NMR Study on the Quantum Effect of Low Dimensional Spin System under High Magnetic Fields

量子自旋效应是低温磁性物理中最有趣的课题之一。其中，量子自旋能隙引起了人们的极大关注。自旋带隙的各种起源是已知的，并且它们被分为两类。一种是由于单自旋或双自旋起源，如单离子各向异性、反铁磁耦合自旋对等;另一种是由于一维反铁磁线性链中的多体量子自旋起源，如Haldron效应、自旋-Peierls跃迁等。其中，一维反铁磁中的多体自旋效应引起的量子自旋带隙已成为一个令人兴奋的问题，当Halfman预言了整数自旋的一维海森堡反铁磁体中自旋能隙的存在时。在此之前，人们一直认为在这样的系统中不存在自旋隙。S=1/2海森堡AF链的精确理论解表明基态和第一激发态之间没有自旋能隙。Si ...更多信息 当人们认为量子自旋效应在S=1/2系统中最明显时，Haldman的预言是耸人听闻的。从那时起，人们做了大量的理论和实验工作来证明Halsberg猜想的正确性，其中S=1/2均匀的Heisenberg AF链不存在自旋能隙，而键交替的链存在自旋能隙。在S=1/2 Heisenberg AF链的量子自旋带隙中，首次在无机材料CuGeO_3中观察到了自旋-Peierls跃迁。SP相是一个一维的AF链，具有自旋能隙，键间交替。为了弄清SP相中的自旋能隙，我们对具有晶体键交替的CuCl_2（γ-甲基吡啶）_2进行了NMR实验，研究了SP相中的自旋能隙。测量并比较了强磁场下自旋回波谱和核自旋-晶格弛豫。少

The quantum spin effect is one of the most interesting subjects in the physics of magnetism at low temperatures. Of these, the quantum spin-gap attracts much attention. Various origins of the spin-gap are known and they are classified into two categories. The one is due to the single or two-spin origin, such as single ion anisotropy, antiferromagnetic (AF) coupled spin pair, etc. The other is due to many body quantum spin origin in one-dimensional AF linear chain, such as the Haldane effect, the spin-Peierls transition etc.Above all, the quantum spin-gap due to the many body spin effect in one-dimensional antiferromagnet has become an exciting problem, when Haldane has predicted the existence of the spin-gap in one-dimensional Heisenberg antiferromagnet with integer spin. Until that time, it was believed that the spin-gap did not exist in such a system. The exact theoretical solution of S=1/2 Heisenberg AF chain shows no spin-gap between the ground state and the first excited state. Si … More nce it was considered that the quantum spin effect should be expected most clearly in S=1/2 system, Haldane's prediction was sensational. Since then, many theoretical and experimental works have been performed to reveal that the Haldane conjecture is true.It should be noted that the S=1/2 uniform Heisenberg AF chain has no spin-gap, while that the chain with bond alternation has a spin-gap. On the quantum spin-gap in the S=1/2 Heisenberg AF chain with bond alternation, the first observation of the spin-Peierls transition has recently been reported in an inorganic material CuGeO_3. The SP-phase, which has a spin-gap, is a one-dimensional AF chain with bond alternation. In order to clarify the spin-gap in SP-phase we have made an NMR experiment on CuCl_2 (gamma-picoline) _2, which has crystallographic bond alternation.These spin-gaps have been studied through NMR experiments. The spin echo spectrum and the nuclear spin-lattice relaxation under high magnetic fields have been performed and compared. Less