Orbital-Order-Induced Metal-Insulator Transition inLa1−xCaxMnO3

We present evidence that the insulator-to-metal transition in ${\mathrm{L}\mathrm{a}}_{1-x}{\mathrm{C}\mathrm{a}}_x{\mathrm{M}\mathrm{n}\mathrm{O}}_3$ near $x\sim 0.2$ is driven by the suppression of coherent Jahn-Teller distortions, originating from $d$-type orbital ordering. The orbital-ordered state is characterized by large long-range $Q2$ distortions below $T_{{\mathrm{O}}^{\prime }\mathrm{\text{−}}{\mathrm{O}}^{*}}$. Above $T_{{\mathrm{O}}^{\prime }\mathrm{\text{−}}{\mathrm{O}}^{*}}$ we find evidence for coexistence between an orbital-ordered and an orbital-disordered state. This behavior is discussed in terms of electronic phase separation in an orbital-ordered insulating and an orbital-disordered metallic state.