Raman Scattering in Charge Ordered Pr0.63Ca0.37MnO3 : Anomalous Temperature Dependence of Linewidth

– We report on the evolution of the Raman-active Ag phonon modes in the chargeordered manganite Pr0.63Ca0.37MnO3 as a function of temperature from 300K to 25K. Our studies reveal that the linewidths of the Ag(2) and Ag(4) phonons increase as temperature decreases. This anomalous temperature dependence of phonon lineshapes, seen for the first time in charge ordered manganites, can be quantitatively understood in terms of a strong spin-phonon coupling involving t2g spins and Ag phonons. The recent upsurge of interest in doped manganites exhibiting colossal magnetoresistance has fuelled a lot of interesting questions regarding the role of spin, orbital and lattice degrees of freedom and the interplay amongst them [1]. These systems of the form A1−xA ′ xMnO3, where A is a trivalent rare earth ion (e.g., La, Pr, Nd) and A is a divalent ion (e.g., Ca, Sr, Pb) show a rich phase diagram depending on the tolerance factor and the amount of doping x which in turn controls the ratio of Mn to Mn. The essential ingredients for understanding their electronic properties are the double-exchange(DE) mechanism [2] and the polaron formation due to the Jahn-Teller(JT) effect [3]. In the case of systems where the weighted average A-site cation radius is small, the system also shows charge ordering (CO), i.e., real space ordering of Mn and Mn ions at low temperatures [4]. The CO state becomes stable when the repulsive Coulomb interaction between the carriers or the cooperative JT effect dominates over the kinetic energy of carriers. In such situations there is a strong competition between the DE interaction which favours ferromagetism (FM), and CO which favours antiferromagnetism (AF) via the coupling between the background t2g (S = 3/2) spins. () Present Address : 104 Davey Laboratory, Pennsylvania State University, University Park, PA 16802, USA (∗∗) Present Address : The Abdus Salam International Centre for Theoretical Physics, Strada Costiera 11, 34100 Trieste, Italy (∗∗∗) Email : [rajeev, venkat, asood, tvrama] @physics.iisc.ernet.in ; [email protected]