料的现状及要求等．也指出了上述诸方面还存在的问题和其发展前途．重点放在表面声波及其器件方面．     

多声子强耦合超导理论

本文提出了多声子过程的强耦合超导理论,给出了能隙方程和T_c公式。结果发现,考虑电子多声子过程,高频声子对提高T_c有重要作用,单胞中对超导贡献的简并度数大,也有利于高T_c的出现。 关键词：     

(Hf0.5Zr0.5V2)Hx的声子谱与超导性的关系

本文报道了用Be过滤探测器谱仪对C-15相的(Hf_0.5Zr_0.5V₂₎H_x等五种样品的声子谱的测量。观察到了它们的声子谱随含氢量而变化以及声学支有规则的软化现象。表明(Hf_0.5Zr_0.5V₂₎H_x的T_c随x的变化,主要是声学支的贡献,声学文软化有利于T_c的提高,而光学支“软化”可能会抑制超导性。 关键词：     

一维介观环中持续电流的电子-声子相互作用非经典效应

基于声子相干态功效和计及声子压缩态非经典效应,研究了电子-磁振子和电子-声子相互作用对一维介观环持续电流的影响. 与自由环比较,由于电子-磁振子相互作用,持续电流的振幅呈现指数减小. 对于正常态电子,电子-声子相互作用导致持续电流以Debye-Waller(D-W)因子衰减.但是计入跳步电子-单声子相干态关联效应导致系统本征态能量大幅度下降,从而持续电流I_n有大幅度增加.另一方面计入双声子相干态行为,由于声子压缩态效应压缩电子-相干(态)声子弹性散射行为,导致电子绕环运 关键词： 持续电流 电子-声子相互作用 声子相干态 声子压缩态效应     

A comment on the W(001) surface stability and reconstruction

Arguments are presented in favour of an additional softening which those changes of surface force constants that lead to a pronounced “soft” surface phonon mode (e.g. M₅) can invoke in the global surface phonon spectrum. This result is illustrated for the W, Mo and Cr (001) surface. Analysis of phonons of an isolated layer is used together with a “perturbation theorem”.     

Hydrogen phonon spectra on transition metal surfaces: infrared reflection-absorption investigations of Mo(l00), W(100), and Pt(111)

Hydrogen phonon spectra at saturation coverage are, reported for Mo(l00), W(100), and Pt(111). Surface infrared reflection absorption Spectroscopy enabled the evaluation of the dipole contribution to each hydrogen phonon over the 800–4000 cm⁻¹ region. A phenomenological line shape analysis, including the effects of broadband absorption by the substrate due to free carrier excitation and surface state absorption, permitted a quantitative comparison of the three surfaces' absorption and an investigation of the role of electron-phonon interaction. We find anomalously broad hydrogen symmetric stretch absorptions, exhibiting relative intensities W(100) > Mo(100) > Pt(111). A sharper Fano absorption on Mo(100) and W(100), assigned to the first overtone of the wag mode and not observed in specular EELS, indicates coupling to surface electronic states of the appropriate symmetry.     

Temperature-dependent Brillouin scattering studies of surface acoustic modes in Nd0.5Sr0.5MnO3

Brillouin scattering experiments are carried out to study the surface acoustic waves in Nd_0.5Sr_0.5MnO₃ as a function of temperature in the range of 40-300 K covering the metal-insulator and charge-ordering phase transitions. The surface modes include surface Rayleigh wave, pseudo-surface acoustic wave (PSAW) and high velocity PSAW. The observed softening of the sound velocities for the surface modes below paramagnetic to ferromagnetic transition, T_c is related to the softening of the C₄₄ elastic constant. The subsequent hardening of the sound velocity below the charge ordering transition temperature T_co is attributed to the coupling of the acoustic phonon to the charge ordered state via long range ordering of the strong Jahn-Teller (JT) distortion.     

On surface phonons and reconstruction: Comparison of the W and the Fe (001) surface

A phenomenological surface phonon model of the (001) surface of W and Fe is used to clarify some questions related to the problem of which high-symmetry surface phonon mode is the most perspective candidate for the tungsten (001) surface reconstruction observed experimentally. Conditions are derived for the appearance and softening of the relevant modes. The ¯M ₁,¯M ₅ and ¯X ₃ modes seem to be the most promising. A few points from the current literature on the reconstruction problem are discussed. The results seem to support the idea about the general instability of the tungsten (001) surface.     

