1,369 research outputs found
Accurate quadratic-response approximation for the self-consistent pseudopotential of semiconductor nanostructures
Quadratic-response theory is shown to provide a conceptually simple but
accurate approximation for the self-consistent one-electron potential of
semiconductor nanostructures. Numerical examples are presented for GaAs/AlAs
and InGaAs/InP (001) superlattices using the local-density approximation to
density-functional theory and norm-conserving pseudopotentials without
spin-orbit coupling. When the reference crystal is chosen to be the
virtual-crystal average of the two bulk constituents, the absolute error in the
quadratic-response potential for Gamma(15) valence electrons is about 2 meV for
GaAs/AlAs and 5 meV for InGaAs/InP. Low-order multipole expansions of the
electron density and potential response are shown to be accurate throughout a
small neighborhood of each reciprocal lattice vector, thus providing a further
simplification that is confirmed to be valid for slowly varying envelope
functions. Although the linear response is about an order of magnitude larger
than the quadratic response, the quadratic terms are important both
quantitatively (if an accuracy of better than a few tens of meV is desired) and
qualitatively (due to their different symmetry and long-range dipole effects).Comment: 16 pages, 20 figures; v2: new section on limitations of theor
Quadratic response theory for spin-orbit coupling in semiconductor heterostructures
This paper examines the properties of the self-energy operator in
lattice-matched semiconductor heterostructures, focusing on nonanalytic
behavior at small values of the crystal momentum, which gives rise to
long-range Coulomb potentials. A nonlinear response theory is developed for
nonlocal spin-dependent perturbing potentials. The ionic pseudopotential of the
heterostructure is treated as a perturbation of a bulk reference crystal, and
the self-energy is derived to second order in the perturbation. If spin-orbit
coupling is neglected outside the atomic cores, the problem can be analyzed as
if the perturbation were a local spin scalar, since the nonlocal spin-dependent
part of the pseudopotential merely renormalizes the results obtained from a
local perturbation. The spin-dependent terms in the self-energy therefore fall
into two classes: short-range potentials that are analytic in momentum space,
and long-range nonanalytic terms that arise from the screened Coulomb potential
multiplied by a spin-dependent vertex function. For an insulator at zero
temperature, it is shown that the electronic charge induced by a given
perturbation is exactly linearly proportional to the charge of the perturbing
potential. These results are used in a subsequent paper to develop a
first-principles effective-mass theory with generalized Rashba spin-orbit
coupling.Comment: 20 pages, no figures, RevTeX4; v2: final published versio
Ab initio studies of phonon softening and high pressure phase transitions of alpha-quartz SiO2
Density functional perturbation theory calculations of alpha-quartz using
extended norm conserving pseudopotentials have been used to study the elastic
properties and phonon dispersion relations along various high symmetry
directions as a function of bulk, uniaxial and non-hydrostatic pressure. The
computed equation of state, elastic constants and phonon frequencies are found
to be in good agreement with available experimental data. A zone boundary (1/3,
1/3, 0) K-point phonon mode becomes soft for pressures above P=32 GPa. Around
the same pressure, studies of the Born stability criteria reveal that the
structure is mechanically unstable. The phonon and elastic softening are
related to the high pressure phase transitions and amorphization of quartz and
these studies suggest that the mean transition pressure is lowered under
non-hydrostatic conditions. Application of uniaxial pressure, results in a
post-quartz crystalline monoclinic C2 structural transition in the vicinity of
the K-point instability. This structure, intermediate between quartz and
stishovite has two-thirds of the silicon atoms in octahedral coordination while
the remaining silicon atoms remain tetrahedrally coordinated. This novel
monoclinic C2 polymorph of silica, which is found to be metastable under
ambient conditions, is possibly one of the several competing dense forms of
silica containing octahedrally coordinated silicon. The possible role of high
pressure ferroelastic phases in causing pressure induced amorphization in
silica are discussed.Comment: 17 pages, 8 figs., 8 Table
Atomic structure and vibrational properties of icosahedral BC boron carbide
The atomic structure of icosahedral BC boron carbide is determined by
comparing existing infra-red absorption and Raman diffusion measurements with
the predictions of accurate {\it ab initio} lattice-dynamical calculations
performed for different structural models. This allows us to unambiguously
determine the location of the carbon atom within the boron icosahedron, a task
presently beyond X-ray and neutron diffraction ability. By examining the inter-
and intra-icosahedral contributions to the stiffness we show that, contrary to
recent conjectures, intra-icosahedral bonds are harder.Comment: 9 pages including 3 figures, accepted in Physical Review Letter
First-principles envelope-function theory for lattice-matched semiconductor heterostructures
In this paper a multi-band envelope-function Hamiltonian for lattice-matched
semiconductor heterostructures is derived from first-principles norm-conserving
pseudopotentials. The theory is applicable to isovalent or heterovalent
heterostructures with macroscopically neutral interfaces and no spontaneous
bulk polarization. The key assumption -- proved in earlier numerical studies --
is that the heterostructure can be treated as a weak perturbation with respect
to some periodic reference crystal, with the nonlinear response small in
