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Diploma thesis

The title of my diploma thesis was

Colossal magnetoresistance: spectral properties of the Holstein double-exchange model and application to manganites

My work was supervised by
Prof. David Edwards (Imperial College London) and
Prof. Wolfgang von der Linden (TU Graz).

The manganites are a special class of transition-metal oxides. These compounds are extremely interesting due to the variety of physical effects which show up, for example, in their phase diagrams. In my thesis I investigated spectral properties of manganites in the doping regime where "colossal magnetoresistance" is observed.

If you are familiar with the physics of manganites the following abstract will give you enough information to decide if you want to read my thesis. The paper containing the main results of my work can be found in the list of publications.


(perovskite structure of LaMnO(3))

Abstract

The many-body coherent potential approximation is used to calculate one-electron spectral functions, optical conductivity and spin-wave energy in the Holstein double-exchange model. The effect of electron-phonon coupling on these properties is studied in detail and comparison is made with experimental data on manganites. Satisfactory agreement is obtained with angle-resolved photoemission results on La(1.2)Sr(1.8)Mn(2)O(7) and optical measurements on Nd(0.7)Sr(0.3)MnO(3). A pseudogap in the one-electron spectrum at the Fermi level plays an important role in both systems, but a small-polaron band is only predicted in the La system. The results suggest that in this bilayered manganite with, unusually strong electron-phonon coupling, small polarons exist in the ferromagnetic state. However, it is found that small-polaron theory does not apply above or below the Curie temperature Tc in a pseudocubic manganite like Nd(0.7)Sr(0.3)MnO(3) with intermediate coupling strength. The observed shift in spectral weight of the optical conductivity to lower energy on going into the ferromagnetic state is found to occur, although it is somewhat suppressed by spurious incoherent scattering at T=0 which is a defect of the theory. A rigorous upper bound on spin-wave energies at T=0 is derived. The spin-wave stiffness constant D decreases with increasing electron-phonon coupling g in a similar way to Tc, but D/(k_B Tc) increases for large g (low Tc) as observed experimentally.