HIGH-FIELD NMR ON LEVEL CROSSOVER IN SPIN-GAP SYSTEMS

The quantum spin gap is one of the most interesting problems in high-f ield magnetism. Various origins of the spin gap are known. The one is due to the single or two-spin origin, such as the single-ion anisotrop y, the exchange-coupled spin pair, etc. The other is due to the many-b ody quantum spin origin in one-dimensional chain. In order to clarify the property of these spin gaps, a systematic NMR experiment up to 15 T below 4.2 K has been performed for spin gap materials of NENP [Ni(C2 H8N2)(2)NO2(ClO4)] (Haldane gap), CuCl2(gamma-picoline)(2) (S = 1/2 li near chain with the bond alternation) and Cu2Cl4(1,4-diazacycloheptane )(2) (S = 1/2 zigzag chain with the bond alternation and the second ne ighbor interaction), and CsFeCl3 (fictitious spin S' = 1 with the sing le-ion anisotropy of easy plane type). The behaviors of the nuclear sp in-lattice relaxation rate around the magnetic field of level crossove r are compared and discussed. (C) 1998 Elsevier Science B.V. All right s reserved.