Temperature- and Magnetic-Field-Dependent Optical Properties of Heavy Quasiparticles in YbIr2Si2

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We report the temperature- and magnetic-field-dependent optical conductivity spectra of the heavy electron metal YbIr₂Si₂. Upon cooling below the Kondo temperature (T K), we observed a typical charge dynamics that is expected for a formation of a coherent heavy quasiparticle state. We obtained a good fitting of the Drude weight of the heavy quasiparticles by applying a modified Drude formula with a photon energy dependence of the quasiparticle scattering rate that shows a similar power-law behavior as the temperature dependence of the electrical resistivity. By applying a magnetic field of 6T below T K, we found a weakening of the effective dynamical mass enhancement by about 12% in agreement with the expected decrease of the 4f-conduction electron hybridization on magnetic field.