  Sr2IrO4 exhibits a novel insulating state driven by spin-orbit interactions.
We report two novel phenomena, namely a large magnetoresistivity in Sr2IrO4
that is extremely sensitive to the orientation of magnetic field but exhibits
no apparent correlation with the magnetization, and a robust metallic state
that is induced by dilute electron (La3+) or hole (K+) doping for Sr2+ ions in
Sr2IrO4. Our structural, transport and magnetic data reveal that a strong
spin-orbit interaction alters the balance between the competing energies so
profoundly that (1) the spin degree of freedom alone is no longer a dominant
force; (2) underlying transport properties delicately hinge on the Ir-O-Ir bond
angle via a strong magnetoelastic coupling; and (3) a highly insulating state
in Sr2IrO4 is proximate to a metallic state, and the transition is governed by
lattice distortions. This work suggests that a novel class of lattice-driven
electronic materials can be developed for applications.
