具有D4h对称性构型的C2+4分子的Jahn-Teller效应与能级分裂

依据量子理论与配位场理论,利用群论和对称性分析的方法探讨了C²⁺₄分子在具有D_4h对称性构型时,E×(b_1g+b_2g)系统的Jahn-Teller效应中的相关问题.研究了C²⁺₄分子的电子态与声子态的对称性及其活跃声子态,讨论了系统声子间的耦合与CG系数,构建了E×(b_1g+b_{2g 关键词： 2+4分子')" href="#">C²⁺4分子 对称性 能级分裂 Jahn-Teller畸变}     

LiNH2 的晶格动力学、介电性质和热力学性质第一性原理研究

采用基于密度泛函理论的平面波赝势方法,在局域密度近似下采用线性响应的密度泛函微扰理论计算了LiNH₂的晶格动力学、介电性质和热力学性质,得到了布里渊区高对称方向上的声子色散曲线和相应的声子态密度,分析了 LiNH₂的红外和拉曼活性声子频率,同时给出它的介电张量和玻恩有效电荷张量. 研究表明,LiNH₂存在小的各向异性,计算所得结果与实验值和其他理论值符合较好.最后,利用得到的声子态密度进一步预测了LiNH₂的热力学性质 关键词： 密度泛函理论 晶格动力学 热力学性质 第一性原理计算     

BaVS3晶格动力学研究

采用基于密度泛函理论的平面波赝势方法,在局域密度近似下采用线性响应的密度泛函微扰理论研究了具有六角结构的BaVS3化合物的品格动力学性质,得到了整个声子谱.计算得到的г点拉曼频率和实验数据进行了比较,其中E22g模、A1g模和实验测量值符合得比较好.对于E32g模,采用线性响应计算的结果与实验值差别较大.对该模应用冻结声子方法研究后认为差异主要是由于E32g模的较强的非谐性引起的.此外V原子在平面内的振动模E12g出现了虚频.虚频的出现预示着六角相的BaVS3结构的不稳定性,从而很好地解释了该材料由六角相到正交相的结构相变.     

第一性原理计算MgB2薄膜拉伸对超导电性的影响

应用全势线性响应线性糕模轨道方法计算MgB₂的电子能带结构、声子谱及电声子耦合常数,并讨论MgB₂的超导电性.通过比较MgB₂薄膜双轴拉伸前后超导电性的变化可以看出，随着a轴晶格常数增大和c轴晶格常数减小,声子谱中硼的E_2g声子频率显著下降，使得电声子耦合强度λ和声子对数平均频率ω_ln增强,提高了MgB₂薄 关键词： 超导电性 能带结构 声子频率 电声子耦合     

Y3Al5O12的热输运性质的第一性原理研究

基于密度泛函微扰理论（DFPT）结合模守恒赝势方法进行晶格动力学模拟.得到了钇铝石榴石（YAG）的声子态密度、分波声子态密度和声子的色散谱.利用第一Brillouin区的特殊点取样方法,计算了YAG的比热容和布局数平均的声子群速度.在非谐相互作用下,利用Fermi黄金公式结合第一Brillouin区的特殊点取样方法,得出了YAG非谐声子平均自由程.综合考虑了两种声子散射机制,得到了YAG陶瓷的热导率.结果表明,对于YAG陶瓷,在低温时,晶界散射将对热阻起主要作用;在高于一定温度时,三声子相互作用对热阻的贡献将占主导地位.同时也从理论上证明了Sato等提出的在室温以上,YAG陶瓷与单晶的热导率的差异可以忽略的观点.所得到的热导率、比热容随温度的变化与实验结果很好地符合. 关键词： 声子平均自由程 密度泛函微扰理论 3Al₅O₁₂声子结构')" href="#">Y₃Al₅O₁₂声子结构 热导率     

