Pressure and magnetic field effects on metal-insulator transitions of bulk and domain wall states in pyrochlore iridates

We have explored metal-insulator phenomena for pyrochlore ${R}_{2}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}$, in which the electron correlation strength and magnetic configuration are systematically controlled by varying the average rare-earth ionic radius $(R={\mathrm{Nd}}_{1\ensuremath{-}x}{\mathrm{Pr}}_{x}$ and ${\mathrm{Sm}}_{y}{\mathrm{Nd}}_{1\ensuremath{-}y})$, external pressure, and magnetic field. Metal-insulator transitions in bulk are caused by increasing $x$ or tuning pressure, indicating that the effective correlation is responsible for the transition. The metallic state intervenes between the paramagnetic insulating and antiferromagnetically ordered insulating phases for $({\mathrm{Sm}}_{y}{\mathrm{Nd}}_{1\ensuremath{-}y}){}_{2}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}$ $(y=0.7--0.9)$, reminiscent of a first-order Mott transition. Furthermore, the metal-to-insulator crossover is observed (around $y=0.7$) for charge transport on magnetic domain walls in the insulating bulk. An application of magnetic field also drives metal-insulator transitions for $({\mathrm{Nd}}_{1\ensuremath{-}x}{\mathrm{Pr}}_{x}){}_{2}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}$, which is likely due to the modulation of Ir $5d$ band structures accompanied by a change in the spin textures. Abundant exotic states are revealed in the vicinity of metal-insulator transitions by finely tuning the physical parameters in ${R}_{2}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}$.