Magnetization process and topological plateau phase induced by circularly polarized laser

One of the fundamental experiments to investigate magnetic properties of materials is a measurement of magnetization curve. Antiferromagnets with large exchange couplings, however, need high external field to achieve their saturated magnetization, and large equipment is required in experiments. We theoretically propose a new and dynamic way to realize magnetization processes of general quantum magnets without any static field. The way is to apply a circularly polarized laser to magnetic systems. We can show that the coupling between the laser and magnets is mapped to an effective static Zeeman term with a longitudinal magnetic field via a time-dependent unitary transformation or Floquet theory. It is hence expected that the magnetization curve of magnets can be realized by applying a suitable laser. We demonstrate dynamical magnetization processes by numerically solving Schr\"odinger equations for concrete quantum spin models under applied lasers. We also show that a laser-induced magnetization plateau state appears in a simple Ferro-Ferro-Antiferro spin chain model under a certain condition and it has a topological nature.