Low-Energy Electrodynamics of Heavy Quasiparticles in ZrZn2

The temperature dependence of the optical conductivity spectrum of an itinerant ferromagnetic material, ZrZn${}_2$, was obtained and the dynamical effective mass and the scattering rate were derived. A renormalized Drude peak with a heavy effective mass developed at low temperatures. The effective mass rapidly increased below $\mathrm{\hslash }\omega$ = 10 meV and at 10 K it was observed to be 3 times heavier than that at 300 K. The scattering rate also rapidly decreased below 10 meV at 10 K. These results indicate the creation of heavy quasiparticles at low temperatures due to spin fluctuations.