Two gaps in (VO)(2)P2O7: Observation using high-field magnetization and NMR

The alternating one-dimensional Heisenberg S = 1/2 chain compound with spin gap, (VO)(2)P2O7, has been studied by the high-field magnetization and P-3 1 NMR measurements. Two critical phenomena at 25 +/- 1 and 46 +/- 2 T are o bserved in the magnetization curve, which are considered to be a crossover between the ground state and the excited state in two kinds of gap systems. P-31 NMR also revealed the coexistence of two magnetic systems with indepe ndent spin gaps. Results obtained from the present and previous studies are well explained by the model in which two kinds of magnetic chains along th e b axis behave as alternating spin chains with independent gap energies.