| speaker: | Hans-Albrecht Krug von Nidda |
| time: | We. 02.04.03, 11:00 - 12:00 |
The dynamic susceptibility of intermetallic compounds containing elements with partially filled 4f or 5f shells (e. g. Ce, Yb or U) carries important microscopic information on the hybridization of the localized f electrons with the conduction electrons (Kondo effect), which determines the low-temperature properties. Below a characteristic temperature T < T*, many of these systems behave like nonmagnetic metals with strongly enhanced effective electronic masses, i. e. heavy fermions like CeCu2Si2, CeNi2Ge2. Furthermore, some intermediate-valence compounds like CePd3 and CeOs2 develop a hybridization gap below T* with a reduced density of states at the Fermi energy. In Kondo insulators, like CeNiSn and Ce3Bi4Pt3, the gap is fully developed, and the density of states vanishes at the Fermi energy, leading to semiconducting behavior. The dynamic susceptibility can be probed by experiments, which are sensitive to the local electronic properties, like neutron scattering, nuclear magnetic resonance (NMR), and electron spin resonance (ESR). Indeed, a lot of Kondo-lattice systems have been investigated by NMR and neutron scattering, however only a few of the investigations were performed by means of ESR. The reason may be that usually the Kondo ion is not an appropriate ESR probe, because it relaxes too fast to give a measurable resonance signal. Hence, it is necessary to dope small amounts of ions with stable magnetic moments (e.g. Gd3+) into the compound under investigation. Nevertheless, ESR allows to probe the different ground states of Kondo systems, i. e. the enhancement of the density of states or the evolution of an energy gap. The talk will concentrate on the ESR experiments in heavy-fermion compounds and related materials, as well as diluted Kondo impurity systems, and compare to the results of NMR and neutron scattering