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| Recent publications: Zhao, Y. J., S. Picozzi,
A. Continenza, W. T. Geng and A. J. Freeman (2002). "Possible
impurity-induced ferromagnetism in II-Ge-V-2 chalcopyrite semiconductors."
Physical Review B 65, 094415. Recently reported room-temperature
ferromagnetic (FM) semiconductors Cd1-xMnxGeP2 and Zn1-xMnxGeP2 point to
a possible important role of II-IV-V-2 chalcopyrite semiconductors in "spintronic" studies.
Here, structural, electronic, and magnetic properties of (i) Mn-doped II-Ge-VI2
(II = Zn or Cd and V = P or As) chalcopyrites and (ii) the role of S as impurity
in Cd1-xMnxGeP2 are studied by first- principles density functional calculations.
We find that the total energy of the anti ferro magnetic (AFM) state is lower
than the corresponding FM state for all systems with Mn composition x = 0.25,
0.50, and 1.0. This prediction is in agreement with a recent experimental
finding that Zn1-xMnxGeP2 experiences a FM to AFM transition for T less than
47 K. Furthermore, a possible transition to the half-metallic FM phase is
predicted in Cd1-xMnxGeP2 due to the electrons introduced by n-type S doping,
which indicates the importance of carriers for FM coupling in magnetic semiconductors.
As expected, the total magnetic moment for the FM phase is reduced by one
mu(B) with each S substituting P. Zhao, Y. J., W. T. Geng,
A. J. Freeman and B. Delley (2002). "Structural, electronic,
and magnetic properties of alpha- and beta-MnAs: LDA and GGA investigations."
Physical Review B 65, 113202. Zinc-blende (alpha-)
and NiAs-type (beta-) MnAs are investigated with a combined first-principles
linearized argumented plane wave and DMol(3) study within both the local
density approximation (LDA) and the generalized gradient approximation (GGA).
First-principles calculations within the GGA predict the lattice volume for
b- MnAs much better than LDA (which underestimates it by 15%) compared with
experiment. The LDA calculated equilibrium lattice volume of alpha-MnAs is
10% smaller than that of GaAs, which is in contradiction to the well-accepted
fact that the lattice volume of Ga1-xMnxAs increases with x. In contrast,
the GGA predicts a reasonable lattice volume for alpha-MnAs. The ferromagnetic
alpha-MnAs is shown to be a metal at a=5.7 Angstrom, and to undergo a transition
to a half-metallic phase when it expands to a>5.8 Angstrom due to the
decreased bandwidth. Further, the calculated cohesive energy of beta-MnAs
is nearly 0.87 eV greater than that of alpha-MnAs, which provides theoretical
support for the instability of alpha-MnAs. Zhao, Y. J. and A. J. Freeman
(2002). "First-principles prediction of a new class of ferromagnetic
semiconductors." Journal of Magnetism and Magnetic Materials
246, 145. Ferromagnetism in I-III-VI2
chalcopyrite semiconductors is predicted to arise from holes provided by
Mn doping in the illustrative case of CuGaSe2. The first-principles calculations
within the generalized gradient approximation to density functional theory
demonstrate that Mn substitutes for Ga sites in CuGaSe2 with a formation
energy that is similar to0.25 eV lower than that for Cu sites. Ferromagnetic
CuGa1-xMnxSe2 is found to be a half-metallic material with a magnetic moment
of 4 mu(B) per Mn for xgreater than or equal to0.25, and its estimated Curie
temperature is more than 110 K. We suggest that higher Curie temperatures
may be achieved for this new class of ferromagnetic semiconductors based
on Mn-doped I-III-VI2 chalcopyrites by employing other materials with a smaller
lattice constant such as CuGaSe2 and CuAlSe2. (C) 2002 Elsevier Science B.V.
All rights reserved. The optical properties
of YNi2B2C are studied by using the first-principles full-potential linearized
augmented plane-wave method within the local density approximation. Anisotropic
behavior is obtained in the optical conductivity, even though the electronic
structure shows three-dimensional character. A large peak in sigma(z) is
obtained at 2.4 eV. The anisotropic optical properties are analyzed in terms
of interband transitions between energy levels, and it is found that the
Ni site plays an important role. The electronic energy loss spectroscopy
spectra are also calculated to help elucidate the anisotropic properties
in this system. Stampfl, C., R. Asahi and
A. J. Freeman (2002). "Surface properties of the refractory
metal-nitride semiconductor ScN: Screened-exchange LDA-FLAPW investigations."
Physical Review B 65, 161204. Density-functional theory
calculations employing the screened- exchange local-density approximation
(SX-LDA) with the full potential linearized augmented plane-wave method have
recently shown that the relatively unexplored refractory nitrides ScN, YN,
and LaN are semiconductors. For the ScN(001) surface, the present calculations
predict that the ideal-relaxed surface has the lowest formation energy for
most of the range of the allowed chemical potentials-and is semiconducting-while
N- deficient structures, which are predicted to form for Sc-rich conditions,
are metallic in nature. Compared to the LDA surface-state band structures,
the SX-LDA selectively pushes down the valence bands for the Sc-terminated
surface and pushes up the conduction bands for the N-terminated structure. Results are presented
of a fully ab initio calculation of impact ionization rates in GaAs within
the density functional theory framework, using a screened-exchange formalism
and the highly precise all-electron full-potential linearized augmented plane
wave method. The calculated impact ionization rates show a marked orientation
dependence in k space, indicating the strong restrictions imposed by the
conservation of energy and momentum. This anisotropy diminishes as the impacting
electron energy increases. A Keldysh type fit performed on the energy- dependent
rate shows a rather soft edge and a threshold energy greater than the direct
band gap. The consistency with available Monte Carlo and empirical pseudopotential
calculations shows the reliability of our approach and paves the way to ab
initio calculations of pair production rates in new and more complex materials. Picozzi, S., A. Continenza,
S. Massidda and A. J. Freeman (2002). "Schottky barrier height
in GaN/Al junctions: an ab-initio study." Physica Status
Solidi a-Applied Research 190, 257. We performed ab-initio
full-potential linearized augmented plane wave (FLAPW) calculations for [0001]-wurtzite
and [111]- zincblende GaN/Al junctions, focusing on the Schottky barrier
height. We propose a procedure to evaluate the potential discontinuity in
the presence of electric fields, showing that their effect is relatively
small (a few tenths of an eV). These calculations assess the rectifying behaviour
of the GaN/Al contact, in agreement with experimental values for the barrier. Picozzi, S., G. Profeta,
A. Continenza, S. Massidda and A. J. Freeman (2002). "Role
of structural relaxations and chemical substitutions on piezoelectric
fields and potential lineup in GaN/Al junctions." Physical
Review B 65, 165316. First-principles full-potential
linearized augmented plane wave calculations are performed to clarify the
role of the interface geometry on piezoelectric fields and potential lineups
in [0001] wurtzite and [111]-zincblende GaN/Al junctions. The electric field
(polarity and magnitude) is found to be strongly affected by atomic relaxations
in the interface region. A procedure is used to evaluate the Schottky-barrier
height in the presence of electric fields, showing that their effect is relatively
small (a few tenths of an eV). These calculations assess the rectifying behavior
of the GaN/Al contact, in agreement with experimental values for the barrier.
