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We study the interplay between collective electronic and lattice modes in
polar metals in an applied magnetic field aligned with the polar axis. Static
spin-orbit coupling leads to the appearance of a particle-hole spin-flip
continuum that is gapped at low energies in a finite field. We find that a weak
spin-orbit assisted coupling between electrons and polar phonons leads to the
appearance of new electronic collective modes. The strength of the applied
magnetic field tunes the number of modes and their energies, which can lie both
above and below the particle-hole continuum. For a range of field values, we
identify Fano-like interference between the electronic continuum and phonons.
We show that signatures of these collective modes can be observed in electron
spin resonance experiments, and we provide the corresponding theoretical
predictions.
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