水热法制备Mo1-XWXS2合金材料及晶体结构的研究

采用水热合成法制备合金半导体材料Mo_1-XW_XS₂（X为浓度）,并应用扫描式电子显微镜（SEM）、X射线衍射（XRD）和拉曼光谱仪（Raman）对其形貌和晶体结构进行表征。实验结果表明,由SEM形貌测试发现,随着掺杂浓度的增加,Mo_1-XW_XS₂片状合金材料表面逐渐粗糙;由XRD晶体结构表征发现,随着掺杂浓度的增加,Mo_1-X W_XS₂的晶格常数逐渐增大;在合金材料的拉曼频移中,随着掺杂浓度的增加,Mo_1-XW_XS₂中A_1g振动模发生蓝移,而E¹_2g振动模发生红移。通过晶格结构和拉曼频移的检测和分析,证实水热合成法可制备不同浓度的Mo_1-XW_XS₂合金半导体材料。本方法可进一步拓展为二硫族化物合金...     

Cs 39D态Rydberg原子Stark光谱的实验研究

本文从理论和实验上对Cs39D态Rydberg原子在弱电场作用下的Stark效应做了详细研究. 理论上利用数值方法计算了Cs原子39D态的Stark结构;实验上,采用两步激发超冷基态原子获得超冷Rydberg原子,通过场电离的方法获得了39D态的Stark光谱,测量获得α^5/2₂,α^5/2₀,α^3/2₂和α^3/2₀相应的极化率分别为：62(7),－146(13), 73(6) 和－106(20) MHz·V^-2cm²,实验结果与理论相符合. 关键词： Rydberg原子 Stark结构 场电离 极化率     

Si掺杂β-Ga2O3的第一性原理计算与实验研究

采用基于密度泛函理论(DFT)的第一性原理平面波超软赝势(USPP)法, 在广义梯度近似(GGA)下计算了本征β-Ga₂O₃和Si掺杂β-Ga₂O₃的能带结构、电子态密度、差分电荷密度和光学特性. 在蓝宝石衬底(0001)晶面上用脉冲激光沉积(PLD)法制备了本征β-Ga₂O₃和Si掺杂β-Ga₂O₃薄膜, 测量了其吸收光谱和反射光 关键词： 第一性原理 超软赝势 密度泛函理论 2O₃')" href="#">Si掺杂β-Ga₂O₃     

Zr4Al2高压下热动力学性质的第一性原理研究

本文主要通过平面波赝势密度泛函理论的第一性原理计算方法,采用广义梯度近似（GGA）交换相关函数,研究了六角结构的Zr₄Al₂的EOS态方程和热动力学性质.通过采用了声子效应的准谐近似德拜模型,我们成功得到了晶胞体积V随温度以及V/V₀、德拜温度（?）、热熔C_V随压强P的变化关系.另外,我们还研究了热膨胀系数α及格林参数γ随温度和压强的变化关系,计算结果表明格林参数和热膨胀系数随温度的升高而增加,但是在较高的温度下,热膨胀系数几乎不受温度的影响.     

插层化合物Ag1/4TiSe2电子结构的第一性原理研究

采用基于密度泛函理论的平面波赝势方法,选用局域密度近似对Ag_1/4TiSe₂及TiSe₂的几何结构进行了优化和总能量计算.计算得到的晶格常量与实验结果符合较好,负的形成能表明有序Ag_1/4TiSe₂系统的稳定性.布居数、键长、能带结构和态密度的计算结果显示：Ag以较强的离子性结合于Ag_1/4TiSe₂中.Ag的插入使得半金属性的TiSe_{2关键词： 1/4}TiSe₂')" href="#">Ag_1/4TiSe₂ 电子结构 插层化合物 第一性原理计算     

Hydrothermal synthesis, characterization and Raman studies of Eu activated Gd2O3 nanorods