We disentangle chemical and structural effects on the relevant properties
(such as the potential discontinuity and the electric field) by studying
unrelaxed ideal nitride/metal systems. Using simple electronegativity arguments,
we outline the leading mechanisms that define the values of the electric
field and Schottky barrier in these ideal systems. Finally, the transitivity
rule is proved to be well satisfied. Picozzi, S., A. Continenza
and A. J. Freeman (2002). "Co2MnX (X=Si, Ge, Sn) Heusler compounds:
An ab initio study of their structural, electronic, and magnetic
properties at zero and elevated pressure." Physical Review
B 66, 094421. The structural, electronic,
and magnetic properties of Co2MnX (X=Si, Ge, Sn) Heusler compounds have been
determined by means of all-electron full-potential linearized augmented plane
wave (FLAPW) calculations. We focus on the effects on the electronic and
magnetic properties induced by: (i) substitution of the X atom, (ii) applied
pressure, and (iii) the use of the local spin density approximation (LSDA)
vs the generalized gradient approximation (GGA) in density functional theory.
A comparison between LSDA and GGA for the exchange-correlation functional
shows that GGA is essential for an accurate description of the equilibrium
volumes and of the electronic and magnetic properties of these systems. We
find that both the energy gap and the spin gap increase as the X atomic number
decreases. As a result of the semiconducting (metallic) character found in
the minority (majority) spin band structure, the Si and Ge based alloys are
predicted to be half-metallic. In contrast, Co2MnSn is found to be a "nearly
half-metallic" compound, since the minority valence band maximum crosses
the Fermi level. The calculated total magnetization of 5 mu(B) is in excellent
agreement with recent experiments. By including a fully self- consistent
treatment of spin-orbit coupling, the GGA calculated orbital moments are
shown to be very small (about 0.008 mu(B) for Mn and about 0.02 mu(B) for
Co), showing that the quenching of the orbital magnetic moment is nearly
complete. The calculated hyperfine fields, both at zero and elevated pressure,
are compared with available experimental data, and show general agreement,
except for Mn. Finally, the calculated Mn 2p exchange splittings, found to
be in good agreement with experiment, are proportional to the Mn magnetic
moments, suggesting a localized nature of ferromagnetism in these Heusler
compounds. Picozzi, S., A. Continenza
and A. J. Freeman (2002). "The Co2MnGe Heusler compound: A
first principles study of the bulk phase and of the interface with
GaAs." Ieee Transactions on Magnetics 38, 2895. Within density functional
theory, full-potential linearized augmented plane wave calculations have
been performed to study the structural, electronic and magnetic properties
of the Co2MnGe Heusler compound. As a result of the semiconducting (metallic)
character found in the minority (majority) band structure, this material
is predicted to be half-metallic, with an integer magnetic moment of 5 mu(B).
This peculiar property, highly desired for spintronics applications, stimulated
the study of the CO2MnGe/GaAs interface. Our calculations show that the predicted
band offset is not suitable for spin-injection and that, due to interface
states, the half-metallicity is lost in proximity to the junction. Picozzi, S., R. Asahi, C.
B. Geller and A. J. Freeman (2002). "Accurate first-principles
detailed-balance determination of Auger recombination and impact
ionization rates in semiconductors." Physical Review Letters
89, 197601. The technologically important
prediction of Auger recombination lifetimes in semiconductors is addressed
by means of a fully first-principles formalism, based on precise energy bands
and wave functions provided by the full-potential linearized augmented plane
wave code. The minority carrier Auger lifetime is determined by two related
approaches: (i) a direct evaluation within Fermi's golden rule, and (ii)
an indirect evaluation, based on a detailed balance formulation combining
Auger recombination and its inverse process, impact ionization, in a unified
framework. Lifetimes determined with the direct and indirect methods show
excellent consistency between them (i) for n-doped GaAs and (ii) with measured
values for GaAs and InGaAs. This indicates the computational formalism as
a new sensitive tool for use in materials performance optimization. Picozzi, S., A. Continenza,
Y. J. Zhao, W. T. Geng and A. J. Freeman (2002). "Structural,
electronic and magnetic properties of chalcopyrite magnetic semiconductors:
A first-principles study." Journal of Vacuum Science &
Technology a-Vacuum Surfaces and Films 20, 2023. Stimulated by recent
experimental observations of room- temperature ferromagnetism of MnxCd1-xGeP2
and MnxZn1-xGeP2, we investigate the structural, electronic, and magnetic
properties of this class of systems (II-Ge-V-2, II=Zn, Cd, and V=As, P) as
a function of Mn concentration and chemical constituents by means of first-principles
density-functional-theory-based codes. Our calculations indicate that, for
Mn substituting the II element, the anti ferromagnetic alignment is the most
stable ordering for all the systems studied. For Zn- and Cd-rich systems,
the total magnetic moments per Mn atom of the ferromagnetic phase is very
close to the ideal value of 5 muB, since the Mn 3d states in the minority
spin channel are nearly empty; on the other hand, for Mn rich compounds,
the stronger p-d hybridization lowers the total magnetic moment to about
4.4 muB. (C) 2002 American Vacuum Society. Nakamura, K., A. J. Freeman,
D. S. Wang, L. P. Zhong and J. Fernandez-de-Castro (2002). "Magnetic
structures at the ferromagnetic NiFe and antiferromagnetic NiMn
interface in exchange-biased films: Role of noncollinear magnetism
and roughness." Physical Review B 65, 012402. The magnetic structures
at the compensated ferromagnetic/antiferromagnetic (FM/AFM) interface of
exchange bias FM NiFe/AFM NiMn films were determined with first- principles
full-potential linearized augmented plane-wave calculations including noncollinear
magnetism. The results predict that the magnetic moments of the FM NiFe layer
lie perpendicular to those of the AFM NiMn layer. The intra-atomic noncollinear
magnetism that arises near the interface is found to play an important role
in stabilizing the perpendicular coupling that leads to a large biquadratic
exchange energy (BEE). The BEE is large enough to require formation of a
magnetic domain wall (with an estimated thickness similar to370 Angstrom)
in the AFM NiMn layers, which may account for the observed large coercivity
and exchange bias. We also discuss magnetic structures at a rough FM/AFM
interface-as simulated in model calculations with the inclusion of line step
defects- which may contribute to a unidirectional magnetic anisotropy. Nakamura, K. and A. J. Freeman
(2002). "Canted ferromagnetism in RuSr2GdCu2O8." Physical
Review B 66, 140405. First principles calculations
using the full-potential linearized augmented plane wave method including
intra-atomic noncollinear magnetism have been performed to determine the
magnetic structures of RuSr2GdCu2O8. The magnetism clearly arises from the
RuO6 octahedra where the moments on neighboring Ru sites order antiferromagnetically
but cant perpendicular to the antiferromagnetic (AFM) axis-and so induce
a weak ferromagnetism. The projected Ru moments along the AFM and FM axes
result in magnetic moments of 1.16 and 0.99mu(B), respectively. The results
are consistent with the possible coexistence of canted ferromagnetism and
superconductivity in the RuSr2GdCu2O8-inferred from experiments. Electronic structures
and magnetism of the nonoxide perovskite MgCNi3-xTx compounds (x=0, 1, 2,
and 3; T=Co and Fe), based on recently found superconducting MgCNi3, have
been investigated with first-principles all-electron full-potential linearized
augmented plane-wave calculations within the local-(spin) density approximation.
