Integrated Nanostructured Systems

Isosurface plot of the defect state charge density associated with a Ga vacancy in GaN showing both the localized nature and the extended tails of the defect wavefunctions. Localization of the defect states favors spin polarization and local moment formation whereas the extended tails of the defect states mediate extremely long range magnetic interaction.

Ab initio calculations of electronic, structural, and magnetic properties of materials; nanostructured materials and their applications; high performance computing.

1. Defect-induced long-range magnetic interaction in wide-gap semiconductors

Cation vacancy induced intrinsic magnetism such as GaN, BN, and ZnO is investigated by employing density functional theory based electronic structure methods. The strong localization of defect states favors spontaneous spin polarization and local moment formation. A neutral cation vacancy in GaN or BN leads to the formation of a net moment of 3u_B with a spin-polarization energy of about 0.5 eV at the low density limit. The extended tails of defect wavefunctions, on the other hand, mediate surprisingly long-range magnetic interactions between the defect-induced moments. This duality of defect states suggests the existence of defect induced or mediated collective magnetism in these otherwise nonmagnetic sp systems.

2. Optical properties of sodium tungsten bronzes

Tungsten oxide based materials have received tremendous interest due to their potential applications in energy related applications. Surprisingly, no first-principles calculations of their optical properties have been done so far. Recently we have started investigating the optical properties (dielectric function) of these technologically important materials. Our preliminary results agree well with available experimental data. We are also investigating the origin of the color change in these materials as a function of Na concentration.

3. Quasiparticle and optical properties of II-oxides and II-nitrides

Optical properties of III-nitrides and II-oxides semiconductors are currently a subject of intensive investigation in response to future demand for energy-related optoelectronic applications. However, our understanding of the optical properties of these materials, including defect-related excited states properties, is still very limited. We have been working on new computational techniques that can substantially improve the predictive power of the theoretical calculation and in the mean time reduce the computational cost.

Density functional theory based first-principle electronic structure methods; quasiparticle properties within the GW approximation; optical properties of semiconductors and metals; phonon structure calculation; magnetic properties.

"Electron-phonon renormalization in cuprates", P. Zhang, S. G. Louie, and M. L. Cohen , Phys. Rev. Lett. 98, 067005 (2007)

"Quasiparticle energy of semicore d-electrons in ZnS: combined LDA U and GW approach", T. Miyake, P. Zhang, M. L. Cohen, and S. G. Louie, Phys. Rev. B, 74, 245213 (2006).

"Nonlocal screening, electron-phonon coupling, and phonon renormalization in metals", P. Zhang, S. G. Louie, and M. L. Cohen, Phys. Rev. Lett. 94, 225502 (2005).

"Fermi surface of NaxCoO2", P. Zhang, W. Luo, S. G. Louie, and M. L. Cohen, Phys. Rev. Lett. 93, 236402 (2004).

"Theory of B2O and BeB2 nanotubes", P. Zhang and V. H. Crespi, Phys. Rev. Lett. 89, 56403 (2002).

"Computational design of direct bandgap semiconductors that lattice match silicon", P. Zhang, V. H. Crespi, E. Chang, S. G. Louie, and M. L. Cohen, Nature 409, 69 (2001).

"Nucleation of carbon nanotubes without pentagonal rings", P. Zhang and V. H. Crespi, Phys. Rev. Lett. 83, 1791 (1999).

"Plastic deformations of carbon nanotubes", P. Zhang, P. E. Lammert, and V. H. Crespi, Phys. Rev. Lett. 81, 5346 (1998).