Field-Induced Dimer Orders In Quantum Spin Chains

Field-induced excitation gaps in quantum spin chains are an interesting phenomenon related to confinements of topological excitations. In this paper I present a type of this phenomenon. I show that an effective magnetic field with a fourfold screw symmetry induces the excitation gap accompanied by dimer orders. The dimer order parameter and the excitation gap exhibit characteristic power-law dependence on the fourfold screw-symmetric field. Moreover, the field-induced dimer order and the field-induced Neel order coexist when the external uniform magnetic field, the fourfold screw-symmetric field, and the twofold staggered field are applied. This situation is in close connection with a compound [Cu(pym)(H2O)(4)]SiF6 center dot H2O [J. Liu et al., Phys. Rev. Lett. 122, 057207 (2019)]. In this paper I discuss a mechanism of field-induced dimer orders by using a density-matrix renormalization group method, a perturbation theory, and quantum field theories.