191 research outputs found
A many-body interatomic potential for ionic systems: application to MgO
An analytic representation of the short-range repulsion energy in ionic
systems is described that allows for the fact that ions may change their size
and shape depending on their environment. This function is extremely efficient
to evaluate relative to previous methods of modeling the same physical effects.
Using a well-defined parametrization procedure we have obtained parameter sets
for this energy function that reproduce closely the density functional theory
potential energy surface of bulk MgO. We show how excellent agreement can be
obtained with experimental measurements of phonon frequencies and temperature
and pressure dependences of the density by using this effective potential in
conjunction with ab initio parametrization.Comment: To appear in Journal of Chemical Physics (Oct 15th 2003
Spectroscopic fingerprints of a surface Mott-Hubbard insulator: the case of SiC(0001)
We discuss the spectroscopic fingerprints that a surface Mott-Hubbard
insulator should show at the intra-atomic level. The test case considered is
that of the Si-terminated SiC(0001) sqrt{3}xsqrt{3} surface, which is known
experimentally to be insulating. We argue that, due to the Mott-Hubbard
phenomenon, spin unpaired electrons in the Si adatom dangling bonds are
expected to give rise to a Si-2p core level spectrum with a characteristic
three-peaked structure, as seen experimentally. This structure results from the
joint effect of intra-atomic exchange, spatial anisotropy, and spin-orbit
coupling. Auger intensities are also discussed.Comment: 4 pages, 2 figures, ECOSS-18 conferenc
How well do Car-Parrinello simulations reproduce the Born-Oppenheimer surface ? Theory and Examples
We derive an analytic expression for the average difference between the
forces on the ions in a Car-Parrinello simulation and the forces obtained at
the same ionic positions when the electrons are at their ground state. We show
that for common values of the fictitious electron mass, a systematic bias may
affect the Car-Parrinello forces in systems where the electron-ion coupling is
large. We show that in the limit where the electronic orbitals are rigidly
dragged by the ions the difference between the two dynamics amounts to a
rescaling of the ionic masses, thereby leaving the thermodynamics intact. We
study the examples of crystalline magnesium oxide and crystalline and molten
silicon. We find that for crystalline silicon the errors are very small. For
crystalline MgO the errors are very large but the dynamics can be quite well
corrected within the rigid-ion model. We conclude that it is important to
control the effect of the electron mass parameter on the quantities extracted
from Car-Parrinello simulations.Comment: Submitted to the Journal of Chemical Physic
CONSOLIDATION AND RESTORATION OF HISTORICAL HERITAGE: THE FLAVIAN AMPHITHEATER IN ROME
Abstract. The recovery and retrofitting techniques adopted for historical structures and archaeological sites face an apparent dichotomy between conservation of constructions and the safety of users. Literatures show several examples where the current day structural safety of historical constructions, gets defined by the nature of past interventions, the compatibility of materials and elements used in retrofitting. The adopted interventions were, in their time, considered innovative, but over the years their compatibility and reversibility leave the historic constructions structurally vulnerable. For these reasons, a careful understanding of the structural systems is fundamental for the implementation of appropriate retrofitting solutions. Especially for monuments and Archaeological sites the objective to be achieved has to be clear, avoiding destructive investigation tests. In this work the instabilities caused by a consolidation intervention on some travertine columns in a sector of the Flavian Amphitheatre, better known as "Colosseum" in Rome, are critically analysed. The current consolidation operations are compared to the previous one. The restoration activity involves in-depth diagnosis process: the historical analysis of the failures and restorations of that area of the Colosseum, a survey of the crack pattern and an indirect investigation on the travertine of the columns. Subsequently the various data coming from the knowledge phase are elaborated, in order to have a correct interpretation of the causes triggering the failure and guide the choice of the most correct retrofitting techniques
The mechanism for the 3 x 3 distortion of Sn/ge (111)
We show that two distinct ground states, one nonmagnetic,
metallic, and distorted, the other magnetic, semimetallic (or insulating) and
undistorted, compete in -phase adsorbates on semiconductor (111)
surfaces. In Sn/Ge(111), LSDA/GGA calculations indicate, in agreement with
experiment, that the distorted metallic ground state prevails. The reason for
stability of this state is analysed, and is traced to a sort of bond density
wave, specifically a modulation of the antibonding state filling between the
adatom and a Ge-Ge bond directly underneath
Disproportionation Phenomena on Free and Strained Sn/Ge(111) and Sn/Si(111) Surfaces
Distortions of the Sn/Ge(111) and Sn/Si(111) surfaces
are shown to reflect a disproportionation of an integer pseudocharge, ,
related to the surface band occupancy. A novel understanding of the
-1U (``1 up, 2 down'') and 2U (``2 up, 1 down'') distortions of
Sn/Ge(111) is obtained by a theoretical study of the phase diagram under
strain. Positive strain keeps the unstrained value Q=3 but removes distorsions.
