arXiv subjects

Samuel Poncé

Publications and source records attributed to Samuel Poncé.

At least 55 records · Page 3Linked to original sources

arXiv2023

Electron-phonon physics from first principles using the EPW code

EPW is an open-source software for $\textit{ab initio}$ calculations of electron-phonon interactions and related materials properties. The code combines density functional perturbation theory and maximally-localized Wannier functions to efficiently compute electron-phonon coupling matrix elements on ultra-fine Brillouin zone grids. This data is employed for predictive calculations of temperature-dependent properties and phonon-assisted quantum processes in bulk solids and low-dimensional materials. Here, we report on significant new developments in the code that occurred during the period 2016-2022, namely: a transport module for the calculation of charge carrier mobility and conductivity under electric and magnetic fields within the $\textit{ab initio}$ Boltzmann transport equation; a superconductivity module for the calculation of critical temperature and gap structure in phonon-mediated superconductors within the $\textit{ab initio}$ anisotropic multi-band Eliashberg theory; an optics module for calculations of phonon-assisted indirect transitions; a module for the calculation of small and large polarons without supercells using the $\textit{ab initio}$ polaron equations; and a module for calculating electron-phonon couplings, band structure renormalization, and temperature-dependent optical spectra using the special displacement method. For each capability, we outline the methodology and implementation, and provide example calculations. We describe recent code refactoring to prepare EPW for exascale architectures, we discuss efficient parallelization strategies, and report on extreme parallel scaling tests.

cond-mat.mtrl-sci↗

arXiv2022

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.

cond-mat.mes-hall↗

arXiv2021

Theory and computation of Hall scattering factor in graphene

The Hall scattering factor, $r$, is a key quantity for establishing carrier concentration and drift mobility from Hall measurements; in experiments it is usually assumed to be 1. In this paper we use a combination of analytical and \textit{ab initio} modelling to determine $r$ in graphene. While at high carrier densities $r \approx 1$ in a wide temperature range, at low doping the temperature dependence of $r$ is very strong with values as high as 4 below 300~K. These high values are due to the linear bands around the Dirac cone and the carrier scattering rates due to acoustic phonons. At higher temperatures $r$ can instead become as low as $0.5$ due to the contribution of both holes and electrons and the role of optical phonons. Finally, we provide a simple analytical model to compute accurately $r$ in graphene in a wide range of temperatures and carrier densities.

cond-mat.mes-hall↗

arXiv2021

Design rule for the emission linewidth of Eu$^{2+}$-activated phosphors

We study from first principles the emission linewidth of Eu$^{2+}$-doped LED phosphors. Based on the one-dimensional configuration coordinate model, an analysis of first principles data obtained for fifteen compounds show that, at working temperature, the linewidth of Eu$^{2+}$ emission band in solids is negligibly affected by quantum effects, and can be extracted from the Franck-Condon energy shifts. For a fixed Stokes shift, the difference of Franck-Condon energy shifts in the excited and ground states is the key factor for the FWHM determination. Narrow emission Eu$^{2+}$-doped LED phosphors are expected for the case with large positive value of such difference.

cond-mat.mtrl-sci↗

arXiv2021

Limits to Electrical Mobility in Lead-Halide Perovskite Semiconductors

Semiconducting polycrystalline thin films are cheap to produce and can be deposited on flexible substrates, yet high-performance electronic devices usually utilize single-crystal semiconductors, owing to their superior electrical mobilities and longer diffusion lengths. Here we show that the electrical performance of polycrystalline films of metal-halide perovskites (MHPs) approaches that of single crystals at room temperature. Combining temperature-dependent terahertz conductivity measurements and ab initio calculations we uncover a complete picture of the origins of charge scattering in single crystals and polycrystalline films of CH$_3$NH$_3$PbI$_3$. We show that Fröhlich scattering of charge carriers with multiple phonon modes is the dominant mechanism limiting mobility, with grain-boundary scattering further reducing mobility in polycrystalline films. We reconcile the large discrepancy in charge diffusion lengths between single crystals and films by considering photon reabsorption. Thus, polycrystalline films of MHPs offer great promise for devices beyond solar cells, including transistors and modulators.

physics.app-ph↗

arXiv2021

Superconducting properties of MoTe$_2$ from the $ab~initio$ anisotropic Migdal-Eliashberg theory

Molybdenum ditelluride (MoTe$_2$) is attracting considerable interest since it is the archetypal type-II Weyl semimetal and a candidate for topological superconductivity. We investigate the superconducting phase diagram of two MoTe$_2$ polymorphs using the $ab~initio$ anisotropic Migdal-Eliashberg theory, and we show that the superconducting dome originates from the synergistic contribution of the density of states at the Fermi level and the transverse acoustic Te modes in the 1T$^\prime$ phase. We find that the electron and hole pockets carry trivial $s$-wave order parameters of slightly different magnitude, reminiscent of a two-gap structure as suggested by recent experiments. We suggest that a possible route for enhancing the superconducting critical temperature, and realizing $s_{+-}$ pairing, in the T$_d$ phase is to exploit its non-trivial band topology via electron doping.

