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

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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