Mechanism of insulator-metal transition in La2/3Ca1/3MnO3: Collapse of small polaron

The insulator-metal (I-M) transition in the colossal magnetoresistance mate rial La2/3Ca1/3MnO3 has been studied by electrical transport, magnetization and electron spin resonance (ESR) measurements. Around the I-M transition, the ESR signal shows a crossover from paramagnetic resonance line to ferro magnetic resonance line, which implies a coexistence of localized small pol arons and itinerant carriers during the I-M transition. Based on the concep t of 'small polaron collapse', a two-phase model is proposed to quantitativ ely describe the electrical transport behaviour across the insulator-metal transition. The calculations of resistivity in different applied magnetic f ields have close trends with the experimental data, which strongly proves t hat the I-M transition results from the collapse of small polarons and the two phases coexist during the transition.