Temperaturabha?ngige elektronische Struktur und Magnetismus von metallischen Systemen mit lokalisierten Momenten Anwendung auf Gadolinium

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This thesis focuses on the theoretical investigation of the temperature dependent electronic and magnetic properties of metallic 4f-systems with localized magnetic moments. The presented theory is based on the Kondo-lattice model, which describes the interaction between a system of 4f-localized magnetic moments and the itinerant conduction band electrons. This interaction is responsible for a remarkable temperature dependence of the electronic structure mainly induced by the subsystem of 4f-localized moments. The many-body problem provoked by the Kondo-lattice model is solved by using a moment conserving Green function technique, which takes care of several special limiting cases. This method reproduces the T = 0-exact solvable limiting case of the ferromagnetically saturated semiconductor. The temperature dependent magnetic properties of the 4f-localized subsystem are evaluated by means of a modified Rudermann-Kittel-Kasuya-Yosida (RKKY) type procedure, which together with the solution of the electronic part allows for a self-consistent calculation of all the electronic and magnetic properties of the model. Results of model calculations allow to deduce the conditions for ferromagnetism in dependence of the electron density n, exchange coupling J and temperature T . The self-consistently calculated Curie temperature TC is presented and discussed in dependence of relevant parameters (J, n, and W) of the model. The second part of the thesis is concerned with the investigation of the temperature dependence of the electronic and magnetic properties of the rare-earth metal Gadolinium (Gd). The original Kondo-lattice model is extended to a multi-band Kondo-lattice model and combined with an ab-initio band structure calculation to take into account for the multi-bands in real systems. The single-particle energies of the model are taken from an augmented spherical wave (ASW) band structure calculation. The proposed method avoids the double counting of relevant interactions by exploiting the T = 0-exact limiting case of the ferromagnetically saturated semiconductor and takes into account the correct symmetry of the atomic orbitals. The only parameter of the theory (inter-band exchange coupling J) is uniquely fixed by the band structure calculation. The self-consistently derived Curie temperature of 294.1 K and the T = 0- magnetic moment of 7.71 µB are surprisingly close to the experimental values. The induced temperature-dependence of the 5d conduction bands explains respective experimental photoemission data.