Answer: Acoustic modes involve same-sub-lattice atoms moving in phase; optical involve opposite sublattice atoms out of phase.
- A Acoustic modes involve same-sub-lattice atoms moving in phase; optical involve opposite sublattice atoms out of phase
- B Acoustic modes actually exhibit a higher vibrational frequency throughout the Brillouin zone than optical modes do
- C Optical phonon modes are actually what carry ordinary sound waves through the crystal lattice
- D Acoustic and optical phonon modes are physically identical in dispersion and frequency in every respect
Correct answer: A. Acoustic modes involve same-sub-lattice atoms moving in phase; optical involve opposite sublattice atoms out of phase
Explanation: Acoustic: atoms in same unit cell move in phase (sound waves). Optical: atoms in different sub-lattices move out of phase (interacts with IR light). Key for understanding thermal conductivity and electron scattering.
Conductors have overlapping bands, semiconductors have a small energy gap that can be bridged by heat or doping, and insulators have a large gap that blocks conduction.
Concept context
Band theory, p-n junction, diodes, transistors, logic gates, and integrated circuits.