Giant Peak of the 1/f Noise at the Metal-Insulator Transitions in Low-Tc CMR Manganites: Evidence of the Percolation Threshold at Tc

We observed a dramatic peak in the 1/f noise at the metal-insulator transition (MIT) in low-Tc manganites. This many-orders-of-magnitude noise enhancement is observed for both polycrystalline and single-crystal samples of La{5/8-y}Pr{y}Ca{3/8}MnO{3} (y=0.35-0.4) in zero magnetic field and Pr{1-x}Ca{x}MnO{3} (x=0.35-0.5) in magnetic field. This observation strongly suggests that the microscopic phase separation in the low-Tc manganites causes formation of a percolation network, and that the observed MIT is a percolation threshold.