Abstract:
The physics of f electron systems is one of the most exciting topics of present day theoretical and experimental solid state research. A number of exciting phenomena can be attributed to the correlations of f electrons. These are strong hybridization, huge magneto-crystalline anisotropy, corn plex magnetic ordering, spin-fluctuation, Kondo effect, mixed valence, metal-insulator transitions, giant and colossal magneto-resistance, orbital ordering, superconductivity and heavy-fermion behavior. The rare earth compounds, having 4f electrons, have always been the topic of research for the condensed matter physicists due to their unusual magnetic and electronic properties. The experimental investigations of the physical properties of the rare earth compounds started about four decades ago when lanthanide elements became available and reasonable good samples (poly-crystalline and single crystal) were successfully prepared.
The theoretical description of these systems is a challenge for the physicists. In order to find the essential physics in these compounds, we have to first decide whether a localized or an itinerant picture is appropriate. The various properties of the f electron systems can be understood using two approaches. One is the model Hamiltonian approach and the other is the band structure approach. A number of model Hamiltonians have been used to study correlation between the f electrons e.g. Kondo model, Anderson model•and Hubbard model. The Kondo model was among the first proposed to describe the physics of f electrons system. An alternative model for magnetic alloys was proposed by Anderson. The Anderson model is more fundamental then the Kondo model. The Hubbard model is perhaps the simplest model which can be used to study the many body aspects of correlated electrons.
