Epitaxial Strain and Superconductivity in ${\mathrm{L}\mathrm{a}}_{2-x}{\mathrm{S}\mathrm{r}}_x{\mathrm{C}\mathrm{u}\mathrm{O}}_4$ Thin Films

The report that $T_c$ was doubled in underdoped ${\mathrm{L}\mathrm{a}}_{2-x}{\mathrm{S}\mathrm{r}}_x{\mathrm{C}\mathrm{u}\mathrm{O}}_4$ films under compressive epitaxial strain has stirred great interest. We show that such films are extremely sensitive to oxygen intake, even at very low temperature, with startling consequences including colossal lattice expansion and a crossover from semiconductor to metallic behavior. We can bring $T_c$ up to 40 K in ${\mathrm{L}\mathrm{a}}_2{\mathrm{C}\mathrm{u}\mathrm{O}}_4$ films on ${\mathrm{S}\mathrm{r}\mathrm{T}\mathrm{i}\mathrm{O}}_3$ substrates—without any Sr doping and under tensile strain. On $\mathrm{L}\mathrm{a}\mathrm{S}\mathrm{r}{\mathrm{A}\mathrm{l}\mathrm{O}}_4$ substrates, we reached $T_c=51.5\mathrm{K}$, the highest so far in ${\mathrm{L}\mathrm{a}}_{2-x}{\mathrm{S}\mathrm{r}}_x{\mathrm{C}\mathrm{u}\mathrm{O}}_4$.

Figure 1

AFM plot (3D) of a 78-nm-thick LSCO film on LSAO. It shows terraces, about 300 nm wide, between one-unit-cell-tall steps, as a consequence of a slight ($0.1°$) miscut of the LSAO substrate. The rms surface roughness of this film was 0.3 nm over the area of $2500\mu {\mathrm{m}}^2$. Thinner films were even smoother.

Figure 2

X-ray data for the LSCO film shown in Fig. 1: (a) finite-thickness oscillations in the vicinity of the (004) reflection; (b) low-angle x-ray reflectance oscillations.

Figure 3

Temperature dependence of resistivity of (a) LCO film, and (b) LSCO film; as grown, annealed at $200°\mathrm{C}$ in vacuum, and annealed at $200°\mathrm{C}$ in ozone, respectively.

Figure 4

Temperature dependence of resistivity of a film consisting of a 20-nm-thick layer of LSCO covered with a 6.5-nm-thick layer of LCO, on a LSAO substrate.