E.Durgun"Department of Materials
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  • E.Durgun"Department of Materials

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Summer 2010 MAT Seminar Series

Polarization Vortices in GeTe Nanoplatelets : Door to New Generation Ferams 
                                                                                                                                        Engin Durgun

Department of Materials Science and Engineering, Massachusetts Institute of
Technology, 77 Massachusetts Avenue, Cambridge, MA USA

Ferroelectrics constitute a class of materials exhibiting a spontaneous macroscopic polarization,
which can be reverted in an external electric field. The remanent and switchable natures of their
polarization make these materials of direct interest for technological applications and are
exploited in non-volatile ferroelectric random access memories (FeRAM). The use of
nanostructures is a promising way of significantly increasing the storage density of FeRAM.
However, this foreseen benefit hinges on whether ferroelectricity still exists in low-dimensional
structures. Albeit significant efforts have been devoted to the characterization and understanding
of ferroelectric nanodots, the field is still at an incipient stage. At the theoretical level,
calculations were based either on a model Hamiltonian approach or on phenomenological
Landau theory but no fully atomistic ab initio calculations, including all degrees of freedoms,
have been performed to date. Furthermore these analyses are limited only to the family of ABO3
perovskite compounds. Here, we discovered that an unusual phase transition exists for an
alternative type of ferroelectric material, namely in zero-dimensional germanium telluride, GeTe
nanoplatelets by using state-of-the-art first-principles calculations based on density functional
theory. We demonstrate, in the interior of sufficiently large dots, the existence of polarization
vortices giving rise to a net and reversible toroidal moment of polarization, G control of which
would opens new opportunities for data-storage applications. The amplitude of G decreases with
the size of the system and is totally suppressed below a critical diameter of 2.7 nm. We revealed
the existence of a surface region within which the atoms behave differently and suppress the
formation of vortex pattern and then highlight the important role of inhomogeneous strain in
stabilizing polarization vortices for smaller sizes. Our results are promising in the sense of
designing ultra-high capacity memory device applications.

Wednesday, 6th of October at 13.40