Light-driven octupolar inverse Faraday effect and multipolar order in Mott insulators

TL;DR

This paper demonstrates how circularly polarized light can control and detect hidden multipolar orders in spin-orbit-coupled Mott insulators, revealing new nonequilibrium phases and structural signatures.

Contribution

It introduces a Floquet-driven approach to generate and manipulate octupolar order and multipolar interactions, expanding the understanding of light-matter coupling in correlated quantum systems.

Findings

Light induces an effective static field coupling to magnetic octupoles.

A bond-dependent anisotropic exchange interaction is generated by light.

Optical control enables access to novel multipolar phases and structural distortions.

Abstract

Hidden multipolar orders in spin-orbit-coupled Mott insulators provide a promising setting for correlated quantum matter, yet their control and detection remain major challenges. Here, we demonstrate that circularly polarized light enables both in $4d^2/5d^2$ systems with edge-sharing octahedra. Using a Floquet Schrieffer-Wolff expansion of a driven Hubbard-Kanamori model, we derive a low-energy multipolar Hamiltonian with two qualitatively new light-driven terms. One is an effective static field that couples linearly to the magnetic octupole, realizing an octupolar inverse Faraday effect. The other is a bond-dependent anisotropic exchange interaction absent in equilibrium. These two couplings are the key result of this work: the first provides a direct optical handle on hidden octupolar order, while the second reorganizes the multipolar exchange landscape and opens an enlarged Kitaev-like multipolar liquid regime. Their interplay produces a nonequilibrium multipolar phase space inaccessible in equilibrium, enabling optical tuning among antiferro-octupolar, ferro-octupolar, partially polarized ferro-quadrupolar, Ising octupolar, and multipolar liquid phases. We further show that the induced multipolar order couples to the lattice, generating reversible trigonal and tetragonal distortions that provide structural fingerprints in pump-probe experiments. Our work establishes a general mechanism for the optical generation, control, and detection of hidden multipolar quantum states.