Role of the exchange-correlation functional on the structural, electronic and optical properties of cubic and tetragonal SrTiO$_3$ including many-body effects
Vijaya Begum, Markus Ernst Gruner, Rossitza Pentcheva
SrTiO$_3$ is a model perovskite compound with unique properties and technological relevance. At 105 K it undergoes a transition from a cubic to a tetragonal phase with characteristic antiferrodistortive rotations of the TiO$_6$ octahedra. Here we study systematically the effect of different exchange correlation functionals on the structural, electronic and optical properties of cubic and tetragonal STO by comparing the recently implemented strongly constrained and appropriately normed (SCAN) meta-GGA functional with the generalized gradient approximation (PBE96 and PBEsol) and the hybrid functional (HSE06). SCAN is found to significantly improve the description of the structural properties, in particular the rotational angle of the tetragonal phase, comparable to HSE06 at a computational cost similar to GGA. The addition of a Hubbard $U$-term (SCAN+$U$, $U=7.45$ eV) allows to achieve the experimental band gap of 3.25 eV with a moderate increase in the lattice constant, whereas within GGA+$U$ the gap is underestimated even for high $U$ values. The effect of the exchange-correlation functional on the optical properties is progressively reduced from 1.5 eV variance in the onset of the spectrum in the independent particle picture to 0.3 eV upon inclusion of many-body effects within the framework of the $GW$ approximation (single-shot $G_0W_0$) and excitonic corrections by solving the Bethe-Salpeter equation (BSE). Moreover, a model BSE approach is shown to reproduce the main features of the optical spectrum at a lower cost compared to $G_0W_0$+BSE. Strong excitonic effects are found in agreement with previous results and their origin is analyzed based on the contributing interband transitions. Last but not least, the effect of the tetragonal distortion on the optical spectrum is discussed and compared to available experimental data.
Hidden structural transition in epitaxial Ca$_{0.5}$Sr$_{0.5}$IrO$_{3}$/SrTiO$_{3}$ thin film
在ABO $_{3}$钙钛矿薄膜中涉及BO $_{6}$八面体旋转模式的变化的结构转变可以隐藏在由衬底施加的全局晶格对称性下，并且经常容易被忽视。利用高分辨X射线衍射实验研究了在SrTiO $_{3}$（STO）和GdScO $_{3}$（GSO）衬底上外延生长的Ca $_{0.5}$ Sr $_{0.5}$ IrO $_{3}$（CSIO）钙钛矿铱酸盐薄膜的结构。虽然CSIO/STO膜层显示出由倒易空间映射证明的全局四斜晶格对称性，但同步加速器X射线数据表明其室温结构是单斜的，这是由于Glazer的IrO $_{6}$八面体的a $^{+}$ a $^{-}$ c $^{-}$型旋转。为了适应较低的对称性结构下的全球tetraxine对称性，膜断裂成四个孪晶域，导致分裂的（半整数，0，整数）超晶格反射。令人惊讶的是，这些超晶格反射的分裂随着温度的增加而减少，最终在T $_{S}$ = 510（5）K时消失，这表明结构转变为具有$^{+}$ a $^{-}$ c $^{0}$八面体旋转的正交相。相比之下，CSIO/GSO薄膜显示出稳定的单斜对称性，具有$^{+}$ b $^{-}$ c $^{-}$八面体旋转，显示出在高达520 K下没有由衬底引起的结构不稳定性。我们的研究说明了在决定外延薄膜的结构中，除了衬底的晶格失配之外，对称性的重要性。
A hybrid-exchange density-functional theory study of the electronic structure of $\mathrm{MnV}_2\mathrm{O}_4$: Exotic orbital ordering in the cubic structure
The role of ionic sizes in inducing the cubic to tetragonal distortion in AV$_{2}$O$_{4}$ and ACr$_{2}$O$_{4}$ (A=Zn, Mg and Cd) compounds
尖晶石型化合物中立方相到四立方相的畸变在过去二十年一直是一个有争议的问题。不同的研究小组提出了不同的机制来理解这些尖晶石中的这种畸变，它们是：（i）称为自旋驱动Jahn-Teller（JT）效应的自旋晶格耦合机制，（ii）强相对论自旋轨道耦合，适度JT畸变和弱V-V相互作用以及（iii）JT效应。现在，为了知道这种扭曲的可能原因，我们通过进行自旋非极化计算来避免这些复杂性（自旋、电子、轨道和晶格自由度之间的各种相互作用）。ZnV $_{2}$ O $_{4}$（ZnCr $_{2}$ O $_{4}$）、MgV $_{2}$ O $_{4}$（MgCr $_{2}$ O $_{4}$）和CdV $_{2}$ O $_{4}$（CdCr $_{2}$ O $_{4}$）的体积模量的计算值分别为$\sim$ 289（$\sim$ 254）、$\sim$ 244（$\sim$ 243）和$\sim$ 230（$\sim$ 233）GPa。对于钒酸盐和铬酸盐，晶格参数$a$的计算值顺序分别为$_{2}$和$_{2}$，与实验结果一致.三次到四次畸变（c/a）的计算值，对于ZnV $_{2}$ O $_{4}$，c/a $<$ 1（ZnCr $_{2}$ O $_{4}$）、MgV $_{2}$ O $_{4}$（MgCr $_{2}$ O $_{4}$）和CdV $_{2}$ O $_{4…
