Mechanism of Insulator-Metal Transition in La_2/3 Ca_1/3MnO₃:Collapse of Small Polaron

The insulator-metal (I-M) transition in colossal magnetoresistance material La_2/3 Ca_1/3MnO₃ 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 ferromagnetic resonance line, which implies a coexistence of localized small polarons and itinerant carriers during the I-M transition. Based on the concept of ‘small polaron collapse’, a two-phase model is proposed to quantitatively describe the electrical transport behaviour across the insulator-metal transition. The calculations of resistivity in different applied magnetic fields have close trends with the experimental data, which strongly proves that the I-M transition results from the collapse of small polarons and two phases coexist during the transition.