Lattice-driven magnetoresistivity and metal-insulator transition in single-layered iridates

Sr${}_2$IrO${}_4$ exhibits an insulating state driven by spin-orbit interactions. We report two phenomena, namely, a large magnetoresistivity in Sr${}_2$IrO${}_4$ 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 (La${}^{3+}$) or hole (K${}^{+}$) doping for Sr${}^{2+}$ ions in Sr${}_2$IrO${}_4$. Our structural, transport, and magnetic data reveal that a strong spin-orbit interaction alters the balance between the competing energies so greatly that (1) the spin degree of freedom alone is no longer a dominant force, (2) the underlying transport properties delicately hinge on the Ir-O-Ir bond angle via a strong magnetoelastic coupling, and (3) a highly insulating state in Sr${}_2$IrO${}_4$ is proximate to a metallic state, and the transition is governed by lattice distortions that can be controlled via either the magnetic field or chemical doping.

Figure 1

The field-cooled magnetization for the $a$ axis and the $c$ axis, $M_a$ and $M_c$, as a function of (a) temperature at μ${}_{o}H$ $=$ 0.1 T, and (b) magnetic field at $T$ $=$ 1.7 and 100 K. (c) The magnetic anisotropy $M_a$/$M_c$ as a function of temperature. (d) The resistivity for the $a$-axis ln ρ${}_a$ as a function of 1/$T$. Inset in (a): Enlarged low-$T{M}_c$. Note that the data in (a) and (d) define regions I, II, and III.

Figure 2

The field dependence at $T$ $=$ 35 K of (a) the magnetization $M_a$ and $M_c$. (b) The $a$-axis resistivity ρ${}_a$ for $H$∥$a$ and $H$∥$c$. (c) The $c$-axis resistivity ρ${}_c$ for $H$∥$a$ and $H$∥$c$. (d) The schematics of the spin configuration for the basal plane (left-hand side) and the $ac$ plane (right-hand side).

Figure 3

The field dependence of the $a$-axis resistivity ρ${}_a$ for $H$∥$a$ and $H$∥$c$ at representative temperatures at (a) $T$ $=$ 10 K (region I), (b) $T$ $=$ 30 K, (c) $T$ $=$ 50 K, and (d) $T$ $=$ 100 K (approaching region III).

Figure 4

(a) The Ir1-O2-Ir1 bond angle θ as a function of La and K doping concentration $x$. The temperature dependence of (b) the $a$-axis resistivity ρ${}_a$, and (c) the $c$-axis resistivity ρ${}_c$ for (Sr${}_{1-x}$La${}_x$)${}_2$IrO${}_4$ with 0 ⩽ $x$ ⩽ 0.04; (d) The temperature dependence of ρ${}_a$ and $M_a$ at μ${}_{o}H$ $=$ 0. 1 T (right-hand scale) for (Sr${}_{0.98}$K${}_{0.02}$)${}_2$IrO${}_4$. Inset in (c): Enlarged low-$T$ ρ${}_c$.