From the results of total-energy calculations, it is expected that (i) the
suppression of superconductivity occurs faster for the Fe-doped case than
for the Co-doped case with increase of x, and (ii) ferromagnetic transitions
occur when xgreater than or equal to2 for the Co- doped cases, while the
Fe-doped cases become ferromagnetic before x=2. From the calculated density
of states, it is found that the superconductor MgCNi3 becomes paramagnetic
and then ferromagnetic, as we increase the number of minority spin d- band
holes via Co and Fe doping at Ni sites. A hypothetical system, MgC(FeCoNi),
has also been calculated and found to be ferromagnetic with magnetic moments
of 0.97, 0.24, and 0.03 mu(B) for Fe, Co, and Ni, respectively, which are
roughly proportional to the number of d-band holes of minority spin. These
features are well explained by d-band spin splitting, as in conventional
metallic ferromagnets. It is now well-recognized
that we are witnessing a golden age of innovation with novel materials, with
discoveries important for both basic science and device applications-some
of which will be treated at this Workshop. In this talk, we discuss the role
of computation and simulation in the dramatic advances of the past and those
we are witnessing today. We will also describe the growing acceptance and
impact of computational materials science as a major component of materials
research and its import for the future. In the process, we will demonstrate
how the well-recognized goal driving computational physics/computational
materials science-simulations of ever- increasing complexity on more and
more realistic models-has been brought into greater focus with the introduction
of greater computing power that is readily available to run sophisticated
and powerful software codes like our highly precise full-potential linearized
augmented plane wave (FLAPW) method, now also running on massively parallel
computer platforms. We will then describe some specific advances we are witnessing
today, and computation and simulation as a major component of quantum materials
design and its import for the future, with the goal-to synthesize materials
with desired properties in a controlled way via materials engineering on
the atomic scale. The theory continues to develop along with computing power.
With the universality and applicability of these methods to essentially all
materials and properties, these simulations are starting to fill the increasingly
urgent demands of material scientists and engineers. (C) 2002 Elsevier Science
B.V. All rights reserved. We investigate InxCd1-xO
materials, where x = 0.0, 0.031, 0.063 and 0.125, to understand their high
electrical conductivity and optical transparency windows, using the full-potential
linearized augmented plane wave (FLAPW) method. In addition, we employ the
screened exchange LDA (sX-LDA) method to evaluate accurate band structures
including band gap that is underestimated by the LDA calculations. The results
show a dramatic Burstein-Moss shift of the absorption edge by the In doping,
reflecting the small effective mass of the Cd 5s conduction band. The calculated
direct band gaps, 2.36 eV for x=0.0 and 3.17 eV for x= 0.063, show excellent
agreement with experiment. The effective mass of the conduction band of CdO
is calculated to be 0.24 in, (in the Delta direction), in good agreement
with an experimental value of 0.27m(e), explaining its high electrical conductivity.
The hybridization between the Cd 5s and the In 5s states yields complex many-body
effects in the conduction bands: a hybridization gap in the conduction bands
and a band-gap narrowing which cancels the further Burstein-Moss shift for
higher In doping. (C) 2002 Elsevier Science B.V. All rights reserved. Zhao, Y. J., W. T. Geng,
A. J. Freeman and T. Oguchi (2001). "Magnetism of chalcopyrite
semiconductors: Cd1-xMnxGeP2." Physical Review B 63,
201202. The recently reported
room-temperature ferromagnetism in Cd1- xMnxGeP2 was investigated for x =
1.0, 0.5, and 0.25 by the local density first-principles full-potential linearized
augmented plane wave (FLAPW) and DMOL3 methods within both local-density
approximation (LDA) and generalized gradient approximation (GGA). We find
that the total energy of the antiferromagnetic (AFM) state is lower than
the corresponding ferromagnetic (FM) state fur all x studied. The GGA gives
a better description of magnetic properties than LDA mainly due to its better
prediction of structure, particularly for high Mn concentrations. The total
spin moment of Cd1-xMnxGeP2 is similar to5.0 mu (B) per Mn atom. The FM alignment
between Mn and P increases the total energy of the Mn-Mn FM coupling and
makes the AFM ordering preferable. Zhao, Y. J., W. T. Geng,
K. T. Park and A. J. Freeman (2001). "Electronic and magnetic
properties of Ga1-xMnxAs: Role of Mn defect bands." Physical
Review B 64, 035207. Ga1-xMnxAs and related
semiconductors are under intense investigation for the purpose of understanding
the ferromagnetism in these materials, pursuing higher T-c, and, finally,
for realizing semiconductor electronic devices that use both charge and spin.
In this work, the electronic and magnetic structures of Ga1-xMnxAs (x=3.125%,
6.25%, 12.5%, 25.0%, 50.0%) are studied by first-principles full-potential
linearized augmented plane wave calculations with the generalized-gradient
approximation. The ferromagnetic state is lower in energy than the paramagnetic
and antiferromagnetic states. It is confirmed that Mn atoms stay magnetic
with well localized magnetic moments. The calculated band structure shows
that Mn doping also forms defect bands, and makes (Ga,Mn)As p- type conducting
by providing holes. Furthermore, an s-d population inversion is found in
the Mn electronic configuration, which results from the strong Mn p-d mixing.
The induced As moments are substantial (about -0.15 mu (B) per Mn atom, and
almost independent of x)-in accord with a recent observed negative As magnetic
circular dichroism signal. Youn, S. J. and A. J. Freeman
(2001). "First-principles electronic structure and its relation
to thermoelectric properties of Bi2Te3." Physical Review
B 63, 085112. The electronic structure
of Bi2Te3. which is a major constituent of the best thermoelectric material
operating at room temperature, was calculated by using the first principles
full-potential linearized-augmented-planewave method with spin- orbit interaction
included by a second variational method. A search of the whole Brillouin
zone shows that the band edges are located off the symmetry lines, with locations
that are in accord with the phenomenological six-band model. In doped Bi2Te3.
Fermi surfaces near the band edges show a nonparabolic behavior. At a high
doping concentration, the Fermi surfaces display elongated features, i.e.,
a knifelike Fermi surface for the valence band and spoonlike Fermi surfaces
for the conduction band. which can be attributed to the layered structure
of Bi2Te3. The effect of the anisotropic electronic structure combined with
a low lattice thermal conductivity of Bi2Te3 gives a large figure of merit. Youn, S. J., W. Mannstadt
and A. J. Freeman (2001). "Analytic spin-orbit coupling matrix
element formulae in FLAPW calculations." Journal of Computational
Physics 172, 387. The full-potential, linearized
augmented, plane wave (FLAPW) method is used widely for accurate
electronic structure calculations. For the electronic structure
of solids with heavy elements, it is necessary to include spin-orbit
coupling interactions. We present simple analytical formulae for
calculating FLAPW spin-orbit matrix elements. These wills erve to
simplify the calculations and save computational time. (C) Academic
Press.
Wang, A., J. R. Babcock,
N. L. Edleman, A. W. Metz, M. A. Lane, R. Asahi, V. P. Dravid, C.
R. Kannewurf, A. J. Freeman and T. J. Marks (2001). "Indium-cadmium-oxide
films having exceptional electrical conductivity and optical transparency:
Clues for optimizing transparent conductors." Proceedings
of the National Academy of Sciences of the United States of America
98, 7113. Materials with high electrical
conductivity and optical transparency are needed for future flat panel display,
solar energy, and other opto-electronic technologies. InxCd1-xO films having
a simple cubic microstructure have been grown on amorphous glass substrates
by a straightforward chemical vapor deposition process. The x = 0.05 film
conductivity of 17.000 S/cm, carrier mobility of 70 cm(2)/Vs, and visible
region optical transparency window considerably exceed the corresponding
parameters for commercial indium-tin oxide. Ab initio electronic structure
calculations reveal small conduction electron effective masses, a dramatic
shift of the CdO band gap with doping, and a conduction band hybridization
gap caused by extensive Cd 5s + In 5s mixing. Stampfl, C., W. Mannstadt,
R. Asahi and A. J. Freeman (2001). "Electronic structure and
physical properties of early transition metal mononitrides: Density-functional
theory LDA, GGA, and screened-exchange LDA FLAPW calculations."
