87 research outputs found
Ab Initio Approach to Second-order Resonant Raman Scattering Including Exciton-Phonon Interaction
Raman spectra obtained by the inelastic scattering of light by crystalline
solids contain contributions from first-order vibrational processes (e.g. the
emission or absorption of one phonon, a quantum of vibration) as well as
higher-order processes with at least two phonons being involved. At second
order, coupling with the entire phonon spectrum induces a response that may
strongly depend on the excitation energy, and reflects complex processes more
difficult to interpret. In particular, excitons (i.e. bound electron-hole
pairs) may enhance the absorption and emission of light, and couple strongly
with phonons in resonance conditions. We design and implement a
first-principles methodology to compute second-order Raman scattering,
incorporating dielectric responses and phonon eigenstates obtained from
density-functional theory and many-body theory. We demonstrate our approach for
the case of silicon, relating frequency-dependent relative Raman intensities,
that are in excellent agreement with experiment, to different vibrations and
regions of the Brillouin zone. We show that exciton-phonon coupling, computed
from first principles, indeed strongly affect the spectrum in resonance
conditions. The ability to analyze second-order Raman spectra thus provides
direct insight into this interaction.Comment: 10 pages, 8 figure
Electron-phonon interaction in Graphite Intercalation Compounds
Motivated by the recent discovery of superconductivity in Ca- and
Yb-intercalated graphite (CaC and YbC) and from the ongoing debate
on the nature and role of the interlayer state in this class of compounds, in
this work we critically study the electron-phonon properties of a simple model
based on primitive graphite. We show that this model captures an essential
feature of the electron-phonon properties of the Graphite Intercalation
Compounds (GICs), namely, the existence of a strong dormant electron-phonon
interaction between interlayer and electrons, for which we
provide a simple geometrical explanation in terms of NMTO Wannier-like
functions. Our findings correct the oversimplified view that
nearly-free-electron states cannot interact with the surrounding lattice, and
explain the empirical correlation between the filling of the interlayer band
and the occurrence of superconductivity in Graphite-Intercalation Compounds.Comment: 13 pages, 12 figures, submitted to Phys. Rev.
Electrons and phonons in the ternary alloy CaAlSi} as a function of composition
We report a detailed first-principles study of the structural, electronic and
vibrational properties of the superconducting C phase of the ternary
alloy CaAlSi, both in the experimental range ,
for which the alloy has been synthesised, and in the theoretical limits of high
aluminium and high silicon concentration. Our results indicate that, in the
experimental range, the dependence of the electronic bands on composition is
well described by a rigid-band model, which breaks down outside this range.
Such a breakdown, in the (theoretical) limit of high aluminium concentration,
is connected to the appearance of vibrational instabilities, and results in
important differences between CaAl and MgB. Unlike MgB, the
interlayer band and the out-of-plane phonons play a major role on the stability
and superconductivity of CaAlSi and related C intermetallic compounds
First-principle study of paraelectric and ferroelectric CsHPO including dispersion forces: stability and related vibrational, dielectric and elastic properties
Using density functional theory (DFT) and density functional perturbation
theory (DFPT), we investigate the stability and response functions of
CsHPO, a ferroelectric material at low temperature. This material
cannot be described properly by the usual (semi-)local approximations within
DFT. The long-range e-e correlation needs to be properly taken into
account, using, for instance, Grimme's DFT-D methods, as investigated in this
work. We find that DFT-D3(BJ) performs the best for the members of the
dihydrogenated alkali phosphate family (KHPO, RbHPO,
CsHPO), leading to experimental lattice parameters reproduced with an
average deviation of 0.5 %. With these DFT-D methods, the structural,
dielectric, vibrational and mechanical properties of CsHPO are globally
in excellent agreement with the available experiments ( 2% MAPE for
Raman-active phonons). Our study suggests the possible existence of a new
low-temperature phase for CsHPO, not yet reported experimentally.
Finally, we report the implementation of DFT-D contributions to elastic
constants within DFPT.Comment: This paper was published in Physical Review B the 25 January 2017 (21
pages, 4 figures
Origin of magnetism and quasiparticles properties in Cr-doped TiO
Combining LSDA+ and an analysis of superexchange interactions beyond DFT,
we describe the magnetic ground states in rutile and anatase Cr-doped TiO.
