arXiv:cond-mat/0403404 · Physical Review B
Incommensurate state in a quasi-one-dimensional S=1∕2 bond-alternating antiferromagnet with frustration in magnetic fields
Takahumi Suzuki, Sei-ichiro Suga
Abstract
We investigate the critical properties of the $S=1∕2$ bond-alternating spin chain with a next-nearest-neighbor interaction in magnetic fields. By the numerical calculation and the exact solution based on the effective Hamiltonian, we show that there is a parameter region where the longitudinal incommensurate spin correlation becomes dominant around the half-magnetization of the saturation. Possible interpretations of our results are discussed. We next investigate the effects of the interchain interaction $({J}^{\ensuremath{'}})$. The staggered susceptibility and the uniform magnetization are calculated by combining the density-matrix renormalization group method with the interchain mean-field theory. For the parameters where the dominant longitudinal incommensurate spin correlation appears in the case ${J}^{\ensuremath{'}}=0$, the staggered long-range order does not emerge up to a certain critical value of ${J}^{\ensuremath{'}}$ around the half-magnetization of the saturation. We calculate the static structure factor in such a parameter region. The size dependence of the static structure factor at $k=2{k}_{F}$ implies that the system has a tendency to form an incommensurate long-range order around the half-magnetization of the saturation. We discuss the recent experimental results for the NMR relaxation rate in magnetic fields performed for pentafluorophenyl nitronyl nitroxide.