Ohtomo A, Muller D A, Grazul J L, Hwang H Y
Bell Laboratories, Lucent Technologies, Murray Hill, New Jersey 07974, USA.
Nature. 2002 Sep 26;419(6905):378-80. doi: 10.1038/nature00977.
The nature and length scales of charge screening in complex oxides are fundamental to a wide range of systems, spanning ceramic voltage-dependent resistors (varistors), oxide tunnel junctions and charge ordering in mixed-valence compounds. There are wide variations in the degree of charge disproportionation, length scale, and orientation in the mixed-valence compounds: these have been the subject of intense theoretical study, but little is known about the microscopic electronic structure. Here we have fabricated an idealized structure to examine these issues by growing atomically abrupt layers of LaTi(3+)O(3) embedded in SrTi(4+)O(3). Using an atomic-scale electron beam, we have observed the spatial distribution of the extra electron on the titanium sites. This distribution results in metallic conductivity, even though the superlattice structure is based on two insulators. Despite the chemical abruptness of the interfaces, we find that a minimum thickness of five LaTiO(3) layers is required for the centre titanium site to recover bulk-like electronic properties. This represents a framework within which the short-length-scale electronic response can be probed and incorporated in thin-film oxide heterostructures.
复杂氧化物中电荷屏蔽的性质和长度尺度对于广泛的系统至关重要，这些系统涵盖陶瓷压敏电阻（变阻器）、氧化物隧道结以及混合价化合物中的电荷有序化。混合价化合物在电荷歧化程度、长度尺度和取向方面存在很大差异：这些一直是深入理论研究的主题，但对于微观电子结构却知之甚少。在此，我们通过生长嵌入SrTi(4+)O(3)中的LaTi(3+)O(3)原子级突变层，构建了一个理想化结构来研究这些问题。利用原子尺度电子束，我们观察到了钛位点上额外电子的空间分布。尽管超晶格结构基于两种绝缘体，但这种分布导致了金属导电性。尽管界面存在化学突变，但我们发现中心钛位点要恢复类似体相的电子性质，需要至少五个LaTiO(3)层的厚度。这代表了一个框架，在其中可以探测短长度尺度的电子响应并将其纳入薄膜氧化物异质结构中。
