1,973 research outputs found
Study of Strain and Temperature Dependence of Metal Epitaxy
Metallic films are important in catalysis, magneto-optic storage media, and
interconnects in microelectronics, and it is crucial to predict and control
their morphologies. The evolution of a growing crystal is determined by the
behavior of each individual atom, but technologically relevant structures have
to be described on a time scale of the order of (at least) tenths of a second
and on a length scale of nanometers. An adequate theory of growth should
describe the atomistic level on very short time scales (femtoseconds), the
formation of small islands (microseconds), as well as the evolution of
mesoscopic and macroscopic structures (tenths of seconds).
The development of efficient algorithms combined with the availability of
cheaper and faster computers has turned density functional theory (DFT) into a
reliable and feasible tool to study the microscopic aspects of growth phenomena
(and many other complex processes in materials science, condensed matter
physics, and chemistry). In this paper some DFT results for diffusion
properties on metallic surfaces are presented. Particularly, we will discuss
the current understanding of the influences of strain on the diffusion (energy
barrier and prefactor) of a single adatom on a substrate.
A DFT total energy calculation by its nature is primarily a static
calculation. An accurate way to describe the spatial and temporal development
of a growing crystal is given by kinetic Monte Carlo (KMC). We will describe
the method and its combination with microscopic parameters obtained from ab
initio calculations. It is shown that realistic ab initio kinetic Monte Carlo
simulations are able to predict an evolving mesoscopic structure on the basis
of microscopic details.Comment: 25 pages, 6 figures, In: ``Morphological Organisation during
Epitaxial Growth and Removal'', Eds. Z. Zhang, M. Lagally. World Scientific,
Singapore 1998. other related publications can be found at
http://www.rz-berlin.mpg.de/th/paper.htm
First-principles studies of kinetics in epitaxial growth of III-V semiconductors
We demonstrate how first-principles calculations using density-functional
theory (DFT) can be applied to gain insight into the molecular processes that
rule the physics of materials processing. Specifically, we study the molecular
beam epitaxy (MBE) of arsenic compound semiconductors. For homoepitaxy of GaAs
on GaAs(001), a growth model is presented that builds on results of DFT
calculations for molecular processes on the beta2-reconstructed GaAs(001)
surface, including adsorption, desorption, surface diffusion and nucleation.
Kinetic Monte Carlo simulations on the basis of the calculated energetics
enable us to model MBE growth of GaAs from beams of Ga and As_2 in atomistic
detail. The simulations show that island nucleation is controlled by the
reaction of As_2 molecules with Ga adatoms on the surface. The analysis reveals
that the scaling laws of standard nucleation theory for the island density as a
function of growth temperature are not applicable to GaAs epitaxy. We also
discuss heteroepitaxy of InAs on GaAs(001), and report first-principles DFT
calculations for In diffusion on the strained GaAs substrate. In particular we
address the effect of heteroepitaxial strain on the growth kinetics of
coherently strained InAs islands. The strain field around an island is found to
cause a slowing-down of material transport from the substrate towards the
island and thus helps to achieve more homogeneous island sizes.Comment: 12 pages, 7 figures, REVTeX, Final version to appear in Appl. Phys. A
(2002). Other related publications can be found at
http://www.fhi-berlin.mpg.de/th/paper.htm
Limit Theorems and Governing Equations for Levy Walks
The Levy Walk is the process with continuous sample paths which arises from
consecutive linear motions of i.i.d. lengths with i.i.d. directions. Assuming
speed 1 and motions in the domain of beta-stable attraction, we prove
functional limit theorems and derive governing pseudo-differential equations
for the law of the walker's position. Both Levy Walk and its limit process are
continuous and ballistic in the case beta in (0,1). In the case beta in (1,2),
the scaling limit of the process is beta-stable and hence discontinuous. This
case exhibits an interesting situation in which scaling exponent 1/beta on the
process level is seemingly unrelated to the scaling exponent 3-beta of the
second moment. For beta = 2, the scaling limit is Brownian motion
Structural stability, magnetic and electronic properties of Co2MnSi(001)/MgO heterostructures: A density functional theory study
A computational study of the epitaxial Co2MnSi(001)/MgO(001) interface
relevant to tunneling magnetoresistive (TMR) devices is presented. Employing ab
initio atomistic thermodynamics, we show that the Co- or MnSi-planes of
bulk-terminated Co2MnSi form stable interfaces, while pure Si or pure Mn
termination requires non-equilibrium conditions. Except for the pure Mn
interface, the half-metallic property of bulk Co2MnSi is disrupted by interface
bands. Even so, at homogeneous Mn or Co interfaces these bands contribute
little to the minority-spin conductance through an MgO barrier, and hence such
terminations could perform strongly in TMR devices.Comment: 4 pages, 3 fig
Density Functional Theory of Epitaxial Growth of Metals
This chapter starts with a summary of the atomistic processes that occur
during epitaxy. We then introduce density functional theory (DFT) and describe
its implementation into state-of-the-art computations of complex processes in
condensed matter physics and materials science. In particular we discuss how
DFT can be used to calculate parameters of microscopic processes such as
adsorption and surface diffusion, and how they can be used to study the
macroscopic time and length scales of realistic growth conditions. This meso-
and macroscopic regime is described by the ab initio kinetic Monte Carlo
approach. We discuss several specific theoretical studies that highlight the
importance of the different diffusion mechanisms at step edges, the role of
surfactants, and the influence of surface stress. The presented results are for
specific materials (namely silver and aluminum), but they are explained in
simple physical pictures suggesting that they also hold for other systems.Comment: 55 pages, 20 figures, to be published "Growth of Ultrathin Epitaxial
Layers", The Chemical Physics of Soild Surfaces, Vol. 8, Eds D. A. King and
D. P. Woodruff (Elsevier Science, Amsterdam, 1997
Effect of the cluster size in modeling the H_2 desorption and dissociative adsorption on Si(001)
Three different clusters, Si_9H_12, Si_15H_16, and Si_21H_20, are used in
density-functional theory calculations in conjunction with ab initio
pseudopotentials to study how the energetics of H_2 dissociativ e adsorption on
and associative desorption from Si(001) depends on the cluster size. The
results are compared to five-layer slab calculations using the same
pseudopotentials and high qu ality plane-wave basis set. Several
exchange-correlation functionals are employed. Our analysis suggests that the
smaller clusters generally overestimate the activation barriers and reaction
energy. The Si_21H_20 cluster, however, is found to predict reaction
energetics, with E_{a}^{des}=56 +- 3 kcal/mol (2.4 +- 0.1 eV), reasonably close
(though still different) to that obtained from the slab calculations.
