Ultrafast dephasing of coherent optical phonons in atomically controlled GeTe/Sb$_{2}$Te$_{3}$ superlattices

TL;DR

This study investigates femtosecond dynamics of coherent optical phonons in GeTe/Sb$_{2}$Te$_{3}$ superlattices, revealing ultrafast dephasing mechanisms influenced by defect scattering and temperature effects.

Contribution

It provides new insights into phonon damping mechanisms in superlattices, highlighting the role of defects and temperature dependence in phonon coherence decay.

Findings

Coherent A$_{1}$ optical modes observed at 5.10 THz and 3.78 THz in as-grown SLs.

Only the 3.68 THz mode observed in annealed SLs.

Decay rate in annealed SLs is strongly temperature dependent, unlike in as-grown SLs.

Abstract

Femtosecond dynamics of coherent optical phonons in GeTe/Sb$_{2}$Te$_{3}$ superlattices (SLs), a new class of semiconductor SLs with three different states, have been investigated by using a reflection-type pump-probe technique at various lattice temperatures. The time-resolved transient reflectivity (TR) obtained in as-grown SLs exhibits the coherent A$_{1}$ optical modes at 5.10 THz and 3.78 THz, while only the single A$_{1}$ mode at 3.68 THz is observed in annealed SLs. The decay rate of the A$_{1}$ mode in annealed SLs is strongly temperature dependent, while that in as-grown SLs is not temperature dependent. This result indicates that the damping of the coherent A$_{1}$ phonons in amorphous SLs is governed by the phonon-defect (vacancy) scattering rather than the anharmonic phonon-phonon coupling.