Raman scattering studies of the temperature- and field-induced melting of charge order in LaxPryCa1-x-yMnO3

We present Raman scattering studies of the structural and magnetic phases that accompany temperature- and field-dependent melting of charge- and orbital-order (COO) in La(0.5)Ca(0.5)MnO(3) and La(0.25)Pr(0.375)Ca(0.375)MnO(3). Our results show that thermal and field-induced COO melting in La(0.5)Ca(0.5)MnO(3) exhibits three stages in a heterogeneous melting process: At low temperatures and fields we observe a long-range, strongly Jahn-Teller (JT) distorted-COO phase; at intermediate temperatures and/or fields, we find a coexistence regime comprising both strongly JT distorted-COO and weakly JT distorted/ferromagnetic metal (FMM) phases; and at high temperatures and/or high fields, we observe a weakly JT distorted homogeneous paramagnetic (PM) or ferromagnetic (FM) phase. In the high field-high temperature regime of La(0.5)Ca(0.5)MnO(3) and La(0.25)Pr(0.375)Ca(0.375)MnO(3), we identify a clear structural change to a weakly JT distorted phase that is associated with either a Imma or Pnma structure. We are able to provide a complete structural phase diagram of La(0.5)Ca(0.5)MnO(3) for the temperature and field ranges 6 <= T <= 170 K and 0 <= H <= 9 T. Significantly, we provide evidence that the field-induced melting transition of La(0.5)Ca(0.5)MnO(3) is first-order, and resembles a crystallization transition of an electronic solid. We also investigate thermal and field-induced melting in La(0.25)Pr(0.375)Ca(0.375)MnO(3) to elucidate the role of disorder in melting of COO. We find that while thermal melting of COO in La(0.25)Pr(0.375)Ca(0.375)MnO(3) is quite similar to that in La(0.5)Ca(0.5)MnO(3), field-induced melting of COO in the two systems is quite different in several respects: The field-induced transition from the COO phase to the weakly JT-distorted-FM phase in La(0.25)Pr(0.375)Ca(0.375)MnO(3) is very abrupt, and occurs at significantly lower fields (H similar to 2 T at T similar to 0 K) than in La(0.5)Ca(0.5)MnO(3) (H similar to 30 T at T=0 K); the intermediate coexistence regime is much narrower in La(0.25)Pr(0.375)Ca(0.375)MnO(3) than in La(0.5)Ca(0.5)MnO(3); and the critical field H(c) increases with increasing temperature in La(0.25)Pr(0.375)Ca(0.375)MnO(3), in contrast to the decrease in H(c) observed with increasing temperature in La(0.5)Ca(0.5)MnO(3). To explain these differences, we propose that field-induced melting of COO in La(0.25)Pr(0.375)Ca(0.375)MnO(3) is best described as the field-induced percolation of FM domains, and we suggest that Griffiths phase physics may be an appropriate theoretical model for describing the unusual temperature- and field-dependent transitions observed in La(0.25)Pr(0.375)Ca(0.375)MnO(3).