cond-mat.supr-con↗

arXiv2021

Importance of long-range channel Sr displacements for the narrow emission in Sr[Li$_2$Al$_2$O$_2$N$_2$]:Eu$^{2+}$ phosphor

The recently discovered Sr[Li$_2$Al$_2$O$_2$N$_2$]:Eu$^{2+}$ red phosphor, candidate for the next generation of eco-efficient white light-emitting diodes, exhibits excellent emission spectral position and exceptionally small linewidth. It belongs to the UCr$_4$C$_4$-structure family of phosphors, containing many potential candidates commercial phosphors, whose small linewidth, tentatively ascribed to the high-symmetry cuboid environment of the doping site, has drawn the attention of researchers in the last five years. We use density functional theory, $Δ$SCF method and configuration coordinate models (CCM) to provide a complete characterization of this material. Using a multi-dimensional CCM, an accurate description of the coupling of the vibronic structure with the electronic 5d$\rightarrow$4f transition is obtained, including the partial Huang-Rhys factors and frequency of the dominant modes. We show that, in addition to the first-coordination shell cuboid deformation mode, low-frequency phonon modes involving chains of strontium atoms along the tetragonal axis shape the emission linewidth in Sr[Li$_2$Al$_2$O$_2$N$_2$]:Eu$^{2+}$. This finding sheds new light on the emission properties of UCr$_4$C$_4$-structure phosphors, possessing similar Ca/Sr/Ba channel. Our approach provides a robust theoretical framework to systematically study the emission spectra of such Eu-doped phosphors, and predict candidates with expected similar or even sharper linewidth.

cond-mat.mtrl-sci↗

arXiv2021

Common workflows for computing material properties using different quantum engines

The prediction of material properties through electronic-structure simulations based on density-functional theory has become routinely common, thanks, in part, to the steady increase in the number and robustness of available simulation packages. This plurality of codes and methods aiming to solve similar problems is both a boon and a burden. While providing great opportunities for cross-verification, these packages adopt different methods, algorithms, and paradigms, making it challenging to choose, master, and efficiently use any one for a given task. Leveraging recent advances in managing reproducible scientific workflows, we demonstrate how developing common interfaces for workflows that automatically compute material properties can tackle the challenge mentioned above, greatly simplifying interoperability and cross-verification. We introduce design rules for reproducible and reusable code-agnostic workflow interfaces to compute well-defined material properties, which we implement for eleven different quantum engines and use to compute three different material properties. Each implementation encodes carefully selected simulation parameters and workflow logic, making the implementer's expertise of the quantum engine directly available to non-experts. Full provenance and reproducibility of the workflows is guaranteed through the use of the AiiDA infrastructure. All workflows are made available as open-source and come pre-installed with the Quantum Mobile virtual machine, making their use straightforward.

cond-mat.mtrl-sci↗

arXiv2020

Structural, electronic, elastic, power and transport properties of $β$-Ga$_2$O$_3$ from first-principles

We investigate the structural, electronic, vibrational, power and transport properties of the $β$ allotrope of Ga$_2$O$_3$ 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$^2$/Vs and 1.2 cm$^2$/Vs, respectively as well as 6300 cm$^2$/Vs and 13 cm$^2$/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$_u$ symmetry are responsible for the electron mobility of $β$- Ga$_2$O$_3$ 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.

cond-mat.mtrl-sci↗

arXiv2019

Beyond the one-dimensional configuration coordinate model of photoluminescence

The one-dimensional configuration coordinate model (1D-CCM) is widely used for the analysis of photoluminescence in molecules and doped solids, and relies on a linear combination of the equilibrium nuclear configurations of ground and excited states. It delivers an estimation of the energy barrier at which ground and excited state curves cross, semi-classically linked to non-radiative transition rate and thermal quenching. To assess its predictive power for the latter properties, we propose a new \textit{optimized configuration path (OCP) method in which} the ground-state and excited-state forces are mixed instead of their configurations. We also define another one-parameter model thanks a double energy parabola hypothesis (DEPH). We compare the OCP method and the DEPH reference with the 1D-CCM for three paradigmatic 4f-5d phosphors Y$_3$Al$_5$O$_{12}$:Ce, Lu$_2$SiO$_5$:Ce, and YAlO$_3$:Ce. We find that the OCP and DEPH methods yield similar results with geometries that have significantly lower ground-state energies than the 1D-CCM for the same 4f-5d energy difference. However, the OCP method suffers from the appearance of multiple local minima, rendering the clear determination of the optimal geometry very difficult in practice. Still the OCP method allows one to quantify the deviations from the 1D-CCM, therefore increasing confidence in the lower bound obtained from the DEPH for the 4f-5d crossing barrier, and its comparison with the energy of the auto-ionization thermal quenching mechanism. We expect the OCP approach to be applicable to other luminescent materials or molecules.

cond-mat.mtrl-sci↗

arXiv2019

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$^2$/Vs at room temperature and 620 cm$^2$/Vs at 100 K.

cond-mat.mtrl-sci↗

arXiv2019

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$_1$ optical phonon through ultrafast infrared optical pulses.

