  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 free carriers in the pyrochlore Eu2Ir2O7 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-insulator transition at T_N = 110 K is
manifested in the Drude spectral weight, which is independent of temperature in
the metallic phase, and which 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=4x10^7 cm/s, a logarithmic renormalization scale
Lambda_L=600 K, and require a Fermi temperature of T_F=100 K associated with
residual unintentional doping to account for the low temperature optical
response and dc resistivity.
