四方相BaTiO3铁电性的第一性原理研究

在广义梯度近似下,利用超软赝势对立方相和四方相BaTiO3晶胞中Ti原子沿c轴位移时体系的能量、原子间电子云重叠布局数和各原子上的净电荷等进行了自洽计算.结果显示,当Ti原子沿c轴位移0.012nm时,四方相BaTiO3体系能量最低,其自发极化强度为0.261C/m2,该结果与实验数据相符合;同时表明,O原子的2p轨道和Ti原子的3d轨道的杂化是BaTiO3晶体出现铁电性的重要原因.     

Phase-shifting point-diffraction interferometer developed by using the electro-optic effect in ferroelectric crystals

A novel and simple phase-shifting point-diffraction interferometer using a z-cut lithium niobate wafer is proposed. The pinhole is realized by an optical lithography process, aluminum deposition, and subsequent lift-off on the surface of the wafer. The phase shifting is obtained by inducing the electro-optic effect along the z crystal axis. We demonstrate experimentally the possibility of retrieving an aberrated wavefront.     

First-principles study of the mechanisms of the pressure-induced dielectric anomalies in ferroelectric perovskites

The pressure-induced giant dielectric anomaly at 0 K of ABO₃ perovskites is investigated at the Hartree-Fock, density-functional theory and hybrid levels. Its mechanism is analyzed in terms of thermodynamic phase stability, structural and phonon contributions and Born effective charges. It is shown that the IR-active soft phonon is responsible for the anomaly. This mode always involves a displacement and a deformation of the oxygen octahedra, while the roles of A and B ions vary among the materials and between high- and low-pressure phase transitions. A sharp increase in the phonon amplitude near the phase transition gives rise to the dielectric anomaly. The use of hybrid functionals is required for agreement with experimental data. The calculations show that the dielectric anomaly in the pressure-induced phase transitions of these perovskites is a property of the bulk material.     

四方相BaTiO3铁电性的第一性原理研究

在广义梯度近似下，利用超软赝势对立方相和四方相BaTiO3晶胞中Ti原子沿c轴位移时体系的能量、原子间电子云重叠布局数和各原子上的净电荷等进行了自洽计算.结果显示，当Ti原子沿c轴位移0012nm时，四方相BaTiO3体系能量最低，其自发极化强度为0261C/m2，该结果与实验数据相符合；同时表明，O原子的2p轨道和Ti原子的3d轨道的杂 化是BaTiO3晶体出现铁电性的重要原因. 关键词： 四方相BaTiO3 铁电性 位移 态密度     

First-principles study of the magnetic properties of nitrogen-doped alkaline earth metal oxides

We present a first-principles study of the magnetic properties of N-doped MgO, CaO and SrO, which have been proposed to constitute a new class of dilute magnetic semiconductors (DMSs) with no magnetic elements. In this study, it was found that under a homogeneously distributed condition, Curie temperatures could reach room temperature at sufficient N concentrations in the range of 20–30 at.%; however, an inhomogeneous N distribution in these DMSs is the favored configuration, which indicates that spinodal decomposition leads to a room-temperature blocking temperature at smaller N concentrations than those estimated for room-temperature ferromagnetism in the homogeneous distribution condition.     

四方铁电体PbFe0.5Nb0.5O3精细结构的第一性原理研究

采用全电势线性缀加平面波法(FPLAPW),计算了PbFe0.5Nb0.5O3的总能量,从而确定了它的四方铁电稳态结构,即B位离子(Fe和Nb)在四方铁电相的平衡构型.计算结果表明,Fe相对于氧八面体沿[001]方向的位移为0.022nm,而Nb的偏心位移为0.0020nm,其中铁电非稳性主要归因于前者 关键词： 铁电体 密度泛函理论 平面波法     

Strain-induced insulator–metal transition in ferroelectric BaTiO_3(001) surface:First-principles study

The electronic properties of TiO_2-terminated BaTiO_3(001) surface subjected to biaxial strain have been studied using first-principles calculations based on density functional theory. The Ti ions are always inward shifted either at compressive or tension strains, while the inward shift of the Ba ions occurs only for high compressive strain, implying an enhanced electric dipole moment in the case of high compressive strain. In particular, an insulator–metal transition is predicted at a compressive biaxial strain of 0.0475. These changes present a very interesting possibility for engineering the electronic properties of ferroelectric BaTiO_3(001) surface.     

The electro-optic effect in proustite

The low-frequency unclamped electro-optic coefficients of proustite have been measured at an optical wavelength of 0.6328m. The results show thatr ₂₂ is comparable to that of lithium niobate,r ₁₃ andr ₃₃ are considerably lower. The maximum theoretical value is calculated at 10.8×10^–12 m/V about three times the largest measured coefficient value.     

