RECENT DEVELOPMENTS IN HIGH-ENERGY SPECTROSCOPIES OF KONDO SYSTEMS

In this paper we review the present status of electron spectroscopy of Ce and Yb Kondo systems, with particular emphasis on photoemission an d inverse photoemission. Our starting point is the situation in the mi d-1980s, characterized by the development of a 'Kondo scenario' for th e thermodynamic and spectral properties, as described in the review by Allen et al. (1986, Adv. Phys., 35, 275). That picture, based on room -temperature results and on a T=0 approach to the impurity Anderson Ha miltonian, has been generally confirmed and better defined over the pa st decade by a number of more elaborate experiments, and by new calcul ational schemes. The use of high-energy resolution and cryogenic tempe ratures has led to the direct observation of the characteristic low-en ergy excitations and has opened the way to stringent tests of the most fundamental aspect of model, namely its scaling properties. Despite t he success of the Kondo approach, more work remains to be done. The im proved quality of the data has revealed some quantitative discrepancie s with the predictions of the model, stressing the need for more reali stic theoretical schemes. Moreover, some recent experiments questionin g the validity of the model have stirred a fierce controversy within t he spectroscopic community. We review those conflicting results and di scuss the possible reasons for the discrepancies, and how new experime nts could solve the present difficulties.