RbNiF_3自旋波量子对的Raman散射谱理论分析

在温度由零度直到Nel温度的范围内讨论了RbNiF_3自旋波量子对的Raman散射谱。由一般的对称性原理及实验结果的分析导出磁性系统与辐射场相互作用哈密顿量的具体形式;应用推迟Green函数运动方程方法,得到散射截面的解析表达式;在自旋波量子对传播子中同时计入能量重正化和寿命的效应。最后,在适当的近似下用计算机作数值计算,在没有可调整参数的情况下得到了在Nel温度以下的Raman峰值位置和半宽度随温度的变化关系及各种温度下的Raman谱形,它们与实验结果符合得较好。     

应用保真度研究两腿XXZ梯子系统的量子相变

本文基于投影纠缠对态的自旋梯子系统的张量网络算法提供了一个有效的方法,来研究两腿自旋1/2的XXZ梯子量子系统的量子临界性,通过与单位格点基态保真度相结合,得到清晰的基态保真度三维图,来探测量子多体系统的量子相变,从而可以绘制出两腿自旋1/2XXZ梯子量子模型的基态相图,在相图中出现铁磁(FM)相、条纹铁磁(SF)相、Néel(N)相、条纹Néel(SN)相、Rung singlet(RS)相、Haldane(H)相、Rung triplet(RT)相、临界XY相(XY1相和XY2相)。在不知道系统是否存在局域序参量或非局域序参量的情况下,可以通过系统的基态波函数,在对称性破缺相中由约化密度矩阵得到局域序参量;或者在对称性相中,找出系统存在的非局域序参量,来完整地描述系统的量子相变和量子临界现象。     

应用积算符理论研究弱耦合双自旋体系的Raman磁共振谱

从积算符理论和实验上详细地研究了弱耦合双自旋体系（CHCl₃的C-H双自旋体系）的Raman磁共振谱．对不同射频场强度及频偏的Raman谱进行了细致的理论和实验研究，表明积算符理论的计算与实验不仅在弱射频场而且在强射频场时都符合得相当好，克服了微扰论的局限，这为进一步研究复杂自旋体系的Raman谱提供了强有力的理论方法．研究还表明，小的射频频偏将有利于多量子跃迁信号的观测，而适当的射频场强则会提高多量子峰的强度 关键词：     

Raman磁共振谱中强单量子信号的压抑

通常单量子信号的强度在Raman磁共振谱中要比其它多量子信号的强度大得多,因而易引起接收机饱和,使信号发生变形,不利于多量子信号的检测,因此压抑强的单量子峰是必要的.积算符理论分析表明,异核体系Raman磁共振谱的单,多量子峰的共振频率及其信号强度关于频偏满足对称关系:当频偏由正变为负时,单量子信号的强度亦随之改变了符号.因此,对正负频偏(数值相等)的两次Raman磁共振实验的FID信号进行累加,就可压抑强的单量子峰,而对于多量子峰则在大的频偏条件下不会相互抵消.因此,这样的实验累加可以保持多量子信号的强度而压抑掉强的单量子峰.作为实验验证,我们给出了异核AX_n体系(CHCl₃,CH₂Cl₂,CH₃OH)的按上述正负频偏循环累加采样方法得到Raman磁共振¹³C实验谱.实验结果同理论预言完全一致.     

铁电磁体Pb(Fe1/2Nb1/2)O3的磁电性能研究

对铁电磁体Pb(Fe1/2Nb1/2)O3单晶样品中的介电和磁性能进行了研究. 认为在其反铁磁相变点观察到的介电常数和损耗的异常来自于自发极化序和自旋序的相互作用引起的磁电耦合. 磁矩与温度的关系曲线在Néel点以下的低温段呈上升趋势,测得的磁滞回线证明有弱铁磁性出现. 对铁电磁体磁电相互作用的Monte Carlo模拟得到与实验类似的结果.     