In parallel, we correct our LSDA+ ground state through GW corrections
(@LSDA+) that reproduce the position of impurity states and the band
gaps in satisfying agreement with experiments. Because of the different
topological coordinations of Cr-Cr bonds in the ground states of rutile and
anatase, superexchange interactions induce either ferromagnetic or
antiferromagnetic couplings of Cr ions. In Cr-doped anatase, this interaction
leads to a new mechanism which stabilizes a ferromagnetic ground state, in
keeping with experimental evidence, without the need to invoke F-center
exchange.Comment: 5<pages, 4 figure
Computationally-driven, high throughput identification of CaTe and LiSb as promising candidates for high mobility -type transparent conducting materials
High-performance -type transparent conducting materials (TCMs) must
exhibit a rare combination of properties including high mobility, transparency
and -type dopability. The development of high-mobility/conductivity -type
TCMs is necessary for many applications such as solar cells, or transparent
electronic devices. Oxides have been traditionally considered as the most
promising chemical space to dig out novel -type TCMs. However, non-oxides
might perform better than traditional -type TCMs (oxides) in terms of
mobility. We report on a high-throughput (HT) computational search for
non-oxide -type TCMs from a large dataset of more than 30,000 compounds
which identified CaTe and LiSb as very good candidates for
high-mobility -type TCMs. From our calculations, both compounds are expected
to be -type dopable: intrinsically for LiSb while CaTe would
require extrinsic doping. Using electron-phonon computations, we estimate hole
mobilities at room-temperature to be about 20 and 70 cm/Vs for CaTe and
LiSb, respectively. The computed hole mobility for
LiSb is quite exceptional and comparable with the electron
mobility in the best -type TCMs.Comment: 10 pages, 5 figure
Band widths and gaps from the Tran-Blaha functional : Comparison with many-body perturbation theory
For a set of ten crystalline materials (oxides and semiconductors), we
compute the electronic band structures using the Tran-Blaha [Phys. Rev. Lett.
102, 226401 (2009)] (TB09) functional. The band widths and gaps are compared
with those from the local-density approximation (LDA) functional, many-body
perturbation theory (MBPT), and experiments. At the density-functional theory
(DFT) level, TB09 leads to band gaps in much better agreement with experiments
than LDA. However, we observe that it globally underestimates, often strongly,
the valence (and conduction) band widths (more than LDA). MBPT corrections are
calculated starting from both LDA and TB09 eigenenergies and wavefunctions.
They lead to a much better agreement with experimental data for band widths.
The band gaps obtained starting from TB09 are close to those from
quasi-particle self-consistent GW calculations, at a much reduced cost.
Finally, we explore the possibility to tune one of the semi-empirical
parameters of the TB09 functional in order to obtain simultaneously better band
gaps and widths. We find that these requirements are conflicting.Comment: 18 pages, 16 figure
Generating and grading 34 Optimised Norm-Conserving Vanderbilt Pseudopotentials for Actinides and Super Heavy Elements in the PseudoDojo
In the last decades, material discovery has been a very active research field
driven by the necessity of new materials for different applications. This has
also included materials incorporating heavy elements, beyond the stable
isotopes of lead. Most of actinides exhibit unique properties that make them
useful in various applications. Further, new heavy elements, taking the name of
super-heavy elements, have been synthesized, filling previously empty space of
Mendeleev periodic table. Their chemical bonding behaviour, of academic
interest at present, would also benefit of state-of-the-art modelling
approaches. In particular, in order to perform first-principles calculations
with planewave basis sets, one needs corresponding pseudopotentials. In this
work, we present a series of fully-relativistic optimised norm-conserving
Vanderbilt pseudopotentials (ONCVPs) for thirty-four actinides and super-heavy
elements. The scalar relativistic version of these ONCVPs is tested by
comparing equations of states for crystals, obtained with \textsc{abinit} 9.6,
with those obtained by all-electron zeroth-order regular approximation (ZORA)
calculations performed with the Amsterdam Modelling Suite BAND code.
-Gauge and -Gauge indicators are used to validate these
pseudopotentials. This work is a contribution to the PseudoDojo project, in
which pseudopotentials for the whole periodic table are developed and
systematically tested. The fully-relativistic pseudopotential files (i.e.
including spin-orbit coupling) are available on the PseudoDojo web-interface
pseudo-dojo.org under the name NC FR (ONCVPSP) v4.x. Pseudopotentials are made
available in psp8 and UPF2 formats, both convenient for \textsc{abinit}, the
latter being also suitable for Quantum ESPRESSO
Limits to Hole Mobility and Doping in Copper Iodide
Over one hundred years have passed since the discovery of the p-type transparent conducting material copper iodide, predating the concept of the âelectronâholeâ itself. Supercentenarian status notwithstanding, little is understood about the charge transport mechanisms in CuI. Herein, a variety of modeling techniques are used to investigate the charge transport properties of CuI, and limitations to the hole mobility over experimentally achievable carrier concentrations are discussed. Poor dielectric response is responsible for extensive scattering from ionized impurities at degenerately doped carrier concentrations, while phonon scattering is found to dominate at lower carrier concentrations. A phonon-limited hole mobility of 162 cm2 Vâ1 sâ1 is predicted at room temperature. The simulated charge transport properties for CuI are compared to existing experimental data, and the implications for future device performance are discussed. In addition to charge transport calculations, the defect chemistry of CuI is investigated with hybrid functionals, revealing that reasonably localized holes from the copper vacancy are the predominant source of charge carriers. The chalcogens S and Se are investigated as extrinsic dopants, where it is found that despite relatively low defect formation energies, they are unlikely to act as efficient electron acceptors due to the strong localization of holes and subsequent deep transition levels
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