arXiv
(422.153 recursos)
This is one of the most extensive subject based repositories in the world in the field of physics, mathematics, astronomy, computer sciences and quantitative biology. This is the principal site with almost 20 mirror versions around the globe. The site is supported by an extensive collection of information and background documentation. An RSS feed is available for anyone interested in keeping up-to-date with newly added materials.
Mostrando recursos 1 - 20 de 262
1.
Many-body and Covalence Effects in the Polarization of Ferroelectric
Perovskites - Resta, R.; Sorella, S.; SISSA; Trieste; Italy
The ferroelectric polarization of perovskite oxides is much larger than
implied by displacement of static ionic charges. We use an explicitly
correlated scheme to investigate the phenomenon; charge transport is evaluated
as a geometric quantum phase. Both covalence and electron-electron interaction
enhance polarization in the weakly correlated regime. At higher values of the
electron-electron interaction, the system undergoes a transition from a band
insulator to a Mott insulator: the static ionic charge is continuous across the
transition, whereas the polarization is discontinuous. Above the transition,
oxygen transports a positive charge.
2.
Gamma phonons and microscopic structure of orthorhombic KNbO3 from
first-principles calculations - Postnikov, A. V.; Borstel, G.
{}From a series of total energy calculations by the full-potential linear
muffin-tin orbital method, the total energy hypersurface as function of atomic
displacements from equilibrium positions has been fitted for different Gamma
phonon modes in orthorhombic KNbO3. Frequencies and eigenvectors of all TO
Gamma phonons have been calculated in the harmonic approximation, and in the
quantum oscillator scheme -- for A2 and B2 modes. The microscopic structure of
the orthorhombic phase has been analyzed in a series of supercell calculations
for different patterns of Nb displacements, providing indications in favour of
the chain structure, with oppositely directed neighboring chains.
3.
Use of the Generalized Gradient Approximation in Pseudopotential
Calculations of Solids - Juan, Yumin; Kaxiras, Efthimios
We present a study of the equilibrium properties of $sp$-bonded solids within
the pseudopotential approach, employing recently proposed generalized gradient
approximation (GGA) exchange correlation functionals. We analyze the effects of
the gradient corrections on the behavior of the pseudopotentials and discuss
possible approaches for constructing pseudopotentials self-consistently in the
context of gradient corrected functionals. The calculated equilibrium
properties of solids using the GGA functionals are compared to the ones
obtained through the local density approximation (LDA) and to experimental
data. A significant improvement over the LDA results is achieved with the use
of the GGA functionals for cohesive energies. For the lattice constant, the
same accuracy as in LDA can...
4.
Theoretical Study of Cubic Structures Based on Fullerene Carbon
Clusters: C$_{28}$C and (C$_{28})_{2}$ - Zeger, Linda M.; Juan, Yu-Min; Kaxiras, Efthimios; Antonelli, A.
We study a new hypothetical form of solid carbon \csc, with a unit cell which
is composed of the \cs \ fullerene cluster and an additional single carbon atom
arranged in the zincblende structure. Using {\it ab initio} calculations, we
show that this new form of solid carbon has lower energy than hyperdiamond, the
recently proposed form composed of \cs \ units in the diamond structure. To
understand the bonding character of of these cluster-based solids, we analyze
the electronic structure of \csc \ and of hyperdiamond and compare them to the
electronic states of crystalline cubic diamond.
5.
Disorder-Driven Pretransitional Tweed in Martensitic Transformations - Kartha, Sivan; Krumhansl, James; Sethna, James; Wickham, Lisa
Defying the conventional wisdom regarding first--order transitions, {\it
solid--solid displacive transformations} are often accompanied by pronounced
pretransitional phenomena. Generally, these phenomena are indicative of some
mesoscopic lattice deformation that ``anticipates'' the upcoming phase
transition. Among these precursive effects is the observation of the so-called
``tweed'' pattern in transmission electron microscopy in a wide variety of
materials. We have investigated the tweed deformation in a two dimensional
model system, and found that it arises because the compositional disorder
intrinsic to any alloy conspires with the natural geometric constraints of the
lattice to produce a frustrated, glassy phase. The predicted phase diagram and
glassy behavior have been verified by numerical simulations, and...
6.
Model for the thermal conductivity of a quasicrystalline alloy - Jagannathan, A.
We present a simple model for the observed temperature dependence of the
thermal conductivity of AlPdMn (Chernikov et al, Phys.Rev.B vol.51,
(1995),153). We account for the low temperature data, upto and including the
plateau of the thermal conductivity, and discuss the similarities and the
differences between the quasicrystal and amorphous solids.
7.
Magnetocrystalline Anisotropy Energy of a Transition Metal Monolayer: A
Non-perturbative Theory - Moos, T. H.; Hübner, W.; Bennemann, K. H.