Eu³⁺ (8 mol%) activated gadolinium oxide nanorods have been prepared by hydrothermal method without and with surfactant, cityl trimethyl ammonium bromide (CTAB). Powder X-ray diffraction (PXRD) studies reveal that the as-formed product is in hexagonal Gd(OH)₃:Eu phase and subsequent heat treatment at 350 and 600 °C transforms the sample to monoclinic GdOOH:Eu and cubic Gd₂O₃:Eu phases, respectively. The structural data and refinement parameters for cubic Gd₂O₃:Eu nanorods were calculated by the Rietveld refinement. SEM and TEM micrographs show that as-obtained Gd(OH)₃:Eu consists of uniform nanorods in high yield with uniform diameters of about 15 nm and lengths of about 50-150 nm. The temperature dependent morphological evolution of Gd₂O₃:Eu without and with CTAB surfactant was studied. FTIR studies reveal that CTAB surfactant plays an important role in converting cubic Gd₂O₃:Eu to hexagonal Gd(OH)₃:Eu. The strong and intense Raman peak at 489 cm⁻¹ has been assigned to A_g mode, which is attributed to the hexagonal phase of Gd₂O₃. The peak at ∼360 cm⁻¹ has been assigned to the combination of F_g and E_g modes, which is mainly attributed to the cubic Gd₂O₃ phase. The shift in frequency and broadening of the Raman modes have been attributed to the decrease in crystallite dimension to the nanometer scale as a result of phonon confinement.     

Laser excited Raman spectra of Gr. VI semiconducting compounds

Raman spectra of hexagonal layer compounds MoS₂, MoSe₂ and MoTe₂ were excited by a HeNe laser (6328Å). All the Raman active long-wave-length phonon frequencies were determined. These are A_1g, E¹_2g, E_1g and E²_g. The frequencies decrease as we pass from sulphur to tellurium and the quasiacoustical mode E²_2g has the lowest frequency. A comparison of the results with infrared spectra shows that the i.r. active mode for E ⊥ C is degenerate with one of the Raman active modes in the basal plane for all the compounds.     

Vibrational spectra of antiferromagnetic FeBO3

Transmission spectra of powdered ferric borate have been measured in the wave number region from 6000 cm^-1 to 30 cm^-1 at room and liq. N₂ temperatures. Eight absorption bands observed are assigned as three A_2u and five E_u phonon modes. Of those modes, the temperature dependence of the E_u mode ar 230 cm^-1 (300 K) and the A_2u mode at 277 cm^-1 (300 K) have been measured in the temperature range containing the Néel temperature. They do not show the influence of magnetic ordering.     

Role of Fe doping on structural and vibrational properties of ZnO nanostructures

In this report, Raman and Fourier Transform Infrared (FTIR) measurements were carried out to study the phonon modes of pure and Fe doped ZnO nanoparticles. The nanoparticles were prepared by sol–gel technique at room temperature. The X-ray diffraction measurements reveal that the nanoparticles are in hexagonal wurtzite structure and doping makes the shrinkage of the lattice parameters, whereas there is no alteration in the unit cell. Raman measurements show both E₂^lowE_{2}^{\mathrm{low}} and E₂^HighE_{2}^{\mathrm{High}} optical phonon mode is shifted towards lower wave number with Fe incorporation and explained on the basis of force constant variation, stress measurements, respectively. In addition, Fe related local vibrational modes (LVM) were observed for higher concentration of Fe doping. FTIR spectra reveal a band at 444 cm⁻¹ which is specific to E ₁ (TO) mode; a red-shift of this mode in Fe doped samples and some surface phonon modes were observed. Furthermore, the observation of additional IR modes, which is considered to have an origin related to Fe dopant in the ZnO nanostructures, is also reported. These additional mode features can be regarded as an indicator for the incorporation of Fe ions into the lattice position of the ZnO nanostructures.     

Raman spectra of MgB2 at high pressure and topological electronic transition

Raman spectra of MgB₂ ceramic samples were measured as a function of pressure up to 32 GPa at room temperature. The spectrum at normal conditions contains a very broad peak at ∼590 cm⁻¹ related to the E _2g phonon mode. The frequency of this mode exhibits a strong linear dependence in the pressure region from 5 to 18 GPa, whereas, beyond this region, the slope of the pressure-induced frequency shift is reduced by about a factor of two. The pressure dependence of the phonon mode up to ∼5 GPa exhibits a change in the slope, as well as a “hysteresis” effect in the frequency vs. pressure behavior. These singularities in the E _2g mode behavior under pressure support the suggestion that MgB₂ may undergo a pressure-induced topological electronic transition.     

