INTRODUCTION
The recent discovery of 40 K superconductivity in
MgB2 has triggered enormous interest in this
intercalated graphite-like system.
CENTRAL QUESTION:
What is the origin of high TC in
MgB2, electron-phonon or other
exotic mechanism?
TO ANSWER THIS QUESTION:
We carried out first principles lattice dynamics calculations
using supercell and direct finite displacement approach. The
calculations are compared with inelastic neutron scattering
measurements. In particular, we studied anharmonicity of the
phonons, their pressure dependencies, and
the electron-phonon coupling in MgB2.
METHOD OF COMPUTATIONS:
The calculations were performed using the pseudopotential plane
wave method (CASTEP) within the generalized gradient approximation.
We employed plane waves with an energy cutoff of 500 eV, and
the ultra-soft pseudopotential for Mg and B. The total energy
and forces converged within 0.5 meV/atom and 0.01 ev/A, respectively.
Brillouin-zone integration were carried out using dk=0.02
A-1, generating 15x15x11 and 9x9xk k-points for
1x1x1 and 2x2x2 supercells, respectively.
METHOD OF EXPERIMENTS:
5 g polycrystalline sample was prepared in the usual way with
the 11B isotope to avoid neutron absorption problems.
Inelastic neutron scattering measurements were made to determine
the generalized phonon density of states (GPDOS), which is the
phonon density of states weighted by the cross section devided
by the mass of each atom. The data were collected from 7 K to
325 K on the Filter Analyzer Spectrometer in the range 5-130 meV,
and on the Fermi chopper instrument for energies of 0.5-30 meV.