Optical evidence for a Weyl semimetal state in pyrochlore Eu2Ir2O7

A Weyl semimetallic state with pairs of nondegenerate Dirac cones in three dimensions was recently predicted to occur in the antiferromagnetic state of the pyrochlore iridates. Here, we show that the THz optical conductivity and temperature dependence of the free carrier response in pyrochlore ${\mathrm{Eu}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ match the predictions for a Weyl semimetal and suggest novel Dirac liquid behavior. The interband optical conductivity vanishes continuously at low frequencies signifying a semimetal. The metal-semimetal transition at $T_N=110$ K is manifested in the Drude spectral weight, which is independent of temperature in the metallic phase and decreases smoothly in the ordered phase. The temperature dependence of the free carrier weight below $T_N$ is in good agreement with theoretical predictions for a Dirac material. The data yield a Fermi velocity $v_F\approx 4\times {10}^7$ cm/s, a logarithmic renormalization scale ${\mathrm{\Lambda }}_L\approx 600$ K, and require a Fermi temperature of $T_F\approx 100$ K associated with residual unintentional doping to account for the low temperature optical response and dc resistivity.