Prediction of superconductivity of Ta2AlC: in situ Raman spectrometry and density functional investigations

In this paper, in situ Raman spectra of Ta₂AlC are measured in the temperature range of 80–500 K at ambient pressure. The frequencies of the Raman modes decrease with increasing temperature, which have been explained by the anharmonic and thermal expansion effects. The line‐width of E_2g (ω₃) mode increases at elevated temperatures, which is found to be due to the anharmonic phonon–phonon scatterings. On the other hand, the line‐widths of E_2g (ω₁) and A_1g (ω₄) modes decrease continuously with increasing temperature, which is explained by the electron–phonon couplings of these two phonon modes with the Ta 5d electrons. The electron–phonon coupling strengths are obtained both in experiments and density functional calculations. Finally, Ta₂AlC is predicted to be a new superconductive MAX phase. Copyright © 2014 John Wiley & Sons, Ltd.     

Physical insight of superconductivity of Nb2AlC: in situ Raman spectrometry investigation

Superconductivity of Nb₂AlC has been previously reported, but the origin is not clear. In this paper, in situ Raman spectra of Nb₂AlC are measured in the temperature range from 80 to 380 K at ambient pressure. The line‐width of E_2g (ω₁) mode increases with temperature which originates from the anharmonic phonon–phonon scattering. On the contrary the line‐widths of E_2g (ω₂) and A_1g (ω₄) modes decrease continuously at elevated temperature. The phenomenon is explained by the electron–phonon coupling. The origin of superconductivity is therefore interpreted by the coupling of Nb 4d electrons with E_2g (ω₂) and A_1g (ω₄) phonon modes. Copyright © 2013 John Wiley & Sons, Ltd.     

Structure and superconductivity in the ternary silicide CaAlSi

Using the linear response-linearized Muffin-tin orbital (LR-LMTO) method, we study the electronic band structure, phonon spectra, electron-phonon coupling and superconductivity for c-axis ferromagnetic-like (F-like) and antiferromagnetic-like (AF-like) structures in ternary silicide CaAlSi. The following conclusions are drawn from our calculations. If Al and Si atoms are assumed to arrange along the c axis in an F-like long-range ordering (-Al-Al-Al-and-Si-Si-Si-), one could obtain the ultrasoft B_1g phonon mode and thus very strong electron-phonon coupling in CaAlSi. However, the appearance of imaginary frequency phonon modes indicates the instability of such a structure. For Al and Si atoms arranging along the c axis in an AF-like long-range ordering (-Al-Si-Al-), the calculated electron-phonon coupling constant is equal to 0.8 and the logarithmically averaged frequency is 146.8 K. This calculated result can correctly yield the superconducting transition temperature of CaAlSi by the standard BCS theory in the moderate electron-phonon coupling strength. We propose that an AF-like superlattice model for Al (or Si) atoms along the c direction may mediate the inconsistency estimated from theory and experiment, and explain the anomalous superconductivity in CaAlSi.      

Morphology of InN nanorods using spectroscopic Raman imaging

We report the vibrational properties of vertical and oblique InN nanorods (NRs) grown by molecular beam epitaxy (MBE). Surface optical (SO) Raman mode at 561 cm⁻¹, belonging to E₁ symmetry [SO(E₁)], is identified along with symmetry allowed Raman modes of E₂(low), E₂(high), and E₁(LO) at 87, 489, and 589 cm⁻¹, respectively, corresponding to wurtzite InN phase. Usually, SO phonon modes arise due to breakdown of translational symmetry of surface potential at surface defects, which are attributed by the surface roughness. Intensity distribution of E₁(LO) and SO(E₁) phonon modes over a specified area have been analysed using Raman area mapping with an optical resolution of 400 nm. Imaging with E₁(LO) phonon mode, originating from the bulk of the sample, distinguishes the vertical NRs alone. We are able to resolve NR morphologies in both vertical and oblique cases with additional Raman mapping analysis of SO(E₁) phonon mode, emerging from the surface irregularities, which are confined to the tip of MBE grown NRs. Copyright © 2013 John Wiley & Sons, Ltd.