The Physics Seminar:Cengiz Sen, University of Tennessee
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  • The Physics Seminar:Cengiz Sen, University of Tennessee

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The Physics of Colossal Magnetoresistance and Multiferroicity in Realistic Models for Manganites                            
                                   Cengiz Sen
                           University of Tennessee
                     Oak Ridge National Laboratory

Abstract:
The interplay between the spin, charge and phononic degrees of freedom in condensed matter systems results in complex phase diagrams where several phases with similar energies are in competition with each other. In the vicinity of such competitions, small external perturbations lead to large responses.

In manganites, colossal magnetoresistance (CMR) is an example where magnetic fields of a few Teslas are enough to change the resistivity by orders of magnitude at the ferromagnetic transition (Curie) temperature. In this talk, we present results on one- and two-orbital spin-fermion-phonon models obtained with real-space Monte Carlo techniques[1,2], where CMR arises due to the competition between ferromagnetic metallic and charge-ordered insulating states. The results show a canonical CMR curve with a resistivity peak of more than 2 orders of magnitude, in excellent agreement with experiments. We investigate Monte Carlo evolutions of spin, charge and phononic degrees of freedom to identify the nature of the state above the Curie temperature, which shows evidence for nanoscale phase separation[2], in conjunction with recent experiments[3].
Furthermore, a recently proposed mechanism of multiferroicity in undoped manganites will also be presented[4]. This mechanism leads to a state with coexisting ferroelectic and magnetic properties, making it a potential candidate for technological applications.
In manganites, colossal magnetoresistance (CMR) is an example where magnetic fields of a few Teslas are enough to change the resistivity by orders of magnitude at the ferromagnetic transition (Curie) temperature. In this talk, we present results on one- and two-orbital spin-fermion-phonon models obtained with real-space Monte Carlo techniques[1,2], where CMR arises due to the competition between ferromagnetic metallic and charge-ordered insulating states. The results show a canonical CMR curve with a resistivity peak of more than 2 orders of magnitude, in excellent agreement with experiments. We investigate Monte Carlo evolutions of spin, charge and phononic degrees of freedom to identify the nature of the state above the Curie temperature, which shows evidence for nanoscale phase separation[2], in conjunction with recent experiments[3].
Furthermore, a recently proposed mechanism of multiferroicity in undoped manganites will also be presented[4]. This mechanism leads to a state with coexisting ferroelectic and magnetic properties, making it a potential candidate for technological applications."
[1] C. S. et al., Phys. Rev. Lett. 98, 127202 (2007).
[2] C. S. et al., in preparation.
[3] J. Tao et al., Phys. Rev. Lett. 103, 097202 (2009).
[4] I. A. Sergienko, C. S., and E. Dagotto, Phys. Rev. Lett. 97, 227204 (2006).
                      
Monday, 29 March 2010,  FENS G029,  13:40