Spin Excitations of the S = 1/2 One-Dimensional Ising-Like Antiferromagnet BaCo2V2O8 in Transverse Magnetic Fields

The high-field multifrequency electron spin resonance (ESR), the magnetization, and the specific heat of single crystals of BaCo2V2O8 (a quasi-one-dimensional Ising-like antiferromagnet with S = 1/2) were measured in two different transverse magnetic fields: H || [100] and H || [110]. A softening of the lowest ESR mode was observed near the saturation field of the magnetization around 40 T in H || [110], whereas it occurs around 10 T in H || [100], far below the saturation field. These observations were reproduced by numerical calculations using the density-matrix renormalization group methods, including the staggered magnetic fields induced by a uniform external magnetic field and the interchain magnetic exchange interaction. We found that the directional dependence of the lowest mode originates mainly from the remarkable angular dependence of the induced magnetic field. The interchain interaction is also quantitatively important for reproducing the experimental results. The directional dependence of the lowest mode is demonstrated by the particle–hole excitation of spinons using the Jordan–Wigner transformation.