Reemergent Metal-Insulator Transitions in Manganites Exposed with Spatial Confinement

The metal-insulator transition is characterized as a single peak in the temperature-dependent resistivity measurements; exceptions to this have never been seen in any single crystal material system. We show that by reducing a single crystal manganite thin film to a wire with a width comparable to the mesoscopic phase-separated domains inherent in the material, a second and robust metal-insulator transition peak appears in the resistivity versus temperature measurement. This new observation suggests that spatial confinement is a promising route for the discovery of emergent physical phenomena in complex oxides.

Figure 1

Transport and magnetic properties of 70 nm ${\mathrm{La}}_{0.325}{\mathrm{Pr}}_{0.3}{\mathrm{Ca}}_{0.375}{\mathrm{MnO}}_3$ thin film and wire with $⟨r_A⟩$ of 1.339 Å on lattice matched ${\mathrm{SrLaGaO}}_4$ on heating and cooling (indicated by arrows) under increasing magnetic fields. (a) Optical image of $10\mu \mathrm{m}$ geometry with expanded view of wire and legend of applied magnetic field direction and 4-point probe configuration. (b) Comparison of temperature dependence of resistivity in film at 0 T (black), 0.4 T (red), 1 T (blue), and 3.5 T (green). The film shows a single MIT peak. (c) Comparison of temperature dependence of resistivity in $10\mu \mathrm{m}$ wire at 1.5 T (black), 1.75 T (red), 2.25 T (blue), and 3.75 T (green). The wire shows a reemergent, low temperature peak in $\rho$ that is not present in the film. (Insets) Show differences in resistivity between warming and cooling curves.

Figure 2

Theoretical model for the ${\mathrm{La}}_{0.325}{\mathrm{Pr}}_{0.3}{\mathrm{Ca}}_{0.375}{\mathrm{MnO}}_3$ wire. (a) Schematic diagram of film and wire with corresponding resistive networks. Black and gray areas represent regions of differing resistivity with independent MIT temperatures. (b) Model data for the LPCMO resistivity in a serial resistive network using LPCMO with constituent $⟨r_A⟩$ of 1.334 and 1.339 Å. The resistivity at 125 K is dominated by the larger $⟨r_A⟩$, while the low-$T$ peak is contributed by the smaller $⟨r_A⟩$.

Figure 3

Transport and magnetic properties of $10\mu \mathrm{m}$ ${\mathrm{La}}_{0.391}{\mathrm{Pr}}_{0.234}{\mathrm{Ca}}_{0.375}{\mathrm{MnO}}_3$ wires with $⟨r_A⟩$ of 1.342 Å on substrates of varying compressive strain. (Top) LPCMO wire on ${\mathrm{SrLaGaO}}_4$ ($<0.1\%$ lattice mismatch) for 0 T (black), 1.5 T (red), and 3 T (blue). Hysteretic behavior is never present at lower temperature and is quickly suppressed with application of magnetic fields. (Center) LPCMO wire on ${\mathrm{NdGaO}}_3$ (+0.5% lattice mismatch) for 2.5 T (black), 2.75 T (red), and 3 T (blue). The slight increase in tensile strain pushes the MIT to a lower temperature but does not spawn a clear separation into two distinct MIT areas, which could be due to overlap. (Bottom) LPCMO wire on ${\mathrm{LaAlO}}_3$ ($-1.5\%$ lattice mismatch) for 0 T (black), 0.2 T (red), and 0.4 T (blue). Independent metal-insulator transitions are visible at both high and low temperatures with some overlap. (Insets) show differences in resistivity between warming and cooling curves.