Electron-doped ${\text{Sr}}_2{\text{IrO}}_{4-\delta }$ $\left(0\le \delta \le 0.04\right)$: Evolution of a disordered $J_{\text{eff}}=\frac{1}{2}$ Mott insulator into an exotic metallic state

Stoichiometric ${\text{Sr}}_2{\text{IrO}}_4$ is a ferromagnetic $J_{\text{eff}}=\frac{1}{2}$ Mott insulator driven by strong spin-orbit coupling. Introduction of very dilute oxygen vacancies into single-crystal ${\text{Sr}}_2{\text{IrO}}_{4-\delta }$ with $\delta \le 0.04$ leads to significant changes in lattice parameters and an insulator-to-metal transition at $T_{\text{MI}}=105\text{K}$. The highly anisotropic electrical resistivity of the low-temperature metallic state for $\delta \approx 0.04$ exhibits anomalous properties characterized by non-Ohmic behavior and an abrupt current-induced transition in the resistivity at $T^{\ast }=52\text{K}$, which separates two regimes of resistive switching in the nonlinear $I\text{−}V$ characteristics. The novel behavior illustrates an exotic ground state and constitutes a new paradigm for devices structures in which electrical resistivity is manipulated via low-level current densities $\sim 10\text{mA}/{\text{cm}}^2$ (compared to higher spin-torque currents $\sim {10}^7–{10}^8\text{A}/{\text{cm}}^2$) or magnetic inductions $\sim 0.1–1.0\text{T}$.

Figure 1

(Color online) A schematic of the Ir1-O2-Ir1 bond angle $\theta$ at $T=90\text{K}$ for (a) $\delta =0$ and (b) $\delta \simeq 0.04$ (the changes in $\theta$ are exaggerated for clarity); a schematic of the band structure corresponding to (c) $\delta =0$ and (d) $\delta \simeq 0.04$. Note that $\theta$ decreases for $\delta =0$ but slightly increases for $\delta \simeq 0.04$ as temperature is lowered.

Figure 2

(Color online) The $a$- and $c$-axis resistivities, ${\rho }_{\text{a}}$ and ${\rho }_{\text{c}}$, as a function of temperature for (a) $\delta =0$ and (b) $\delta \simeq 0.04$; (c) $c$-axis thermoelectric power, $S_{\text{c}}\left(T\right)$ for $\delta =0$ and $\delta \simeq 0.04$. Inset in (b): ${\rho }_{\text{a}}$ and ${\rho }_{\text{c}}$ vs $T$ for $1.7<T\le 20\text{K}$; note a downturn near 5 K in ${\rho }_{\text{c}}$ and linear temperature dependence in ${\rho }_{\text{a}}$.

Figure 3

(Color online) Temperature dependence of (a) $\text{log}{\rho }_{\text{a}}$ for several representative values of $\delta$, (b) the $a$- and $c$-axis magnetization, $M_{\text{a}}$ and $M_{\text{c}}$ for $\delta =0$ (thin lines) $\delta \simeq 0.04$ (thick lines) and ${\rho }_{\text{a}}$ (right scale) for $\delta \simeq 0.04$, and (c) ${\rho }_{\text{c}}$ at 0 and 7 T. Inset in (c): $\text{log}{\rho }_{\text{a}}$ vs $T^{-1/4}$.

Figure 4

(Color online) (a) Temperature dependence of ${\rho }_{\text{c}}$ for several representative values of current $I$. (b) $I\text{−}V$ curves at several representative temperatures and (c) temperature dependence of ${\rho }_{\text{c}}$ at $I=5$ and 14 mA and threshold $V_{\text{th}}$ (right scale).