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Samuel Poncé

Publications and source records attributed to Samuel Poncé.

At least 19 recordsLinked to original sources
arXiv2026

Importance of nonlinear long-range electron-phonon interaction for the carrier mobility of anharmonic halide perovskites

The interaction between the electrons and the lattice vibrations in a solid is responsible for various important effects, such as formation of polarons, temperature dependent bandgaps, phonon-limited carrier transport, and conventional superconductivity. Most works assume a linear electron-phonon interaction, where the electron only interacts with one phonon at a time. However, the validity of this assumption has not been verified in polar anharmonic materials, where large ionic displacements may invalidate the assumption of linear interaction. Here, we show that nonlinear electron-phonon interactions contribute significantly to the finite-temperature electron mobility of the inorganic lead halide perovskite CsPbI$_3$. The effect of nonlinear interaction is taken into account using the recently derived expression for the long-range part of the one-electron-two-phonon matrix element. We calculate the electron mobility from first principles within the self-energy relaxation-time approximation, treating the electron-phonon coupling in the long-range approximation. Despite these approximations, the calculated mobilities are in good agreement with the available experimental data, while enabling us to isolate and quantify the contribution of nonlinear electron-phonon interactions relative to the conventional linear coupling. We find that the one-electron-two-phonon interaction modifies the temperature dependence of the mobility in CsPbI$_3$ and reduces its room-temperature value by about 10\%. This sizeable contribution results from the combined effects of strong lattice anharmonicity and large thermal phonon populations, the latter being enhanced by the low phonon frequencies associated with the heavy constituent atoms. These results show when nonlinear electron-phonon interactions become relevant, and indicate they should be considered for finite-temperature properties of halide perovskites.

cond-mat.mtrl-sci↗
arXiv2026

Higher-order nonadiabaticity governs the temperature dependence of the phonon spectrum

Nonadiabatic effects determine the frequency and linewidth of coupled phonon modes, shape of Kohn anomalies and have important implications on many material properties. State-of-the-art ab-initio nonadiabatic phonon self-energy relies on an infinite electron lifetime approximation, which cannot capture the temperature dependence of the phonon spectrum, neglects long-wavelength intraband phonon decay, and exhibits exaggerated phonon splitting. In MgB$_2$, we show how the higher-order nonadiabatic phonon corrections mitigate these deficiencies, yielding linewidths in a closer experimental agreement and a dome-like coupling strength temperature dependence.

cond-mat.mtrl-sci↗
arXiv2026

First-Principles Spin-Lattice Coupling from Downfolded Electron-Phonon Interaction

We present a method to calculate spin-phonon coupling parameters from first-principles perturbation theory by downfolding the electron-phonon coupling (EPC). We exploit the localized nature of magnetic moments and atomic displacements by working in the Wannier representation of the electronic Hamiltonian and the EPC matrix. The spin system is mapped to a classical Heisenberg Hamiltonian, whose parameters are obtained by treating local spin rotations as a perturbation within a Green's-function formalism. The spin and phonon perturbations are connected through the EPC parameters, which enter as lattice-induced perturbations to the tight-binding Hamiltonian. By combining these lattice perturbations with local spin rotations, we obtain real-space derivatives of magnetic exchange parameters without performing displaced magnetic supercell calculations. We illustrate the method on SrMnO$_3$ and show that it can be integrated directly into standard workflows.

cond-mat.mtrl-sci↗
arXiv2026

In search of novel ductile superconductors

We performed a first-principles high-throughput screening of the mechanical properties of phonon-mediated superconductors selected from the recent experimentally synthesized superconducting materials database PRX Energy 4, 033012 (2025). We developed the workflows that combine first-principles calculations of elastic constants and generalized stacking fault energies to assess the ductility of superconducting candidates. Starting from the 250 materials identified with promising superconducting critical temperatures, we computed their elastic tensors to evaluate bulk and shear moduli, Pugh's ratio, and Pettifor's ratio from first principles. To further characterize their plastic deformation behavior, we calculated the stacking fault energy and surface energy for selected materials and slip directions, allowing the estimation of Rice's ratio and ductility indicators. We found that several new materials simultaneously exhibit high-T$_c$ and ductility including HfPd$_2$Al, TiRuSb, and ZrNi$_2$Ga with predicted isotropic T$_c$= 6.80K, 12.88K, and 8.23K, respectively. This work offers a quantitative mapping of mechanical performance across a wide range of superconductors and provides a reference to identify new mechanically promising superconductors.

cond-mat.supr-con↗
arXiv2026

Optimal transition states for polaron hopping transport without supercells

Polaron formation localizes charge carriers and drives a crossover from band-like to hopping transport in materials. Hopping dynamics can be obtained from DFT supercell calculations of transition states, but these suffer from polaron self-interaction, spurious electrostatics, and poor scaling with polaron size. We introduce a supercell-free framework for ab initio polaron hopping transport based on the ab initio polaron equations formalism, its variational formulation, and the string method. The approach optimizes transition states between self-trapped polaron states directly in reciprocal space and provides the polaron configurations along the path, enabling evaluation of adiabatic hopping rates and mobilities. We apply the method to LiF and rutile TiO$_2$, revealing multi-step and anisotropic hopping mechanisms. In rutile TiO$_2$, the computed electron-polaron mobility agrees with experiment, whereas band-like Boltzmann transport substantially overestimates the mobility. Our results establish a scalable route to first-principles polaron-hopping dynamics in materials in which charge motion is governed by self-trapping.

