Enhancing phonon moments through framework flexibility

When circularly polarized light excites two degenerate optic phonons, the induced excitation can possess a magnetic moment and pseudoangular momentum due to the circular motions of ions [1]. These moments are generally fairly small, and are difficult to detect in normal materials, let alone use in applications. Nonetheless, flexible framework materials (e.g. metal organic frameworks) possess qualities which potentially allow for large phonon moments, as they can sustain large amplitude atomic displacements without loss of stability. To take an initial step towards the rational design of these materials with enhanced anomalous phononic properties, theoretical maximum phonon moments in flexible framework materials will be discussed.

[1] D. M. Juraschek and N. A. Spaldin, Orbital magnetic moments of phonons, Phys. Rev. Materials 3, 064405.