Physical Review B 63, 155106. The desirable physical
properties of hardness, high temperature stability, and conductivity make
the early transition metal nitrides important materials for various technological
applications. To learn more about the nature of these materials, first-principles
density-functional theory calculations using the full-potential linearized
augmented plane wave (FLAPW) method within the local-density approximation
(LDA) and with the generalized gradient approximation (GGA) have been performed.
We investigate the bulk electronic and physical properties of a series of
early transition metal mononitrides, namely, those formed with 3d metals
(ScN, TiN, VN), 4d metals (YN, ZrN, NbN), and 5d metals (LaN, HfN, TaN) in
the rocksalt structure. In particular, lattice constants, bulk moduli, heats
of formation, and cohesive energies as well as bulk band structures and densities
of states are reported, and trends discussed. We find that the GGA yields
1%-2% larger lattice constants, 10%-20% smaller bulk moduli, and 10%-30%
lower heats of formation compared to the LDA. The GGA slightly overestimates
lattice constants, but leads overall to an improved agreement with experiment
compared to the LDA, for which the values rue too small. These materials
are metallic with the exception of ScN, YN, and LaN, which appear to be semimetals
within the LDA (and GCA), but are in fact semiconductors with indirect band
gaps of 1.58, 0.85, and 0.75 eV, respectively, as revealed by calculations
performed using the screened-exchange LDA approach. These last, relatively
unexplored. refractory III-V nitrides may therefore have potential use in
device applications; in particular, ScN is well lattice matched to GaN, a
wide-band-gap semiconductor that is of great current interest in relation
to optoelectronic devices, and high temperature and high power electronic
applications. Shishidou, T., A. J. Freeman
and R. Asahi (2001). "Effect of GGA on the half-metallicity
of the itinerant ferromagnet CoS2." Physical Review B
64, 180401. The half-metallicity
of CoS2, important for its possible use in spintronic applications. is investigated
by means of density functional full-potential linearized augmented plane
wave calculations within both the local spin-density approximation (LSDA)
and the generalized gradient approximation (GGA). In contrast with earlier
and our own predictions employing the LSDA, we find that CoS2 is clearly
half-metallic with use of the GGA for the exchange-correlation functional.
While the GGA induced modifications of the band structure are significant,
they are limited to only the e(g down arrow) state which is responsible for
producing the half-metallicity, while leaving all other features the same
as those obtained by LSDA. We address the recently measured reflectivity
spectra and rectify the assignments given for their underlying optical transitions. Profeta, G., A. Continenza
and A. J. Freeman (2001). "Energetics and bonding properties
of the Ni/beta-SiC (001) interface: An ab initio study." Physical
Review B 64, 045303. We investigate the adsorption
of a Ni monolayer on the beta - SiC (001) surface by means of highly precise
first-principles all-electron full-potential linearized augmented plane wave
calculations. Total-energy calculations for the Si- and C- terminated surfaces
reveal high Ni-SiC adsorption energies, with respect to other metals, confirmining
the strong reactivity and the stability of the transition metal/SiC interface.
These high binding energies, about 7.3-7.4 eV, are shown to be related to
strong p-d hybridization, common to both surface terminations and different
adsorption sites and despite the large mismatch, can stabilize overlayer
growth. A detailed analysis of the bonding mechanism, hybridization of the
surface states, charge transfer, and surface core level shifts reveals the
strong covalent character of the bonding. After a proper accounting of the
Madelung term. the core-level shift is shown to follow the charge-transfer
trend. Profeta, G., A. Blasetti,
S. Picozzi, A. Continenza and A. J. Freeman (2001). "Termination
effects at metal/ceramic junctions: Schottky barrier heights and
interface properties of the beta- SiC(001)/Ni systems." Physical
Review B 64, 235312. First-principles full-potential
linearized augmented plane wave calculations for the beta -SiC[001]/Ni interface
are presented, focused on the effects of different termination,, on the structural
and electronic properties We find a strong reactivity of the interface, as
confirmed by the high adhesion energies that are larger for the C-terminated
junction than for the Si-terminated junction, in agreement with that previously
found fur Ti and Al. The metal-induced gap state, are efficiently screened
(Angstrom)in b(o)th termination,,, resulting in a decay length of about 1
. The calculated dependence of the Schottky barrie(r) height on different
termination,, is not very strong and we investigate the observed differences
between Si- and C-terminated junctions in terms of Born effective charges,
electronegativity arguments, interface geometries, and screening effects.
The agreement with available experimental data is excellent. thus confirming
the strong rectifying behavior of this metal/ceramic contact. Nakamura, K., K. T. Park,
A. J. Freeman and J. D. Jorgensen (2001). "Magnetic and electronic
structures of superconducting RuSr2GdCu2O8." Physical Review
B 63, 024507. The coexistence of ferromagnetism
and superconductivity in RuSr2GdCu2O8 was reported both from experiments
(by Tallon et al.) and first-principles calculations (by Pickett et al.).
Here we report that our first principles full-potential linearized augmented
plane wave calculations, employing the precise crystal structure with structural
distortions (i.e., RuO6 rotations) determined by neutron diffraction, demonstrate
that antiferromagnetic ordering of the Ru moments is energetically favored
over the previously proposed ferromagnetic ordering. Our results are consistent
with recently performed magnetic neutron diffraction experiments (Lynn et
al.). Ru t(2g) states, which are responsible for the magnetism, have only
a very small interaction with Cu e (g) states, which results in a small exchange
splitting of these states. The Fermi surface, characterized by strongly hybridized
dp sigma orbitals, has nesting features similar to those in the two-dimensional
high-T-c cuprate super conductors. Kim, I. G., J. I. Lee, B.
I. Min and A. J. Freeman (2001). "Surface electronic structures
of superconducting thin film MgB2(0001)." Physical Review
B 64, 020508. The surface electronic
structures of superconducting MgB2 were investigated using the all-electron
full-potential linearized augmented plane-wave method. Hexagonal (0001) surfaces
with both B terminated (B-Term) and Mg terminated (Mg-Term) were considered.
Due to the nearly-free-electron nature of the Mg surface layer, the vacuum
screening range of Mg-Term is shorter than that of B-Term, which shows the
covalent bonding nature of the B surface layer. Considerably enhanced densities
of states near the Fermi level are found at the surface layers especially
for B-Term, which is expected to yield an enhanced superconductivity in the
surface of thin film MgB2 over that in bulk-assuming no large changes in
the electron-ion matrix elements and phonon frequency contributions. While
this expectation is contrary to the weakened superconductivity observed in
surface-oriented experiments, we attribute this discrepancy to extrinsic
surface effects. Ingram, B. J., T. O. Mason,
R. Asahi, K. T. Park and A. J. Freeman (2001). "Electronic
structure and small polaron hole transport of copper aluminate."
Physical Review B 64, 155114. High-temperature electrical
property measurements (electrical conductivity, thermoelectric coefficient)
on polycrystalline CuAlO2 exhibited characteristic small polaron features,
i.e., low mobilities (0.1-0.4 cm(2)/V s) and an activation energy of similar
to0.14 eV. The thermopower was p type (similar to 440 muV/K) and roughly
temperature independent. The local-density full-potential linearized augmented-plane-wave
method was used to calculate the band structure, densities of states, and
optical properties (within the electric-dipole approximation) in order to
account for the unique electronic and optical properties of this potential
transparent conducting oxide. Geng, W. T., A. J. Freeman
and R. Q. Wu (2001). "Magnetism at high-index transition-metal
surfaces and the effect of metalloid impurities: Ni(210)."
