A. Ramakanth

Self-energy approach to the correlated Kondo lattice model

We develop an interpolating self-energy approach to the correlated Kondo lattice model. The correlation of the band electrons is taken into account by a Hubbard interaction. The method is based on a self-energy ansatz, the structure of which allows one to fulfill a maximum number of exactly solvable limiting cases. The parameters of the ansatz are fitted to spectral moments via high-energy expansion of the self-energy. The band electron correlations are taken into account by an effective medium approach, being correct in the strong coupling (U) regime. The theory is considered reliable for all temperatures, band occupations, and exchange couplings. Results are presented for the respective dependences of spectral densities, quasiparticle densities of states, and characteristic correlation functions, and interpreted in terms of elementary spin exchange processes between itinerant conduction electrons and localized magnetic moments. The appearance of magnetic polarons, the typical quasiparticle of Kondo lattices, in the energy spectrum is worked out. Spin exchange processes prevent a total spin polarization of the band electrons even for arbitrarily strong exchange couplings as long as the local moments are represented by quantum mechanical spins.

Thermal expansion of mixed valence compounds

Abstract The anamolous thermal expansion of mixed valent systems is studied using the periodic Anderson model (PAM). The coefficient of thermal expansion is related to the temperature derivative of the 4f-level occupancy. The 4f-level occupancy and its temperature derivative have been evaluated self-consistently using the PAM in the mean-field approximation for a wide range of temperatures. Thereby, the interpolation, the earlier authors employed to obtain a peak from the low and high temperature results, is avoided. The order of magnitude and the qualitative behaviour obtained here are in agreement with earlier results.

Carrier Induced Ferromagnetism in Concentrated and Diluted Local-Moment Systems

For modeling the magnetic properties of concentrated and diluted magnetic semiconductors, we use the Kondo-lattice model. The magnetic phase diagram is derived by inspecting the static susceptibility of itinerant band electrons, which are exchange coupled to localized magnetic moments. It turns out that rather low band occupations favor a ferromagnetic ordering of the local moment systems due to an indirect coupling mediated by a spin polarization of the itinerant charge carriers. The disorder in diluted systems is treated by adding a CPA-type concept to the theory. For almost all moment concentrations

Ferromagnetism within the periodic Anderson model : A new approximation scheme

x

On the semiconductor-metal transition in a model for mixed valence systems

, ferromagnetism is possible, however, only for carrier concentrations

Non-linear magnetic response of intermediate valence state

n

Interplay of magnetic order and Jahn–Teller distortion in a model with strongly correlated electron system

distinctly smaller than

Ground state properties of the periodic Anderson model

x

Re-entrant and spin-glass-like behaviour of the Anderson lattice

. The charge carrier compensation in real magnetic semiconductors (in

REENTRANT-LIKE BAND JAHN–TELLER EFFECT AND ITS FIELD DEPENDENCE

{\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{As}