Quantum phase transitions in a quasi-one-dimensional Ising quantum magnet in transverse fields

We report neutron scattering and thermodynamic measurements of the quasi-one-dimensional Ising magnet $\mathrm{Sr}{\mathrm{Co}}_{2}{\mathrm{V}}_{2}{\mathrm{O}}_{8}$ under transverse fields along the tetragonal $a/b$ direction. Our experiments reveal a N\'eel-type magnetic order in zero field, which successively changes into a coplanar antiferromagnetic order in applied fields. The detailed evolution of the noncollinear magnetic order in magnetic fields can be understood by considering the competition between the intrinsic magnetic interactions, uniform transverse field, and effective staggered field. Moreover, a series of gapped discrete confined-spinon modes are observed in zero field. With increasing field, the spin gap progressively softens, reaching a minimum value at $\ensuremath{\sim}7\phantom{\rule{0.16em}{0ex}}\mathrm{T}$ where the N\'eel-type magnetic ordering moment is completely suppressed. This corresponds to the three-dimensional quantum critical point (QCP). The implications for multiple QCPs in this class of materials are discussed.