High field polarization and magnetoelectric coupling in chiral [Cu(pym)(H₂O)₄]SiF₆H₂O

The Heisenberg antiferromagnetic chain is a canonical model for understanding many-body gaps that emerge in quantum magnets and, as a result, there has been significant work on this class of materials for much of the past century. Chiral chains, on the other hand, have received markedly less attention. [Cu(pym)(H₂O)₄]SiF₆H₂O is an S = 1/2 chiral antiferromagnet with an unconventional spin gap and no long-range ordering at zero field, features that distinguish it from more conventional spin chains. Here, we report high field electrical polarization measurements, strong magnetoelectric coupling, and extraordinary magnetic field - temperature phase diagrams for this system. In addition to three low-field transitions, we observe a series of phase transitions near magnetic saturation that depend upon the field direction. These results imply long-range ordering across the entire field range, underscoring the importance of Dzyaloshinskii–Moriya and transverse interactions when the field is strong enough to compete with Heisenberg exchange in the intrachain direction. In particular, electrical polarization perpendicular to the chiral chain requires symmetry breaking in the formerly square ab plane - an effect that requires adjustments to the working model. These findings open the door to the study of quantum magnetism in new settings and provide exciting opportunities to reveal properties and states of matter under external stimuli that capitalize on the symmetry breaking that only chirality can deliver.