Study of the competition between the multipole interaction and Kondo effect in magnetic ion doped RB_6 (R=rare-earthion)

本研究的目的是阐明RB_6（R=稀土离子）中多极相互作用和近藤效应之间竞争的机制。我们得到了以下结果。(1)对Ce_xR_<1-x>B_6的研究表明Ce-Nd相互作用很弱，Ce-Pr相互作用很强。在前者中，Ce和Pr协同形成有序状态，近藤效应被破坏；有序态主要由Ce离子构成，Nd离子具有较强的顺顺性，近藤效应在有序态中仍然存在。(2)对Ce_<0.7>La_<0.3>B_6的第四相进行了中子衍射实验，发现了直接观察到的八极序。然而，由于八极子序列所期望的O_<xy>-FQ顺序与CeB_6的内在相互作用O_<xy>-AFQ相互作用相竞争，Ce_xLa_<1-x>B_6的整体性质尚未明确。(3)从r掺杂对Ce_<0.7>La_<0.3>B_6的IV相的影响来看，发现少量Nd和Pr掺杂容易破坏IV相。然而，在Ce_<0.6>La_<0.4>B_6的情况下，nd掺杂的IV相较强，得到了以下意想不到的结果。钕的掺入提高了从IV相到顺磁相的转变温度。(4)考察了压力对PrB_6的影响。我们发现，在T_<10>=7K以上，施加0.3GPa的小压力，出现了一个新的相。我们还发现，施加压力大大增强了相称(C)相。这表明，近程相互作用敏感地依赖于Pr-Pr距离。(5)在压力作用下CeB_6在P=1.3GPa以上出现了铁磁（FM）阶。该调频基态是由O_<xyz>-AFQ和T_<xyz>-AFO两阶序共存构成的。据我们所知，这是没有交换相互作用的FM基态的第一个例子。

The purposes of this study are to clarify the mechanisms of the competition between the multipole interaction and Kondo effect in RB_6(R=rare-earth ion). We obtained the following results.(1) The investigation on Ce_xR_<1-x>B_6 showed that the Ce-Nd interaction is very weak but the Ce-Pr one is strong. In the latter, Ce and Pr ion cooperatively form the ordered state and Kondo effect is destroyed but in the former; the ordered state is constructed mainly by Ce ions and Nd ions have a strong paramagetnic character and Kondo effect remains even in the ordered state.(2) By the neutron diffraction experiment on phase IV of Ce_<0.7>La_<0.3>B_6, the direct observation of the octupole order was discovered. However, as the O_<xy>-FQ order expected from the proposed octupole order competes with the O_<xy>-AFQ interaction which is the intrinsic interaction in CeB_6, the overall nature of Ce_xLa_<1-x>B_6 has not yet been clarified.(3) From the investigation of the R-doping effect on phase IV of Ce_<0.7>La_<0.3>B_6, phase IV is found to be easily destroyed by a small amount of Nd and Pr dopings. However, in the case of Ce_<0.6>La_<0.4>B_6, phase IV is strong for the Nd-doping and the following unexpected results was obtained. The transition temperature from phase IV to paramagnetic one is enhanced by Nd doping.(4) The pressure effect on PrB_6 was investigated. We discovered that a new phase appears above T_<10>=7K by applying only a small pressure of 0.3GPa. We also found that commensurate(C) phase is largely enhanced by applying pressure. These indicate that the short range interaction depending sensitively on the Pr-Pr distance.(5) In the pressure effect on CeB_6 we discovered the appearance of the ferromagnetic(FM) order above P=1.3GPa. This FM ground state is constructed from the coexistence of the O_<xyz>-AFQ and T_<xyz>-AFO orders. To our knowledge, this is the first example of the FM ground state without the exchange interaction.