中文摘要：

系统研究了Nd0.5Ca0.5Mn0.97Co0.03O3单相多晶样品在低温下的电磁性质和超声特性．超声测量结果表明，体系在低温下超声声速出现异常，超声声速从室温开始随着温度的降低逐渐减小，并在220K附近声速达到最小，其相对变化（△V／V）超过2％．之后，随着温度的进一步降低，声速急剧硬化，其相对变化达到14．5％．分析表明这是由于体系中电-声子相互作用导致的电荷有序态转变的结果，该电-声子耦合来源于Mn^3＋的Jahn-Teller效应．同时，该体系在100K左右出现了金属-绝缘体转变同时伴随着铁磁相变，分析表明在低温下Co离子与Mn离子之间存在着铁磁交换作用，因此微量的Co掺杂会在反铁磁区域内形成部分铁磁小团簇，当铁磁团簇连通后，体系由反铁磁电荷有序态（AFM-CO）向铁磁金属态（FM-M）转变．研究结果表明了样品在电荷有序相变温度TCO与金属-绝缘体转变温度TMI之间（TMI〈T〈TCO）发生了铁磁金属态与反铁磁电荷有序态共存的相分离现象．TMI〈T〈TCO间的电荷输运遵循可变程跃迁（VRH）模型．

英文摘要：

The magnetization, resistivity and ultrasonic sound velocity have been measured in polycrystalline Nd0.5Ca0.5Mn0.97Co0.03O3. A charge ordering transition was observed near 220K in the temperature dependence of magnetization and resistivity curves. It was found that the longitudinal sound velocity shows a dramatic softening and stiffening around charge ordering transition Tco. The ultrasonic anomaly near Tco indicates the existence of strong el ectron-phonon interaction in Nd0.5Ca0.5Mn0.97Co0.03O3 which originates from Jahn-Teller effect. Around TMI-100K, a insulator-metal transition coincident with the FM transition was observed. The transport mechanism of system in the temperature ranges TMI〈 T 〈 TCO obey the variable-range hopping （VRH） model.