Prof Paul G Evans, Materials Science and Engineering, University of Wisconsin-Madison, USA

Picosecond-Scale Structural Dynamics in Ferroelectrics and Multiferroics: Exotic Polarization States and Structural Transformations
When Apr 23, 2018
from 02:00 PM to 03:00 PM
Where LR8
Contact Name
Contact Phone 01865-283446
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The electronic, optical, and magnetic properties of emerging ferroelectric and multiferroic materials depend on fascinating balance between atomic-scale effects associated with chemical bonding and longer-range interactions arising from mechanical stress and electrical polarization.  At the nanoscale the overall sizes of structures select the distances of elastic and polarization interactions and introduces novel phenomena such as the edge-induced alignment of polarization nanodomains. It is also rapidly becoming possible to vary the balance among competing effects dynamically, and to probe materials at the natural timescale of the fundamental processes associated with their variation. In nanoscale structures, the relevant times set by the speed of sound and the size of the devices are in the picosecond regime, where precise structural characterization is only now becoming available. I will discuss the development of structural probes using coherent x-rays produced by synchrotron x-ray light sources and free-electron lasers that now make it possible to discover how transformations in nanoscale multiferroics and ferroelectrics can be induced and to learn about their fundamental origins. I focus on the dynamics of two effects: a transformation between competing structural phases of the multiferroic compound BiFeO3 and the reconfiguration of an exotic few-nanometer-width polarization domain pattern in a ferroelectric/dielectric PbTiO3/SrTiO3 superlattice heterostructure. In both cases, the dynamics can be driven electrically, using the coupling between the electrical polarization and electric field, or optically at a much faster scale via ultrafast above-bandgap excitation. Resolving outstanding challenges in this area, including identifying the effects following the initial optical excitation is a multiscale problem that will require resolving how key mechanical, optical, and electronic effects are related.