Melting of an antiferromagnetic Heisenberg spin chain by external magnetic fields

We study the magnetic-field-induced insulator - metal transition in the chain model at quarter filling. In this model the nearest-neighbour hopping energy t competes with the next-nearest-neighbour exchange energy , and for a small the system is an insulator accompanied by a charge ordering. Response to external magnetic fields is investigated numerically by the density-matrix renormalization-group method. We have found that a transition from the charge ordered insulating phase to a metallic phase occurs at a finite strength of the magnetic field for small . The transition is an example of magnetic-field-induced melting transition of a quantum antiferromagnetic Heisenberg spin chain system.