Phonons, phasons and atomic dynamics in quasicrystals.

Area of Science:

• Condensed matter physics
• Materials science
• Solid-state physics

Background:

• Quasicrystals exhibit unique atomic and dynamical properties distinct from traditional crystals.
• Phason modes are characteristic of quasicrystals and are predicted to be diffusive in the hydrodynamic theory.
• Diffuse scattering is a key experimental signature linked to quenched-in phason modes in icosahedral quasicrystals.

Purpose of the Study:

• To review and analyze phason, phonon, and atomic dynamics in quasicrystals.
• To investigate the nature of phason modes (diffusive vs. ballistic) and their experimental manifestations.
• To understand lattice dynamics and its relation to quasicrystal stability.

Main Methods:

• Analysis within the framework of hydrodynamic theory.
• Interpretation of diffuse scattering experiments.
• Study of lattice dynamics using inelastic neutron and X-ray scattering.
• Modeling with oscillating pair potentials.

Main Results:

• Long-wavelength phason modes in quasicrystals are confirmed as diffusive.
• Equilibrium phason modes in AlPdMn icosahedral phase are diffusive above 500 °C.
• Well-defined acoustic modes are observed only for wavevectors below 0.3 Å(-1), broadening significantly beyond this.
• Results are interpretable using the pseudo-Brillouin zone boundary concept.
• Oscillating pair potentials successfully reproduce experimental observations, enabling atomic-scale dynamics analysis.
• Exceptional dynamical flexibility is observed in the 1/1 approximant.

Conclusions:

• The hydrodynamic theory accurately describes phason dynamics in quasicrystals.
• Lattice dynamics is strongly influenced by wavevector magnitude and interactions.
• Oscillating pair potentials provide a powerful tool for understanding quasicrystal dynamics and stability.