VOLUME 84, NUMBER 19 PHYSICAL REVIEW LETTERS 8MAY 2000 Jahn-Teller Dynamics in Charge-Ordered Manganites from Raman Spectroscopy V. Dediu, C. Ferdeghini,* F.C. Matacotta, P. Nozar, and G. Ruani Istituto di Spettroscopia Molecolare, CNR, via Gobetti 101, 40129 Bologna, Italy (Received 8 October 1999) The Raman spectra of the charge-ordered manganite Pr 0.65 Ca 0.35 MnO 3 were studied as functions of temperature and excitation energy and compared to magnetic moment and electrical conductivity behav- iors. Both the charge ordering (T co  225 K) and the antiferromagnetic transitions (T N  175 K) affect the spectral shape and intensity, indicating strong charge-lattice and spin-lattice couplings. Below T co a transition from dynamic Jahn-Teller distortions to a collective static distortion takes place. A change of the spectra is observed on increasing the excitation energy above 2.5 eV and it is attributed to a resonant polaron excitation. PACS numbers: 75.30.Vn, 71.38.+i, 78.30.–j Doped praseodymium manganites Pr 12x Sr, Ca x MnO 3 have the most unusual magnetic, electrical, and optical properties among the colossal magnetoresistance per- ovskites. The optimally doped Pr manganites (x  0.3 0.5) are not ferromagnetic under usual conditions, showing insulating behavior down to low temperatures. Below 230 K, charge ordering (CO) and orbital ordering take place, giving rise to a superstructure (a 0  2a) with the ordering of Mn 31 and Mn 41 cations along the a axis with one Mn 41 -O and two Mn 31 -O bond lengths [1]. Pr 12x Sr, Ca x MnO 3 is a Jahn-Teller (JT) system, where the JT distortion leads to a cubic-orthorhombic transition [2]. The JT distortion takes place on the Mn 31 sites, where the e g level is twofold degenerate. The partial localization of the e g electrons due to the JT effect or charge order- ing is responsible for the “high” resistance values above the Curie temperature in manganites [3], while their de- localization leads to a ferromagnetic metallic phase via the double-exchange mechanism. Thus, the lattice-carrier coupling plays a crucial role to determine the magnetic and electrical properties of these manganites. Although the Raman spectroscopy is a powerful tool for studying such effects, the Raman spectra of manganites are not yet well understood. In this Letter, we present the Raman spectra of Pr 0.65 Ca 0.35 MnO 3 single-phase polycrystalline samples along with their electrical and magnetic data across the charge-ordering transition, where structural, electrical, and magnetic properties change sharply. The magnetization curve taken at 1 T by the SQUID magnetometer (Fig. 1) is in good agreement with available data [4] showing several features related to the magnetic and electrical transitions in this compound. A maximum below 250 K (T co  225 K from derivative analysis) indi- cates the charge-ordering transition, which leads to a strong carrier localization; a second maximum near 175 K coin- cides with the onset of antiferromagnetism (T N ), while a broad minimum above 100 K corresponds to the transition to canted spin order. These values are in good agreement with neutron diffraction data [5]. At low temperatures, a deviation from the canted spin behavior might be due to Pr-ion spin ordering [1]. Above T N , the material is paramagnetic at both sides of the charge-ordering transition, but the nature of magnetic interaction changes drastically at T co : It is ferromagnetic at T . T co , with a Curie-Weiss behavior M  1T 2 160 K and antiferromagnetic at T , T co M  1T 1 205 K. The superexchange interaction, which competes with double-exchange interaction in the manganites [6], becomes dominant below T co due to the strong carrier localization, driving the transition from a ferromagnetic to an antiferromagnetic interaction. The resistance shows insulating behavior (inset in Fig. 1), with a sharp increase near T co . Obviously, the Mn 31 -Mn 41 ordering decreases the frequency of the carrier hopping between the two manganese states. Micro-Raman studies at different temperatures (T  4.2 400 K) were recorded in backscattering geometry. Three different excitation energies were used in our stud- ies: 488 nm (blue), 514 nm (green), and 632.8 nm (red). FIG. 1. Magnetization MT  curve measured at 1 T magnetic field. The solid lines near T co are guides for the eye. The temperature dependence of electrical resistance is shown in the inset. 0031-9007 00  84(19)  4489(4)$15.00 © 2000 The American Physical Society 4489