Differences in the calculated activation energies are discussed in relation to
the efficiency of clusters to describe the properties of the clean Si(001)-2x1
surface.Comment: 10 pages, 6 figures, submitted to J. Chem. Phy
First-principles statistical mechanics study of the stability of a sub-nanometer thin surface oxide in reactive environments: CO oxidation at Pd(100)
We employ a multiscale modeling approach to study the surface structure and
composition of a Pd(100) model catalyst in reactive environments. Under gas
phase conditions representative of technological CO oxidation (~1 atm, 300-600
K) we find the system on the verge of either stabilizing sub-nanometer thin
oxide structures or CO adlayers at the surface. Under steady-state operation
this suggests the presence or continuous formation and reduction of oxidic
patches at the surface, which could be key to understand the observable
catalytic function.Comment: 4 pages including 2 figures; related publications can be found at
http://www.fhi-berlin.mpg.de/th/th.htm
Terahertz conductivity of the heavy-fermion compound UNi2Al3
We have studied the optical properties of the heavy-fermion compound UNi2Al3
at frequencies between 100 GHz and 1 THz (3 cm^-1 and 35 cm^-1), temperatures
between 2 K and 300 K, and magnetic fields up to 7 T. From the measured
transmission and phaseshift of radiation passing through a thin film of
UNi2Al3, we have directly determined the frequency dependence of the real and
imaginary parts of the optical conductivity (or permittivity, respectively). At
low temperatures the anisotropy of the optical conductivity along the a- and
c-axes is about 1.5. The frequency dependence of the real part of the optical
conductivity shows a maximum at low temperatures, around 3 cm^-1 for the a-axis
and around 4.5 cm^-1 for the c-axis. This feature is visible already at 30 K,
much higher than the Neel temperature of 4.6 K, and it does not depend on
external magnetic fields as high as 7 T. We conclude that this feature is
independent of the antiferromagnetic order for UNi2Al3, and this might also be
the case for UPd2Al3 and UPt3, where a similar maximum in the optical
conductivity was observed previously.Comment: 7 pages, 9 figure
First-principles, atomistic thermodynamics for oxidation catalysis
Present knowledge of the function of materials is largely based on studies
(experimental and theoretical) that are performed at low temperatures and
ultra-low pressures. However, the majority of everyday applications, like e.g.
catalysis, operate at atmospheric pressures and temperatures at or higher than
300 K. Here we employ ab initio, atomistic thermodynamics to construct a phase
diagram of surface structures in the (T,p)-space from ultra-high vacuum to
technically-relevant pressures and temperatures. We emphasize the value of such
phase diagrams as well as the importance of the reaction kinetics that may be
crucial e.g. close to phase boundaries.Comment: 4 pages including 2 figure files. Submitted to Phys. Rev. Lett.
Related publications can be found at
http://www.fhi-berlin.mpg.de/th/paper.htm
Density-functional theory study of half-metallic heterostructures: interstitial Mn in Si
Using density-functional theory within the generalized gradient
approximation, we show that Si-based heterostructures with 1/4 layer
-doping of {\em interstitial} Mn (Mn) are
half-metallic. For Mn concentrations of 1/2 or 1 layer, the
states induced in the band gap of -doped heterostructures still display
high spin polarization, about 85% and 60%, respectively. The proposed
heterostructures are more stable than previously assumed -layers of
{\em substitutional} Mn. Contrary to wide-spread belief, the present study
demonstrates that {\em interstitial} Mn can be utilized to tune the magnetic
properties of Si, and thus provides a new clue for Si-based spintronics
materials.Comment: 5 pages, 4 figures, PRL accepte
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