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Photoinduced Pseudospin Polarization in a Three-Orbital Hubbard Model

In a Hubbard model for the Kitaev spin-liquid candidate material $α$-RuCl$_3$ with three $t_{2g}$ orbitals per Ru site, we calculate photoinduced dynamics based on the exact diagonalization method and interpret them with the help of a high-frequency expansion in quantum Floquet theory. The high-frequency expansion shows two types of effective magnetic fields during the application of a circularly polarized light field. One of them originates from spin-orbit coupling and is within the honeycomb lattice. The other is of purely kinetic origin and perpendicular to the lattice. The former fields are antiparallel at the two sites within a unit cell and rotate in accordance with the momentum distribution of holes that follow the light field. When the light field is weak, pseudospin dynamics are governed by the former fields; thus, the average of the pseudospins almost vanishes. The latter fields are parallel at the two sites within a unit cell and produce nonzero perpendicular components of the pseudospins when the light field is strong. Numerically obtained perpendicular components are consistent with the latter fields when the frequency of the light field is well below the Mott gap. The relevance to the inverse Faraday effect recently observed in $α$-RuCl$_3$ is discussed.