Incommensurate state in a quasi-one-dimensional $S=1/2$ bond-alternating antiferromagnet with frustration in magnetic fields

TL;DR

This study explores the incommensurate magnetic order in a quasi-one-dimensional $S=1/2$ bond-alternating antiferromagnet with frustration, revealing conditions for incommensurate correlations and their stability against interchain interactions.

Contribution

It provides a detailed analysis of incommensurate spin correlations in a frustrated spin chain under magnetic fields, combining numerical methods and effective Hamiltonian approaches.

Findings

Incommensurate spin correlations dominate near half-magnetization.

Interchain interactions suppress long-range order up to a critical strength.

System shows a tendency to form incommensurate long-range order.

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 ($J^{\prime}$). 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 $J^{\prime}=0$, the staggered long-range order does not emerge up to a certain critical value of $J^{\prime}$ 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 $k=2k_{\rm F}$ 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.