Surface octahedral distortions and atomic design of perovskite interfaces

Progress in understanding and exploiting the properties of complex oxide heterostructures requires advances in stateof- the-art growth and characterization techniques for these materials. Currently, atomic control of their synthesis is demonstrably inferior to that of their semiconductor counterparts, [ 1 ] despite recent progress for example in creating a high mobility two-dimensional electron gas in semiconducting ZnO [ 2 , 3 ] and in exploring novel interface effects in perovskites. [ 4 ] This superiority in growth control can be attributed in part to the development of quantitative, in situ refl ection high-energy electron diffraction (RHEED) possible in the UHV environment typical of semiconductor fi lm growth. [ 5 ] Oxides with the perovskite structure (chemical formula ABO 3 ) have attracted enormous interest as they display a wide range of exotic properties driven by strong spin-charge-lattice coupling, [ 6 ] such as quantum critical behavior in superconducting cuprates, colossal magnetoresistance in phase-separated manganites, and magnetoelectric coupling in BiFeO 3 . This coupling underlies the strong response of the electronic structure to small distortions of the BO 6 octahedra away from simple cubic coordination, distortions driven either by a Jahn-Teller (J-T) mechanism (removing energy level degeneracy by symmetry reduction) or by octahedral rotations to accommodate imperfect packing within the cubic unit cell that depends on cationic radii. These distortions are signifi cantly infl uenced in a stochastic way by disorder, unavoidably introduced by random A-site doping [ 7 ] or off-stoichiometry (ratio A:B ? 1, or oxygen vacancies) often seen in epitaxial films. In the search for novel electronic properties at interfaces, additional defect mechanisms such as cation intermixing and segregation must be addressed and their precise distribution near the interface known. Ultimately, identifying new electronic properties will depend critically on eliminating these forms of disorder and defects, or precisely controlling them if desired, through atomic design. Here we report evidence of signifi cant improvements in the atomic design of perovskite interfaces made possible by advances in in situ RHEED control of surfaces that exploit the detection of the characteristic octahedral distortions in the surface layer as it is being deposited. This method allows optimization of the phase diagram versus doping of films, and eliminates intermixing and anomalous lattice dilations at interfaces that have previously been observed. Careful structural analysis shows an unusual evolution of octahedral distortions including both J-T type and rotations near the interface not seen in bulk. These new results should be included in electronic structure calculations modeling the properties of real heterointerfaces.