90 research outputs found
A many-body perturbation theory approach to the electron-phonon interaction with density-functional theory as a starting point
The electron-phonon interaction plays a crucial role in many fields of
physics and chemistry. Nevertheless, its actual calculation by means of modern
many-body perturbation theory is weakened by the use of model Hamiltonians that
are based on parameters difficult to extract from the experiments. Such
shortcoming can be bypassed by using density-functional theory to evaluate the
electron-phonon scattering amplitudes, phonon frequencies and electronic bare
energies. In this work, we discuss how a consistent many-body diagrammatic
expansion can be constructed on top of density-functional theory. In that
context, the role played by screening and self-consistency when all the
components of the electron-nucleus and nucleus-nucleus interactions are taken
into account is paramount. A way to avoid over-screening is notably presented.
Finally, we derive cancellations rules as well as internal consistency
constraints in order to draw a clear, sound and practical scheme to merge
many-body perturbation and density-functional theory.Comment: 25 pages, 13 figure
Structural, electronic, elastic, power and transport properties of -GaO from first-principles
We investigate the structural, electronic, vibrational, power and transport
properties of the allotrope of GaO from first-principles. We
find phonon frequencies and elastic constants that reproduce the correct band
ordering, in agreement with experiment. We use the Boltzmann transport equation
to compute the intrinsic electron and hole drift mobility and obtain a room
temperature values of 258 cm/Vs and 1.2 cm/Vs, respectively as well as
6300 cm/Vs and 13 cm/Vs at 100 K. Through a spectral decomposition of
the scattering contribution to the inverse mobility, we find that multiple
longitudinal optical modes of B symmetry are responsible for the electron
mobility of - GaO but that many acoustic modes also contributes,
making it essential to include all scattering processes in the calculations.
Using the von Hippel low energy criterion we computed the breakdown field to be
5.8 MV/cm at room temperature yielding a Baliga's figure of merit of 1250 with
respect to silicon, ideal for high-power electronics. This work presents a
general framework to predictively investigate novel high-power electronic
materials.Comment: With respect to the published version, this version includes the
correct citation to F. Macheda and N. Bonini, Phys. Rev. B 98, 201201 (2018)
- Ref. [64
First-principles study of Ce doped lanthanum silicate nitride phosphors: Neutral excitation, Stokes shift, and luminescent center identification
We study from first principles two lanthanum silicate nitride compounds,
LaSiN and LaSiN, pristine as well as doped with
Ce ion, in view of explaining their different emission color, and
characterising the luminescent center. The electronic structures of the two
undoped hosts are similar, and do not give a hint to quantitatively describe
such difference. The neutral excitation of the Ce
ions is simulated through a constrained density-functional theory method
coupled with a SCF analysis of total energies, yielding absorption
energies. Afterwards, atomic positions in the excited state are relaxed,
yielding the emission energies and Stokes shifts. Based on these results, the
luminescent centers in LaSiN:Ce and LaSiN:Ce are
identified. The agreement with the experimental data for the computed
quantities is quite reasonable and explains the different color of the emitted
light. Also, the Stokes shifts are obtained within 20\% difference relative to
experimental data.Comment: 12 pages, 10 figure
First-principles Study of the Luminescence of Eu2+-doped Phosphors
The luminescence of fifteen representative Eu-doped phosphors used for
white-LED and scintillation applications is studied through a Constrained
Density Functional Theory. Transition energies and Stokes shift are deduced
from differences of total energies between the ground and excited states of the
systems, in the absorption and emission geometries. The general applicability
of such methodology is first assessed: for this representative set, the
calculated absolute error with respect to experiment on absorption and emission
energies is within 0.3 eV. This set of compounds covers a wide range of
transition energies that extents from 1.7 to 3.5 eV. The information gained
from the relaxed geometries and total energies is further used to evaluate the
thermal barrier for the crossover, the full width at half-maximum of
the emission spectrum and the temperature shift of the emission peak, using a
one-dimensional configuration-coordinate model. The former results indicate
that the crossover cannot be the dominant mechanism for the thermal
quenching behavior of Eu-doped phosphors and the latter results are
compared to available experimental data and yield a 30 mean absolute
relative error. Finally, a semi-empirical model used previously for
Ce-doped hosts is adapted to Eu-doped hosts and gives the
absorption and emission energies within 0.9 eV of experiment, underperforming
compared to the first-principles calculation.Comment: 17 pages, 13 figures, (Phys. Rev. B 2017 Accept
Assessment of First-Principles and Semiempirical Methodologies for Absorption and Emission Energies of Ce-Doped Luminescent Materials
In search of a reliable methodology for the prediction of light absorption
and emission of Ce-doped luminescent materials, 13 representative
materials are studied with first-principles and semiempirical approaches. In
the first-principles approach, that combines constrained density-functional
theory and SCF, the atomic positions are obtained for both ground and
excited states of the Ce ion. The structural information is fed into
Dorenbos' semiempirical model. Absorption and emission energies are calculated
with both methods and compared with experiment. The first-principles approach
matches experiment within 0.3 eV, with two exceptions at 0.5 eV. In contrast,
the semiempirical approach does not perform as well (usually more than 0.5 eV
error). The general applicability of the present first-principles scheme, with
an encouraging predictive power, opens a novel avenue for crystal site
engineering and high-throughput search for new phosphors and scintillators.Comment: 12 pages, 3 figure
Hole mobility of strained GaN from first principles
Nitride semiconductors are ubiquitous in optoelectronic devices such as LEDs
and Blu-Ray optical disks. A major limitation for further adoption of GaN in
power electronics is its low hole mobility. In order to address this challenge,
here we investigate the phonon-limited mobility of wurtzite GaN using the ab
initio Boltzmann transport formalism, including all electron-phonon scattering
processes, spin-orbit coupling, and many-body quasiparticle band structures. We
demonstrate that the mobility is dominated by acoustic deformation-potential
scattering, and we predict that the hole mobility can significantly be
increased by lifting the split-off hole states above the light and heavy holes.
