Lee Ho Nyung, Christen Hans M, Chisholm Matthew F, Rouleau Christopher M, Lowndes Douglas H
Condensed Matter Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA.
Nature. 2005 Jan 27;433(7024):395-9. doi: 10.1038/nature03261.
Theoretical predictions--motivated by recent advances in epitaxial engineering--indicate a wealth of complex behaviour arising in superlattices of perovskite-type metal oxides. These include the enhancement of polarization by strain and the possibility of asymmetric properties in three-component superlattices. Here we fabricate superlattices consisting of barium titanate (BaTiO3), strontium titanate (SrTiO3) and calcium titanate (CaTiO3) with atomic-scale control by high-pressure pulsed laser deposition on conducting, atomically flat strontium ruthenate (SrRuO3) layers. The strain in BaTiO3 layers is fully maintained as long as the BaTiO3 thickness does not exceed the combined thicknesses of the CaTiO3 and SrTiO3 layers. By preserving full strain and combining heterointerfacial couplings, we find an overall 50% enhancement of the superlattice global polarization with respect to similarly grown pure BaTiO3, despite the fact that half the layers in the superlattice are nominally non-ferroelectric. We further show that even superlattices containing only single-unit-cell layers of BaTiO3 in a paraelectric matrix remain ferroelectric. Our data reveal that the specific interface structure and local asymmetries play an unexpected role in the polarization enhancement.
受外延工程领域最新进展的启发，理论预测表明，钙钛矿型金属氧化物超晶格中会出现大量复杂行为。这些行为包括应变导致的极化增强以及三元超晶格中出现不对称特性的可能性。在此，我们通过高压脉冲激光沉积技术，在具有原子级平整度的导电钌酸锶（SrRuO₃）层上，以原子尺度控制制备了由钛酸钡（BaTiO₃）、钛酸锶（SrTiO₃）和钛酸钙（CaTiO₃）组成的超晶格。只要BaTiO₃层的厚度不超过CaTiO₃层和SrTiO₃层的总厚度，BaTiO₃层中的应变就能完全保持。通过保持完全应变并结合异质界面耦合，我们发现，尽管超晶格中一半的层名义上是非铁电的，但相对于同样生长的纯BaTiO₃，超晶格的整体全局极化增强了50%。我们进一步表明，即使在顺电基质中仅包含单胞层BaTiO₃的超晶格也仍然是铁电的。我们的数据表明，特定的界面结构和局部不对称性在极化增强中发挥了意想不到的作用。
