title: Optic phonons and anisotropic thermal conductivity in hexagonal Ge2Sb2Te5.
abstract: The lattice thermal conductivity (κ) of hexagonal Ge2Sb2Te5(h-GST) is studied via direct first-principles calculations. We find significant intrinsic anisotropy (κa/κc~2) of κ in bulk h-GST, with the dominant contribution to κ from optic phonons, ~75%. This is extremely unusual as the acoustic phonon modes are the majority heat carriers in typical semiconductors and insulators. The anisotropy derives from varying bonding along different crystal directions, specifically from weak interlayer bonding along the c-axis, which gives anisotropic phonon dispersions. The phonon spectrum of h-GST has very dispersive optic branches with higher group velocities along the a-axis as compared to flat optic bands along the c-axis. The large optic mode contributions to the thermal conductivity in low-κ h-GST is unusual, and development of fundamental physical understanding of these contributions may be critical to better understanding of thermal conduction in other complex layered materials.
authors.affiliation: Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831 USA.
authors.forename: Saikat
authors.initials: S
authors.lastname: Mukhopadhyay
authors.affiliation: Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831 USA.
authors.forename: Lucas
authors.initials: L
authors.lastname: Lindsay
authors.affiliation: Department of Physics and Astronomy, University of Missouri, Columbia, MO 65211-7010 USA.
authors.forename: David J
authors.initials: DJ
authors.lastname: Singh