Dynamics of heavy fermions: Drude response in UPd2Al3 and UNi2Al3

While the effective mass of heavy fermions governs their thermodynamics, the optical properties are dominated by the characteristic relaxation rate which is expected to scale inversely with the effective mass. At the relaxation rate clear features, the so-called Drude response occur in the real and imaginary parts of the complex conductivity. Conventional optical spectroscopy can only indirectly probe the Drude response; thus we use novel broadband microwave spectroscopy to directly measure the frequency-dependent conductivity of UPd2Al3 and UNi2Al3 in the relevant frequency range and unambiguously observe the full low-energy electrodynamics of the heavy fermions including the Drude response, and we find a relaxation rate much lower than previously determined for other heavy-fermion compounds. r 2006 Elsevier B.V. All rights reserved. PACS: 71.27.þa; 72.15.Lh; 78.20. e