Critical Phenomena and Dynamics of Orbital Degrees of Freedom

我们研究了（1）轨道自由度与其它自由度强耦合的3d过渡金属氧化物体系和（2）轨道自由度自发有序的4f稀土金属化合物体系。对于（1），我们主要研究了层状锰氧化物La_<2-2x>Sr_（1+2x）Mn_2O_7。在0.5 [小于或等于] x [小于或等于] 0.6中，我们已经成功地用FZ方法生长单晶，并且发现反铁磁（AF）金属态随着掺杂率x的增加而变得稳定。我们的中子散射研究表明，A型AF金属态和CE型AF电荷有序态竞争共存于电子相分离态，该电子相分离态随温度和x急剧变化。我们还利用共振X射线散射、极化中子衍射、磁康普顿剖面、自旋波测量和分析等技术，定量地阐明了0.3 × 0.5中的轨道状态及其对磁性和输运的影响。我们首次用共振x射线散射方法研究了DyB_2C_2的反铁四极（AFQ）有序化过程和结构。我们还证实了HoB_2C_2在T_Q = 5.0K以下经历了AFQ和AFM有序化，并发现在T_Q以上仍存在短程磁关联，并具有不同于低温态的奇异无公度周期。我们还通过共振X射线散射揭示了AFQ有序化过程的细节和CeB_6的状态，CeB_6是一种长期以来被认为是AFQ材料的原型。

We have studied (1) systems with orbital degrees of freedom which are strongly coupled to the other degrees of freedom - 3d transition metal oxides and (2) systems with orbital degrees of freedom which spontaneously order - 4f rare-earth metal compounds. As for (1), we have focused on layered Mn oxide La_<2-2x>Sr_<1+2x>Mn_2O_7. In 0.5 【less than or equal】 x 【less than or equal】 0.6, we have succeeded in growing single crystals with the FZ method, and found that an antiferromagnetic (AF) metallic state becomes stable with increasing the doping rate x. Our neutron scattering studies have revealed that the A-type AF metallic state and the CE-type AF charge-ordered state competitively coexist in an electronically phase separated state, which drastically changes with temperature and x. We have also employed other techniques such as resonant x-ray scattering, polarized neutron diffraction, magnetic Compton profile, spin wave measurement and analysis, and clarified quantitatively the orbital state and its effect to the magnetism and transport in 0.3 【less than or equal】 x 【less than or equal】 0.5.As for (2), we have scrutinized the antiferro-quadrupole (AFQ) ordering process and structure in DyB_2C_2 by resonant x-ray scattering, which is the first example of resonant x-ray scattering measurement of AFQ state. We have also confirmed that HoB_2C_2 undergoes AFQ and AFM ordering below T_Q = 5.0 K, and found that there remains short-range magnetic correlations, well above T_Q, with an exotic incommensurate periodicity different from that of the low temperature state. We also revealed the details of AFQ ordering process and state hi CeB_6, known for a prototypical AFQ material for a long time, by resonant x-ray scattering.