SUMMARY OF RESULTS OF THE EPSRC FUNDED RESEARCH EP/D005191/1
Phonon Engineering of Nanocomposite Thermoelectric Materials
Aims and objectives of the project:
Ab initio investigations of phonon modes and
The main objectives of the project were:
- Studies of phonon modes on compound semiconductor surfaces, such
as GaAs(001)-beta_2(2x4), by employing ab initio
calculations based upon the pseudopotential method, density
functional theory, and a linear response technique.
- Generalisation, and application, of an adiabatic bond charge
model to investigate phonon modes on compound semiconductor
surfaces, such as GaAs(001)-beta_2(2x4). Assessment of
accuracy of the method by comparing results with the results from
the ab initio method in (i) above.
- Studies of electron-phonon interaction in Mg based
antiperovskite superconductors and semiconductors by employing the
ab initio method in (1) above.
All the three objectives of this project have been achieved. A brief
- Zone-centre atomic vibrational modes on the
GaAs(001)-beta_2(2x4) surface have been determined by
applying a combination of the ab initio plane wave
pseudopotential method and a linear response scheme. Seven
significant modes have been identified and their energy locations
and polarisation characteristics explained. It is found that the
highest surface frequency mode lies at 8.74 THz and originates from
the three-fold coordinated Ga atom in the sub-surface layer
vibrating against its neighbouring As atoms in the top and third
layers. The As-As dimer stretch mode is found to lie at 6.33 THz.
Energy locations and polarisation characteristics of As--As dimer
rocking and swinging phonon modes have also been identified and
discussed. The zone-centre results have been published in Phys.
Rev. B 74 (2006) 073307:1-3. Full dispersion results are nearly
complete and will soon be analysed and submitted for publication.
- Our previously developed adiabatic bond charge model for
calculating phonons on Si(001) and zinc-blende(110) surfaces has
been generalised and applied to deal with complex surface
geometries. Full dispersion relation and the density of states for
phonon branches on the GaAs(001)-beta_2(2x4) surface have
been calculated. The results have been compared and contrasted with
the zone-centre results obtained by applying the much more
computationally demanding ab initio method adopted in part (1)
of the project.
The results will soon be submitted for publication in an
- Ab initio calculations have been performed of the
electronic structure, phonon dispersion relations, and
electron-phonon interaction in cubic MgXNi_3 (X=B, C and N).
Our work suggests that, in contrast to previous works, there is no
dip in the dispersion of the lowest acoustic branch along the
symmetry direction Gamma-X, but predicts a shallow dip in the
dispersion of this branch along the other two principal symmetry
directions, viz. Gamma-M and Gamma-R. The lowest phonon
branch in MgNNi_3 is characterised by instability over a large
region in the reciprocal space, suggesting that this compound is
structurally unstable in the cubic phase. For all stable phonon
modes the electron-phonon coupling parameter lambda is higher for
MgCNi_3 than for the other two materials. The strong
electron-phonon coupling interaction in MgCNi_3 clearly indicates
that this material can be described as a BCS-type superconductor.
This work has been published in J. Phys.: Condensed
Matter, 18 (2006) 11089-11101.
- In addition to the main objectives discussed in (1-3) above, the
visitor and the PI collaborated on the number of on-going projects.
Some of these works have been published and/or submitted for
publication. The PI also made an oral presentation, and a poster
presentation, at the European Conference on Surface Science