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

对(La_&lt；1-z&gt；Pr_z)_&lt；1.2&gt；Sr_&lt；1.8&gt；的导热系数、热膨胀系数、温差电势和弹性常数随温度和磁场的变化进行了系统的研究Mn2O7(z=0，0.2，0.4和0.6)单晶：(1)在金属-绝缘体转变温度附近观察到巨磁热效应、超磁致伸缩效应和巨磁弹性效应，表现出CMR效应.(2)由于极化子跃迁引起的动态晶格畸变干扰了平面声子的传导.另一方面，在端元样品Sr3Mn2O7中观察到的典型声子传导是由于与小极化子强耦合的局域晶格扭曲的不均匀性的减少。基于Medarde等人提出的金属态和绝缘态局域晶格畸变的不同驰豫模型，讨论了Pr替代引起的双层锰氧化物的晶格畸变和各向异性热导率。(3)将巨磁热效应作为磁化强度的函数，并用Pr替代引起的e_g电子轨道态的系统变化来讨论巨磁热效应。(4)我们研究了(La_&lt；1-z&gt；Pr_z)_&lt；1.2&gt；Sr_&lt；1.8&gt；中剩余磁致伸缩的时间依赖性Mn2O7单晶，以考察双层结构锰氧化物晶格畸变的慢动力学。双交换和Jahn-Teller轨道-晶格相互作用之间的竞争导致观察到的晶格轮廓遵循伸展的指数函数。这一发现表明，局域晶格畸变与e_g电子轨道耦合的空间增长与场诱导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.