cond-mat.mtrl-sci↗

arXiv2019

First-principles calculations of charge carrier mobility and conductivity in bulk semiconductors and two-dimensional materials

One of the fundamental properties of semiconductors is their ability to support highly tunable electric currents in the presence of electric fields or carrier concentration gradients. These properties are described by transport coefficients such as electron and hole mobilities. Recently, advances in electronic structure methods for real materials have made it possible to study these properties with predictive accuracy and without resorting to empirical parameters. Here, we review the most recent developments in the area of ab initio calculations of carrier mobilities of semiconductors. In the first part, we offer a brief historical overview of approaches to the calculation of carrier mobilities, and we establish the conceptual framework underlying modern ab initio approaches. We summarize the Boltzmann theory of carrier transport and we discuss its scope of applicability, merits, and limitations in the broader context of many-body Green's function approaches. We discuss recent implementations of the Boltzmann formalism within the context of density functional theory and many-body perturbation theory calculations, placing an emphasis on the key computational challenges and suggested solutions. In the second part, we discuss recent investigations of classic materials such as silicon, diamond, GaAs, GaN, Ga2O3, and lead halide perovskites as well as low-dimensional semiconductors such as graphene, silicene, phosphorene, MoS2, and InSe. We also review recent efforts toward high-throughput calculations of carrier transport. In the last part, we discuss the extension of the methodology to study spintronics and topological materials and we comment on the possibility of incorporating Berry-phase effects and many-body correlations beyond the standard Boltzmann formalism.

cond-mat.mtrl-sci↗

arXiv2019

Wannier90 as a community code: new features and applications

Wannier90 is an open-source computer program for calculating maximally-localised Wannier functions (MLWFs) from a set of Bloch states. It is interfaced to many widely used electronic-structure codes thanks to its independence from the basis sets representing these Bloch states. In the past few years the development of Wannier90 has transitioned to a community-driven model; this has resulted in a number of new developments that have been recently released in Wannier90 v3.0. In this article we describe these new functionalities, that include the implementation of new features for wannierisation and disentanglement (symmetry-adapted Wannier functions, selectively-localised Wannier functions, selected columns of the density matrix) and the ability to calculate new properties (shift currents and Berry-curvature dipole, and a new interface to many-body perturbation theory); performance improvements, including parallelisation of the core code; enhancements in functionality (support for spinor-valued Wannier functions, more accurate methods to interpolate quantities in the Brillouin zone); improved usability (improved plotting routines, integration with high-throughput automation frameworks), as well as the implementation of modern software engineering practices (unit testing, continuous integration, and automatic source-code documentation). These new features, capabilities, and code development model aim to further sustain and expand the community uptake and range of applicability, that nowadays spans complex and accurate dielectric, electronic, magnetic, optical, topological and transport properties of materials.

cond-mat.mtrl-sci↗

arXiv2018

Carrier Lifetimes and Polaronic Mass Enhancement in the Hybrid Halide Perovskite CH$_3$NH$_3$PbI$_3$ from Multiphonon Fröhlich Coupling

We elucidate the nature of the electron-phonon interaction in the archetypal hybrid perovskite CH$_3$NH$_3$PbI$_3$ 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.

cond-mat.mtrl-sci↗

arXiv2018

Electron-plasmon and electron-phonon satellites in the angle-resolved photoelectron spectra of n-doped anatase TiO2

We develop a first-principles approach based on many-body perturbation theory to investigate the effects of the interaction between electrons and carrier plasmons on the electronic properties of highly-doped semiconductors and oxides. Through the evaluation of the electron self-energy, we account simultaneously for electron-plasmon and electron-phonon coupling in theoretical calculations of angle-resolved photoemission spectra, electron linewidths, and relaxation times. We apply this methodology to electron-doped anatase TiO2 as an illustrative example. The simulated spectra indicate that electron-plasmon coupling in TiO2 underpins the formation of satellites at energies comparable to those of polaronic spectral features. At variance with phonons, however, the energy of plasmons and their spectral fingerprints depends strongly on the carrier concentration, revealing a complex interplay between plasmon and phonon satellites. The electron-plasmon interaction accounts for approximately 40% of the total electron-boson interaction strength and it is key to improve the agreement with measured quasiparticle spectra.

cond-mat.mtrl-sci↗

arXiv2017

First-principles Study of the Luminescence of Eu2+-doped Phosphors

The luminescence of fifteen representative Eu$^{2+}$-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 $4f-5d$ 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 $4f-5d$ crossover cannot be the dominant mechanism for the thermal quenching behavior of Eu$^{2+}$-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$^{3+}$-doped hosts is adapted to Eu$^{2+}$-doped hosts and gives the absorption and emission energies within 0.9 eV of experiment, underperforming compared to the first-principles calculation.

cond-mat.mtrl-sci↗

arXiv2017

Assessment of First-Principles and Semiempirical Methodologies for Absorption and Emission Energies of Ce$^{3+}$-Doped Luminescent Materials

In search of a reliable methodology for the prediction of light absorption and emission of Ce$^{3+}$-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 $Δ$SCF, the atomic positions are obtained for both ground and excited states of the Ce$^{3+}$ 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.

cond-mat.mtrl-sci↗