Competing Orders and Disorder-Induced Insulator to Metal Transition in Manganites

Effects of disorder on the two competing phases, i.e., the ferromagnetic metal and the commensurate charge/lattice ordered insulator, are studied by Monte Carlo simulation. The disorder suppresses the charge/lattice ordering more strongly than the ferromagnetic order, driving the commensurate insulator to the ferromagnetic metal near the phase boundary in the pure case. Above the ferromagnetic transition temperature, on the contrary, the disorder makes the system more insulating, which might cause an enhanced colossal magnetoresistance as observed in the half-doped or Cr-substituted manganites. No indication of the percolation or the cluster formation is found, and there remains the charge/lattice fluctuations instead which are enhanced toward the transition temperature.

Figure 1

Phase diagram of ${\mathrm{L}\mathrm{n}}_{1/2}{\mathrm{B}\mathrm{a}}_{1/2}{\mathrm{M}\mathrm{n}\mathrm{O}}_3$ from Ref. 8. $T_{\mathrm{C}}$, $T_{\mathrm{C}\mathrm{O}}$, $T_{\mathrm{N}}$, and $T_{\mathrm{S}\mathrm{G}}$ denote the ferromagnetic, charge-ordering, antiferromagnetic, and spin-glass transition temperatures, respectively. The superscript (order) is for the ($\mathrm{L}\mathrm{n},\mathrm{B}\mathrm{a}$)-ordered materials. The shaded area shows the charge-ordered insulating state in the ordered materials. The hatched area represents the disorder-induced ferromagnetic phase.

Figure 2

(a)–(c) Phase diagram of model (1) for different strengths of the random potential: (a) $\Delta =0.0$; (b) $\Delta =0.1$; and (c) $\Delta =0.2$. The lines are guides for the eyes. The gray data and the lines in (b) and (c) are the results in (a) for comparison. (d)–(g) Density of states at $T=0.016$ for $L=8$ clusters: (d) $\Delta =0.0$ for various $g$; (e) $g=0.4$; (f) $g=1.6$; and (g) $g=0.8$ by varying $\Delta$. The Fermi energy is at $\omega =0$. Only the typical error is shown for each case.

Figure 3

Optical conductivity for $g=0.8$ at (a) $T=0.02$ and (b) $T=0.06$ ($L=6$). (c) Average of the square of the lattice distortion ($L=8$). See the text for details. (d) Schematic picture of the temperature dependence of the resistivity.

Figure 4

(a) Temperature dependence of the standard deviation of the square of the local lattice distortion at $\Delta =0.2$. Transition temperatures for FM (COI) in the absence of the disorder are indicated by the gray (black) arrows. (b) Susceptibility for the ($\pi ,\pi$) lattice ordering at $g=1.0$. Both are for $L=8$ clusters.