First-order density-wave-like transitions in surface-doped ${\mathbf{Na}}_{\mathbf{2}}{\mathbf{IrO}}_{\mathbf{3}}$

We demonstrate that the surface of the honeycomb lattice iridate ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ is extremely tunable by plasma etching. We have succeeded in turning the surface of ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ metallic by argon plasma etching which leads to the removal of Na from the surface. The surface structure does not change in this process as revealed by grazing incidence small-angle x-ray scattering. The sheet resistance $R_s$ can be reduced by several orders of magnitude by varying the etching duration. Temperature-dependent $R_s\left(T\right)$ for the metallic samples shows signatures of spin- or charge-density-wave transitions with abrupt changes in $R_s$. Thermal hysteresis between cooling and warming measurements across the transition indicates a first-order transition. For the most metallic sample $R_s\left(T\right)$ data at low temperatures follow a $T^2$ behavior suggesting normal Fermi-liquid behavior.

Figure 1

A comparison of grazing incidence small-angle x-ray scattering patterns for ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ before and after varying periods of etching using an Ar plasma.

Figure 2

A semilogarithmic plot of the sheet resistance $R_s$ versus temperature $T$ for ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ after varying periods of etching using an Ar plasma.

Figure 3

A semilogarithmic plot of the sheet resistance $R_s$ versus temperature $T$ for two samples of ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ after 10 min of plasma etching.

Figure 4

Sheet resistance $R_s$ versus temperature $T$ for ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ after 20 min of Ar plasma etching. (a) $R_s$ versus $T$ measured in a zero magnetic field while cooling from $T=305\mathrm{K}$. The inset shows the cooling and warming data to highlight the thermal hysteresis indicating the first-order nature of the transition. (b) $R_s\left(T\right)$ measured while cooling from $T=305\mathrm{K}$ in various applied magnetic fields $H$. The inset shows the data close to the transition to highlight the field dependence at and below the transition.

Figure 5

Sheet resistance $R_s$ versus temperature $T$ for two ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ xtals after 30 min of Ar plasma etching. (a) $R_s$ versus $T$ of one xtal measured in a zero magnetic field while cooling from $T=305\mathrm{K}$. Inset I shows the cooling and warming data to highlight the thermal hysteresis indicating the first-order nature of the transition. Inset II shows the low-temperature data below $T=20\mathrm{K}$. The curve through the data is a fit to a $T^2$ dependence suggesting Fermi-liquid behavior. Inset III shows the $R_s\left(T\right)$ data measured in two magnetic fields to highlight the absence of any $H$ dependence. (b) $R_s$ versus $T$ of the second xtal measured in a zero magnetic field while cooling from $T=305\mathrm{K}$. The inset shows the low-temperature data below $T=20\mathrm{K}$. The curve through the data is a fit to a $T^2$ dependence suggesting Fermi-liquid behavior.