Physical Review B 63, 064427. Structural, electronic,
and magnetic properties near the Ni(210) surface and the effect of Li, B,
P, and Ca impurities are determined by means of the all-electron total energy/atomic
force full-potential linearized augmented plane wave method with the generalized
gradient approximation. For the Ni(210) clean surface, simulated with an
11-layer slab, multilayer relaxation is found to be confined to the top three
layers. The magnetic moment of the surface Ni layer, 0.79/mu (B), is enhanced
by 27%, compared with its bulk value. This result confirms the well-accepted
understanding that the reduced coordination number at a clean transition-metal
surface with a high index leads to enhanced magnetic moments. Boron and P
strongly alter the atomic structure at the Ni(210) surface, whereas Li and
Ca have only a slight influence, due to the weak, chemical bonding with the
nickel substrate. it is found that all four selected elements exert detrimental
effects on the Ni(210) surface magnetism. The effects of B and P are stronger
than those of Li and Ca, mainly due to their stronger hybridization with
the nickel d stares. An analysis of the results for B and P suggests that
it is the stronger magnetization of its free standing monolayer that makes
P more detrimental than B on the nickel surface magnetism. Geng, W. T., A. J. Freeman
and G. B. Olson (2001). "Influence of alloying additions on
grain boundary cohesion of transition metals: First-principles determination
and its phenomenological extension." Physical Review B
63, 165415. The toughness and ductility
of ultrahigh-strength alloys is often limited by intergranular embrittlement,
particularly under conditions of unfavorable environmental interactions such
as hydrogen embrittlement and stress corrosion cracking. Here we investigated
the mechanism by which the segregated substitutional additions cause intergranular
embrittlement. An electronic level phenomenological theory is proposed to
predict unambiguously the effect of a substitutional alloying addition on
grain boundary cohesion of metallic alloys, based on first- principles full-potential
linearized augmented plane-wave method (FLAPW) calculations on the strengthening
and embrittling effects of the metals Mo and Pd on the Fe grain boundary
cohesion. With the bulk properties of substitutional alloying addition A
and the matrix element M as inputs, the strengthening or embrittling effect
of A at the grain boundary of M can be predicted without carrying out first-principles
calculations once the atomic structure of the corresponding clean grain boundary
is determined. Predictions of the embrittlement potency of a large number
of metals, including the 3d, 4d, and 5d transition metals, are presented
for the Fe Sigma3 (111) and the Ni Sigma5 (210) grain boundaries. Rigorous
FLAPW calculations on the effect of Co, Ru, W, and Re on the Fe Sigma3 (111)
grain boundary and Ca on the Ni Sigma5 (210) grain boundary cohesion confirm
the predictions of our model. This model is expected to be applicable to
other high-angle boundaries in general and instructive in the quantum design
of ultrahigh-strength alloys with resistance to intergranular fracture. Geng, W. T., M. Kim and A.
J. Freeman (2001). "Multilayer relaxation and magnetism of
a high-index transition metal surface: Fe(310)." Physical
Review B 63, 245401. Structural, electronic,
and magnetic properties of the Fe(310) surface are studied using first-principles
full-potential linearized augmented plane wave method within the generalized
gradient approximation. Sizable multilayer relaxation is found to extend
to the seventh layer from the surface. While low- energy electron diffraction
(LEED) and first-principles calculations on multilayer relaxations generally
agree for low- index surfaces, there is a disagreement for this high-index
surface. We conjecture that this disagreement might be due to the small data
set and variational freedom in the LEED analysis. The spin magnetic moment
of the Fe(310) surface and subsurface atoms is enhanced to 2.85 mu (B) and
2.65 mu (B), from a bulk value of 2.23 mu (B). The surface layer enhancement
is smaller than that in Fe(100) and larger than that in Fe(111), although-all
three surfaces have the same coordination number. Subsurface layers are found
to play an important role in the magnetization of the surface atoms in the
case of an open surface, where the vacuum affects more atomic layers. Geng, W. T., A. J. Freeman
and G. B. Olson (2001). "Influence of alloying additions on
the impurity induced grain boundary embrittlement." Solid
State Communications 119, 585. We have investigated
the influence of Mo as a substitutional alloying addition on the grain boundary
embrittlement induced by interstitial impurity P by using the first-principles
full- potential linearized augmented plane wave total energy/atomic force
method within the generalized gradient approximation. With Mo segregated
to a clean Fe Sigma3(111) grain boundary, the embrittling potency of P increases
from +0.19 to +0.88 eV, suggesting a strong detrimental ternary effect in
P-Mo couple. Since Mo has a direct strengthening effect of -0.90 eV, the
combined effect of a P-Mo couple is -0.02 eV, which confirms the experimental
evidence that Mo overcompensates the P embrittlement. Since Mo has a larger
atomic size than Fe, it reduces the volume available for P at the grain boundary
and hence an increased elastic energy. On the other hand, the strong chemical
bonding between Mo and the Fe surface makes the top Fe layers more saturated
and hence a weakened vertical P-Fe bonding. Together, they induce a strong
embrittling ternary effect in a P-Mo couple. This understanding can also
explain the embrittling ternary effect of Mn on P embrittlement and is expected
to be applicable to more general cases and instructive in quantum design
of ultrahigh-strength alloys. (C) 2001 Elsevier Science Ltd. All rights reserved. Geng, W. T. and A. J. Freeman
(2001). "Multilayer relaxation of Cu(331)." Physical
Review B 64, 115401. Both quantitative low-energy
electron diffraction (LEED) analysis and theoretical predictions based on
the embedded-atom method suggested that Cu(331) has a sequence of interlayer
spacing contractions and expansions dissimilar to that expected for a terrace
with three atomic layers. To understand the anomalous behavior of Cu(331),
we have performed a comparative study of Cu(331) and Cu(211) by using the
first-principles full-potential linearized augmented plane-wave method within
the generalized gradient approximation. For both surfaces, our calculations
reproduce the multilayer relaxation sequences given by LEED measurements.
The electronic structure analysis indicates that the seemingly anomalous
multilayer relaxation in Cu(331) can also be understood in the charge smoothing
picture. However. the trend of charge smoothing in such transition metal
surfaces depends not only on how many atomic layers a terrace has, but also
on the bond-length-bond-order interplays. Geller, C. B., W. Wolf, S.
Picozzi, A. Continenza, R. Asahi, W. Mannstadt, A. J. Freeman and
E. Wimmer (2001). "Computational band-structure engineering
of III-V semiconductor alloys." Applied Physics Letters
79, 368. Accurate band structures
of binary semiconductors AB (A=Al, Ga, In and B=P, As, Sb) and selected ternary
III-V semiconductors were calculated using an all-electron screened exchange
approach within the full potential linearized augmented plane- wave method.
Fundamental band gaps and Gamma -L and Gamma -X separations in higher-lying
conduction bands are predicted with an accuracy of a few tenths of 1 eV.
Screened exchange also performs better than the local density approximation
for calculating conduction-band effective masses. Highly n-doped InPAs materials
with compositions near InP0.2As0.8 offer lower effective masses, greater
optical band-gap shifts, and potentially higher electron mobility than n-doped
InGaAs materials with comparable band gaps. (C) 2001 American Institute of
Physics. Continenza, A., S. Picozzi,
W. T. Geng and A. J. Freeman (2001). "Coordination and chemical
effects on the structural, electronic, and magnetic properties in
Mn pnictides." Physical Review B 64, 085204. Simple structures of
MnX binary compounds, namely hexagonal NiAs and zincblende, are studied as
a function of the anion (X=Sb,As, P) by means of the full-potential linearized
augmented plane wave (FLAPW) method within the local spin density and generalized
gradient approximations. An accurate analysis of the structural, electronic,
and magnetic properties reveals that the cubic structure greatly favors the
magnetic alignment in these compounds leading to high magnetic moments and
nearly half-metallic behavior for MnSb and MnAs. The effect of the anion
chemical species is related to both its size and the possible hybridization
with the Mn d states; both contributions are seen to hinder the magnitude
of the magnetic moment for small and light anions. Our results are in very
good agreement with experiment, where available, and show that the generalized
gradient approximation is essential to correctly recover both the equilibrium
volume and magnetic moment. Cho, S. L., S. J. Youn, Y.
