title: Phonon modes of MgB2: super-lattice structures and spectral response.
abstract: Micrometre-sized MgB2 crystals of varying quality, synthesized at low temperature and autogenous pressure, are compared using a combination of Raman and infra-red (IR) spectroscopy. These data, which include new peak positions in both spectroscopies for high quality MgB2, are interpreted using DFT calculations on phonon behaviour for symmetry-related structures. Raman and IR activity additional to that predicted by point group analyses of the P6/mmm symmetry are detected. These additional peaks, as well as the overall shapes of calculated phonon dispersion (PD) models are explained by assuming a double super-lattice, consistent with a lower symmetry structure for MgB2. A 2× super-lattice in the c-direction allows a simple correlation of the pair breaking energy and the superconducting gap by activation of corresponding acoustic frequencies. A consistent physical interpretation of these spectra is obtained when the position of a phonon anomaly defines a super-lattice modulation in the a-b plane.
authors.affiliation: Institute for Future Environments, Queensland University of Technology, 2 George St., Brisbane, QLD, 4001 Australia. jose.alarco@qut.edu.au.
authors.forename: Jose A
authors.initials: JA
authors.lastname: Alarco
authors.forename: Alison
authors.initials: A
authors.lastname: Chou
authors.forename: Peter C
authors.initials: PC
authors.lastname: Talbot
authors.forename: Ian D R
authors.initials: ID
authors.lastname: Mackinnon