We have investigated the electronic and optical properties of (Sr$_{1-x}$Ca$_{x}$)$_2$IrO$_4$ (x= 0 - 0.375) and (Sr$_{1-y}$Ba$_y$)$_2$IrO$_4$ (y= 0 - 0.375) epitaxial thin-films, in which the bandwidth is systematically tuned via chemical substitutions of Sr ions by Ca and Ba. Transport measurements indicate that the thin-film series exhibits insulating behavior, similar to the J$_{eff}$= 1/2 spin-orbit Mott insulator Sr$_2$IrO$_4$. As the average A-site ionic radius increases from (Sr$_{1-x}$Ca$_{x}$)$_2$IrO$_4$ to (Sr$_{1-y}$Ba$_y$)$_2$IrO$_4$, optical conductivity spectra in the near-infrared region shift to lower energies, which cannot be explained by the simple picture of well-separated J$_{eff}$= 1/2 and J$_{eff}$= 3/2 bands. We suggest that the two-peak-like optical conductivity spectra of the layered iridates originates from the overlap between the optically-forbidden spin-orbit exciton and the inter-site optical transitions within the J$_{eff}$= 1/2 band. Our experimental results are consistent with this interpretation as implemented by a multi-orbital Hubbard model calculation: namely, incorporating a strong Fano-like coupling between the spin-orbit exciton and inter-site d-d transitions within the J$_{eff}$= 1/2 band.
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。
Nanoscale Sr$_2$IrO$_4$ Freestanding Thin-Films for Flexible Electronics
Department of Physics and Astronomy, University of Kentucky（肯塔基大学物理与天文学系）；Department of Materials Science and Engineering, The Ohio State University（俄亥俄州立大学材料科学与工程系）；Department of Physics, Pusan National University（釜山国立大学物理系）；Advanced Photon Source, Argonne National Laboratory（阿贡国家实验室先进光子源）
Spin-orbit excitons and electronic configuration of the $5d^4$ insulator Sr$_3$Ir$_2$O$_7$F$_2$
在这里，我们报告内的低能量激发的顺磁自旋轨道绝缘体Sr $_3$ Ir $_2$ O $_7$ F $_2$研究通过共振非弹性X射线散射，\textit{ab initio}量子化学计算，模型哈密顿模拟。该材料是Sr $_3$ Ir $_2$ O $_7$的独特$d^{4}$ Ir $^{5+}$类似物，当F离子插入Sr $_3$ Ir $_2$ O $_7$的方形晶格IrO $_{6}$双层的SrO层中时形成。由于Ir $^{5+}$离子的大的畸变，我们的计算表明，一个大的单离子各向异性产生$S$ =1（$L{\approx}$ 1，$J{\approx}$ 0）基态波函数。弱耦合，激子模式的$S_z$ =0的基态观察和良好的描述由一个现象学的自旋轨道激子模型先前开发的$3d$和$4d$过渡金属离子。我们的研究结果的解释有关以前的研究接近$d^{4}$填料的空穴掺杂虹彩酸盐的影响进行了讨论。