Kim, A. DiVenere, G. K. L. Wong, A. J. Freeman and J. B. Ketterson
(2001). "Polarity inversion in polar-nonpolar-polar heterostructures."
Physical Review Letters 87, 126403. We have observed an epilayer-thickness-dependent
polarity inversion for the growth of CdTe on Sb(Bi)/CdTe(111)B. For films
with Sb(Bi) thicknesses of less than 40 Angstrom (15 Angstrom), the CdTe
layer shows a B (Te-terminated) face, but it switches to an A (Cd-terminated)
face for thicker layers. On the other hand, a CdTe layer grown on Bi(Sb)/Me(111)A
always shows the A face regardless of Sb or Bi layer thicknesses. In order
to address the observations we have performed ab initio calculations, which
suggest that the polarity of a polar material on a nonpolar one results from
the binding energy difference between the two possible surface configurations. Impurity-induced reduction
of intergranular cohesion-a major factor limiting the usable strength level
of ultrahigh-strength steels-is particularly severe when aggravated by mobile
hydrogen through environmental interaction, as in the case of hydrogen stress
corrosion cracking. As an aid in establishing an understanding on the electronic
level, the influence of hydrogen on the cohesion of an iron grain boundary
was determined using the full-potential linearized augmented plane wave (FLAPW)
method with the generalized gradient approximation. Through precise calculations
on both grain boundary and free surface environments, we found that hydrogen
is a strong embrittler. Analysis of the results in terms of structural relaxation,
bonding character, and magnetic interactions shows that the hydrogen-iron
chemical bond is stronger on the free surface and a charge-transfer mechanism
is found to play a dominant role for the hydrogen-induced reduction of cohesion
across the iron grain boundary. These results provide a quantitative explanation
from first principles for the technologically important phenomenon of hydrogen-induced
intergranular embrittlement. Smith, R. W., W. T. Geng,
C. B. Geller, R. Wu and A. J. Freeman (2000). "The effect of
Li, He and Ca on grain boundary cohesive strength in Ni." Scripta
Materialia 43, 957. Profeta, G., A. Continenza,
L. Ottaviano, W. Mannstadt and A. J. Freeman (2000). "Structural
and electronic properties of the Sn/Si(111)root 3x root 3R30 degrees
surface." Physical Review B 62, 1556. The structural and electronic
properties of the Sn/Si(111)root 3x root 3 surface are determined by means
of all-electron local density full-potential augmented plane wave thin film
calculations. We find strong similarities with the more extensively studied
isolectronic systems (Sn/Ge and Pb/Ge) as far as the relaxed structure and
the electronic properties are concerned. In analogy with these systems, we
find, within the local density approximation (LDA), two surface states weakly
dispersed in the Brillouin zone which are responsible for the hexagonal patterns
observed in scanning tunneling microscopy (STM) experiments. When our calculated
results, including STM images, are compared with available structural data
and STM images, we find that the LDA predictions well reproduce the electron
distribution at the surface and the structural properties, leading to a complete
description of this system at room temperature. Picozzi, S., A. Continenza,
R. Asahi, W. Mannstadt, A. J. Freeman, W. Wolf, E. Wimmer and C.
B. Geller (2000). "Volume and composition dependence of direct
and indirect band gaps in ordered ternary III-V semiconductor compounds:
A screened-exchange LDA study." Physical Review B 61,
4677. Quasiparticle electronic
band structures for ternary InGaAs and InGaSb systems, modeled by a luzonite
structure, have been obtained using the screened-exchange local-density approximation
approach. We focus our attention on energy transitions relevant for the electron-hole
pair lifetime and, in particular, on the direct (E<(Gamma)over bar>-<(Gamma)over
bar>) and lowest indirect (E<(Gamma)over bar>-(L) over bar) band
gaps as a function of volume, common anionic species, and composition. All
these degrees of freedom can be used to tune the transitions considered:
as a first result, a large range of band gaps (almost 1 eV) can be obtained
by varying the lattice constants within a few percent with respect to their
equilibrium values. Moreover, the lowest indirect transition can be strongly
reduced by replacing As with Sb. Finally, the composition dependence of these
transitions shows that, due to symmetry properties of the potential in ternary
luzonites, In- rich systems have the smallest indirect band gaps of all compounds
investigated in this work. Picozzi, S., A. Continenza,
G. Satta, S. Massidda and A. J. Freeman (2000). "Metal-induced
gap states and Schottky barrier heights at nonreactive GaN/noble-metal
interfaces." Physical Review B 61, 16736. We present ab initio
local density FLAPW calculations on nonreactive N-terminated [001] ordered
GaN/Ag and GaN/Au interfaces and compare the results (such as metal induced
gap states and Schottky barrier heights) with those obtained for GaN/Al,
in order to understand the dependence of the relevant electronic properties
on the deposited metal. Our results show that the density-of-gap states is
appreciable only in the first semiconductor laver close to the interface.
The decay length of the gap states in the semiconductor side is about 2.0+/-0.1
Angstrom and is independent of the deposited metal, therefore being to a
good extent a bulk property of GaN. Our calculated values of the Schottky
barrier heights are Phi(Bp) (GaN/Ag) = 0.87 eV and Phi(Bp) (GaN/Au) = 1.08
eV; both values are smaller than the GaN/Al Value [Phi(Bp) (GaN/Al) = 1.51
eV] and this quite large spread of values excludes the possibility of a Fermi-level
pinning within the GaN band gap. Because of the low screening in GaN, the
potential barrier at the junction is strongly affected by the structural
arrangement of the first metal layer at the interface. This leads to quite
large variations of the Schottky barrier height as a function of the metal.
in contrast with the behavior of GaAs/metal interfaces. First-principles local
density full-potential linearized augmented plane wave (FLAPW) calculations
have been performed to investigate the magnetism and magnetic anisotropy
in ferromagnetic NiFe, antiferromagnetic NiMn and their interface. We found
that the magnetic moments of the Fe atom in NiFe and that of the Mn atom
in NiMn are enhanced at their surface compared to their bulk states, The
magneto-crystalline anisotropy (MCA) is also drastically influenced by the
environment. Bulk NiFe and NiMn show slightly perpendicular and in-plane
easy magnetic axes, respectively, However, the effects of the surface change
their MCA direction and strength. For the interface between NiFe and NiMn,
where the magneto-crystalline anisotropy is dominated by the NiMn layer,
the interface effects may be important for discussing the exchange bias. Lee, J. I., S. C. Hong, W.
