THE ELECTRODYNAMIC RESPONSE OF HEAVY-ELECTRON MATERIALS WITH MAGNETICPHASE-TRANSITIONS

We have investigated the electrodynamic response of the heavy-electron compounds U2Zn17, UPd2Al3, UCu5 and URu2Si2. Particular emphasis has been devoted to the optical evidence of the antiferromagnetic phase tr ansitions at T-N = 9.7 K, 14 K, 15 K and 17 K for U2Zn17, UPd2Al3, UCu 5 and URu2Si2, respectively. In the excitation spectrum of UCu5 and UR u2Si2, we found an absorption in the far-infrared, which develops belo w T-N and is ascribed to the excitation across a spin-density-wave typ e gap, suggesting that the antiferromagnetic phase transition might be itinerant in nature, and invokes a Fermi surface instability. Since t his gap-like feature is absent in U2Zn17 and UPd2Al3, we argue that th ese latter compounds belong to a characteristically different class of antiferromagnets, representative of the heavy-electron compounds with an ordering of essentially localized magnetic moments. The antiferrom agnetic ordering then leads to a suppression of the spin-flip mechanis m below T-N At low temperatures, we observe for all compounds the form ation of a narrow Drude-like resonance in the optical conductivity, wh ich is ascribed to the electrodynamic response of the heavy-quasiparti cles, and is indicative of the progressive development of the many-bod y coherent Kondo state, coexisting with both types of magnetic orderin g. In this review, we also present the evolution of the optical proper ties due to Ni- and Re-doping in UCu5 and URu2Si2, respectively. The o ptical evidence of the itinerant antiferromagnetic ordering is suppres sed in both compounds upon doping and particularly for the URu2Si2 com pound this is consistent with a crossover to a ferromagnetic ground st ate upon Re-doping.