503 research outputs found
Structure maps for hcp metals from first principles calculations
The ability to predict the existence and crystal type of ordered structures
of materials from their components is a major challenge of current materials
research. Empirical methods use experimental data to construct structure maps
and make predictions based on clustering of simple physical parameters. Their
usefulness depends on the availability of reliable data over the entire
parameter space. Recent development of high throughput methods opens the
possibility to enhance these empirical structure maps by {\it ab initio}
calculations in regions of the parameter space where the experimental evidence
is lacking or not well characterized. In this paper we construct enhanced maps
for the binary alloys of hcp metals, where the experimental data leaves large
regions of poorly characterized systems believed to be phase-separating. In
these enhanced maps, the clusters of non-compound forming systems are much
smaller than indicated by the empirical results alone.Comment: 7 pages, 4 figures, 1 tabl
Thermodynamic stabilities of ternary metal borides: An ab initio guide for synthesizing layered superconductors
Density functional theory calculations have been used to identify stable
layered Li--B crystal structure phases derived from a recently proposed
binary metal-sandwich (MS) lithium monoboride superconductor. We show that the
MS lithium monoboride gains in stability when alloyed with electron-rich metal
diborides; the resulting ordered LiB ternary phases may form
under normal synthesis conditions in a wide concentration range of for a
number of group-III-V metals . In an effort to pre-select compounds with the
strongest electron-phonon coupling we examine the softening of the in-plane
boron phonon mode at in a large class of metal borides. Our results
reveal interesting general trends for the frequency of the in-plane boron
phonon modes as a function of the boron-boron bond length and the valence of
the metal. One of the candidates with a promise to be an MgB-type
superconductor, LiAlB, has been examined in more detail: according to
our {\it ab initio} calculations of the phonon dispersion and the
electron-phonon coupling , the compound should have a critical
temperature of K.Comment: 10 pages, 9 figures, submitted to PR
Optical properties of the Ce and La di-telluride charge density wave compounds
The La and Ce di-tellurides LaTe and CeTe are deep in the
charge-density-wave (CDW) ground state even at 300 K. We have collected their
electrodynamic response over a broad spectral range from the far infrared up to
the ultraviolet. We establish the energy scale of the single particle
excitation across the CDW gap. Moreover, we find that the CDW collective state
gaps a very large portion of the Fermi surface. Similarly to the related rare
earth tri-tellurides, we envisage that interactions and Umklapp processes play
a role in the onset of the CDW broken symmetry ground state
Chemical pressure and hidden one-dimensional behavior in rare earth tri-telluride charge density wave compounds
We report on the first optical measurements of the rare-earth tri-telluride
charge-density-wave systems. Our data, collected over an extremely broad
spectral range, allow us to observe both the Drude component and the
single-particle peak, ascribed to the contributions due to the free charge
carriers and to the charge-density-wave gap excitation, respectively. The data
analysis displays a diminishing impact of the charge-density-wave condensate on
the electronic properties with decreasing lattice constant across the
rare-earth series. We propose a possible mechanism describing this behavior and
we suggest the presence of a one-dimensional character in these two-dimensional
compounds. We also envisage that interactions and umklapp processes might play
a relevant role in the formation of the charge-density-wave state in these
compounds.Comment: 8 pages, 5 figure
Covalent bonding and the nature of band gaps in some half-Heusler compounds
Half-Heusler compounds \textit{XYZ}, also called semi-Heusler compounds,
crystallize in the MgAgAs structure, in the space group . We report a
systematic examination of band gaps and the nature (covalent or ionic) of
bonding in semiconducting 8- and 18- electron half-Heusler compounds through
first-principles density functional calculations. We find the most appropriate
description of these compounds from the viewpoint of electronic structures is
one of a \textit{YZ} zinc blende lattice stuffed by the \textit{X} ion. Simple
valence rules are obeyed for bonding in the 8-electron compound. For example,
LiMgN can be written Li + (MgN), and (MgN), which is isoelectronic
with (SiSi), forms a zinc blende lattice. The 18-electron compounds can
similarly be considered as obeying valence rules. A semiconductor such as
TiCoSb can be written Ti + (CoSb); the latter unit is
isoelectronic and isostructural with zinc-blende GaSb. For both the 8- and
18-electron compounds, when \textit{X} is fixed as some electropositive cation,
the computed band gap varies approximately as the difference in Pauling
electronegativities of \textit{Y} and \textit{Z}. What is particularly exciting
is that this simple idea of a covalently bonded \textit{YZ} lattice can also be
extended to the very important \textit{magnetic} half-Heusler phases; we
describe these as valence compounds \textit{ie.} possessing a band gap at the
