Evidence of A Pressure-Induced Orbital Transition in A Layered Manganite

The charge density distribution of a bilayer manganite La1.04Sr1.96Mn2O7 has been investigated by means of the maximum entropy method using the synchrotron radiation x-ray powder data. La1.04Sr1.96Mn2O7 shows a layered-type (A-type) antiferromagnetic spin-ordering below T(N)approximate to200 K. We directly observed that application of pressure of several GPa at 17 K changes the orbital state from the two-dimensional d(x)(2)-y(2) state to the three-dimensionally fluctuating state. This orbital transition, which is probably driven by the pressure-induced modification of the oxygen environment around the Mn site, causes the metallic conductivity as well as the ferromagnetism of the system.