Negative strain attracts pseudocharge from the valence band causing first a
-2U distortion (Q=4) on both Sn/Ge and Sn/Si, and eventually a
-3U (``all up'') state with Q=6. The possibility of a
fluctuating phase in unstrained Sn/Si(111) is discussed.Comment: Revtex, 5 pages, 3 figure
SiC(0001): a surface Mott-Hubbard insulator
We present ab-initio electronic structure calculations for the Si-terminated
SiC(0001) surface. While local density approximation
(LDA) calculations predict a metallic ground state with a half-filled narrow
band, Coulomb effects, included by the spin-polarized LDA+U method, result in a
magnetic (Mott-Hubbard) insulator with a gap of 1.5 eV, comparable with the
experimental value of 2.0 eV. The calculated value of the inter-site exchange
parameter, J=30K, leads to the prediction of a paramagnetic Mott state, except
at very low temperatures. The observed Si 2p surface core level doublet can
naturally be explained as an on-site exchange splitting.Comment: RevTex, 4 pages, 4 eps-figure
First Principles Calculations of Charge and Spin Density Waves of sqr3-Adsorbates on Semiconductors
We present ab-initio electronic structure results on the surface of sqr3
adsorbates. In particular, we address the issue of metal-insulator
instabilities, charge-density-waves (CDWs) or spin-density-waves (SDWs), driven
by partly filled surface states and their 2D Fermi surface, and/or by the onset
of magnetic instabilities. The focus is both on the newly discovered
commensurate CDW transitions in the Pb/Ge(111) and Sn/Ge(111) structures, and
on the puzzling semiconducting behavior of the Pb/Ge(111), K/Si(111):B and
SiC(0001) surfaces. In all cases, the main factor driving the instability
appears to be an extremely narrow surface state band. We have carried out so
far preliminary calculations for the Si/Si(111) surface, chosen as our model
system, within the gradient corrected local density (LDA+GC) and local spin
density (LSD+GC) approximations, with the aim of understanding the possible
interplay between 2D Fermi surface and electron correlations in the surface +
adsorbate system. Our spin- unrestricted results show that the sqr3
paramagnetic surface is unstable towards a commensurate SDW with periodicity
3x3 and magnetization 1/3.Comment: 9 pages, 4 Postscript figures, to be published in Surf. Sc
Local structure of liquid carbon controls diamond nucleation
Diamonds melt at temperatures above 4000 K. There are no measurements of the
steady-state rate of the reverse process: diamond nucleation from the melt,
because experiments are difficult at these extreme temperatures and pressures.
Using numerical simulations, we estimate the diamond nucleation rate and find
that it increases by many orders of magnitude when the pressure is increased at
constant supersaturation. The reason is that an increase in pressure changes
the local coordination of carbon atoms from three-fold to four-fold. It turns
out to be much easier to nucleate diamond in a four-fold coordinated liquid
than in a liquid with three-fold coordination, because in the latter case the
free-energy cost to create a diamond-liquid interface is higher. We speculate
that this mechanism for nucleation control is relevant for crystallization in
many network-forming liquids. On the basis of our calculations, we conclude
that homogeneous diamond nucleation is likely in carbon-rich stars and unlikely
in gaseous planets
- …