电光晶体研究究进进展及其对称性性研研究

由电场引起的晶体折射率的变化称为晶体的电光效应,具有电光效应的晶体称为电光晶体。利用电光效应可以制作电光Q开关等重要光电器件。晶体的电光效应及其有效利用,均与其对称性密切相关。本文在综述电光晶体研究进展的基础上,推导了不同晶系中各晶类电光效应类型及其特点,并以电光Q开关的要求为例讨论了晶体的电光效应及其对称性之间的关系。从讨论结果得出:立方晶系Td-ˉ43m,三方晶系C3v-3m,四方晶系D2d-4ˉ2m,D4v-4mm和六方晶系D3h-6ˉm2,C6v-6mm点群中相应电光系数的横向或纵向效应可能有实用价值。在考虑旋光性影响后,立方晶系T-23,三方晶系D3-32,四方晶系中S4-4ˉ,六方晶系中C3h-6ˉ等也有可能应用。但是,电光晶体的应用与许多因素相关,对称性只是其中一个基本条件。     

层状氧化钼的电子结构、磁和光学性质第一原理研究

按照基于自旋密度泛函理论的赝势平面波第一原理计算方法,理论研究了两种层堆叠结构氧化钼(正交和单斜MoO_3)的电子结构、磁性和光学特性,探讨其作为电致变色材料或电磁材料在光电子器件中的技术应用.采用先进的半局域GGA-PW91和非局域HSE06交换相关泛函精确计算晶体结构和带隙宽度.计算得出较低密排面解离能,表明两种层状氧化钼的单片层很容易从体材料上剥落.能带结构和投影态密度分析表明:导带底和价带顶电子态主要来自于层平面方向成键的原子轨道,呈现典型的二维电子结构特征.无缺陷的MoO_3块体材料具有明显的磁矩,O空位会导致磁矩增加;由Mo原子和顶点氧原子产生的亚铁磁耦合磁矩是MoO_3层状材料磁性的主要来源;层状氧化钼在可见光区具有明显的光吸收响应,光吸收谱表现出显著的各向异性并在带电时发生明显的蓝移或形成新的低频可见光吸收峰.计算结果证明层状氧化钼具有明显的电致变色和磁控性能,为设计高性能电磁或光电子功能材料提供了理论依据和技术数据.     

BiFeO3中各离子在铁电相变中作用本质的第一性原理研究

采用基于密度泛函理论的广义梯度近似方法和赝势平面波法,对多铁材料BiFeO3的铁电反铁磁相和可能的高温顺电相的电子结构进行了第一性原理研究.计算验证了BiFeO3基态为G型反铁磁有序,Fe离子的理论磁矩与实验值相符.铁电相变发生后,Bi-6s态和6p态发生了电荷转移,Bi-6s电子的作用更加明显.Born有效电荷的研究表明铁电畸变主要表现为Bi原子的位移,并且电极化强度计算值很好地符合薄膜实验结果.部分态密度的计算表明Bi-6p态的成键轨道与反键轨道间的能量劈裂大于其他电子态,使得Bi-6p态与O-2p态的共价作用选择性增强,这是铁电畸变的根本引发机理.Bi-6s电子因静电斥力而发生了极化,且没有参与Bi-6p轨道与O-2p轨道间的杂化,其电荷转移缘于其与O-2s,2p轨道发生了微弱的共价作用,并因此而强化了6s态电子的极化,这种共价作用更利于Bi-O间的相对位移,也是体系强电极化产生的原因.     

Cr二维单层薄片中Jahn-Teller效应的第一性原理研究

采用基于密度泛函理论的第一性原理计算,本文对Cr单层薄片的一系列二维结构（长方、正方、六角、斜方和中心长方晶格）进行了结构稳定性和电子性质研究.结果表明,在Cr的二维体系中,对称性较低的斜方晶格和中心长方晶格是稳定的,对称性较高的正方和六角晶格是不稳定的,而长方晶格的形成能很小.Cr二维原子薄片的两种稳定结构都是六角结构畸变的结果,六角晶格的键角减小时会形变成斜方晶格,键角增大时会形变成中心长方晶格,这是由于Jahn-Teller效应使简并能级自发破缺,继而结构产生降低对称性的形变,最终使得体系变得稳定.     

First-principles study on structural,electronic and ferroelectric properties of Bi2VO5.5 compound

A first-principles investigation of the origin of ferroelectricity in the Aurivillius compound Bi₂VO_5.5 is presented. Calculations with the density functional theory, in conjunction with the modern theory of polarization, allowed us to study the structural, electronic, and polar properties of two configurations built with oxygen vacancies, causing a charge imbalance and a subsequent displacement of the ions, generating two maximum polarizations, one of 14.75 μC/cm² and one of 4.31 μC/cm² along [011] direction. Electron localization function schemes were used to identify the asymmetry of charges in (001), (010) and (100) planes. The results obtained in this study establish that the origin of ferroelectricity is due to the displacement of the ions caused by oxygen vacancies and the asymmetric distribution of the isolated pair of Bi ions. On the other hand, the bandgap calculations and the results of P_s establish that Bi₂VO_5.5 is a candidate ferro-photovoltaic material.     

