  Field-induced excitation gaps in quantum spin chains are an interesting
phenomenon related to confinements of topological excitations. In this paper, I
present a novel 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 gap and dimer orders induced so exhibit characteristic
power-law dependence on the fourfold screw-symmetric field. Moreover, the
field-induced dimer order and the field-induced N\'eel 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)(H$_2$O)$_4$]SiF$_6$ [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.