高阶多量子跃迁的一维直接检测——Raman磁共振方法

研究了异核AX₃自旋体系的初始态为最高阶量子相干态时的Raman磁共振谱.研究表明,唯有制备高阶多量子相干的初始态时才有可能对高阶多量子跃迁进行有效检测.积算符理论的精确性使得理论上准确地预言了Raman磁共振谱中各阶多量子跃迁频率、谱线强度,使得各阶多量子跃迁频率相互交错时仍有可能解析谱图.在Raman磁共振实验中采用了正负频偏交替变化采样的方法,对单量子信号进行压制,使其更有效地观察高阶多量子跃迁.     

一维自旋1键交替XXZ链中的量子纠缠和临界指数

利用基于张量网络表示的矩阵乘积态算法以及无限虚时间演化块抽取方法,本文研究了一维无限格点自旋1的键交替反铁磁XXZ海森伯模型中的量子相变.分别计算了系统的von Neumann熵、单位格点保真度和序参量,从而得到了系统随键交替强度的变化从拓扑有序Néel相到局域有序二聚化相的量子相变点.我们用矩阵乘积态方法拟合出了相变的中心荷c?0.5,表明此相变属于二维经典的Ising普适类.另外,通过对拓扑Néel序的数值拟合,我们得到了相变点处的特征临界指数β′=0.236和γ′=0.838.     

包含硅量子点的富硅SiNx 薄膜结构与发光特性

采用等离子体增强化学气相沉积法, 以NH₃与SiH₄为反应气体, n型单晶硅为衬底, 低温(220 ℃)沉积了富硅氮化硅(SiN_x)薄膜. 在N₂氛围中, 于500–1100 ℃ 范围内对样品进行了热退火处理. 采用Raman 光谱技术分析了薄膜内硅量子点的结晶情况, 结果表明, 当退火温度低于950 ℃时, 样品的晶化率低于18%, 而当退火温度升为1100 ℃, 晶化率增加至53%, 说明大部分硅量子点都由非晶态转变为晶态. 实验通过Fourier 变换红外吸收(FTIR)光谱检测了样品中各键的键合结构演变, 发现Si–N键和Si–H键随退火温度升高向高波数方向移动, 说明了薄膜内近化学计量比的氮化硅逐渐形成. 实验还通过光致发光(PL)光谱分析了各样品的发光特性, 发现各样品中均有5个发光峰, 讨论了它们的发光来源, 结合Raman光谱与FTIR光谱表明波长位于500–560 nm的绿光来源于硅量子点, 其他峰则来源于薄膜内的缺陷态. 研究了硅量子点的分布和尺寸对发光带移动的影响, 并根据PL峰位计算了硅量子点的尺寸, 其大小为1.6–3 nm, 具有良好的限域效应. 这些结果有助于制备尺寸不同的硅量子点和基于硅量子点光电器件的实现. 关键词： 硅量子点 氮化硅薄膜 光致发光 Fourier 变换红外吸收     

Fe3Si在低温下的自旋波研究

用中子相干非弹性散射测量了Fe₃Si在14K的自旋波色散关系。从自旋波二次方色散关系得到低温下自旋波劲度系数D₀=270meV·?²。用Heisenberg模型可进一步导出Fe₃Si的有效交换积分J_eff。从磁化强度的温度依赖关系也获得了一个自旋波劲度系数D_m(O)。从两种不同方法获得的劲度系数的比较中发现在Fe₃Si中可能存在着Stoner激发,与早先工作相联系,本文还给出了从14K直到居里温度自旋波劲度系数D的温度依赖关系。 关键词：     

关于海森堡反铁磁链材料LiVGe2O6有限温度相变的理论研究

自旋s=1的海森堡反铁磁链材料LiVGe₂O₆的磁化率以及 核磁共振实验表明该材料在临界温度约为22 K时由顺磁相转变为反铁磁Néel相, 且低温磁激发谱存在能隙. 本文在已有模型哈密顿量的基础上提出了一个低能场论模型——Ginzburg-Landau理论来描述 这一反铁磁链材料, 并运用这一理论讨论了LiVGe₂O₆由于自发对称性破缺导致的有限温度相变及 相应的磁化率变化情况, 理论计算很好地解释了现有的实验结果. 关键词： 海森堡模型 σ模型')" href="#">O(3)非线性σ模型 有限温度相变     

