High field NMR on level crossover in spin-gap systems

The quantum spin gap is one of the most interesting problems in high-field 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 anisotropy, the exchange-coupled spin pair, etc. The other is due to the many-body 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(C2H8N2)(2)NO2(ClO4)] (Haldane gap), CuCl2(gamma-picoline)(2) (S = 1/2 linear chain with the bond alternation) and Cu2Cl4(1,4-diazacycloheptane)(2) (S = 1/2 zigzag chain with the bond alternation and the second neighbor interaction), and CsFeCl3 (fictitious spin S' = 1 with the single-ion anisotropy of easy plane type). The behaviors of the nuclear spin-lattice relaxation rate around the magnetic field of level crossover are compared and discussed. (C) 1998 Elsevier Science B.V. All rights reserved.