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Posted: Apr 20, 2012

Polarons in 2D

(Nanowerk News) In a collaborative work between the LCN, University of York and the National Renewable Energy Laboratory, scientists have discovered that the binary metal-oxide materials m-HfO2 and m-ZrO2 exhibit unexpected two-dimensional (2D) polaron behaviour. This discovery broadens the field of quasi-2D systems which often exhibit novel electronic properties.
Spin density distribution
Spin density distribution of a hole in a perfect monoclinic HfO2 (left) and after formation of a small polaron (right). The hole is trapped on 3-coordinated oxygen ions in both cases.
Polarons are quasiparticles comprised of a charge carrier (e.g. an electron or hole) and the accompanying polarisation of surrounding medium. In dielectric materials, such as oxides, the polarisation can be strong enough to cause the charge carrier to localise on a single anion (the charge is said to be self-trapped). The formation and properties of polarons in 2D systems is thought to play an important role in effects such as high temperature superconductivity, (photo-)catalysis and magnetism. However, predicting polaron formation in a particular material as well as their structure and properties remains extremely challenging for both experiment and computer simulations.
In their work published in Physical Review Letters ("Two-Dimensional Polaronic Behavior in the Binary Oxides m-HfO2 and m-ZrO2"), Alex Shluger, Keith McKenna, Matthew Wolf and colleagues have simulated the behaviour of polarons formed by holes in m-HfO2 and m-ZrO2 oxides. These materials contains two types of oxygen anions, three and four coordinated, that are separated from each other and respectively arranged in 2D layers within the crystal lattice.
Using improved versions of density functional theory they showed that holes self-trap only at three-coordinated oxygen anions and that the material exhibits 2D polaronic mobility. Such effects were previously thought only to occur in more complex oxide materials, such as high-temperature superconducting oxides, and at surfaces or interfaces. Investigation of the correlated dynamics and interaction of holes confined in these types of material may reveal interesting effects such as superconductivity, hole crystallisation, or magnetism which may deepen our understanding of these interesting and important phenomena.
Source: London Centre for Nanotechnology
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