First-Order Metal-Insulator Transitions in Manganites: Are They Universal?

Abstract

Conductivity data for ${\mathrm{La}}_{2-2x}{\mathrm{Sr}}_{1+2x}{\mathrm{Mn}}_2{\mathrm{O}}_7$ ($x=0.6$) show a first-order transition from an orbital- or charge-ordered insulator to a metal as the temperature falls below $\sim 160\mathrm{K}$. The change in conductivity is 100 times larger than that seen previously in any single-phase manganite in zero field. The metallic low-temperature state is similar to $x=0.58$, but $x=0.58$ shows no evidence of orbital or charge order. This result supports a conclusion that strongly coupled magnetic-conductive transitions are first order.