Delicate competing electronic states in ultrathin manganite films

The coupling between the electrical transport properties of $\mathrm{L}{\mathrm{a}}_{2/3}\mathrm{S}{\mathrm{r}}_{1/3}\mathrm{Mn}{\mathrm{O}}_3$ (LSMO) thin films and structural phase transitions of $\mathrm{SrTi}{\mathrm{O}}_3$ (STO) substrates at $T_{\mathrm{s}}=105\mathrm{K}$ has been investigated. We found that the electrical resistivity of LSMO films exhibit a “cusp” at $T_{\mathrm{s}}$, which is greatly amplified by tuning films to the verge of metallic and insulating phases, i.e., to the boundary of two delicate competing electronic states. Our results demonstrate that small amounts of strain can tip the subtle balance of competing interactions and tune the electronic properties in correlated electron materials.

Figure 1

(a, b) Temperature ($T$) dependence of the electrical resistivity (a) of the 6-u.c. LSMO film grown at 130 mTorr and its derivative (b). The arrows indicate the transition temperature $T_{\mathrm{s}}$. (c) Temperature dependence of the electrical resistivity in the vicinity of $T_{\mathrm{s}}=105\mathrm{K}$ for films with different thickness grown at 130 mTorr.

Figure 2

$T$-dependent resistivity of (a) 6-u.c. LSMO films grown at different oxygen pressure; (b) different thickness LSMO films capped by 2-u.c. STO grown at 130 mTorr; and (c) 5-u.c. and 6-u.c. LSMO films under different magnetic fields. The magnetic field was normal to the film surface.

Figure 3

(a) Definition of the turning angle Θ. $k_1$ and $k_2$ are the first derivative of resistivity over temperature from below $\left({\frac{d\rho }{dT}|}_{0-}\right)$ and above $\left({\frac{d\rho }{dT}|}_{0+}\right)T_{\mathrm{s}}$, respectively. Θ is the angle between $k_2$-normalized $k_1$ and $k_2$ slope lines, i.e., $\mathrm{\Theta }\equiv {45}^{\circ }-\mathrm{ta}{\mathrm{n}}^{-1}\left(k_1/k_2\right)$. (b) The definition of β, which is the slope of the curve in the Arrhenius plot ln ρ vs 1/$T$. The sample (6, 130, 0) is used for a demonstration for (a) and (b). (c) Arrhenius plot for a series of insulating LSMO samples. (d) Θ as a function of $k_{\mathrm{B}}\beta$, where $k_{\mathrm{B}}$ is the Boltzmann constant.

Figure 4

(a) ln(ρ) vs 1000/$T$ near $T_{\mathrm{s}}=105\mathrm{K}$ for 6-u.c LSMO film grown at $P_{\mathrm{O}}=130\mathrm{m}\mathrm{Torr}$. (b) Lattice constants of STO versus temperature (after Ref. [25]).