Optical selection rules of the magnetic excitation in the S=1/2 one-dimensional Ising-like antiferromagnet BaCo2V2O8

In this study, we clarify the selection rules of optical transitions from the ground state to the magnetic excited states in the quasi-one-dimensional quantum Ising-like antiferromagnet BaCo2V2O8 with spin S = 1/2 using the high-field electron spin resonance measurement with illuminating polarized electromagnetic waves. We demonstrate that the unconventional magnetic excitation via the pair creation of quasipaticles at wave numbers q(z) = pi/2 and pi in the field-induced quantum critical state can couple with both oscillating magnetic and electric fields. Our density matrix renormalization group calculations indicate that the observed selection rules can be explained by two- and four-fold periodical spin interactions, originating from the screw chain structure in BaCo2V2O8, in addition to the magnetoelectric coupling between spins and electric dipoles.