Crossover from Large to Small Polarons across the Metal-Insulator Transition in Manganites

We report Mn $K$-edge extended x-ray absorption fine structure spectra on ${\mathrm{La}}_{0.75}{\mathrm{Ca}}_{0.25}{\mathrm{MnO}}_3$ up to high momentum transfer across the metal-insulator $\left(M-I\right)$ transition. The data show compelling evidence for (i) large or intermediate Jahn-Teller polarons (IJTP), characterized by an anomalous longer Mn-O bond $(\Delta R=0.09\mathrm{}Å)$ in the metallic phase $(T<\mathrm{}170\mathrm{}\mathrm{K})$, and (ii) appearance of small JT polarons (SJTP) at $T>\mathrm{}170\mathrm{}\mathrm{K}$, characterized by a longer Mn-O bond $(\Delta R=0.21\mathrm{}Å)$, which coexist with the IJTP above the $M-I$ transition and has equal probability in the temperature range of colossal magnetoresistance.