YTiO_3中偶极激活光学声子的椭圆偏振研究和晶格动力学计算
Dipole-active optical phonons in YTiO_3: ellipsometry study and lattice-dynamics calculations
摘要：
利用椭圆偏振光谱法对正交晶系（Pbnm）YTiO_3单晶的三个主晶向进行了声子频率测量，准确地提取了各向异性复介电响应。我们确定了所有25个红外激活声子模式允许对称性，7B_1u，9B_2u和9B_3u，沿c-，b-和a-轴，分别极化。从复介电函数\tilde ε（ω）及其逆-1/ \tilde ε（ω）的经典色散分析中，我们定义了横向（TO）和纵向（LO）声子模的谐振频率、宽度和振子强度。我们计算了B_1u、B_2u和B_3u简正模的本征频率和本征矢量，并通过比较它们的频率和振子强度与目前晶格动力学研究的结果，建议了在我们的椭圆偏振光谱中观察到的TO声子模的归属。在此基础上，我们估算了YTiO_3晶格中原子的动态有效电荷。我们发现，在一般情况下，YTiO_3晶格中的动态有效电荷是典型的钙钛矿氧化物家族。与铁电BaTiO_3相比，氧的动态有效电荷与沿c轴的位移有关，并没有显示出异常大的值。同时，Y和ab面氧的动态有效电荷表现出各向异性，表明沿a轴的强杂化。
英文摘要：
The anisotropic complex dielectric response was accurately extracted from spectroscopic ellipsometry measurements at phonon frequencies for the three principal crystallographic directions of an orthorhombic (Pbnm) YTiO_3 single crystal. We identify all twenty five infrared-active phonon modes allowed by symmetry, 7B_1u, 9B_2u, and 9B_3u, polarized along the c-, b-, and a-axis, respectively. From a classical dispersion analysis of the complex dielectric functions \tildeε(ω) and their inverses -1/\tildeε(ω) we define the resonant frequencies, widths, and oscillator strengths of the transverse (TO) and longitudinal (LO) phonon modes. We calculate eigenfrequencies and eigenvectors of B_1u, B_2u, and B_3u normal modes and suggest assignments of the TO phonon modes observed in our ellipsometry spectra by comparing their frequencies and oscillator strengths with those resulting from the present lattice-dynamics study. Based on these assignments, we estimate dynamical effective charges of the atoms in the YTiO_3 lattice. We find that, in general, the dynamical effective charges in YTiO_3 lattice are typical for a family of perovskite oxides. By contrast to a ferroelectric BaTiO_3, the dynamical effective charge of oxygen related to a displacement along the c-axis does not show the anomalously large value. At the same time, the dynamical effective charges of Y and ab-plane oxygen exhibit anisotropy, indicating strong hybridization along the a-axis.
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