The magnetocrystalline anisotropy energy $E_{anis}$ for a monolayer of Fe and
Ni is determined using a fully convergent tight-binding calculation including
$s$-$d$ hybridization. The spin-orbit interaction $\lambda_{so}$ is treated
non-perturbatively. Remarkably, we find $E_{anis}\propto\lambda_{so}^2$ and
important contributions to $E_{anis}$ due to the lifting of degeneracies near
the Fermi-level. This is supported by the calculated decrease of the anisotropy
energy with increasing temperature on a scale of several hundred K. Our results
clarify the present debate on the origin of $E_{anis}$.
8.
Microscopic Modeling of the Growth of Order in an Alloy: Nucleated and
Continuous Ordering - Chakraborty, Bulbul; Xi, Zhigang
We study the early-stages of ordering in $Cu_3 Au$ using a model Hamiltonian
derived from the effective medium theory of cohesion in metals: an approach
providing a microscopic description of interatomic interactions in alloys. Our
simulations show a crossover from a nucleated growth regime to a region where
the ordering does not follow any simple growth laws. This mirrors the
experimental observations in $Cu_3 Au$. The kinetics of growth, obtained from
the simulations, is in semi-quantitative agreement with experiments. The
real-space structures observed in our simulations offer some insight into the
nature of early-stage kinetics
9.
Temperature dependence of surface reconstructions of Au on Pd(110) - Nieminen, J. A.
Surface reconstructions of Au film on Pd(110) substrate are studied using a
local Einstein approximation to quasiharmonic theory with the Sutton-Chen
interatomic potential. Temperature dependent surface free energies for
different coverages and surface structures are calculated. Experimentally
observed transformations from $(1\times1)$ to $(1 \times 2)$ and $(1 \times 3)$
structures can be explained in the framework of this model. Also conditions for
Stranski-Krastanov growth mode are found to comply with experiments. The domain
of validity of the model neglecting mixing entropy is analyzed.
10.
Surface States, Surface Metal-Insulator, and Surface Insulator-Metal
Transitions - TOSATTI, E.
I present an informal discussion of various cases where two-dimensional
surface metal-insulator structural and charge-density-wave instabilities driven
by partly filled surface states have been advocated. These include
reconstructions of clean semiconductor surfaces and of W(100) and Mo(100), as
well as anomalies on the hydrogen-covered surfaces H/W(110) and H/Mo(110), and
possibly alkali-covered surfaces such as K/Cu(111). In addition I will also
discuss the opposite type of phenomena, namely surface insulator-metal
transitions, which can be argued to occur on $\alpha -$% Ga(001),
high-temperature Ge(111), and probably Be(0001).
11.
Projection of plane-wave calculations into atomic orbitals - Sanchez-Portal, Daniel; Artacho, Emilio; Soler, Jose M.
The projection of the eigenfunctions obtained in standard plane-wave
first-principle electronic-structure calculations into atomic-orbital basis
sets is proposed as a formal and practical link between the methods based on
plane waves and the ones based on atomic orbitals. Given a candidate atomic
basis, ({\it i}) its quality is evaluated by its projection into the plane-wave
eigenfunctions, ({\it ii}) it is optimized by maximizing that projection, ({\it
iii}) the associated tight-binding Hamiltonian and energy bands are obtained,
and ({\it iv}) population analysis is performed in a natural way. The proposed
method replaces the traditional trial-and-error procedures of finding
appropriate atomic bases and the fitting of bands to obtain tight-binding
Hamiltonians. Test...
12.
Existence of long-range order in the steady state of a two dimensional,
two-temperature XY model - Bassler, Kevin E.; Racz, Zoltan
Monte Carlo simulations are used to show that the steady state of the d=2,
two-temperature, diffusive XY model displays a continuous phase transition from
a homogeneous disordered phase to a phase with long-range order. The long-range
order exists although both the dynamics and the interactions are local, thus
indicating the failure of a naive extension of the Mermin-Wagner theorem to
nonequilibrium steady states. It is argued that the ordering is due to
effective dipole interactions generated by the nonequilibrium dynamics.
13.
Hohenberg-Kohn theorem is valid - Mazin, I. I.; Cohen, R. E.
In a recent PRL, Gonze et al claim that the density functional theory in
Hohenberg-Kohn formulation is not valid for crystalline solids in a homogenious
electric field. They introduce another theory, density-and-polarization
functional theory, which has to substitute the DFT in the calculations of the
long-wave dielectric response. We show that the theorem proven by Gonzo et al
holds in addition to, and not instead of, the Hohenberg-Kohn theorem.
14.
