High field NMR on level crossover in spin-gap systems

Abstract

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(C 2H 8N 2) 2NO 2(ClO 4)] (Haldane gap), CuCl 2(γ-picoline) 2 ( S= 1 2 linear chain with the bond alternation) and Cu 2Cl 4(1,4-diazacycloheptane) 2 ( S= 1 2 zigzag chain with the bond alternation and the second neighbor interaction), and CsFeCl 3 (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.