Zero-temperature magnetic-field-induced phase transition between two ordered gapped phases in spin ladders with ferromagnetic legs

摘要

We suggest that under an increase of magnetic field, a spin ladder with ferromagnetic legs does not pass without fail through an incommensurate phase, but possibly in a straight way turns into a fully polarized ferromagnetic phase. The spin gap remains finite at the transition point. This scenario of zero-temperature first-order phase transition is demonstrated for two special solvable spin-ladder models.