Study of low-energy magnetic excitations with high energy-resolution neutron spectroscopy

在KEK脉冲中子源（KENS）上安装了一台高分辨率超导磁体非弹性中子谱仪，首次利用该谱仪系统研究了外磁场作用下稀释海森堡反铁磁系统的非弹性中子散射。用能量分辨率为10 μ eV的中子非弹性散射观测到了有限尺寸团簇中的磁激发。用高能量分辨率的中子非弹性散射观测到了近自旋Ising反铁磁体Rb_2Co_Mg_F_4中自旋的反常扩散运动<0.6><0.4>。在顺磁相的动量空间上积分的散射函数揭示了具有叠加在洛伦兹线形上的E^&lt; -（0.35 0.06）&gt;形式的长尾谱<plus-minus>，其源自正常临界磁散射。利用中子非弹性散射技术，我们研究了稀释的Heisenberg反铁磁体（RbMn_cMg_<1-c>F_3，c=0.39）在渗流阈值c_p= 0.312附近的磁激发。在布里渊区没有观察到自旋波的传播。但观察到叠加在伊辛团簇激发上的宽峰。这一宽峰的起源归因于在一个分形网络中的分形子的激发。

A high-resolution inelastic neutron spectrometer equipped with a superconducting magnet has been installed at the pulsed spallation neutron source (KENS) at KEK.For the first experiment using this spectrometer system, we investigated inelastic neutron scattering from diluted Heisenberg antiferromagnetic systems with applied field. Magnetic excitations in finite-size clusters were observed with energy-resolution of 10 mueV.The anomalous diffusive motions of spins in a near-percolating Ising antiferromagnet, Rb_2Co_<0.6>Mg_<0.4>F_4, have been observed by high energy-resolution neutron inelastic scattering. The scattering function integrated over momentum space in the paramagnetic phase reveals a long-tailed spectrum having the form of E^< -(0.35<plus-minus>0.06) > superimposed on the Lorentzian lineshape, which originates from the normal critical magnetic scattering. This observation of a power-law decay function is a direct measurement of the anomalous diffusion in a system with fractal geometry.Using neutron inelastic scattering techniques we investigated the magnetic excitations in a diluted Heisenberg antiferromagnet (RbMn_cMg_<1-c>F_3 with c=0.39) close to the percolation threshold of c_p=0.312.No indication of propagating spin waves was observed throughtout the Brillouin zone, but a broad peak superimposed on Ising-cluster excitations was observed. The origin of this broad peak is attributed to the excitation of fractons in a percolating network.