comparison to the linear response. Quadratic response theory is then used in
conjunction with k.p perturbation theory to develop a multi-band effective-mass
Hamiltonian (for slowly varying envelope functions) in which all interface
band-mixing effects are determined by the linear response. To within terms of
the same order as the position dependence of the effective mass, the quadratic
response contributes only a bulk band offset term and an interface dipole term,
both of which are diagonal in the effective-mass Hamiltonian. Long-range
multipole Coulomb fields arise in quantum wires or dots, but have no
qualitative effect in two-dimensional systems beyond a dipole contribution to
the band offsets.Comment: 25 pages, no figures, RevTeX4; v3: final published versio
Hole dynamics in a quantum antiferromagnet beyond the retraceable path approximation
The one-hole spectral weight for two chains and two dimensional lattices is
studied numerically using a new method of analysis of the spectral function
within the Lanczos iteration scheme: the Lanczos spectra decoding method. This
technique is applied to the model for , directly in the
infinite size lattice. By a careful investigation of the first 13 Lanczos steps
and the first 26 ones for the two dimensional and the two chain cases
respectively, we get several new features of the one-hole spectral weight. A
sharp incoherent peak with a clear momentum dispersion is identified, together
with a second broad peak at higher energy. The spectral weight is finite up to
the Nagaoka energy where it vanishes in a non-analytic way. Thus the lowest
energy of one hole in a quantum antiferromagnet is degenerate with the Nagaoka
energy in the thermodynamic limit.Comment: RevTeX 3.0, SISSA preprint 156/93/CM/MB, 10 pages + postscript file
appended, contains more accurate calculations in Fig.
Temperature-dependent Raman study of CeFeAsO0.9F0.1 Superconductor: Crystal field excitations, phonons and their coupling
We report temperature-dependent Raman spectra of CeFeAsO0.9F0.1 from 4 K to
300 K in spectral range of 60 to 1800 cm-1 and interpret them using estimates
of phonon frequencies obtained from first-principles density functional
calculations. We find evidence for a strong coupling between the phonons and
crystal field excitations; in particular Ce3+ crystal field excitation at 432
cm-1 couples strongly with Eg oxygen vibration at 389 cm-1 . Below the
superconducting transition temperature, the phonon mode near 280 cm-1 shows
softening, signaling its coupling with the superconducting gap. The ratio of
the superconducting gap to Tc thus estimated to be ~ 10 suggests CeFeAsO0.9F0.1
as a strong coupling superconductor. In addition, two high frequency modes
observed at 1342 cm-1 and 1600 cm-
Magnons in real materials from density-functional theory
We present an implementation of the adiabatic spin-wave dynamics of Niu and
Kleinman. This technique allows to decouple the spin and charge excitations of
a many-electron system using a generalization of the adiabatic approximation.
The only input for the spin-wave equations of motion are the energies and Berry
curvatures of many-electron states describing frozen spin spirals. The latter
are computed using a newly developed technique based on constrained
density-functional theory, within the local spin density approximation and the
pseudo-potential plane-wave method. Calculations for iron show an excellent
agreement with experiments.Comment: 1 LaTeX file and 1 postscript figur
Theoretical evidences for enhanced superconducting transition temperature of CaSi in a high-pressure AlB phase
By means of first-principles calculations, we studied stable lattice
structures and estimated superconducting transition temperature of CaSi at
high pressure. Our simulation showed stability of the AlB structure in a
pressure range above 17 GPa. In this structure, doubly degenerated optical
phonon modes, in which the neighboring silicon atoms oscillate alternately in a
silicon plane, show prominently strong interaction with the conduction
electrons. In addition there exists a softened optical mode (out-of-plan motion
of silicon atoms), whose strength of the electron-phonon interaction is nearly
the same as the above mode. The density of states at the Fermi level in the
AlB structure is higher than that in the trigonal structure. These findings
and the estimation of the transition temperature strongly suggest that higher
is expected in the AlB structure than the trigonal structures
which are known so far.Comment: 6 pages and 11 figure
Unraveling the molecular mechanisms of color expression in anthocyanins
Anthocyanins are a broad family of natural dyes, increasingly finding application as substitutes for artificial colorants in the food industry. In spite of their importance and ubiquity, the molecular principles responsible for their extreme color variability are poorly known. We address these mechanisms by computer simulations and photoabsorption experiments of cyanidin-3-O-glucoside in water solution, as a proxy for more complex members of the family. Experimental results are presented in the range of pH 1-9, accompanied by a comprehensive systematic computational study across relevant charge states and tautomers. The computed spectra are in excellent agreement with the experiments, providing unprecedented insight into the complex behavior underlying color expression in these molecules. Besides confirming the importance of the molecule's charge state, we also unveil the hitherto unrecognized role of internal distortions in the chromophore, which affect its degree of conjugation, modulating the optical gap and in turn the color. This entanglement of structural and electronic traits is also shared by other members of the anthocyanin family (e.g. pelargonidin and delphinidin) highlighting a common mechanism for color expression across this important family of natural dyes
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