Sub-Atomic Movements of Domain Walls in Crystal-Lattice Potential
Monday, October 25, 2004 - 11:00am - 11:50am
Andre Geim (University of Manchester)
The discrete nature of the crystal lattice bears on virtually every material property but it is only when the size of condensed-matter objects - e.g., dislocations, vortices in superconductors, domain walls in magnetic materials - becomes comparable to the lattice period, that the discreteness reveals itself explicitly. The associated phenomena are usually described in terms of the Peierls (?atomic washboard?) potential, which was first introduced for the case of dislocations at the dawn of the condensed-matter era. Since then, it has been invoked in many situations to explain certain features in bulk properties of materials but never observed directly. We have succeeded for the first time to monitor experimentally how a single domain wall moves through individual peaks and troughs of the atomic landscape. The wall becomes trapped between adjacent crystalline planes, which results in its propagation by distinct jumps matching the periodicity of the Peierls potential (Nature 426, 812, 2003). As a domain wall moves from one Peierls valley to another, it becomes unexpectedly flexible at Peierls ridges, which we attribute to atomic-size kinks propagating along a wall in the transient bistable position. The physics of topological defects at this true atomic scale is badly understood and seems to require a theory beyond the existing models.