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Surface Structure of Correlated Electron Materials: An inverse problem
The exotic behavior exhibited by many of the transition metal compounds, like
superconductivity [1,2], colossal magnetoresistance [3], ferroelectric [4] and multiferroic
behavior, are due to the close coupling between the many degrees of freedom in these
materials. Spin, charge, orbital and lattice parameters are coupled in an interesting
nonlinear fashion. Breaking the symmetry by creating a surface is an opportunity to probe
the details of the coupling in a controlled fashion and is of immense technological
importance, since any device will be created from thin film interfaces with broken
symmetry. As a consequence of their exotic and diverse physical properties, transition metal
oxide are considered to be the frontier of research on “emergent research device materials”
[5].
The lack of translational symmetry at a solid surface can result in lattice distortions
involving the topmost atomic layers. As a consequence these atomic rearrangements can
turn the surface into a completely new environment with different physical and chemical
properties. In the case of transition metal oxides, due to the subtle lattice-charge-orbitalspin balance, even very small lattice distortions can create different “surface phases”, with
surprising new physical properties very distinct from bulk behavior. Understanding the
relation between structure and physical properties (structural functionality) at the surface is
of both fundamental and technological interest. The LASiGMA programs is aimed at solving
the inverse problem, i.e. determining the surface structure of these complex materials from
the quantitative measurement of the diffraction of electrons from the surface [6]. There are
three computational problems that must be solved.
1) The use of non-spherical potentials in multiple scattering calculations. Present
calculations use spherical potentials which are not appropriate for the complex materials.
2) High performance LEED Theoretical calculations: The complexity of these materials,
i.e. the number of atoms in the unit cell requires advances in programming, such as
parallel processing of the analysis.
3) Global search methods for structure determination: The large size and complexity of
the structure of these materials means that there are many parameters required in
the search process. New codes need to be developed for this search process.
4)
References
[1] “The Theory of Superconductivity in the High-Tc Cuprate Superconductors”, P. W. Anderson,
Princeton University Press (August 4, 1997).
{2} New Iron based superconductors
[3] “Interplane Tunneling Magnetoresistance in a Layered Manganite Crystal.”, T. Kimura, Y. Tomioka, H.
Kuwahara, A. Asamitsy, M. Tamura, and Y. Tokura., Science 274, 1698-1701 (1996)
[4] “Enhanced of Ferroelectricity in Strained BaTiO3 Thin Films”, K. J. Choi, M. Biegalski, Y.L. Li, A.
Sharan, J. Schubert, R. Uecker, P. Reiche, Y.B. Chen, X.Q. Pan, V. Gopalan, L.-Q.Chen, D.G. Schlom,
C.B. Eom, Science 306, 1005 (2004).
[5] “An Emergent Change of Phase for Electronics”, Hidenori Takagi and Harold Y. Hwang, Science 327,
1601 (2010).
[6] “Low-Energy Electron Diffraction Experiment, Theory, and Surface Structure Determination”, M. A.
Van Hove, W. H. Weinberg, and C.-M Chan, (Springer Verlag, Berlin, 1986).