Raman scattering studies of field- and temperature-dependent melting of charge order in La$_{0.25}$Pr$_{0.375}$Ca$_{0.375}$MnO$_{3}$ and La$_{0.5}$Ca$_{0.5}$MnO$_{3}$*

The La$_{0.25}$Pr$_{0.375}$Ca$_{0.375}$MnO$_{3}$ system provides an interesting opportunity to study the effects of chemical disorder---introduced by replacing La$^{3+}$ with Pr$^{3+}$ having a smaller ionic radius---on the charge-ordered (CO) state and the complex field- and temperature-dependent phase behavior observed in the manganese perovskite La$_{1-x}$Ca$_{x}$MnO$_{3}$. We report field- and temperature-dependent Raman studies of La$_{0.25}$Pr$_{0.375}$Ca$_{0.375}$MnO$_{3}$ and La$_{0.5}$Ca$_{0.5}$MnO$_{3}$, in which we are able to carefully study the effects of disorder on both field-induced melting of CO and on the field-induced evolution of novel structural and magnetic phases in the (La,Pr,Ca)MnO$_{3}$ system. Among other results we will discuss, La$_{0.25}$Pr$_{0.375}$Ca$_{0.375}$MnO$_{3}$ exhibits a quantum melting transition from a CO state to a ferromagnetic metal phase at fields less than 7 T, which is a much lower value than that observed in La$_{0.5}$Ca$_{0.5}$MnO$_{3}$ ($\sim $20T).