Optical spectroscopy of intermediate valence and heavy fermion materials

Abstract

Intermediate vaience materials form a class of Compounds which always contain ions of rare earth or actinide elements. The hybridization of the 4/or 5/ states of such elements with the conduction electrons, which generally have d character, results in a stable quantum—mechanic configuration which is intermediate between the two integer valent configurations 4(5)/n 5d° and 4(5)/s1 bd1. Narrow bands develop at the Fermi energy; they possess strongly / character. The strong electron-electron interaction, characteristic of / electrons, is in competition with the magnetic interaction between the periodically arranged ions. The metallic Systems called heavy fermions constitue a limiting case for which the interactions enhance the effective density of states of the narrow bands around the Fermi level up to values hundreds or thousands times larger than in normal metals. One can also speak of quasiparticles (where the quasiparticle is an electron dressed by all many body interactions) eharaeterized by correspondly enhanced effective masses. We present the results of optical reflectivity measurements performed in a wide energy range and at low as well as at room temperature on some of these Compounds: CePd3 (which is an intermediate valent metal), UPt3 and CeCu6 (heavy fermion Systems with a linear coefficient of the specific heat rising, respectively, to 450 and 1550 mJ/K2mol). In order to compare the results of CeCu6r reflectivity measurements on the reference Compound LaCu6 are also presented. The analysis of the infrared part of the spectrum (down to about 1 meV, 1 mm wavelength) gives us the values of the plasma frequency of the carriers in the narrow quasiparticle bands and the energy of the first electronic excitations together with the effective mass of the quasiparticles and the contribution to the dielectric funetion due to the electron-electron Screening. CePd3 at low temperature exibits a relatively small plasma frequency o^ = 0.34 eV. The mass enhancement has been evaluated to be about 40 times the electronic mass. This value is still small compared to m*= 250 m or m*= 700 m, observed, respectively for UPt3 and CeCu6. These numbers are obtained in the approximation of one narrow (4 meV or 2 meV) parabolic band at the Fermi level. However the plasma frequencies a£= 280 meV for UPt3 and o^= 150 meV for CeCu6 agree very well with the data of de Haas—van Alphen experiments.