Light-induced Faraday effect originates from antisymmetric third-order susceptibility due to dynamical Kleinman symmetry breakdown, shown in an sp tight-binding model without requiring magnetization.
Nonadiabatic Theory of Phonon Magnetic Moments in Insulators and Metals
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abstract
We develop a nonadiabatic theory of phonon magnetic moments applicable to both insulators and metals. By relating the phonon magnetic moment to the force-velocity response of ions in a magnetic field, we derive a gauge-invariant expression using a gauge-covariant Wigner expansion. The formalism naturally separates Fermi-sea and Fermi-surface contributions and captures the full dependence on phonon frequency. In gapped systems, our theory reduces to previous adiabatic expressions in the low-frequency limit. Beyond this limit, it reveals additional contributions arising from resonant interband processes and the Fermi surface. Applying our theory to Pb$_{1-x}$Sn$_x$Te, we find that the Fermi-surface contribution substantially enhances the phonon magnetic moment, reproducing the same order of magnitude as the experimental observation. Our results provide a unified framework for describing phonon magnetic moments beyond the adiabatic regime.
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Light-induced Faraday effect from dynamical breakdown of Kleinman symmetry
Light-induced Faraday effect originates from antisymmetric third-order susceptibility due to dynamical Kleinman symmetry breakdown, shown in an sp tight-binding model without requiring magnetization.