New polaronic excitons in ferroelectric oxides: phenomenological theory

It is shown that three types of new polaronic excitons exist in ferroelectric oxides-charge transfer vibronic excitons (CTVEs). The electron-hole bipolaron pairs are common in all these cases, the difference having to do with the origin of charge transfer-lattice instabilities. These are the cases of the charge transfer and lattice distortions and their vibronic interactions induced in the harmonic (i) and anharmonic (ii) regimes both in the electronic excited state on the one hand, and the (iii) case of the CTVE excited vibronic state induced in the anharmonic regime but related with electronic ground state on the other. All these CTVEs are characterized by strong coupling with good self-localization conditions in separate orientation wells of their multi-well potentials. The relevant phenomenological theory is developed. While the (i) type CTVE is confirmed by luminescence experiments, the (iii) type CTVE with low excitation energy and with metastable intrinsic defect behaviour can be a candidate for the explanation of an order-disorder effect in the classical ferroelectric oxides discovered recently [Phys. Rev. Lett. 90 (2003) 037601].     

Effect of lateral fluorination in antiferroelectric and ferroelectric mesophases: synchrotron X-ray diffraction,dielectric spectroscopy and electro-optic study

Effect of lateral fluorination in the rigid core on several macroscopic and microscopic properties of a terphenyl based mesogenic chiral ester has been studied by synchrotron X-ray, dielectric and electro-optic techniques. Correlation lengths across the smectic planes, in para-, ferro- and antiferroelectric phases, are found to be significantly less in the fluorinated compound. Para to ferroelectric transition is found to be tricritical in nature in both the compounds. Fluorination resulted in slower response under a square pulse. Collective mode relaxation behaviour, with and without bias field, in all the phases are also found to be different in the fluorinated compound.     

Dielectric and electro-optic behavior of pure ferroelectric liquid crystal material and the isomeric mixtures

We report here the dielectric relaxation spectroscopy and electro-optical properties of a ferroelectric liquid crystal (FLC) CB470 (R) and isomeric mixtures containing CB470 (R) and its optical isomer CB480 (S) in two different concentrations. We have studied with two isomeric mixtures: mixing CB470 (R) and CB480 (S) in the ratio 95:5 (w/w) and 90:10 (w/w). With increase of percentage of CB480 (S) in the mixture the spontaneous polarization (P_S) value has been decreased due to cancellation of a part of the dipolar contribution, since the direction of P_S of the two isomers are in opposite direction. The tilt angles of the pure FLC materials and the isomeric mixtures remain almost unchanged. The dielectric strength of Goldstone mode is almost proportional to the spontaneous polarization (P_S). An additional collective mode has been observed in the SmC¹ phase due to the formation of domain with large periodicity in pure FLC material and in one of the isomeric mixtures by unwinding the helical structures, which is known as domain mode. The existence of domain mode has been studied extensively with the variation of P_S in the isomeric mixtures. The result has been discussed.     

Integrated electro-optic platform using ferroelectric crystals with the point symmetry of orthorhombic mm2

We theoretically and numerically investigate the modulation efficiencies of integrated electro-optic (EO) modulators based on ferroelectric crystals with the point symmetry of orthorhombic mm2. An analytical expression describing the index-ellipsoid under the EO effect is derived, where all 5 non-zero linear EO coefficients of mm2-crystals are considered. As practical candidates for integrated EO platform, three mm2-crystals (KNbO₃, KTiOPO₄, and Ba₂NaNb₅O₁₅) that are well-established and commercially available are chosen for numerical simulations. The refractive-index (RI) changes caused by an external electric field applied to each crystal with an arbitrary direction are analyzed in detail for the given optical polarization states of input light wave. For each crystal, the modulation efficiency is evaluated in terms of the half-wave voltage required for a π-phase shift of input wave, which is compared with the established counterpart based on LiNbO₃. The results indicate that higher modulation efficiencies are achievable in the two schemes using KNbO₃ and Ba₂NaNb₅O₁₅ than those for other cases, meaning that either lower power-consuming or larger beam-window integrated EO devices are feasible.     

Ge掺杂对InI导电性能影响的第一性原理研究

采用密度泛函理论框架下的第一性原理平面波超软赝势方法,在相同环境条件下对不同浓度Ge掺杂的In I导电性能进行了研究.建立了由不同浓度的Ge原子替代In原子的In1-x Ge x I(x=0,0.125,0.25)模型.对低温下高掺杂Ge原子的In1-x Ge x I半导体的优化参数、总态密度、能带结构进行了计算.结果表明:Ge的掺入使In1-x Ge x I材料的体积减小,总能量升高,稳定性降低;Ge原子浓度越大,进入导带的相对电子数量越多,In1-x Ge x I电子迁移率减小,电阻率增大,同时最小光学带隙也增大,有利于改善体系的核探测性能.