Dynamical theory for fluctuations and phase transitions in orbitally degenerate electron systems

考虑轨道涨落的理论框架我们扩展了平均场理论，考虑了涨落效应。新理论在无限维空间的极限下是精确的，被认为是经典自旋球模型近似的量子力学推广。我们的理论比用定域费米子表示自旋的理论更进了一步，解释了轨道简并电子系统（如CeB_6和Ce_<0.75>La_<0.25>B_6）中的奇异磁序我们通过假设次近邻格点之间的赝偶极相互作用解释了CeB_6第III相中奇异磁序的起源。此外，我们还讨论了Ce_ La_ B_6中新发现的第IV相的八极有序的可能性<0.75><0.25>。我们发现了一种新的相，它在能量上接近于已经观察到的相，并且可以在外场（如单轴压力）的存在下实现。我们开发了新的功能型 ...更多信息 轨道简并电子系统中的自旋波和轨道波激发我们从复杂的自旋轨道有序态出发，导出了元激发的色散关系。含16个Ce离子的大晶胞产生48个分支。我们导出了所有的分支，并找到了谱与多极相互作用之间的特征关系。特别是某些分支在波矢的某些方向上几乎是沿着平的，这可以用中子衍射来检验。轨道简并一维系统的精确动力学到目前为止，我们解释了三维系统的结果。在低维中，涨落的作用要重要得多。为了更好地理解这种情况，我们研究了一个一维正解模型。该模型具有SU（4）对称性来表示简并性，并且相互作用与距离的平方（1/γ^2）成反比。我们成功地精确地导出了动力学关联函数。少

Formulation of theoretical framework with account of orbital fluctuations We extended the mean-field theory to take account of fluctuation effects. The new theory becomes exact in the limit of infinite spatial dimensions, and is regarded as a quantummechanical extension of the spherical model approximation for classical spins. Our theory goes one step beyond the other ones that use localized fermions to represent spins.Explanation of the strange order in orbitally degenerate electron systems such as CeB_6 and Ce_<0.75>La_<0.25>B_6We explained origin of strange magnetic order in the phase III of CeB_6 by assuming a pseudo-dipolar interaction between next-nearest-neighbor sites. Furthermore, we discussed possibility of an octupole order for the newly found phase IV in Ce_<0.75>La_<0.25>B_6. We found a new plase which lies energetically close to the already observed phases, and which may be realized in the presence of external field such as uniaxial pressure. We developed new functional i … More ntegral formalism to perform the Ginzburg-Landau expansion microscopically.Spin and orbital wave excitations in orbitally degenerate electron systemsWe derived the dispersion relations of elementary excitations from the complicated spinorbital ordered state. The large unit cell with 16 Ce ions give rise to 48 branches. We derived all branches and found characteristic relation between the spectrum and the multipolar interaction. In particular some branches are almost flat along certain directions of wave vector, which may be checked by neutron diffraction.Exact dynamics of orbitally degenerate one-dimensional systemsSo far we explained results for three-dimensional systems. In low dimensions, the role of fluctuations is much more important. In order to see the situation most clearly, we studied an exactly soluble model in one dimension. This model has the SU (4) symmetry to represent the degeneracy, and the interaction is inversely proportional to the square of the distance (1/γ^2). We succeeded in deriving the dynamical correlation function exactly. Less