paper

Negative Refraction of Terahertz Phonons via Interfacial Momentum Compensation

arXiv:2511.03599 · doi:10.1103/n9zv-9kc5

Abstract

Negative refraction provides a route to steer and focus wave energy flow, but it remains difficult to realize for coherent terahertz (THz) phonons. The difficulty stems from conventional dispersion-based mechanisms, which require strongly anisotropic or negative-curvature dispersions, while the long-wavelength acoustic phonons most favorable for coherent transport are nearly isotropic. Here we overcome this limitation by introducing a momentum compensation mechanism mediated by discrete translational symmetry. Discrete translational symmetry parallel to the interface supplies a compensating tangential momentum, reopening transmitted channels beyond the conventional critical condition and enabling negative refraction when this compensation reverses the tangential component. Mode-resolved calculations for hBN/graphene heterostructures establish this mechanism in laterally stitched in-plane interfaces and show how twisted van der Waals moire superlattices shift the negative-refraction window to lower frequencies. These results identify periodic crystalline interfaces as symmetry-engineered elements for THz phonon momentum conversion and wavefront control.

Accepted for publication in Physical Review Letters. Revised to match the accepted manuscript; the title, author list, text, figures, and references have been updated

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