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

用散射技术观察单个粒子在介质上的扩散是困难的。这类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尾等。由非弹性中子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。这个量很难用常规的中子散射法得到。然而，脉冲中子能谱的最新发展使我们能够从能谱的角度观察到自相关函数，从而可以阐明上述未解决的问题。自相关函数在均匀介质上的自旋扩散中呈指数衰减。在渗流网络中，扩散是反常的，自相关函数呈幂律。在二维渗透磁体Rb_2Co_<0.6>Mg_<0.4>F_4的自旋扩散过程中，我们用幂律观测了其能谱，以确定只有近渗透体系的谱线是反常的，而对应的纯化合物Rb_2CoF_4的谱线是洛伦兹谱线。观测到的能谱可以用洛伦兹量很好地描述。我们还尝试观察了CsMnBr_3一维磁体的长时间高。我们在很小的能量区域用幂律观察能谱。此外，我们对RbMn_<0.39>Mg_<0.61>F_3的三维渗透磁体进行了高能量分辨率测量，并首次观察到长波长极限下自旋波激发向随波长减小的分数子激发的交叉。

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.