Mannstadt and A. J. Freeman (2000). "Rippled surface structure
and electronic and magnetic properties of Ni3Al(001)." Physical
Review B 62, 6982. Structural, electronic,
and magnetic properties of Ni3Al(001) are investigated by the all-electron
thin him full-potential linearized augmented plane wave method based on the
local density approximation. A stable rippled surface atomic geometry is
determined by atomic force and total energy calculations. The surface Ni
atoms contract down to the bulk region by 4.2% of the bulk interlayer spacing
while all of the other atoms including the surface AI atoms remain close
to their bulk positions. The amount of rippling found by calculation (0.06
Angstrom) is almost within experimental error (0.02+/-0.03 Angstrom). Charge
densities, calculated work functions and densities of states for the relaxed
rippled and unrelaxed surfaces are reported. The spin polarized calculation
predicts that the Ni3Al(001) surface is "magnetically dead," unlike
the bulk region. The first-principles
calculations within the local density approximation using the full potential
linearized augmented plane wave (FLAPW) method were performed to investigate
the structural and magnetic properties of the Ta/NiFe interface for both
clean NixFe1-x (001) thin films and with a Ta overlayer. To study the composition
dependence, we adopted x = 0.5 (L1(0) structure with either Fe or Ni layers
on the surface) and 0.75 (L1(2) structure with mixed Fe-Ni layers or with
Ni on the surface). The equilibrium overlayer/substrate distance and the
preferred site position of Ta were obtained by tructural optimization employing
atomic-force calculations and total energy comparisons for several possible
adsorption sites of Ta. By comparing with results for the clean surface of
five-layer NixFe1-x (001) films, we found that Ta has a significant detrimental
effect on the magnetic properties of NiFe with its induced magnetic moment
coupled ferro- or antiferro- magnetically with the substrate depending sensitively
on the surface layer. (C) 2000 American Institute of Physics. [S0021- 8979(00)73908-9]. The effects of Mo and
Pd segregation on the cohesion of the Fe Sigma 3(111) grain boundary an investigated
by using the first- principles full-potential linearized augmented-plane-wave
total-energy-atomic-force method with the generalized gradient approximation.
Based on the Rice-Wang model, our total energy calculations show that Mo
has a significant beneficial effect on the Fe grain-boundary cohesion, while
Pd behaves as a weak embrittler. An analysis of the geometry optimization
indicates that Mo has a moderate atomic size to fit well in the grain- boundary
hole, whereas Pd introduces a larger perturbation on the atomic structure
near the grain boundary. The elastic energy associated with the Mo and Pd
segregation is estimated with a rigid environment approximation. It is found
that both Mo and Pd introduce a beneficial volume effect. Studies of the
electronic structures show that its strong bonding capability makes Mo a
cohesion enhancer (-0.90 eV) for the Fe Sigma 3(111) grain-boundary. By comparison,
its weak bonding capability leads Pd to be a weak embrittler (+0.08 eV).
Our first- principles quantum-mechanical results support the main idea of
the atomistic theories in that the elemental cohesive energy difference between
the substitutional element and the host element plays an important role in
determining its effect on the grain-boundary cohesion. However, the numerical
results for Pd, which has a similar elemental cohesive energy to that of
Fe, point to the importance of the role played by the volume effect. It is
expected that in a lower-angle Fe grain boundary which has a larger grain-boundary
volume expansion, Pd can possibly become a cohesion enhancer. Asahi, R., Y. Taga, W. Mannstadt
and A. J. Freeman (2000). "Electronic and optical properties
of anatase TiO2." Physical Review B 61, 7459. First-principles calculations
using the full-potential linearized augmented plane-wave method have been
performed to investigate detailed electronic and optical properties of TiO2
in the anatase structure. The fully optimized structure, obtained by minimizing
the total energy and atomic fords, are in good agreement with experiment.
Stabilization of the structure by the trade off between a favorable coordination
in the sp(2) hybridization and the Coulomb repulsion among oxygen atoms is
also demonstrated. We calculate band structure, densities of states and charge
densities, and interpret their features in terms of the bonding structure
in the molecular orbital picture. The optical properties, calculated within
the dipole approximation, are found to agree with recent experiments on single
crystals of anatase TiO2. Near the absorption edge, the results show a significant
optical anisotropy in the components parallel and perpendicular to the c
axis. We demonstrate that this large dichroism results from the existence
of nonbonding d(xy) orbitals located at the bottom of the conduction bands,
which allows direct dipole transitions dominantly for the perpendicular component. Asahi, R., W. Mannstadt and
A. J. Freeman (2000). "Screened-exchange LDA methods for films
and superlattices with applications to the Si(100)2X1 surface and
InAs/InSb superlattices." Physical Review B 62,
2552. We have developed efficient
screened-exchange local-density approximation (sX-LDA) methods for films
and superlattices (FLM/SL) with which to calculate self-consistent electronic
structures for both occupied and unoccupied states. Considering nonuniform
charge densities and local-field effects in the z direction for FLM/SL, we
have employed nonlocal Thomas-Fermi wave vectors to define the screened-exchange
interaction. Three methods, for bulk, superlattice, and film, have been implemented
in the full-potential linearized augmented plane wave method. The film sX-LDA
method was then applied to the Si(100)2x1 surface. The calculated occupied
surface states show very good agreement with experiment. On the other hand,
an underestimation of the correction to the unoccupied surface states, by
about 0.2 eV, was estimated in comparison with available GW calculations.
The ionization energy of Si was evaluated with the film geometry to be 5.35
eV by virtue of the quasiparticle corrections, showing good agreement with
the experimental value of 5.15 +/- 0.08 eV. Rie also present an application
of the superlattice sX-LDA method to [001] ordered InAs/InSb heterojunctions
and superlattices. Band gaps and band offsets under strained conditions were
directly calculated by sX-LDA without any experimental data as input. Slightly
larger valence-band offsets than the LDA results. by about 0.08 eV, agree
with the consequence of the GW calculations, indicating an increase of the
potential negativity in the InAs region. This potential change along with
the charge redistribution at the interface is found to be crucial to evaluate
accurate band gaps of the superlattices. Wu, R. Q. and A. J. Freeman
(1999). "Spin-orbit induced magnetic phenomena in bulk metals
and their surfaces and interfaces." Journal of Magnetism
and Magnetic Materials 200, 498. First-principles electronic
structure studies based on local spin density functional theory and performed
on extremely complex simulations of ever increasingly realistic systems,
play a very important role in explaining and predicting surface and interface
magnetism. This review deals with what is a major issue for first-principles
theory, namely the theoretical/computational treatment of the weak spin-orbit
coupling in magnetic transition metals and their alloys and its important
physical consequences: magneto-crystalline anisotropy, magnetostriction,
magneto-optical Kerr effects and X-ray magnetic circular dichroism. As is
demonstrated, extensive first-principles calculations and model analyses
now provide simple physical insights and guidelines to search for new magnetic
recording and sensor materials. (C) 1999 Elsevier Science B.V. All rights
reserved. Shick, A. B., Y. N. Gornostyrev
and A. J. Freeman (1999). "Magnetoelastic mechanism of spin-reorientation
transitions at step edges." Physical Review B 60,
3029. The symmetry-induced
magnetic anisotropy due to monoatomic steps at strained Ni films is determined
using results of first-principles relativistic full-potential linearized
augmented-plane-wave calculations and an analogy with the Neel model. We
show that there is a magnetoelastic anisotropy contribution to the uniaxial
magnetic anisotropy energy in the vicinal plane of a stepped surface. In
addition to the known spin-direction reorientation transition at a flat Ni/Cu(001)
surface, we propose a spin-direction reorientation transition in the vicinal
plane for a stepped Ni/Cu surface due to the magnetoelastic anisotropy. We
show that with an increase of Ni- film thickness, the magnetization in the
vicinal plane turns perpendicular to the step edge at a critical thickness
calculated to be in the range of 16-24 Ni layers for the Ni/Cu(1,1,13) stepped
surface. [S0163-1829(99)13629-4]. Shick, A. B., J. B. Ketterson,
D. L. Novikov and A. J. Freeman (1999). "Electronic structure,
phase stability, and semimetal- semiconductor transitions in Bi."
Physical Review B 60, 15484. The structural stability
of bulk Bi is studied using the local- density full-potential linear muffin-tin
orbital method. The effect of both the trigonal shear angle and internal
displacement on the electronic structure is determined. It is shown that
the internal displacement changes the Bi electronic structure from a metal
to a semimetal, in qualitative agreement with a Jones-Peierls-type transition.
