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 electron-phonon interaction

The main objectives of the project were:

1. 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.
2. 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.
3. 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 summary follows:

1. 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.
2. 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 international journal.

3. 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.

4. 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 (ECOSS24).