Derek A. Stewart

Intrinsic lattice thermal conductivity of semiconductors from first principles

The original version of this article may be found at the Applied Physics Letters website: http://dx.doi.org/10.1063/1.2822891 Copyright (2007) American Institute of Physics

Interface effects in spin-dependent tunneling

Abstract In the past few years the phenomenon of spin-dependent tunneling (SDT) in magnetic tunnel junctions (MTJs) has aroused enormous interest and has developed into a vigorous field of research. The large tunneling magnetoresistance (TMR) observed in MTJs garnered much attention due to possible application in random access memories and magnetic field sensors. This led to a number of fundamental questions regarding the phenomenon of SDT. One such question is the role of interfaces in MTJs and their effect on the spin polarization of the tunneling current and TMR. In this paper we consider different models which suggest that the spin polarization is primarily determined by the electronic and atomic structure of the ferromagnet/insulator interfaces rather than by their bulk properties. First, we consider a simple tight-binding model which demonstrates that the existence of interface states and their contribution to the tunneling current depend on the degree of hybridization between the orbitals on metal and insulator atoms. The decisive role of the interfaces is further supported by studies of spin-dependent tunneling within realistic first-principles models of Co/vacuum/Al, Co/Al 2 O 3 /Co, Fe/MgO/Fe, and Co/SrTiO 3 /Co MTJs. We find that variations in the atomic potentials and bonding strength near the interfaces have a profound effect resulting in the formation of interface resonant states, which dramatically affect the spin polarization and TMR. The strong sensitivity of the tunneling spin polarization and TMR to the interface atomic and electronic structure dramatically expands the possibilities for engineering optimal MTJ properties for device applications.

Role of light and heavy embedded nanoparticles on the thermal conductivity of SiGe alloys

We have used an atomistic {\it ab initio} approach with no adjustable parameters to compute the lattice thermal conductivity of Si

First-principles calculation of the isotope effect on boron nitride nanotube thermal conductivity.

_{0.5}

Twisting phonons in complex crystals with quasi-one-dimensional substructures

Ge

Ab Initio Thermal Transport

_{0.5}

Two-Dimensional Intrinsic Half-Metals With Large Spin Gaps

with a low concentration of embedded Si or Ge nanoparticles of diameters up to 4.4 nm. Through exact Greens function calculation of the nanoparticle scattering rates, we find that embedding Ge nanoparticles in

Contribution of d-band electrons to ballistic transport and scattering during electron-phonon nonequilibrium in nanoscale Au films using an ab initio density of states

\text{Si}_{0.5}\text{Ge}_{0.5}

Ab initio investigation of phonon dispersion and anomalies in palladium

provides 20% lower thermal conductivities than embedding Si nanoparticles. This contrasts with the Born approximation which predicts an equal amount of reduction for the two cases, irrespective of the sign of the mass difference. Despite these differences, we find that the Born approximation still performs remarkably well, and it permits investigation of larger nanoparticle sizes, up to 60 nm in diameter, not feasible with the exact approach.

Enhanced oxygen vacancy diffusion in Ta2O5 resistive memory devices due to infinitely adaptive crystal structure

Isotopic composition can dramatically affect thermal transport in nanoscale heat conduits such as nanotubes and nanowires. A 50% increase in thermal conductivity for isotopically pure boron ((11)B) nitride nanotubes was recently measured, but the reason for this enhancement remains unclear. To address this issue, we examine thermal transport through boron nitride nanotubes using an atomistic Greens function transport formalism coupled with phonon properties calculated from density functional theory. We develop an independent scatterer model for (10)B defects to account for phonon isotope scattering found in natural boron nitride nanotubes. Phonon scattering from (10)B dramatically reduces phonon transport at higher frequencies and our model accounts for the experimentally observed enhancement in thermal conductivity.