Scanning Tunneling Microscopy of Atomic Scale Phonon Standing Waves in Quasi-freestanding WSe2 Monolayers

Abstract

Using scanning tunneling microscopy (STM) we observed atomic scale interference patterns on quasi-freestanding WSe 2 islands grown on top of graphene. The bias-independent double atomic size periodicity of these patterns and the sharp Brillouin zone edge revealed by 2D STM Fourier analysis indicate formation of optical phonon standing waves due to scattering on intercalating defects supporting these islands. Standing wave patterns of both synchronized and non-synchronized optical phonons, corresponding to resonant and non-resonant phonon scattering regimes, were experimentally observed. We also found the symmetry breaking effect for individual phonon wave packets, one of the unique features distinguishing phonon standing waves. We show that vibrational and electronic anharmonicities are responsible for STM detection of these patterns. A significant contribution to the interference contrast arises from quantum zero-point oscillations.

Scanning Tunneling Microscopy Observation of Phonon Condensate

“…It seems very likely that the two types of interference patterns on STM images are associated with two different kinds of boundary conditions, decay laws, and scattering regimes 32 imposed by intercalation defects. The observed difference could be induced by two types of intercalating molecules with different masses, for example CO vs. H 2 O molecules, or it could be caused by two preferential attachment sites for intercalation defects 33 34 (see left inset of Fig. 3e ).…”