Raman scattering from phonons and magnons in antiferromagnetic Fe3BO6

Raman scattering by phonons and by magnon pairs has been observed in Fe₃BO₆. Of the predicted 60 Raman-active modes, 39 have been identified and classified according to their symmetries. The two-magnon band shows a strong decrease in intensity with increasing temperature, and almost vanishes close to T_N = 508 K. The origin of this effect is attributed to the existence of a nearly dispersionless magnon branch.     

Rotational CARS N2 thermometry: validation experiments for the influence of nitrogen spectral line broadening by hydrogen

Rotational coherent anti‐Stokes Raman spectroscopy (CARS) in fuel‐rich hydrocarbon flames, with a large content of hydrogen in the product gases (∼20%), has in previous work shown that evaluated temperatures are raised several tens of Kelvin by taking newly derived N₂ H₂ Raman line widths into account. To validate these results, in this work calibrated temperature measurements at around 300, 500 and 700 K were performed in a cell with binary gas mixtures of nitrogen and hydrogen. The temperature evaluation was made with respect to Raman line widths either from self‐broadened nitrogen only, N₂ N₂ [energy‐corrected‐sudden (ECS)], or by also taking nitrogen broadened by hydrogen, N₂ H₂ [Robert–Bonamy (RB)], Raman line widths into account. With increased amount of hydrogen in the cell at constant temperature, the evaluated CARS temperatures were clearly lowered with the use of Raman line widths from self‐broadened nitrogen only, and the case with inclusion of N₂ H₂ Raman line widths was more successful. The difference in evaluated temperatures between the two different sets increases approximately linearly, reaching 20 K (at T ∼ 300 K), 43 K (at T = 500 K) and 61 K (at T = 700 K) at the highest hydrogen concentration (90%). The results from this work further emphasize the importance of using adequate Raman line widths for accurate rotational CARS thermometry. Copyright © 2010 John Wiley & Sons, Ltd.     

Magnon Raman scattering in CoBr2

The one-magnon Raman spectrum of CoBr₂ has been investigated as a function of temperature, and peak frequency, integrated intensity and width parameters obtained. The results obtained for the band energy at low temperature (22.2 ± 0.2 cm^-1 at 5.7.K) are in good agreement with AFMR and neutron scattering results. The one-magnon energy renormalises relatively slowly with increasing temperature and is about 15 cm^-1 at T_N = 19 K, whereas the integrated intensity approaches zero like the magnetization at T_N and the width diverges. A low intensity band at 26.8 ± 1 cm^-1 (7.6K) may be due to two-magnon scattering from spin waves along the c-axis.     

Possible interplay between a two phonon mode and high energy magnetic excitations in BiFeO<Subscript>3</Subscript>

We have performed Raman measurements on high energy excitations in BiFeO₃ single crystals as a function of both temperature and laser excitation lines. A strong feature observed at 1250 cm^-1 in the Raman spectra has been previously assigned to two phonon overtone. This peak exhibits an unusual frequency shift with the laser lines and the temperature dependence of its Fano lineshape shows two singularities at 150 K and 200 K which can be related to magnetic excitations. In the same energy range, we have also identified the two-magnon excitation with a temperature dependence very similar to the one measured for the one-magnon modes.     

Calculation of nuclear spin-lattice relaxation rate due to two-magnon scattering in YBa2Cu3O6

YBa₂Cu₃O₆ is known to be an antiferromagnetic insulator with high Neel temperature (T_N≈400≈500 K). The nuclear spin-lattice relaxation rate 1/T₁ due to two-magnon scattering is calculated, using values of ecchange integrals and anisotropy gap published in literature. We found approximately quadratic dependence of relaxation rate 1/T₁ on temperature for temperatures above 100 K. The value of 1/T₁, determined by low-energy magnons, is found to be inversely proportional to magnitude of gap in magnon spectrum.     

