National Science Foundation Disorder Makes Materials Slower Paul G. Evans, University of Wisconsin-Madison, DMR Explanation: Electronic materials based on transition metal oxides can have highly desirable properties for electronics, but little is known about how quickly these materials can respond to applied electric fields. The UW-Madison team lead by Professor Paul Evans has used x-ray diffraction to study operating electronic devices consisting of alternating atomic layers of materials with different properties. The disorder in the structure of these materials dramatically slows their response to applied electric fields. University of Wisconsin-Madison graduate students Margaret Cosgriff and Pice Chen are using x-ray diffraction measurements at synchrotron light sources to study how the structure of materials changes in operating electronic devices. (courtesy of P. G. Evans) Outcome: Researchers at the University of Wisconsin-Madison have showed that the speed at which materials respond electrically depends the disorder in their structure. Impact: The development of a new generation of electronic devices based on exotic “complex oxide” materials depends on being able to understand and control how these materials respond to applied electric fields.

National Science Foundation Origin of Slow Piezoelectric Dynamics in Ferroelectric/Dielectric Superlattices Paul G. Evans, University of Wisconsin-Madison, DMR The properties of complex oxide superlattices depend on the in the interactions between the atomic-scale layers of their chemical components. Graduate students Margaret Cosgriff and Pice Chen have shown using synchrotron x-ray diffraction that superlattices composed of alternating layers of barium titanate (BaTiO 3 ) and calcium titanate (CaTiO 3 ) have large distortions of the oxygen octahedral building blocks within the CaTiO 3 layers. Crucially, this rotation pattern is highly disordered, with a coherence length equal to the superlattice period, resulting in broadened octahedral rotation diffraction peaks. Together the rotations and their disorder combine to slow the piezoelectricity of the superlattice. Understanding oxygen octahedral rotations and their dynamics can lead to a new degree of control of the properties of complex oxides in electronic devices. Oxygen octahedral rotations are located in reciprocal space at locations with half-integer indices. The breadth of these reflections indicates that the rotations are highly disordered. (courtesy of P. G. Evans)

National Science Foundation Field-Induced Structural Phase Transitions Paul G. Evans, University of Wisconsin-Madison, DMR Compressively strained thin films of the multiferroic material bismuth ferrite (BiFeO 3 ) can exhibit a number of competing structural phases. The synchrotron x-ray diffraction patterns of these films exhibit reflections from each phase, with intensities that correspond to their relative population. Graduate students Margaret Cosgriff and Pice Chen studied how the structure of BiFeO 3 thin films changes in applied electric fields and found that the resulting transitions between phases are (i) readily reversible and (ii) can occur with characteristic time constants of 100 ns. The results show that structural phase transitions in BiFeO 3 can be both fast and reversible. Possible applications of this effect include devices that can exploit the large changes in magnetic and dielectric properties accompanying the phase transition. BiFeO 3 diffraction pattern exhibiting reflections from several phases. Relative populations of phases are reversibly and rapidly changed by applied electric fields. (courtesy of P. G. Evans)