Adaptive coordinate, real-space electronic structure calculations on
parallel computers - Zumbach, Gil; Modine, N. A.; Kaxiras, Efthimios
We present a method for electronic structure calculations that retains all of
the advantages of real space and addresses the inherent inefficiency of a
regular grid, which has equal precision everywhere. The computations are
carried out on a {\em regular} mesh in {\em curvilinear space}, which allows
natural and efficient decomposition on parallel computers, and effective use of
iterative numerical methods. A novel feature is the use of error analysis to
optimize the curvilinear grid for highly inhomogeneous electronic
distributions. We report accurate all-electron calculations for H$_2$, O, and
O$_{2}$.
15.
Roughness of Randomly Crumpled Elastic Sheets - Tzschichholz, F.; Hansen, A.; Roux, S.
We study the roughness of randomly crumpled elastic sheets. Based on
analytical and numerical calculations, we find that they are self affine with a
roughness exponent equal to one. Such crumpling occurs {\it e.g.\/} when wet
paper dries. We also discuss the case of correlated crumpling, which occurs in
connection with flattening of randomly folded paper.
16.
Swelling Kinetics of Layered Structures: Triblock Copolymer Mesogels - Chen, C. -M.; MacKintosh, F. C.; Williams, D. R. M.
We consider the swelling kinetics of layered structures. We focus on the case
of triblock copolymer mesogels, although our results are applicable to other
layered structures including clays. We assume the mesogels are swollen by a
solvent that is good for the bridging block but poor for the non-bridging
block. At long times the penetration front moves as in ordinary diffusion,
i.e., as $t^{1/2}$. At short times, however, the bending elasticity of the
non-bridging layers becomes important. This bending elasticity leads to a
$t^{1/6}$ relaxation of the penetration front at early times. The crossover
length between these two regimes is approximately the width of a single layer.
However, for...
17.
Prediction of an undimerized, insulating, antiferromagnetic ground-state
in halogen-bridged linear-chain Ni compounds - Anisimov, V. I.; Albers, R. C.; Wills, J. M.; Alouani, M.; Wilkins, J. W.
A parameter-free, mean-field, multi-orbital Hubbard model with nonspherical
Coulomb and exchange interactions, implemented around all-electron
local-density approximation (LDA) calculations, correctly predicts the band-gap
energy, the absence of dimerization, and the antiferromagnetic ground state of
halogen-bridged linear-chain Ni compounds. This approach also reproduces the
insulating ground state and dimerization in PtX linear-chain compounds in
agreement with experiment and previous calculations. Three "ps" figures are
included at the end of the RevTex file and compressed using uufiles.
18.
Diffusion processes and growth on stepped metal surfaces - Merikoski, J.; Ala-Nissila, T.
We study the dynamics of adatoms in a model of vicinal (11m) fcc metal
surfaces. We examine the role of different diffusion mechanisms and their
implications to surface growth. In particular, we study the effect of steps and
kinks on adatom dynamics. We show that the existence of kinks is crucially
important for adatom motion along and across steps. Our results are in
agreement with recent experiments on Cu(100) and Cu(1,1,19) surfaces. The
results also suggest that for some metals exotic diffusion mechanisms may be
important for mass transport across the steps.
19.
Structure and Magnetism of Fe/Si Multilayers Grown by Ion-Beam
Sputtering - Chaiken, A.; Michel, R. P.; Wall, M. A.
Ion-beam sputtering has been used to prepare Fe/Si multilayers on a variety
of substrates and over a wide range of temperatures. Small-angle x-ray
diffraction and transmission electron microscopy experiments show that the
layers are heavily intermixed although a composition gradient is maintained.
When the spacer layer is an amorphous iron silicide, the magnetic properties of
the multilayers are similar to those of bulk Fe. When the spacer layer is a
crystalline silicide with the B2 or DO$_3$ structure, the multilayers show
antiferromagnetic interlayer coupling like that observed in
ferromagnet/paramagnet multilayers such as Fe/Cr and Co/Cu. Depending on the
substrate type and the growth temperature, the multilayers grow in either...
20.
First-principles molecular dynamics of liquid alkali metals based on the
quantal hyper-netted chain theory - Kambayasi, Shaw; Chihara, Junzo
The density-functional theory proves that an ion-electron mixture can be
treated as a one-component liquid interacting only via a {\it pairwise}
interaction in the evaluation of the ion-ion radial distribution function
(RDF), and provides a set of integral equations: one is an integral equation
for the ion-ion RDF and another for an effective ion-ion interaction, which
depends on the ion-ion RDF. After some approximations are introduced to the
integral equation (QHNC) for the effective potential, the MD simulation and the
procedure to determine the effective interaction from the QHNC equation are
performed iteratively to be self-consistent (the QHNC-MD method). This method
provides a first-principles calculation of structures of simple...
1.
