Control of the Orbital Degrees of Freedem by the Magnetic Field and Pressure

该项目揭示了3d、4f和5 f电子系统中轨道自由度的以下新方面。我们指出，CeB_6在低温和低磁场下表现出反铁四极有序的反常性质的根源是四极、八极和交换三种相互作用的共存和竞争。本文用Ce_xR_ B_6（R=Pr，Nd，Gd）体系研究了磁掺杂对CeB_6中多极有序的影响<1-x>，得到了同时具有自旋和四极矩的Pr和Nd离子与只有自旋自由度的Gd离子对多极有序的不同影响。我们研究了磁性离子掺杂对CeB 6近藤效应的影响，发现Ce_xNd_ B_6体系中Ce和Nd离子之间的相互作用非常小<1-x>。为了阐明Ca_ La_xB_6中弱铁磁性的起源<1-x>，我们研究了Yb_<1-x>La_xB_6系统，发现该系统在T_c= 200 K时表现出弱铁磁性，这很难归因于Fe杂质效应。对于Kondo绝缘体系统，我们研究了Yb_<1-x>Lu_xB_<12>系统，并提出了以下模型。掺Lu离子不改变200 K的能隙尺寸，但在费米能级形成了带隙内态，并出现了通常的Kondo效应。对于三维电子系统，我们研究了Y_<1-x>Ca_xTiO_3系统。之前报道的x=0.4处的金属-绝缘体转变的起源归因于具有不同晶体结构的绝缘相和金属相之间的相分离。对于5 f电子系统，我们通过磁弹性质的研究，研究了UCu_2Sn的长程有序性，发现其有序度参数未知。

This project has revealed the following new aspects on the orbital degrees of freedom in the 3d, 4f and 5f electron systems. We have shown that the origin of the unusual properties of CeB_6 showing thel antiferro-quadrupolar ordering at low temperatures and low magnetic fields is the coexistence and competition between the three interactions of the quadrupolar, octupolar and exchange. We have investigated the magnetic doping effect on the multipolar ordering in CeB_6 by using the Ce_xR_<1-x>B_6(R=Pr, Nd, Gd) systems and obtained the different affects between Pr and Nd ions with both spin and quadrupolar moments and Gd ion with only spin degrees of freedom. We have investigated the magnetic ion doping effect on the Kondo effect in CeB6 we found the extremely small interaction between Ce and Nd ions in Ce_xNd_<1-x>B_6 system. In order to clarify the origin of the weak ferromagnetism in Ca_<1-x>La_xB_6, we have investigated the Yb_<1-x>La_xB_6 system and found that this system exhibits the weak ferromagnetism at T_c=200K which is difficult to ascribe to the Fe impurity effect. As for the Kondo insulator system, we have investigated the Yb_<1-x>Lu_xB_<12> system and proposed the following model. Lu ion doping does not change the energy gay size of 200K but forms the in-gap state at the Fermi level and finally the usual Kondo effect becomes to be seen. As for the 3d electron system, we have investigated the Y_<1-x>Ca_xTiO_3 system. The origin of the metal-insulator transition at x=0.4 reported before has been ascribed to the phase separation between the insulating and metallic phases with different crystal structures. As for the 5f electron system, we have studied Ucu_2Sn exhibiting the long range order whose order parameter has not known by investigating the magnetoelastic properties and found that the ferro-quadrupolar order takes place.