Electronic and optical properties of La-doped $\mathrm{S}{\mathrm{r}}_3\mathrm{I}{\mathrm{r}}_2{\mathrm{O}}_7$ epitaxial thin films

We have investigated structural, transport, and optical properties of tensile strained ${\left(\mathrm{S}{\mathrm{r}}_{1-x}\mathrm{L}{\mathrm{a}}_x\right)}_3\mathrm{I}{\mathrm{r}}_2{\mathrm{O}}_7$ ($x=0$, 0.025, 0.05) epitaxial thin films. While high-$T_{\mathrm{c}}$ superconductivity is predicted theoretically in the system, we have observed that all of the samples remain insulating with finite optical gap energies and Mott variable-range hopping characteristics in transport. Cross-sectional scanning transmission electron microscopy indicates that structural defects such as stacking faults appear in this system. The insulating behavior of the La-doped $\mathrm{S}{\mathrm{r}}_3\mathrm{I}{\mathrm{r}}_2{\mathrm{O}}_7$ thin films is presumably due to disorder-induced localization and ineffective electron doping of La, which brings to light the intriguing difference between epitaxial thin films and bulk single crystals of the iridates.

Figure 1

(a) X-ray θ-2θ scans of the epitaxial ${\left({\mathrm{Sr}}_{1-x}{\mathrm{La}}_x\right)}_3{\mathrm{Ir}}_2{\mathrm{O}}_7$ ($x=0$, 0.025, 0.05) thin films grown on STO substrates, where only the ($00l$)-diffraction peaks of the thin films ($l=4$, 6, 10, 12, 14) are visible. The enlarged scans near ($00\underline{10}$) reflections of the thin films and the (002) reflections of the substrates are shown on the right. The peaks from the substrates are labeled with asterisk (*) symbols. (b) Reciprocal space map around the (103) reflection of the STO substrates with the ($10\underline{16}$) reflection of ${\left({\mathrm{Sr}}_{1-x}{\mathrm{La}}_x\right)}_3{\mathrm{Ir}}_2{\mathrm{O}}_7$ ($x=0$, 0.025, 0.05) thin films. (c) The rocking curve scan of ${\left({\mathrm{Sr}}_{1-x}{\mathrm{La}}_x\right)}_3{\mathrm{Ir}}_2{\mathrm{O}}_7$ thin films’ ($10\underline{16}$) reflections, which have a full width at half maximum of 0.2° for all thin films. (d) The out-of-plane lattice parameters of the ${\left({\mathrm{Sr}}_{1-x}{\mathrm{La}}_x\right)}_3{\mathrm{Ir}}_2{\mathrm{O}}_7$ thin films obtained from the x-ray diffraction are shown as a function of La concentration. The open circle, square, and diamond indicate the out-of-plane lattice parameters of ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7$ single crystals from Refs. [21, 29, 34], respectively.

Figure 2

(a) Temperature-dependent resistivity of the ${\left(\mathrm{S}{\mathrm{r}}_{1-x}\mathrm{L}{\mathrm{a}}_x\right)}_3\mathrm{I}{\mathrm{r}}_2{\mathrm{O}}_7$ thin films, which indicates that all of the thin films are insulators. The resistivities of $\mathrm{S}{\mathrm{r}}_2\mathrm{Ir}{\mathrm{O}}_4$ thin film [20] and $\mathrm{SrIr}{\mathrm{O}}_3$ thin film [37] on STO substrates are shown for comparison. (b) Logarithmic resistivities vs $T^{-1/4}$, which are offset along the $y$ axis for clarity, show that all of the ${\left(\mathrm{S}{\mathrm{r}}_{1-x}\mathrm{L}{\mathrm{a}}_x\right)}_3\mathrm{I}{\mathrm{r}}_2{\mathrm{O}}_7$ thin films display a 3D Mott-VRH conduction mechanism. The dashed lines indicate the linear fit.

Figure 3

Optical absorption spectra [α(ω)] of ${\left(\mathrm{S}{\mathrm{r}}_{1-x}\mathrm{L}{\mathrm{a}}_x\right)}_3\mathrm{I}{\mathrm{r}}_2{\mathrm{O}}_7$, $\mathrm{S}{\mathrm{r}}_2\mathrm{Ir}{\mathrm{O}}_4$ [20] and $\mathrm{SrIr}{\mathrm{O}}_3$ [37] thin films at room temperature. The plots are shifted vertically by ${10}^5\mathrm{c}{\mathrm{m}}^{-1}$ for clarity. The dotted black curves are the fit curves using Lorentz oscillators, which match well with the experimental spectra. The inset shows the fitted absorption spectra at low energy using the Wood-Tauc method.

Figure 4

(a) A cross-sectional Z-contrast STEM image of $\mathrm{S}{\mathrm{r}}_3\mathrm{I}{\mathrm{r}}_2{\mathrm{O}}_7$. The brightest spots are Ir atoms; Sr, Ti, and O atoms are faint due to their small atomic (Z) number. (b) The orange dashed rectangular region of the STEM image in panel (a) with ideal $\mathrm{S}{\mathrm{r}}_3\mathrm{I}{\mathrm{r}}_2{\mathrm{O}}_7$ unit cell. The schematic diagram of the unit cell is shown for clarity. Red dots represent Ir atoms in this and following images. (c) The yellow dashed rectangular region of the STEM image and schematic diagram showing the intergrowth of one single layer of SrO, shown by blue arrows, which causes a structural change from $\mathrm{S}{\mathrm{r}}_3\mathrm{I}{\mathrm{r}}_2{\mathrm{O}}_7$ to $\mathrm{S}{\mathrm{r}}_2\mathrm{Ir}{\mathrm{O}}_4$. (d) The blue dashed rectangular region of the STEM image and schematic diagram that indicates three separate atomic layers sequentially stacked into the page. The foremost layer (red) is the ideal structure, while the second (blue) and third (green) layers contain vertical intergrowth of single SrO layers as shown by the red arrows. These layers show a nonzero overlap as indicated in the rightmost diagram.