The electrodynamic response of heavy-electron materials with magnetic phase 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 transitions at T-N = 9.7 K, 14 K, 15 K and 17 K for U2Zn17, UPd2Al3, UCu5 and URu2Si2, respectively. In the excitation spectrum of UCu5 and URu2Si2, we found an absorption in the far-infrared, which develops below T-N and is ascribed to the excitation across a spin-density-wave type gap, suggesting that the antiferromagnetic phase transition might be itinerant in nature, and invokes a Fermi surface instability. Since this gap-like feature is absent in U2Zn17 and UPd2Al3, we argue that these 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 antiferromagnetic ordering then leads to a suppression of the spin-flip mechanism below T-N At low temperatures, we observe for all compounds the formation of a narrow Drude-like resonance in the optical conductivity, which is ascribed to the electrodynamic response of the heavy-quasiparticles, and is indicative of the progressive development of the many-body coherent Kondo state, coexisting with both types of magnetic ordering. In this review, we also present the evolution of the optical properties due to Ni- and Re-doping in UCu5 and URu2Si2, respectively. The optical evidence of the itinerant antiferromagnetic ordering is suppressed in both compounds upon doping and particularly for the URu2Si2 compound this is consistent with a crossover to a ferromagnetic ground state upon Re-doping.