New excitement with crystal-field excitations

The crystal field is one of the major interactions in rare-earth compounds. Neutron spectroscopy has become the key tool to measure the crystal-field transitions in metallic systems. This has been demonstrated for almost 1000 metallic rare-earth compounds in the past 30 years which resulted in a det ailed understanding of the various physical effects caused by the crystal-f ield splitting. One may conclude that the determination and description of crystal fields in metallic rare-earth systems is now well established and h as become a standard technique. Yet the past years have seen exciting devel opments in different applications where the crystal-field concept attained increasing and sometimes even crucial importance. This is exemplified for t wo applications: Firstly, the novel principle for cooling by adiabatic pres sure application which is based on the occurrence of a pressure-induced str uctural and/or magnetic phase transition where the point symmetry at the ra re-earth site is changed involving a change in the degeneracy of the crysta l-field states. Secondly, the observation of anomalies in the linewidth of crystal-field transitions in high-temperature superconductors which reveals direct information on the doping and isotope dependence of the pseudogap. (C) 2001 Elsevier Science B.V. All rights reserved.