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Lattice and ultrasonic effects of orbital ordering and fluctuation

轨道排序和涨落的晶格和超声波效应

基本信息

批准号：
11220205
负责人：
GOTO Terutaka
金额：
$ 27.33万
依托单位：
Niigata University
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research on Priority Areas (B)
财政年份：
1999
资助国家：
日本
起止时间：
1999 至 2001
项目状态：
已结题

项目摘要

In the present project we have investigated on orbital orderings and fluctuations by use of the coupling of the elastic strain of sound waves to the quadrupole of the orbital degrees of freedom. By the ultrasonic measurements, we have investigated the magnetic phase diagram of Ce_xLa_<1-x>B_6 with ヲ」_8 ground state. In the compound x=0.75 the ordered phase IV changes into the antiferro-magnetic phase III. In more diluted compounds x=0.70 and 0.65, however, the phase IV is stable down to absolute zero. The ultrasonic measurements show significant difference in low-temperature limit of C_<44>ofx=0.70, 0.65 and that of x=0.60, 0.50. We have deduced, therefore, that the compounds x=0.60 and 0.50 falls into a non-magnetic ground state showing no distinct phase transition. This is consistent with the previous specific heat measurements by Nakamura et al. In order to examine the order parameter of the phase IV, we have made thermal expansion measurements showing an expansion along the (001) d … More irection and a shrink along the(111) direction in phase IV. This result is a proof for a trigonal distortion in phase IV indicating a quadrupole ordering O_<yz>=O_<zx>=O_<xy>。・.We。。have。。found that tetragonal ternary compounds HoB_2C_2 and DyB_2C_2 exhibit softenings in (C_<11>-C_<12>)/2, C_<44>, C_<66>. These results indicate a pseudo-triplet model consisting of E-doublet and B or A singlet is responsible for HoB_2C_2 and a pseudo-quartet model with E_<1/2> doublet and E_<3/2> doublet is for DyB_2C_2. We have also performed ultrasonic investigation on La_<1-x>Sr_xMnO_3 and Pr_xCa_<1-x>MnO_3 to investigate their orbital orderings. We have commonly observed the softening in (C_<11>-C_<12>)/2, in para-phase, where the Jahn-Teller coupling of quadrupoles O^0_2 and O^2_2 of E_g doublet is responsible. We obtained the energy gain 10。チ100K, which is considerable smaller than that in Mills' theory. The elastic softening in C_<44> due to charge ordering in Pr_xCa_<1-x>MnO_3 with x=0.50 has been found. We have successfully applied the Landau theory to explain the charge ordering of the system associated with the softening of C_<44>. Less

在本项目中，我们利用声波弹性应变与轨道自由度四极的耦合研究了轨道的有序和波动。通过超声测量，我们研究了cu ' _8基态下Ce_xLa_<1-x>B_6的磁相图。在x=0.75的化合物中，有序相IV转变为反铁磁相III。然而，在更稀释的化合物x=0.70和0.65中，第四相稳定到绝对零度。x=0.70、0.65时C_<44>的低温极限与x=0.60、0.50时的低温极限有显著差异。因此，我们推断，化合物x=0.60和0.50落入无磁性基态，没有明显的相变。这与Nakamura等人先前的比热测量结果一致。为了检验第四相的有序参数，我们对第四相进行了热膨胀测量，结果表明第四相沿（001）d方向膨胀，沿（111）方向收缩。这一结果证明了第四相存在三角畸变，表明四极有序O_<yz>=O_<zx>=O_<xy>。·我们。。有。。发现三元四方化合物HoB_2C_2和DyB_2C_2在(C_<11>-C_<12>)/2、C_<44>、C_<66>中表现出软化。这些结果表明，HoB_2C_2是由e -双重态和B或a单重态组成的伪三重态模型，DyB_2C_2是由E_<1/2>双重态和E_<3/2>双重态组成的伪四重奏模型。我们还对La_<1-x>Sr_xMnO_3和Pr_xCa_<1-x>MnO_3进行了超声研究，以确定它们的轨道序。我们通常观察到在(C_<11>-C_<12>)/2中，在准相中，E_g重态的四极O^0_2和O^2_2的Jahn-Teller耦合是造成软化的原因。我们得到了能量增益10。这比米尔斯的理论要小得多。在x=0.50时，发现在Pr_xCa_<1-x>MnO_3中，电荷排序导致C_<44>的弹性软化。我们成功地应用朗道理论解释了与C_<44>的软化有关的系统的电荷顺序。少