Soft phonons and structural phase transition in superconducting Ba0.59K0.41BiO3

We have observed a softening of phonons and a structural phase transition in a superconducting Ba_0.59K_0.41BiO₃ (T_c = 31 K) single crystal using elastic and inelastic neutron scattering measurements. The soft phonon occurs for the [1 1 1] transverse acoustic mode at the zone boundary. The phonon energies in this vicinity are found to continuously decrease with decreasing temperature from above room temperature. This softening stops at a temperature close to T_s, where a structural phase transition from cubic to tetragonal symmetry occurs. The overall results are consistent with previous data that reported phonon softening and a (0.5, 0.5, 0.5) type superstructure in several Ba_1?xK_xBiO₃ systems. However, we also find weaker (0.5, 0.5, 0) type superstructure peaks that reveal an additional component to the modulation. No significant change related to the superconductivity was observed for these soft phonon energies or linewidths.     

Soft phonon behavior and magnetism at the low temperature structural phase transition of La1.65Nd0.35CuO4

We report a neutron scattering investigation of a La_1.65Nd_0.35CuO₄ single crystal which undergoes a second order structural transition with decreasing temperature from a high temperature orthorhombic phase (space group Bmab) to a distinct low temperature orthorhombic phase (space group Pccn) atT _s =76 K. The transition is induced by the softening of the phonon mode that involves rotations of the tilt axis of the CuO₆ octahedra. Antiferromagnetic long range order is established belowT _N =316 K, and a hysteretic reorientation to a noncollinear spin structure occurs belowT _s . The low energy spin dynamics of La_1.65Nd_0.35CuO₄ aboveT _s are closely similar to those of stoichiometric La₂CuO₄; at 100 K the gaps for in-plane and out-of-plane polarized spin waves are 2.3 meV and 5 meV, respectively. Out-of-plane polarized magnons are little affected by the structural transition, but the gap for in-plane polarized magnons increases markedly belowT _s .     

Raman scattering in intermediate valent SmB6

We have measured the Raman spectra of intermediate valent (IV) SmB₆ and compared it to LaB₆ and EuB₆. Beside the three high energy Raman active phonons we found additional features in the spectra. Most prominent is a peak at 172 cm^-1 for SmB₆, at 214 cm^-1 for LaB₆ and at ～220 cm^-1 for EuB₆. The spectra are analysed in terms of defect induced phonon excitations yielding a weighted phonon density of states. The softening of the line in IV SmB₆ compared to the other hexaborides is unusual since even in the trivalent MB₆ compounds the Raman active phonon modes normally are at lower frequencies than in semiconducting, divalent samples. This phonon softening is explained in analogy with the phonon anomalies in other IV compounds like Sm_1?xY_xS and TmSe.     

The Debye-Waller factor in helium scattering by a crystalline surface

On the dense face of a metallic crystal or the (001) face of lithium fluoride, the distance between nearest neighbours is of the same order of magnitude as the helium diameter. The hypothesis of simultaneous interaction between incident helium atoms of thermal kinetic energy and surface atoms belonging to a surface unit cell is therefore introduced. Then the exponent of the Debye-Waller factor contains mean square and mean correlated displacement between atoms of the cell. This gives for the (00) peak an apparent surface Debye temperature Θ_sa higher than the corresponding value usually deduced for instance from LEED measurements. For the (001) face of copper the calculated value is: 323 < Θ_sa < 354 K according to the value ascribed to bulk Debye temperature. On this face, recent experimental results show that helium atoms are coherently scattered in the specular (00) peak. From its intensity analysis is deduced an attractive well depth of 0.009 ± 0.002 eV and Θ_s = 370 ± 10 K, very close to the calculated result. The simultaneous interaction implies that the one phonon exchange can only take place with phonons of long wavelength. This theoretical expectation seems to be in qualitative agreement with experimental data.     

Synthesis and optical properties of La1−XCeXMnO3 studied by infrared reflectivity measurements

The bulk polycrystalline La_1−xCe_xMnO₃ (0.0 ≤ x ≤ 0.8) manganites were synthesized through citrate sol-gel auto-combustion method for a wide range of composition 0.0 ≤ x ≤ 0.8. The x-ray diffraction (XRD) analysis suggests the single phase rhombohedral (R-3c) symmetry for parent LaMnO₃ compound, however a secondary cubic (Fm-3 m) phase of cerium oxide (CeO₂) is appeared when Ce is incorporated into the LaMnO₃. A detailed XRD study of these materials were discussed, and specifically the infrared reflectivity measurements on La_1−xCe_xMnO₃ materials are discussed in this study for the first time. The phonon softening and hardening was observed for different modes but softening was found to be more pronounced which implied strong electron–phonon coupling. The most affected phonon mode was appeared for the doping range 0.2 ≤ x ≤ 0.4, in which the largest softening was observed. The Lyddane–Sachs–Teller relation is verified by experimental FTIR data.