The advent of light scattering being applied to the spectroscopic investigation of metals occurred about a decade ago. Case studies have focused on layered charge-density-wave materials, superconductors, graphite intercalation com- pounds, thin metallic layers and metallic superlattices. Reviews of these topics can be found in the books edited by Cardona and Giintherodt (1982b, 1989).
In this article we give a status report on the progress that has been made in applying light scattering to various rare-earth (R) intermetallic compounds with the emphasis on Kondo- and intermediate valence-type materials. In this field of research, light scattering can now provide detailed information about localized electronic excitations [such as crystalline electric field (CEF) excitations, spin- orbit-split (J) multiplet excitations], phononic and elastic properties. This has not been an easy task, since for the materials under investigation the penetration depth of the light is of the order of about 100 A, yielding a rather small scattering volume. In particular we want to show how light (Raman and Brillouin) scattering has become a valuable complementary tool compared to neutron scattering with the advantages of high resolution (~<0.5 meV), strict symmetry selection rules, high (<1000meV) as well as extremely small (>0.03 meV) observable energy losses, small usable sample size (~<lmm 3) and independence of rare-earth isotope. Moreover, Brillouin scattering from surface wave excitations of metals offers the possibility of determining the elastic constants of small (<1 mm 3) or irregularly shaped samples, which otherwise are not accessible by ultrasonic measurements. The disadvantage of light scattering being limited to small wavevectors of the excitations, is irrelevant in the case of localized electronic excitations. Difficulties owing to the small penetration depth of the light can be overcome by careful surface preparation techniques, such as cleaving or fracturing under an inert-gas atmosphere.
The paper will be organized according to the types of excitations in rare-earth intermetallics that have been investigated successfully by light scattering. Starting with intraconfigurational and interconfigurational localized electronic excitations in sect. 2.2 we turn to crystal-field excitations in sect. 2.3. Section 3 is devoted to lattice dynamics in rare-earth intermetallics, that is the investigation of optical phonon modes by Raman scattering and the investigation of elastic constants by Brillouin scattering. Finally, in sect. 4 we shall deal with modes of electronic as well as phononic character resulting from strong electron-phonon coupling.
LIGHT SCATTERING IN INTERMETALLIC COMPOUNDS 165 2. Light scattering from 4f localized electronic excitations
2.1. Introduction
Among the various classes of lanthanide intermetallic compounds those ex- hibiting intermediate-valence or Kondo behavior have received much attention over the last decade. The occurrence of two different energetically degenerate configurations of the lanthanide atoms (4fn5d16s 2 and 4fn+16s2), together with the strongly localized character of the 4f shell are responsible for this seemingly irregular pattern of properties exhibited by these compounds. Despite the near degeneracy of the 4f, 5d and 6s binding energies allowing for 4f interconfigura- tional excitations (4fn+16s2--~4fn5d16s2), the spatial localization of the 4f elec- trons inside the filled 5s and 5p orbitals of the Xe cores, as discussed, e.g., by Goldschmidt (1978), is the reason why the 4f states largely retain their highly correlated, atomic nature after compound formation. These particular properties of the 4f shell, apart from being the cause of some of the most interesting aspects of lanthanide research, are also the source of the problems posed towards a quantitative description of the electronic structure of these materials. These problems arise because of the inadequacy of the conventional methods of band structure calculation for handling correlated many-electron states. On the other hand, descriptions of the intermediate valence 4f shell on the basis of an ionic picture, characterized by singlet-ion intraconfigurational J-multiplet level excita- tions and crystal-field excitations, taking into account hybridization perturbatively is the appropriate way to describe Kondo and intermediate-valence (IV) behavior (M/iller-Hartmann et al. 1984, Kasuya 1985, Gupta and Malik 1987). The ionic picture was promoted for the first time in the interconfigurational fluctuation (ICF) model (Hirst 1970).
In this model the interaction between two 4f configurations, both described by their J-multiplet level structure, is parameterized first by the interconfiguration excitation energy (Ex), denoting the energy difference between the ground states of the two J-multiplet systems, and second by the fluctuation temperature (T 0 or interconfigurational mixing width. This model has been applied to the analysis and interpretation of experimental data (Wohlleben 1984, R6hler et al. 1982a, Wittershagen and Wohlleben 1985). A schematic picture of this model is given in fig. 1. In any case these modified low lying many-particle electronic excitations (J multiplet level excitations, crystal-field excitations, interconfigurational excita- tions) profoundly influence the thermodynamic and transport properties of IV and Kondo compounds. Their experimental determination serves not only as a spectroscopic test of microscopic theories but is an indispensable prerequisite of any understanding of the fascinating and numerous macroscopic properties of these compounds.
A more comprehensive presentation of the many experimental and theoretical aspects of IV and Kondo compounds as well as a compilation of the original references can be found in a number of conference proceedings and review articles as given, e.g., by Wachter and Boppart (1982), M/iller-Hartmann et al.
(1984), Kasuya (1985) and Gupta and Malik (1987).
166 E. ZIRNGIEBL and G. G U N T H E R O D T
J' T
L f n ~ \ 4.f ~-1 + e (EF)
Fig. 1. Schematic energy level diagram of the in- terconfigurational fluctuation (ICF) model describ- ing valence fluctuations between two 4f configura- tions (4f", 4f"-1), characterized by their Jmultiplet level structure. The basic parameters of the ICF model E x and Tf denote the interconfigurational excitation energy and interconfigurational mixing width, respectively.
The application of Raman scattering to the lanthanide Kondo and IV com- pounds has resulted in a fruitful area of research as judged from its contributions to the understanding of these materials and from the large number of publica- tions. In this section we review Raman scattering experiments that deal with the determination of intraconfigurational excitations (J-multiplet and crystal-field- level excitations), as well as interconfigurational excitation energies of valence unstable lanthanide compounds.
2.2. Electronic Raman scattering in Sml_~ Y~Se and Sm a-~ Y~ S
Raman scattering in rare-earth chalcogenides has been reviewed by G/intherodt and Merlin (1984b). The Sm chalcogenides will be mentioned in this section for the sake of comparison with stable valence reference compounds and for demon- strating the configurational crossover from stable to fluctuating valence. Spin- orbit- and crystal-field-split levels of 4f states of lanthanide ions in insulating hosts have been extensively studied by Koningstein (1967) and Koningstein and Gr/inberg (1971) by means of electronic Raman scattering. In this subsection we discuss electronic Raman scattering from 4f spin-orbit-split levels in Sml_xRxSe and Sml_zRxS solid solutions in the vicinity of the 4f configuration crossover from stable valence to the intermediate-valence (IV) state. Such investigations have a direct bearing not only on the valence fluctuation problem, but also on the fundamentals of Raman scattering.
The ground state of S m 2+ in the Sm monochalcogenides is 4f6(7Fj = 0, 1 , . . . , 6), with a 0.6 eV wide spin-orbit-split
7Fj
multiplet. Electronic Raman scattering from the different J-multiplet levels of cleaved (100) faces of semicon- ducting SmSe (G/intherodt et al. 1981a) is shown in fig. 2. The odd-Y levels show up for perpendicular incident (El) and scattered (Es) polarization vectors, where-LIGHT SCATI'ERING IN INTERMETALLIC COMPOUNDS 167
i i II i ifl~ ,l~i i Ill ,
N Sm Se 80 I<
- - E i / E s J=3 J=A 1 ... Ei II Es . i l/ t