title: Observation of T₂-like coherent optical phonons in epitaxial Ge₂Sb₂Te₅/GaSb(001) films.
abstract: The phonon spectrum of Ge₂Sb₂Te₅ is a signature of its crystallographic structure and underlies the phase transition process used in memory applications. Epitaxial materials allow coherent optical phonons to be studied in femtosecond anisotropic reflectance measurements. A dominant phonon mode with frequency of 3.4 THz has been observed in epitaxial Ge₂Sb₂Te₅ grown on GaSb(001). The dependence of signal strength upon pump and probe polarization is described by a theory of transient stimulated Raman scattering that accounts for the symmetry of the crystallographic structure through use of the Raman tensor. The 3.4 THz mode has the character of the 3 dimensional T₂ mode expected for the O(h) point group, confirming that the underlying crystallographic structure is cubic. New modes are observed in both Ge₂Sb₂Te₅ and GaSb after application of large pump fluences, and are interpreted as 1 and 2 dimensional modes associated with segregation of Sb.
authors.affiliation: School of Physics and Astronomy, University of Exeter, Stocker Road, Exeter, EX4 4QL, UK.
authors.forename: A
authors.initials: A
authors.lastname: Shalini
authors.forename: Y
authors.initials: Y
authors.lastname: Liu
authors.forename: U A S
authors.initials: UA
authors.lastname: Al-Jarah
authors.forename: G P
authors.initials: GP
authors.lastname: Srivastava
authors.forename: C D
authors.initials: CD
authors.lastname: Wright
authors.forename: F
authors.initials: F
authors.lastname: Katmis
authors.forename: W
authors.initials: W
authors.lastname: Braun
authors.forename: R J
authors.initials: RJ
authors.lastname: Hicken