Pressure-induced insulator-metal transition in a bilayer manganite: Pressure control of orbital stability

Pressure effects on transport as well as structural properties have been investigated for a bilayer manganite, ${\mathrm{La}}_{2-2x}{\mathrm{Sr}}_{1+2x}{\mathrm{Mn}}_2{\mathrm{O}}_7$ $\mathrm{}(x=0.48\mathrm{}),$ near the phase boundary between the ferromagnetic metallic and the A-type antiferromagnetic states. An insulator-metal (IM) transition is found to be induced by applying pressure of $\sim 0.7\hspace{0.5em}\mathrm{GPa}.$ On the basis of the synchrotron radiation x-ray structural analysis under high pressures, we have ascribed the IM transition to the variation of the stability of the $e_g$ orbitals.