Molecular Foundry Seminar

Domain Wall Functionality in a Complex Oxide: BiFeo₃

Dr. Jan Seidel, Materials Sciences Division of LBL and Department of Physics, UC Berkeley,

Tuesday, June 1st at 1:30 pm, Bldg. 67-Room 3111

Abstract:

Understanding and eventually manipulating electronic states in complex oxides is a major goal of contemporary condensed matter physics. Considerable attention is focused on spin-charge-lattice interactions, which provide a platform for control of various degrees of freedom, e.g. by applied currents, strain or electric/magnetic fields. Despite this attention, many aspects of these interactions are not fully understood, particularly at interfaces and topological boundaries, such as domain walls, where the same electronic properties are linked to the inherent order parameters of the material, its structure and symmetry.

I will present recent results on electrical and optical properties of ferroelectric domain walls in multiferroic BiFeO3 [1]. The origin and nature of the observed electrical conductivity at certain wall types is probed using a combination of conductive atomic force microscopy, high resolution transmission electron microscopy and first-principles density functional computations. I show that a structurally driven change in both the electrostatic potential and local electronic structure (i.e., a decrease in band gap) at the domain wall is linked to the observed electrical conductivity which can also be tuned by chemical doping.

In addition, I will discuss a mechanism for photovoltaic charge separation, which operates over a distance of 1–2 nm and produces voltages that are significantly higher than the bandgap in photovoltaic devices [2]. The separation happens at previously unobserved nanoscale steps of the electrostatic potential that naturally occur at such domain walls. Electric-field control over domain structure allows the photovoltaic effect to be reversed in polarity or turned off. This new degree of control, and the high voltages produced, may find application in optoelectronic devices.

[1] J. Seidel et al., Conduction at domain walls in oxide multiferroics, Nature Materials 8, 229 (2009)
[2] S.-Y. Yang et al., Above-bandgap voltages from ferroelectric photovoltaic devices, Nature
     Nanotechnology 5, 143 (2010)