Metal-Insulator Transition in Ultrathin ${\mathrm{LaNiO}}_3$ Films

Transport in ultrathin films of ${\mathrm{LaNiO}}_3$ evolves from a metallic to a strongly localized character as the film’s thickness is reduced and the sheet resistance reaches a value close to $h/e^2$, the quantum of resistance in two dimensions. In the intermediate regime, quantum corrections to the Drude low-temperature conductivity are observed; they are accurately described by weak localization theory. Remarkably, the negative magnetoresistance in this regime is isotropic, which points to magnetic scattering associated with the proximity of the system to either a spin-glass state or the charge ordered antiferromagnetic state observed in other rare earth nickelates.

Figure 1

(a) Sheet resistance versus temperature for different film thicknesses. The dashed line corresponds to $\sim 25\mathrm{k}\Omega$, the quantum of resistance in 2D. Arrows mark the temperatures where upturns in the sheet resistance occur. (b) Resistivity versus temperature for a metallic 9 u.c. film. The dotted line is an extrapolation of the high temperature linear behavior. The inset shows the resistivity versus $T^2$ for temperatures below 25 K. (c) Logarithm of conductance as a function of $1/T^{1/3}$ for a 5 u.c. film for temperatures from 1.5 to 250 K. The black line is the linear fit to the data between 1.5 and 5 K.

Figure 2

Sheet conductance versus the logarithm of temperature for a 7 u.c. film. The linear fit to the data (black line) gives a slope of $1.1\times {10}^{-5}\mathrm{S}\approx e^2/\pi h$, hence $p=1$. If $p=3$, the data points should follow the gray dotted line. The inset shows the resistivity of the same sample on a linear scale.

Figure 3

(a) Perpendicular magnetic field dependence of the MR, $[R(H)-R(0)]/R(0)$, of a 7 u.c. film for several fixed temperatures. (b) Magnetoconductance as a function of magnetic field including fits to the data discussed in the text. The number of points and temperature curves were reduced for clarity. (c) Temperature behavior of the inelastic scattering length, extracted from the fits of the magnetoconductance for three different samples showing weak localization. The solid and dotted lines are theoretical curves for the case where $p=1$ and 3, respectively.

Figure 4

Magnetoresistance with the magnetic field applied parallel and perpendicular to the film plane, measured at 1.5 K for a 7 u.c. LNO film.