WFU Physics Colloquium

TITLE: A unified approach to nanoscale charge and energy transport

SPEAKER: Dr. Derek Stewart,

TIME: Thursday Jan. 31, 2008 at 4:00 PM

PLACE: Room 101 in Olin Physical Laboratory

With the development of nanoscale electronics and the push towards molecular devices, the line delineating device and contacts has effectively disappeared. The development of an atomistic framework for charge and heat transport is essential. I will discuss a unified approach to nanoscale transport that combines density functional theory and non-equilibrium Green's function (NEGF) formalism. The synergy of these fields provides a natural language for the flow of electrons and phonons in nanostructures. I will highlight the power of this combined approach with two examples: the role of interfaces in magnetic tunnel junctions and a new ab-initio approach to thermal conductivity.

Magnetic tunnel junctions consist of two magnetic leads separated by a thin oxide layer. Switching the magnetic orientation of the leads from parallel to antiparallel can result in dramatic changes in resistance. In traditional models, spin polarization in these devices is determined by the bulk properties of the magnetic leads. However, recent experiments for junctions with identical magnetic leads and different oxide regions indicate that the spin polarization can differ and even flip sign[1]. Using a first principles NEGF transport approach, I will show that the traditional model fails and that changes in the spin polarization can be traced to the electrode-oxide interfaces and oxide band structure[2].

Many present thermal transport models still rely on semi-classical models that lack predictive power. I will introduce the first ab-initio approach for thermal conductivity where harmonic and anharmonic interatomic force constants (IFCs) are calculated using density functional perturbation theory. For bulk materials, these IFCs are used to determine phonon scattering in the phonon Boltzmann equation. Excellent agreement is found with experiment for silicon and germanium for temperatures from 100K to 300K[3]. The ab-initio IFCs are also used in a NEGF formalism to study phonon transport in nanoscale heat conduits such as carbon nanotubes[4].

1. J. M. De Teresa et al., Phys. Rev. Lett. 82, 4288 (1999); Science, 286, 507 (1999).
2. J. P. Velev et al., Phys. Rev. Lett., 95, 216601 (2005)
3. D. A. Broido et al., Appl. Phys. Lett., 91, 231922 (2007)
4. N. Mingo et al., Phys. Rev. B (accepted for publication) (2008)