Crossover behavior of magnetic excitations in the anisotropic haldane antiferromagnet under high magnetic fields

Field-induced magnetic phases are investigated in the one-dimensional S = 1 Haldane system with the easy-plane anisotropy under external magnetic fields along an arbitrary direction, using the density-matrix renormalization group method. At a critical field the system exhibits the quantum phase transition from the nonmagnetic Haldane phase to a magnetic phase. It is found that the quantum analog of the classical soliton bearing system is induced except for the case where the field is applied perpendicular to the easy plane. In the exceptional case, Tomonaga-Luttiger spin-fluid state is realized. The angle dependence of the critical field is discussed in connection with two consecutive field-induced phase transitions observed in NDMAP. Possible implications for such novel transitions are given.