Study of diffusion phenomena of spin particles by a new technique of high resolution neutron spectroscopy

用散射技术很难观察到单个粒子在介质上的扩散。在这些现象中，尚未解决的问题有分形晶格上的反常扩散、低维晶格上自旋扩散的长时间拖尾等，中子非弹性散射得到的散射函数的波数积分等于自相关函数的傅里叶变换。这个量很难通过常规的中子散射获得。然而，最近的发展，脉冲中子能谱，使我们能够观察到的自相关函数的能量谱，和上述列出的未解决的问题可以得到澄清。自相关函数在均匀介质中的自旋扩散中呈指数衰减。在扩散网络上，扩散是反常的，自相关函数呈幂律。我们以前在二维超导磁体Rb_2Co_Mg_F_4中观察到自旋扩散服从幂律的能谱<0.6><0.4>，以确定只有近超导系统的线型是反常的，而相应的纯化合物Rb_2CoF_4的线型是Lorentzian的。所观察到的能谱很好地描述了一个洛伦兹。我们还尝试了在一维磁体CsMnBr_3中观察到的长时间tall。在很小的能量范围内，我们观察到了符合幂律的能谱。在此基础上，我们对三维超导磁体RbMn_ Mg_ F_3进行了高能量分辨率的测量<0.39><0.61>，首次观察到了随着波长的减小，从长波长极限的自旋波激发到分形子激发的交叉现象。

It was difficult to observe the diffusion of single particles on a medium by scattering techniques. Unsolved subjects among such phenomena are anomalous diffusion on a fractal lattice, the long time tail in spin diffusion on a low-dimensional lattice, and so on. The wavenumber-integration of the scattering function obtained by inelastic neutron scattering is equal to the Fourier transform of the self correlation function. This quantity was hardly obtained by a conventional neutron scattering. However, recent developments on pulsed-neutron spectroscopy enables us to observe the self correlation function in the terms of the energy spectrum, and the above listed unsolved subjects can be elucidated. The self-correlation function decays exponentially in the spin diffusion on a homogeneous medium. On a percolating network, the diffusion is anomalous and the self correlation function shows a power law. We previously observed the energy spectrum with a power law in the spin diffusion in a two-dimensional percolating magnet, Rb_2Co_<0.6>Mg_<0.4>F_4, In order to make sure that the line shape of only the near-percolating system is anomalous and that of a corresponding pure compound Rb_2CoF_4 takes the form of a Lorentzian. The observed energy spectrum was well described by a Lorentzian. We also tried to observe the long time tall in a one-dimensional magnet, CsMnBr_3. We observed energy spectrum with a power law at very small energy region. Further, we performed a high energy resolution measurement on a three-dimensional percolating magnet, RbMn_<0.39>Mg_<0.61>F_3, and firstly observed the crossover from spin wave excitations at long wavelength limit to a fracton excitations with decreasing the wavelength.