Nonreciprocal electron hydrodynamics under magnetic fields: Applications to nonreciprocal surface magnetoplasmons

Recent experiments have elucidated that novel nonequilibrium states inherent in the so-called hydrodynamic regime are realized in ultrapure metals with sufficiently strong momentum-conserving scattering. In this Letter, we formulate a theory of electron hydrodynamics with broken inversion symmetry under magnetic fields and find that novel terms emerge in hydrodynamic equations which play a crucial role for the realization of the nonreciprocal responses. Specifically, we clarify that there exists a novel type of nonreciprocal collective modes dubbed nonreciprocal surface magnetoplasmons arising from an interplay between magnetic fields and the orbital magnetic moment. We reveal that these nonreciprocal collective modes indeed give rise to the nonreciprocity in magneto-optical responses such as the reflectivity. The physics discussed here will bridge the two important notions magnetoplasmonics and nonreciprocity in electron hydrodynamic materials with inversion symmetry breaking and magnetic fields.