Paramagnetic labeling as a method for the soft spectroscopy of electronic states

Abstract

details of both the crystal-field states of the impurity ion and the electronic band spectrum of the metal. A new method for the soft spectroscopy of electronic states based on measurements of the temperature dependence of the width G MM 8 ( T) of transitions between the crystal-field states uM& of a paramagnetic ion implanted in the compound being studied is proposed. To make specific use of this method in neutron and optical spectroscopy, a classification of the types of temperature dependence of the natural relaxation width g M(T) of the levels is devised, and procedures for possible experimental methods are proposed. A nonzero value of the natural relaxation width g G(T) of the crystal-field ground state uG& of an impurity ion at zero temperature is obtained within the proposed self-consistent model, but is beyond the scope of perturbation theory. It is shown that the widely accepted estimate of the characteristic temperature of Kondo systems T*5G G(T50)/2 from the quasielastic scattering width at zero temperature G G(T50)/2 is incorrect in the case of strong relaxation in a system with soft crystal fields. The proposed model is applied to the quantitative analysis of the relaxation of the crystal-field levels of paramagnetic Pr 31 ions implanted in CeAl3 and LaAl3. The results of the calculations are in quantitative agreement with the experimental data. © 1998 American Institute of Physics.@S1063-7761~98!02005-8# The methods that have been developed for studying electronic states in metals ~angle-resolved photoemission spectroscopy; 1 quantum oscillations of the magnetic susceptibility, 2 conductivity, 3 magnetostriction, 4 and elastic moduli 5 associated with the de Haas‐van Alphen effect; infrared spectroscopy; 6 Raman scattering; 7 etc.! provide complementary information regarding the structure of electron spectra. A comparison of the experimental data obtained by different methods with the results of band calculations of the electronic structure provides fairly reliable data on the properties of the compounds studied. The methods for investigating electronic states can be divided into ‘‘hard’’ and ‘‘soft’’ methods. In the case of hard spectroscopy, the influence of the measurement process on the system exceeds the scales W* of the characteristic interactions forming the electronic spectrum of the system ~in Kondo systems W* is of the order of the Kondo temperature TK ; in variable-valence systems W* is of the order of the valence fluctuations !. Therefore, compounds with strong electron correlations, which have low-energy modes in the spectrum of elementary excitations, can be investigated most effectively by soft spectroscopic methods, in which the measurement process does not destroy the eigenstates of the system being investigated. The conditions imposed on spectroscopic measurements by the softness of the elementary excitations in variable-valence and Kondo systems greatly restricts the set of methods that are applicable to the investigation of highly correlated systems. For example, the interpretation of photoemission measurements ~because of the large energy transfers in the measurement process! and data from methods based on de Haas‐van Alphen oscillations ~because of the large magnetic fields, which can destroy the structure of soft excitations! requires a special investigation of the influence of the measurement process on the lowenergy properties of the compound being studied. Therefore, the development of new soft spectroscopic methods for highly correlated electronic systems is an important undertaking. This paper proposes a method for analyzing the electronic structure based on measurements of the temperature dependence of the relaxation of crystal-field levels of an impurity ion which has special properties ~a paramagnetic label! and is implanted in the compound being investigated. A similar idea for investigating semiconductor compounds by an electron paramagnetic resonance technique was proposed