KRUGER1	Ferroelectricity in Manganites
Type	Theoretical
#students	1
Orientation	Manganite transition-metal oxides are remarkable materials that show complex ordering of charge, orbital, and magnetic degrees of freedom. A prominent example is the complex CE superstructure in La(1/2)Sr(3/2)MnO(4) [1], which is characterised by a simultaneous charge checkerboard ordering and formation of ferromagnetic zig-zag chains [2,3]. It has been suggested [4] that for doping levels slightly less than x=1/2 the chains deform, giving rise to a net electric polarisation. Such a state would break inversion symmetry and is predicted to be magnetic and ferroelectric at the same time. Compounds with an interdependence of magnetism and ferroelectricity could be very useful: they would open up a plethora of new applications, such as switching of magnetic memory elements by electric fields.
How	The aim of the project is to theoretically check this prediction. The student will analyse the doping dependence of the degenerate double exchange model in a mean-field approximation. This model, which correctly predicts the CE structure at x=1/2, describes local moments that are coupled to the electronic bands of the two degenerate e_g orbitals.
What	This project will form part of a larger collaboration with the experimental groups led by Prof Toby Perring and Dr Robin Perry at the ISIS pulsed neutron and muon source at the Rutherford Appleton Laboratory.
References	[1] D. Senff, F. Kruger, S. Scheidl, M. Benomar, Y. Sidis, F. Demmel, and M. Braden, Phys. Rev. Lett. 96, 257201 (2006) [2] E.O. Wollan and W.C. Koehler, Phys. Rev. 100, 545 (1955) [3] J.B. Goodenough, Phys. Rev. 100, 564 (1955) [4] D.V Efremov, J. van den Brink, and D.I. Khomskii, Nature Materials 3, 853 (2004)
Supervisor	Dr Frank Kruger, f.kruger@ucl.ac.uk