We have spectroscopically investigated the thermally and doping-induced metal-insulator transitions for pyrochlore-type ${\mathrm{Nd}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ as well as its Rh-doped analogs ${\mathrm{Nd}}_2({\mathrm{Ir}}_{1-x}{\mathrm{Rh}}_x{)}_2{\mathrm{O}}_7$, where the spin-orbit interaction as well as the electron correlation is effectively tuned by the doping level ($x$). The charge dynamics dramatically changes on an energy scale of 1 eV in the course of thermally and doping-induced metal-insulator transitions, while the insulating ground state shows a small but well-defined charge gap of 45 meV. Anomalous doping variation of the low-energy ($<0.3\mathrm{eV}$) optical-conductivity spectra at the ground state can be interpreted in terms of the phase changes among the narrow-gap Mott insulator, Weyl semimetal, and correlated metal.

• Received 12 June 2012

DOI:https://doi.org/10.1103/PhysRevLett.109.136402