This can be achieved by reversing the sign of the crystal-field splitting via
strain or via coherent excitation the A optical phonon through ultrafast
infrared optical pulses.Comment: 17 pages and 11 figure
Route to high hole mobility in GaN via reversal of crystal-field splitting
A fundamental obstacle toward the realization of GaN p-channel transistors is
its low hole mobility. Here we investigate the intrinsic phonon-limited
mobility of electrons and holes in wurtzite GaN using the ab initio Boltzmann
transport formalism, including all electron-phonon scattering processes and
many-body quasiparticle band structures. We predict that the hole mobility can
be increased by reversing the sign of the crystal-field splitting, in such a
way as to lift the split-off hole states above the light and heavy holes. We
find that a 2% biaxial tensile strain can increase the hole mobility by 230%,
up to a theoretical Hall mobility of 120 cm/Vs at room temperature and 620
cm/Vs at 100 K.Comment: Submitted to PRL on 3 Nov 2018. Update to cross-reference with
arXiv:1908.0207
Carrier Lifetimes and Polaronic Mass Enhancement in the Hybrid Halide Perovskite CHNHPbI from Multiphonon Fr\"ohlich Coupling
We elucidate the nature of the electron-phonon interaction in the archetypal
hybrid perovskite CHNHPbI using ab initio many-body calculations
and an exactly solvable model. We demonstrate that electrons and holes near the
band edges primarily interact with three distinct groups of
longitudinal-optical vibrations, in order of importance: the stretching of the
Pb-I bond, the bending of the Pb-I-Pb bonds, and the libration of the organic
cations. These polar phonons induce ultrafast intraband carrier relaxation over
timescales of 6-30 fs and yield polaron effective masses 28% heavier than the
bare band masses. These findings allow us to rationalize previous experimental
observations and provide a key to understanding carrier dynamics in halide
perovskites
General invariance and equilibrium conditions for lattice dynamics in 1D, 2D, and 3D materials
The long-wavelength behavior of vibrational modes plays a central role in
carrier transport, phonon-assisted optical properties, superconductivity, and
thermomechanical and thermoelectric properties of materials. Here, we present
general invariance and equilibrium conditions of the lattice potential; these
allow to recover the quadratic dispersions of flexural phonons in
low-dimensional materials, in agreement with the phenomenological model for
long-wavelength bending modes. We also prove that for any low-dimensional
material the bending modes can have a purely out-of-plane polarization in the
vacuum direction and a quadratic dispersion in the long-wavelength limit. In
addition, we propose an effective approach to treat invariance conditions in
crystals with non-vanishing Born effective charges where the long-range
dipole-dipole interactions induce a contribution to the lattice potential and
stress tensor. Our approach is successfully applied to the phonon dispersions
of 158 two-dimensional materials, highlighting its critical relevance in the
study of phonon-mediated properties of low-dimensional materials.Comment: 14 pages, 7 figures for main text, 52 pages with supplementary
informatio
Dynamical and anharmonic effects on the electron-phonon coupling and the zero-point renormalization of the electronic structure
The renormalization of the band structure at zero temperature due to
electron-phonon coupling is investigated in diamond, BN, LiF and MgO crystals.
We implement a dynamical scheme to compute the frequency-dependent self-energy
and the resulting quasiparticle electronic structure. Our calculations reveal
the presence of a satellite band below the Fermi level of LiF and MgO. We show
that the renormalization factor (Z), which is neglected in the adiabatic
approximation, can reduce the zero-point renormalization (ZPR) by as much as
40%. Anharmonic effects in the renormalized eigenvalues at finite atomic
displacements are explored with the frozen-phonon method. We use a
non-perturbative expression for the ZPR, going beyond the Allen-Heine-Cardona
theory. Our results indicate that high-order electron-phonon coupling terms
contribute significantly to the zero-point renormalization for certain
materials
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