CaVO $_3$薄膜中的金属-绝缘体转变：外延应变、尺寸限制和表面效应之间的相互作用
Metal-insulator transition in CaVO$_3$ thin films: interplay between epitaxial strain, dimensional confinement, and surface effects
摘要：
本文利用密度泛函理论和动态平均场论（DFT+DMFT）研究了$_3$薄膜金属-绝缘体相变（MIT）的多个调控参数。我们专注于分离基板引起的外延应变从那些相关的薄膜厚度减少的影响。我们表明，约3-4%的拉伸外延应变是足够的诱导过渡到一个顺磁莫特绝缘相。这对应于可以在SrTiO $_3$衬底上实现的应变水平。使用独立的平板模型，我们然后证明，减少膜厚度也可以导致MIT在CaVO $_3$，然而，只有厚度小于4钙钛矿单位。我们的计算表明，MIT在这样的超薄膜的结果主要来自表面诱导的晶体场分裂之间的$t_{2g}$ -轨道，有利于形成轨道极化的莫特绝缘体。这种表面诱导的晶场分裂与拉伸外延应变引起的晶场分裂是同一类型的，因此这两种效应也可以合作。此外，我们的计算证实了在膜表面的相关效应的增强，导致在最外层的准粒子的光谱重量减少，而批量样的性能恢复在只有几层远离表面。
英文摘要：
We use density functional theory plus dynamical mean-field theory (DFT+DMFT) to study multiple control parameters for tuning the metal-insulator transition (MIT) in CaVO$_3$ thin films. We focus on separating the effects resulting from substrate-induced epitaxial strain from those related to the reduced thickness of the film. We show that tensile epitaxial strain of around 3-4% is sufficient to induce a transition to a paramagnetic Mott-insulating phase. This corresponds to the level of strain that could be achieved on a SrTiO$_3$ substrate. Using free-standing slab models, we then demonstrate that reduced film thickness can also cause a MIT in CaVO$_3$, however, only for thicknesses of less than 4 perovskite units. Our calculations indicate that the MIT in such ultra-thin films results mainly from a surface-induced crystal-field splitting between the $t_{2g}$-orbitals, favoring the formation of an orbitally-polarized Mott insulator. This surface-induced crystal-field splitting is of the same type as the one resulting from tensile epitaxial strain, and thus the two effects can also cooperate. Furthermore, our calculations confirm an enhancement of correlation effects at the film surface, resulting in a reduced quasiparticle spectral weight in the outermost layer, whereas bulk-like properties are recovered within only a few layers away from the surface.
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