Fermi energy albeit only in one spin direction. The \textit{local} moment in
these magnetic compounds resides on the \textit{X} site.Comment: 18 pages and 14 figures (many in color
Pressure dependence of the charge-density-wave gap in rare-earth tri-tellurides
We investigate the pressure dependence of the optical properties of CeTe,
which exhibits an incommensurate charge-density-wave (CDW) state already at 300
K. Our data are collected in the mid-infrared spectral range at room
temperature and at pressures between 0 and 9 GPa. The energy for the single
particle excitation across the CDW gap decreases upon increasing the applied
pressure, similarly to the chemical pressure by rare-earth substitution. The
broadening of the bands upon lattice compression removes the perfect nesting
condition of the Fermi surface and therefore diminishes the impact of the CDW
transition on the electronic properties of Te.Comment: 5 pages, 4 figure
Nonlocal extension of the dispersive-optical-model to describe data below the Fermi energy
Present applications of the dispersive-optical-model analysis are restricted
by the use of a local but energy-dependent version of the generalized
Hartree-Fock potential. This restriction is lifted by the introduction of a
corresponding nonlocal potential without explicit energy dependence. Such a
strategy allows for a complete determination of the nucleon propagator below
the Fermi energy with access to the expectation value of one-body operators
(like the charge density), the one-body density matrix with associated natural
orbits, and complete spectral functions for removal strength. The present
formulation of the dispersive optical model (DOM) therefore allows the use of
elastic electron-scattering data in determining its parameters. Application to
Ca demonstrates that a fit to the charge radius leads to too much
charge near the origin using the conventional assumptions of the functional
form of the DOM. A corresponding incomplete description of high-momentum
components is identified, suggesting that the DOM formulation must be extended
in the future to accommodate such correlations properly. Unlike the local
version, the present nonlocal DOM limits the location of the deeply-bound hole
states to energies that are consistent with (\textit{e,e}\textit{p})
and (\textit{p,2p}) data.Comment: 14 pages, 10 figures, submitted to Physical Review
Evidence for coupling between collective state and phonons in two-dimensional charge-density-wave systems
We report on a Raman scattering investigation of the charge-density-wave
(CDW), quasi two-dimensional rare-earth tri-tellurides Te (= La, Ce,
Pr, Nd, Sm, Gd and Dy) at ambient pressure, and of LaTe and CeTe under
externally applied pressure. The observed phonon peaks can be ascribed to the
Raman active modes for both the undistorted as well as the distorted lattice in
the CDW state by means of a first principles calculation. The latter also
predicts the Kohn anomaly in the phonon dispersion, driving the CDW transition.
The integrated intensity of the two most prominent modes scales as a
characteristic power of the CDW-gap amplitude upon compressing the lattice,
which provides clear evidence for the tight coupling between the CDW condensate
and the vibrational modes
First principles modelling of magnesium titanium hydrides
Mixing Mg with Ti leads to a hydride Mg(x)Ti(1-x)H2 with markedly improved
(de)hydrogenation properties for x < 0.8, as compared to MgH2. Optically, thin
films of Mg(x)Ti(1-x)H2 have a black appearance, which is remarkable for a
hydride material. In this paper we study the structure and stability of
Mg(x)Ti(1-x)H2, x= 0-1 by first-principles calculations at the level of density
functional theory. We give evidence for a fluorite to rutile phase transition
at a critical composition x(c)= 0.8-0.9, which correlates with the
experimentally observed sharp decrease in (de)hydrogenation rates at this
composition. The densities of states of Mg(x)Ti(1-x)H2 have a peak at the Fermi
level, composed of Ti d states. Disorder in the positions of the Ti atoms
easily destroys the metallic plasma, however, which suppresses the optical
reflection. Interband transitions result in a featureless optical absorption
over a large energy range, causing the black appearance of Mg(x)Ti(1-x)H2.Comment: 22 pages, 9 figures, 4 table
Surface-peaked medium effects in the interaction of nucleons with finite nuclei
We investigate the asymptotic separation of the optical model potential for
nucleon-nucleus scattering in momentum space, where the potential is split into
a medium-independent term and another depending exclusively on the gradient of
the density-dependent g matrix. This decomposition confines the medium
sensitivity of the nucleon-nucleus coupling to the surface of the nucleus. We
examine this feature in the context of proton-nucleus scattering at beam
energies between 30 and 100 MeV and find that the pn coupling accounts for most
of this sensitivity. Additionally, based on this general structure of the
optical potential we are able to treat both, the medium dependence of the
effective interaction and the full mixed density as described by
single-particle shell models. The calculated scattering observables agree
within 10% with those obtained by Arellano, Brieva and Love in their
momentum-space g-folding approach.Comment: 16 pages, 8 figures, submitted to PR
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