DOE OSTI2011
When the size of materials is comparable to the characteristic length scale of their physical properties, novel functionalities can emerge. For semiconductors, this is exemplified by the 'superlattice' concept of Esaki and Tsu, where the width of the repeated stacking of different semiconductors is comparable to the 'size' of the electrons, resulting in novel confined states now routinely used in opto-electronics. For metals, a good example is magnetic/non-magnetic multilayer films that are thinner than the spin-scattering length, from which giant magnetoresistance (GMR) emerged, used in the read heads of hard disk drives. For transition metal oxides, a similar research program is currently underway, broadly motivated by the vast array of physical properties that they host. This long-standing notion has been recently invigorated by the development of atomic-scale growth and probe techniques, which enables the study of complex oxide heterostructures approaching the precision idealized in Fig. 1(a). Taking the subset of oxides derived from the perovskite crystal structure, the close lattice match across many transition metal oxides presents the opportunity, in principle, to develop a 'universal' heteroepitaxial materials system. Hand-in-hand with the continual improvements in materials control, an increasingly relevant challenge is to understand the consequences of the electrostatic boundary conditions which arise in these structures. The essence of this issue can be seen in Fig. 1(b), where the charge sequence of the sublayer 'stacks' for various representative perovskites is shown in the ionic limit, in the (001) direction. To truly 'universally' incorporate different properties using different materials components, be it magnetism, ferroelectricity, superconductivity, etc., it is necessary to access and join different charge sequences, labelled here in analogy to the designations 'group IV, III-V, II-VI' for semiconductors. As we will review, interfaces between different families creates a host of electrostatic issues. They can be somewhat avoided if, as in many semiconductor heterostructures, only one family is used, with small perturbations (such as n-type or p-type doping) around them. However, for most transition metal oxides, this is greatly restrictive. For example, LaMnO{sub 3} and SrMnO{sub 3} are both insulators in part due to strong electron correlations, and only in their solid solution does 'colossal magnetoresistance' emerge in bulk. Similarly, the metallic superlattice shown in Fig. 1(c) can be considered a nanoscale deconstruction of (La,Sr)TiO{sub 3} to the insulating parent compounds. Therefore the aspiration to arbitrarily mix and match perovskite components requires a basic understanding of, and ultimately control over, these issues. In this context, here we present basic electrostatic features that arise in oxide heterostructures which vary the ionic charge stacking sequence. In close relation to the analysis of the stability of polar surfaces and semiconductor heterointerfaces, the variation of the dipole moment across a heterointerface plays a key role in determining its stability. Different self-consistent assignments of the unit cell are presented, allowing the polar discontinuity picture to be recast in terms of an equivalent local charge neutrality picture. The latter is helpful in providing a common framework with which to discuss electronic reconstructions, local-bonding considerations, crystalline defects, and lattice polarization on an equal footing, all of which are the subject of extensive current investigation.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