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Condensed Matter, Mesoscale and Nanoscale Physics, cond-mat.mes-hall
Abstract:
The Valley Hall effect provides a popular route to engineer robust waveguides
for bosonic excitations such a photons and phonons. The almost complete absence
of backscattering in many experiments has its theoretical underpinning in a
smooth-envelope approximation that neglects large momentum transfer and is
accurate only for small bulk band gaps and/or smooth domain walls. For larger
bulk band gaps and hard domain walls backscattering is expected to become
significant. Here, we show that in this experimentally relevant regime, the
reflection of a wave at a sharp corner becomes highly sensitive on the
orientation of the outgoing waveguide relative to the underlying lattice.
Enhanced backscattering can be understood as being triggered by resonant
tunneling transitions in quasimomentum space. Tracking the resonant tunneling
energies as a function of the waveguide orientation reveals a self-repeating
fractal pattern that is also imprinted in the density of states and the
backscattering rate at a sharp corner.