The total energy is calculated to have a double-well dependence on the internal
displacement, and to provide a stabilization of the trigonal phase. We show
that an increase of trigonal shear angle (towards the cubic value of 60 degrees)
leads to a semimetal- semiconductor transition in Bi. Using coherency strain
arising from a film/substrate lattice constant mismatch, this may provide
a way to induce semiconducting behavior in Bi films, and with it to control
their thermoelectric properties. [S0163- 1829(99)02947-1]. Ab initio electronic
structure calculations have been performed for the clean GaN surface (both
N- and Ga-terminated) and for the GaN/Al 1-ML system, obtained from the clean
surface by addition of an Al monolayer. In the most stable configuration,
Al is found to occupy the bridge site at the clean unreconstructed N-terminated
surface. All the systems studied show an appreciable surface charge density
due to midgap states strongly localized in the surface region; this localization
is particularly enhanced for the N-terminated case, showing the high instability
of the unreconstructed clean surface. On the other hand, the Ga-terminated
surface is found to be more stable and to behave very similarly to the Al-covered
system. The Fermi level E-F is shifted toward the conduction band with respect
to the clean N-terminated surface. Additional coverage of Al layers does
not significantly affect the initial position of E-F within the GaN band
gap, showing that the Schottky barrier height is well established after the
first Al layer. [S0163-829(98)02643-5]. Lee, J. I., W. Mannstadt
and A. J. Freeman (1999). "Multilayer-relaxed structure of
the (1 x 2) Pt(110) surface." Physical Review B 59,
1673. The multilayer-relaxed
structure and electronic properties of the (1 x 2) Pt(110) surface have been
investigated by the self- consistent all-electron full-potential linearized
augmented plane-wave method. The relaxed geometry, determined by total energy
and atomic force calculations, shows large contractions in the first and
second interlayer spacings, significant buckling in the third layer, and
a lateral displacement in the fourth (center) layer of the slab. In general,
our calculated results are consistent with experimental data. The microscopic
origin of the relaxed structure is discussed using the calculated electronic
structures. The large inward relaxation of the surface atoms is attributed
to the more localized nature of their 5d electrons, which weakens the d-d
hybridization. [S0163-1829(99)06804-6]. Kim, M., A. J. Freeman and
R. Q. Wu (1999). "Surface effects and structural dependence
of magneto-optical spectra: UItrathin Co films and CoPtn alloys
and multilayers." Physical Review B 59, 9432. First-principles calculations
of the optical conductivities and magneto-optical Kerr effects for bulk and
thin films of Co and for CoPtn alloys (n = 1 and 3) and multilayer (n = 3)
are carried out using the full potential linearized augmented plane wave
method with the local spin density approximation. For fee bulk Co, we obtain
a negligible dependence of the polar Kerr rotation on the direction of magnetization.
By contrast, the calculated Kerr spectra of CoPtn are found to be very sensitive
to structural changes. For Co(001) and Co(111) ultrathin films, the surface
effects cause a redshift for the high-energy peaks in the magneto-optic Kerr
effect (MOKE) spectra, mainly due to band narrowing. Except for the free
standing Co(001) monolayer, the profile of their MOKE spectra, however, is
very close to that of bulb Co. [S0163-1829(99)04313-1]. Geng, W. T., A. J. Freeman,
R. Wu, C. B. Geller and J. E. Raynolds (1999). "Embrittling
and strengthening effects of hydrogen, boron, and phosphorus on
a Sigma 5 nickel grain boundary." Physical Review B
60, 7149. The embrittling and strengthening
effects of hydrogen, boron, and phosphorus on a Sigma 5(210) [100] nickel
grain boundary are investigated by means of the full-potential linearized
augmented plane-wave method with the generalized-gradient approximation formula.
Optimized geometries for both the free surface and grain-boundary systems
are obtained by atomic-force calculations. The results obtained show that
hydrogen and phosphorus are embrittlers and that boron acts as a cohesion
enhancer. An analysis of the atomic, electronic, and magnetic structures
indicates that atomic size and the bonding behavior of the impurity with
the surrounding nickel atoms play important roles in determining its relative
embrittling or cohesion enhancing behavior. [S0163-1829(99)02926-4]. A major issue for first-principles
theory in magnetism is the treatment of the weak spin-orbit coupling (SOC)
and its subsequent effects in magnetic transition metal bulk, surfaces and
multilayers. Using either a perturbative or a self- consistent approach for
SOC, we have recently investigated important phenomena such as magnetic crystalline
anisotropy (MCA), magnetostriction, and magneto-optical Kerr effects (MOKE)
in various transition metal systems using the highly precise local density
full-potential linearized augmented plane wave (FLAPW) method. With the aid
of accurate total energy and atomic force approaches based on the LDA and
GGA formalisms, the atomic structures of all the surfaces and interfaces
are fully relaxed. Excellent agreement with experiment has been achieved
for most of the systems investigated in terms of their equilibrium geometries,
MCA energies, magnetostrictive coefficients and MOKE spectra. (C) 1999 Elsevier
Science B.V. All rights reserved. Charache, G. W., D. M. DePoy,
J. E. Raynolds, P. F. Baldasaro, K. E. Miyano, T. Holden, F. H.
Pollak, P. R. Sharps, M. L. Timmons, C. B. Geller, W. Mannstadt,
R. Asahi, A. J. Freeman and W. Wolf (1999). "Moss-Burstein
and plasma reflection characteristics of heavily doped n-type InxGa1-xAs
and InPyAs1-y." Journal of Applied Physics 86,
452. Degenerately doped (> 10(19)
cm(-3)) n-type InxGa1-xAs (x similar to 0.67) and InPyAs1-y (y similar to
0.65) possess a number of intriguing electrical and optical properties relevant
to electro-optic devices and thermophotovoltaic devices in particular. Due
to the low electron effective mass of these materials (m*< 0.2) and the
demonstrated ability to incorporate n-type dopants into the high 10(19) cm(-3)
range, both the Moss-Burstein band gap shift and plasma reflection characteristics
are particularly dramatic. For InGaAs films with a nominal undoped band gap
of 0.6 eV and N=5x10(19) cm(- 3), the fundamental absorption edge increased
to 1.27 eV. InPAs films exhibit a shorter plasma wavelength (lambda(p)similar
to 5 mu m) in comparison to InGaAs films (lambda(p)similar to 6 mu m) with
similar doping concentrations. The behavior of the plasma wavelength and
the fundamental absorption edge are investigated in terms of conduction band
nonparabolicity and Gamma-L valley separation using detailed band structure
measurements and calculations. (C) 1999 American Institute of Physics. [S0021-8979(99)01612-6]. Asahi, R., W. Mannstadt and
A. J. Freeman (1999). "Optical properties and electronic structures
of semiconductors with screened-exchange LDA." Physical
Review B 59, 7486. Results of first-principles
calculations of the optical properties and electronic structure, determined
by employing the screened-exchange LDA method to obtain accurate electronic
structures as well as self-consistent eigenfunctions, are presented for some
cubic semiconductors (Si, Ge, InSb, and GaAs). As implemented in the full-potential
linearized augmented-plane-wave method, this approach does not require any
adjustable parameters to calculate optical properties. The inadequacy of
using the transverse expression of the dielectric function (involving ep
matrix elements) is shown numerically for the optical spectra and dielectric
constants even when the renormalization factor associated with the LDA eigenvalues
is included. This is because the nonlocality of the Hamiltonian and the difference
of the self-consistent eigenfunctions from the LDA ones are not negligible.
In contrast, the optical properties evaluated using the longitudinal expression
(with e(iq.r) matrix elements) yield excellent agreement with experiment
considering the neglect of lifetime, local-field, and excitonic effects.
[S0163-1829(99)01708-7].
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