Physical properties of phonons and dynamical John-Teller effect in colossal-magnetoresistance bilayer-manganite single-crystals

本文系统地研究了（La_<1-z>Pr_z）_<1.2>Sr_Mn_2O_7<1.8>（z=0，0.2，0.4和0.6）单晶的热导率、热膨胀系数、热电势和弹性常数随温度和磁场的变化。(1)在金属-绝缘体转变温度附近观察到巨磁热效应、巨磁致伸缩效应和巨磁弹性效应，表明巨磁电阻效应。(2)ab-平面声子传导被与Mn位点上的极化子跳跃相关联的动态晶格畸变所干扰。另一方面，在端元样品Sr_3Mn_2O_7中观察到的典型的声子传导归因于与小极化子强耦合的局域晶格畸变中的不均匀性的减少。基于Medarde等人提出的金属态和绝缘态局域晶格畸变的不同弛豫模型，讨论了双层锰氧化物中Pr替代引起的晶格畸变和各向异性热导率。(3)巨磁热效应作为磁化强度的函数进行缩放，并讨论了由于Pr取代的e_g-电子轨道状态的占据的系统变化。(4)本文研究了（La_<1-z>Pr_z）_<1.2>Sr_<1.8>Mn_2O_7单晶的弛豫磁致伸缩随时间的变化，以探讨双层锰氧化物晶格畸变的慢动力学。双交换和Jahn-Teller型轨道-晶格相互作用之间的竞争导致所观察到的晶格轮廓遵循拉伸指数函数。这一发现表明，与e_g电子轨道耦合的局域晶格畸变的空间增长与场致CMR效应的出现密切相关。

We have undertaken a systematic study of thermal conductivity, thermal expansion, thermoelectric power and elastic constant as a function of temperature and magnetic field for (La_<1-z>Pr_z)_<1.2>Sr_<1.8>Mn_2O_7 (z=0, 0.2, 0.4 and 0.6) single crystals.(1) Giant magnetothermal effect, giant magnetostriction effect and giant magnetoelastic effect are observed near the metal-insulator transition temperature showing the CMR effect.(2) The ab-plane phonon conduction is disturbed by the dynamic lattice distortion associated with polaron hopping on Mn sites. On the other hand, the typical phonon conduction observed for the end member sample Sr_3Mn_2O_7 is attributed to a reduction of the inhomogeneities in the local lattice distortions strongly coupled with small polarons. The lattice distortion due to Pr substitution and anisotropic thermal conductivity in bilayer manganites are discussed on the basis of different relaxation models of local lattice distortions in metal and insulating states proposed by Medarde et al.(3) The giant magnetothermal effect is scaled as a function of magnetization and discussed in terms of a systematic variation of the occupation of the e_g-electron orbital states due to Pr substitution.(4) We have investigated the time dependence of remanent magnetostriction in a (La_<1-z>Pr_z)_<1.2>Sr_<1.8>Mn_2O_7 single crystal, in order to examine the slow dynamics of lattice distortion in bilayered manganites. A competition between double exchange and Jahn-Teller type orbital-lattice interactions results in the observed lattice profile following a stretched exponential function. This finding suggests that spatial growth of the local lattice distortions coupled with e_g-electron orbital strongly correlates with the appearance of the field-induced CMR effect.