  Sr3Ir2O7 exhibits a novel Jeff=1/2 insulating state that features a splitting
between Jeff=1/2 and 3/2 bands due to spin-orbit interaction. We report a
metal-insulator transition in Sr3Ir2O7 via either dilute electron doping (La3+
for Sr2+) or application of high pressure up to 35 GPa. Our study of
single-crystal Sr3Ir2O7 and (Sr1-xLax)3Ir2O7 reveals that application of high
hydrostatic pressure P leads to a drastic reduction in the electrical
resistivity by as much as six orders of magnitude at a critical pressure, PC =
13.2 GPa, manifesting a closing of the gap; but further increasing P up to 35
GPa produces no fully metallic state at low temperatures, possibly as a
consequence of localization due to a narrow distribution of bonding angles
{\theta}. In contrast, slight doping of La3+ ions for Sr2+ ions in Sr3Ir2O7
readily induces a robust metallic state in the resistivity at low temperatures;
the magnetic ordering temperature is significantly suppressed but remains
finite for (Sr0.95La0.05)3Ir2O7 where the metallic state occurs. The results
are discussed along with comparisons drawn with Sr2IrO4, a prototype of the
Jeff = 1/2 insulator.