Raman scattering from magnons in CoCl2 and FeCl2

One-magnon Raman scattering has been observed in the metamagnets CoCl₂ and FeCl₂. The k = 0 magnon energies are 16 ± 1 cm^-1 at 21 K and 16.4 ± 0.4 cm^-1 at 12 K, respectively and these values are in good agreement with previous AFMR and neutron scattering results. A search for two-magnon scattering in both compounds was unsuccessful, largely because of masking from nearby first-order phonons and a weak temperature dependent broad band at 140 cm^-1 in CoCl₂, which is assigned to two-phonon scattering from acoustic phonons.     

压力下的CeBi电子传导特性

在4.2—273K温区内,0—18kbar压强下,测量了CeBi单晶的电阻率。实验表明,尼尔温度随压强增大而增大,且温度T>T_N后,磁性电阻率ρ_s与lnΤ成正比。计及p-f作用,我们用Anderson模型计算了与压强有关的CeBi磁性电阻率,并将计算结果与实验比较。结果表明,4f电子能级与费密能级的距离随压强增大而减小,压强增大导致p-f作用的增强。 关键词：     

Temperature-dependent light scattering from magnetic excitons in TlCoF3

Raman spectra of antiferromagnetic thallium cobaltous fluoride have been obtained with 4579A argon ion laser excitation at temperatures from 4°K to T_N = 94 ± 2°K. The features observed consist of six Co²⁺ excitons ranging in energy from 325 to 1070 cm^-1, at two-magnon peak with low-temperature energy of 315 cm^-1, and a one-magnon feature whose 4°K energy is 37 cm^-1. The energy and linewidth of the one-magnon scattering has been measured from 4°K to about 0.8 T_N; it is found that the magnon becomes critically damped at about 0.8 T_N, in good agreement with our previous observations on RbCoF₃. The Co²⁺ excitons observed at 325, 380, 410, 730 (weak), 960, and 1070 cm^-1 agree in energy quite well with the KCoF₃ levels calculated by Buyers, Holden et al. as 340, 400, 467, 767, 967 and 1050 cm^-1.     

μSR investigation of the magnetic ordering in EuAs3

We have investigated the magnetic ordering and the incommensurate-commensurate phase transition in EuAs₃ by zero-field (ZF) and longitudinal-field μSR. In the commensurate phase, stable at temperatures below T_L=10.3 K, the ZF muon signal exhibits oscillations corresponding to four muon precession frequencies the lowest of which behaves anomalously. The muon signal shows no oscillation but exponential decay in the incommensurate phase stable in temperature range from T_L≈10.3 K up to T_N≈11 K. The temperature dependence of the fitted relaxation rate shows divergence-like behaviour at the ordering temperature T_N≈11 K and also at the lock-in transition T_L≈10.3 K. The results are in qualitative agreement with those previously obtained by neutron and X-ray magnetic scattering investigations except for the anomalous temperature dependence of the lowest frequency in the commensurate phase. We propose a model for this anomalous behaviour.     

Detailed infrared spectroscopic study of thermal transitions in new hybrid [(CH3)2CHNH3]4Cd3Cl10 crystal

Phase transitions of tetra(isopropylammonium)decachlorotricadmate(II) [(CH₃)₂CHNH₃]₄Cd₃Cl₁₀ crystal have been studied by infrared, far infrared and Raman measurements in wide temperature range, between 11 K and 388 K. The temperature changes of wavenumber, center of gravity, width and intensity of the bands were analyzed to clarify cationic and anionic contributions to the phase transitions mechanism. The results of investigation showed earlier by differential scanning calorimetry (DSC), thermal expansion and dielectric measurements clearly confirmed the sequence of phase transitions at T₁=353 K, T₂=294 K and T₃=260 K. The current results derived from DSC and infrared measurements revealed additional phase transition at T₄=120 K.