Anomalous Lattice Softening at<i>X</i>=0.19 and 0.82 in La_1-<i>X</i>Ca_<i>X</i>MnO₃

Abstract

The longitudinal sound velocity v s ( T ) has been measured for La 1- X Ca X MnO 3 crystals, and anomalous softenings of v s ( T ) have been observed at the peculiar Ca concentrations of X =0.19 and X =0.82. For the X =0.19 sample, in which the ferromagnetic (FM) metallic phase competes with the FM insulating phase with a polaron ordering, the anomalous v s ( T ) softening occurs immediately below the FM transition temperature T c . For the X =0.82 sample, in which the charge-ordered (CO) phase competes with the canted antiferromagnetic (CAF) phase, the deep local minimum of v s ( T ) occurs around the CO transition temperature T CO . These anomalous v s ( T ) softenings result from the giant lattice fluctuations which originate from phase competitions and suggest strong charge-lattice and spin-lattice coupling in this system.