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

该项目揭示了3d、4f和5f电子系统中轨道自由度的新方面。结果表明，CeB_6在低温低磁场条件下呈现反铁四极有序的异常性质是由于四极、八极和交换三种相互作用的共存和竞争。利用Ce_xR_<1-x>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，得到了具有自旋和四极矩的Pr和Nd离子与仅具有自旋自由度的Gd离子对CeB_6中多极有序的不同影响。我们研究了CeB6中磁离子掺杂对近道效应的影响，发现Ce_xNd_<1-x>B_6体系中Ce和Nd离子之间的相互作用极小。为了弄清Ca_<1-x>La_xB_6中弱铁磁性的来源，我们对Yb_<1-x>La_xB_6体系进行了研究，发现该体系在T_c=200K时表现出弱铁磁性，这很难归因于Fe杂质效应。对于Kondo绝缘子系统，我们研究了Yb_<1-x>Lu_xB_<12>系统，并提出了以下模型：Lu离子掺杂不改变200K的能基大小，而是在费米能级形成隙内态，最终出现通常的近藤效应。对于三维电子体系，我们研究了Y_<1-x>Ca_xTiO_3体系。以前报道的x=0.4处金属-绝缘体转变的原因是不同晶体结构的绝缘相和金属相之间的相分离。对于5f电子体系，我们通过对其磁弹性性质的研究，研究了Ucu_2Sn在序参量未知的情况下表现出长程序，发现其存在铁-四极序。

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.