Pressure-induced colossal piezoresistance effect and the collapse of the polaronic state in the bilayer manganite (La0.4Pr0.6)1.2Sr1.8Mn2O7

We have investigated the effect of hydrostatic pressure as a function of temperature on the resistivity of a single crystal of the bilayer manganite (La0.4Pr0.6)1.2Sr1.8Mn2O7. Whereas a strong insulating behaviour is observed at all temperatures at ambient pressure, a clear transition into a metallic-like behaviour is induced when the sample is subjected to a pressure (P) of ∼1.0 GPa at T < 70 K. A huge negative piezoresistance ∼106 in the low temperature region at moderate pressures is observed. When the pressure is increased further (5.5 GPa), the high temperature polaronic state disappears and a metallic behaviour is observed. The insulator to metal transition temperature exponentially increases with pressure and the distinct peak in the resistivity that is observed at 1.0 GPa almost vanishes for P > 7.0 GPa. A modification in the orbital occupation of the eg electron between 3dx2-y2 and 3dz2-r2 states, as proposed earlier, leading to a ferromagnetic double-exchange phenomenon, can qualitatively account for our data. (paper)