From superlattices to supercrystals

The concept of order is a cornerstone of condensed matter physics. Atoms order to form crystals, which, in turn, play host to other fascinating and technologically useful ordering phenomena. For example, in ferroelectric crystals, tiny displacements of ions create electric dipoles that order cooperatively to produce a net electric polarization. The orientation of this polarization can be reversed by applying an electric field and serves as the basis for the operation of ferroelectric random access memories that exploit the different orientations of the polarization to store the 1 and 0 bits of information.

Thanks to advances in physical vapour deposition techniques, today, it is possible to create artificially ordered materials, such as superlattices, that consist of very thin sheets of different crystalline materials stacked periodically on top of each other. Artificially layered crystals that combine materials with similar structure but different properties in this way often lead to the emergence of new and unexpected behaviour. For example, in superlattices consisting of periodically alternating ferroelectric and dielectric layers, each just a few nanometres thick, electric dipoles often arrange themselves into complex nanoscale patterns of stripes and whorls. Such dipole patterns are extremely responsive to applied electric fields and give rise to unusual dielectric properties that may prove useful in reducing the power consumption of the billions of transistors in our everyday electronics.

Writing in the journal Nature Materials, a collaboration involving researchers from the LCN and the Department of Physics and Astronomy, and colleagues from France, Ireland and Czech Republic, have found that in some superlattices composed of layers of ferroelectric lead titanate (PbTiO3) separated by thin spacers of metallic or insulating oxides, the electric dipoles form an unusual pattern of nanoscale domains—regions with uniform orientation of the dipoles—that order in three dimensions to create a ‘domain supercrystal’. Under the influence of applied electric field, small displacements of the boundaries between the different domains give rise to large changes in the net electric polarization and thereby enhance the dielectric response of the material along all three spatial directions.

Perhaps an even more interesting aspect of the domain supercrystals is their highly inhomogeneous structure.  The complex domain patterns in the ferroelectric layers are accompanied by very strong distortions of the crystalline lattice that, in turn, set up a periodic modulation with very large local curvatures in the metallic or insulating spacers. This curvature modifies the local symmetry of the spacer layers, inducing additional polarity that could lead to interesting changes in their properties. Importantly, the curvature can be tuned by application of electric fields and its periodicity can be engineered on demand by adjusting the thicknesses of the ferroelectric layers, making such superlattices an ideal system for exploring curvature-induced phenomena in a variety of insulating, conducting and magnetic materials

Other contributors
Marios Hadjimichael, Gilbert A. Chahine, Michele Conroy, Kalani Moore, Eoghan N. O’ Connell, Petr Ondrejkovic, Pavel Marton, Jiri Hlinka, Ursel Bangert, Steven Leake
Attached image
Schematic of the polarization arrangement in a domain supercrystal and its corresponding X-ray diffraction pattern