ELECTRONIC AND MAGNETIC PROPERTIES OF HEAVY FERMIONS AND MIXED-VALENCE SYSTEMS

Abstract

Some of the rare earth (RE) and actinide compounds show a variety of anomalous electronic and magnetic properties (For example Sm-compounds like SmB6,SmS; Ce-compounds like CePd3, CeAl2' CeA13' CeCu6; U-compounds like UA12' UPt3, UCd11 etc.). These compounds have got a broad conduction band and atomic-like f-orbitals lying within the conduction band. The f-states hybridize with the conduction states thus giving rise to the itinerant character to f-eiectrons. This s-f mixing gives rise to the anomalous phenomenon. These materials behave like Curie paramagnets above a temperature T*, showing the existence of well defined local f-moments on RE and actinide ions. Below T* the materials show a variety of behaviour. (a) The materials like a-Ce, Sm chalcogenides etc., in which RE ion coexists in two different valence states (termed as mixed valence (MV) systems), do not show magnetic ordering down to the lowest temperature. At low temperature these MV materials behave like Fermi liquids. Some MV materials like TmSe and UNi5-xCux for (4 < x < 5) in which Tm and U ions are in mixed valent states do show magnetic ordering at low temperature. (iv) We have studied variation of average valence and spin susceptibility with temperature for different values of hybridization and for different effective values of the f-level. When the f-level lies deep below the Fermi level, the susceptibility diverges in most of the temperature region for large U, while for weak interaction the divergence is reduced to a small temperature region. When the f-level lies near the Fermi energy, the susceptibility is finite at T=OK, increases linearly at low temperature and shows Curie-Weiss behavior at higher temperature. Further we have studied the temperature variation of electronic specific heat and electrical resistivity for . different values of hybridization and different positions of f-level. In the low temperature region, the specific heat increases sharply and shows a peak. The peak is quite sharp for small V, while the peak broadens for large V. The sharp increase in Cv and appearance of a peak are charac-teristic of large y and appearance of a gap near Fermi energy. As f-level is shifted down in the conduction band, the gap becomes more narrower and we have a more sharp peak at low temperature. We have used Cox formula to calculate the resistivity and find that the resistivity increase rapidly with increasing temperature in the low temperature region (Known as Fermi liquid region of coherence region), reaching a maxima and then decreasing slowly at higher temperatures. We conclude further that when f-level lies deep below the (v) Fermi level or hybridization V is reduced, the peak in low-temperatures resistivity becomes more sharp and moves towards low -T regime. It may be noted here that we have not consi-dered phonon contribution to resistivity, which is appreciable towards the higher temperature. Finally, in order to consider the possibility of magnetic ordering in these systems, we introduce a phenomeno-logical ferromagnetic exchange coupling among the nearest f-spins in the periodic Anderson model. Here one finds nonmagnetic as well as magnetic solution depending upon the relative strengths of exchange interaction J and mixing potential V. Here the exchange and hybridization compete in determining the magnetic-nonmagnetic ground state of these systems. This conclusion comes out while performing J-V phase diagrams. The effect of the exchange interaction J on the magnetic susceptibility is to increase the suscepti-bility without changing the overall characteristic of the susceptibility curves.