cond-mat.mtrl-sci↗
arXiv2026

From Symmetry to Stability: Structural and Electronic Transformation in Cs$_2$KInI$_6$

Cs$_2$KInI$_6$ is a promising lead-free halide double perovskite with a calculated direct band gap of 1.94 eV, ideal for solar cell applications. Our first-principles calculations reveal that its cubic phase (Fm$\bar{3}$m) is dynamically unstable. Using an accelerated machine learning approach, we identify 42 dynamically stable structures and further validate these findings using first-principles calculations on 11 of these. The most stable phase has Cmc$2_1$ symmetry with 20 atoms/unit cell. It lies 13 meV/atom above the convex hull but lacks octahedral cation coordination. The most stable perovskite-like structure has P$\bar{3}$ symmetry with 10 atoms/unit cell and low octahedral connectivity. Structure-property trade-offs are highlighted, with calculated distortions generally widening the band gap, shifting it from direct to indirect, and flattening the band edges. This work showcases the synergy of genetic algorithms, machine-learned potentials, and first-principles validation for discovering stable, complex materials.

cond-mat.mtrl-sci↗
arXiv2026

Micro-environment of the Eu interstitial in $β$-SiAlON:Eu$^{2+}$ green phosphor

The precise atomic-scale structure around Eu$^{2+}$ activators in the $β$-Si$_{6-z}$Al$_z$O$_z$N$_{8-z}$:Eu$^{2+}$ commercial green phosphor remains elusive. We use the first-principles $Δ$SCF excited-state method, embedding of the interatomic force constants for supercells up to 3501 atoms, and Huang-Rhys theory to clarify this issue. Monte Carlo exploration is used to identify representative low-energy structural models spanning different levels of Al/O concentration $z$. For the lowest-energy structure at low $z$, our computed photoluminescence spectrum reproduces the experimental vibronic peaks at 6~K with excellent agreement in peak positions and intensities, validating the Eu-N$_9$ coordination model with Al, O, and Eu confined to the same crystallographic plane. Analysis of the low-energy structures reveals that the electron-phonon coupling is weak ($S \approx 2.15$) with a robust characteristic phonon signature across different Al/O arrangements, explaining the surprising persistence of resolved phonon replicas with increasing $z$. We explain the experimentally observed red-shift of emission with increasing $z$ through systematic trends in zero-phonon line energies, modest increases in Huang-Rhys factors, and larger configurational diversity at higher compositions.

cond-mat.mtrl-sci↗
arXiv2026

Extraction of the self energy and Eliashberg function from angle resolved photoemission spectroscopy using the xARPES code

Angle-resolved photoemission spectroscopy is a powerful experimental technique for studying anisotropic many-body interactions through the electron spectral function. Existing attempts to decompose the spectral function into non-interacting dispersions and electron-phonon, electron-electron, and electron-impurity self-energies rely on linearization of the bands and manual assignment of self-energy magnitudes. Here, we show how self-energies can be extracted consistently for curved dispersions. We extend the maximum-entropy method to Eliashberg-function extraction with Bayesian inference, optimizing the parameters describing the dispersions and the magnitudes of electron-electron and electron-impurity interactions. We compare these novel methodologies with state-of-the-art approaches on model data, then demonstrate their applicability with two high-quality experimental data sets. With the first set, we identify the phonon modes of a two-dimensional electron liquid on TiO$_2$-terminated SrTiO$_3$. With the second set, we obtain unprecedented agreement between two Eliashberg functions of Li-doped graphene extracted from separate dispersions. We release these functionalities in the novel Python code xARPES.

cond-mat.mtrl-sci↗
arXiv2026

Elastic Constants and Bending Rigidities from Long-Wavelength Perturbation Expansions

Mechanical and elastic properties of materials are among the most fundamental quantities for many engineering and industrial applications. Here, we present a formulation that is efficient and accurate for calculating the elastic and bending rigidity tensors of crystalline solids, leveraging interatomic force constants and long-wavelength perturbation theory. Crucially, in the long-wavelength limit, lattice vibrations induce macroscopic electric fields which further couple with the propagation of elastic waves, and a separate treatment on the long-range electrostatic interactions is thereby required to obtain elastic properties under the appropriate electrical boundary conditions. A cluster expansion of the charge density response and dielectric screening function in the long-wavelength limit has been developed to efficiently extract multipole and dielectric tensors of arbitrarily high order. We implement the proposed method in a first-principles framework and perform extensive validations on silicon, NaCl, GaAs and rhombohedral BaTiO$_3$ as well as monolayer graphene, hexagonal BN, MoS$_2$ and InSe, obtaining good to excellent agreement with other theoretical approaches and experimental measurements. Notably, we establish that multipolar interactions up to at least octupoles are necessary to obtain the accurate short-circuit elastic tensor of bulk materials, while higher orders beyond octupole interactions are required to converge the bending rigidity tensor of 2D crystals. The present approach greatly simplifies the calculations of bending rigidities and will enable the automated characterization of the mechanical properties of novel functional materials.

cond-mat.mtrl-sci↗
arXiv2026

Beyond-quasiparticle transport with vertex correction: self-consistent ladder formalism for electron-phonon interactions

We present a self-consistent many-body framework for computing phonon-limited electronic transport from first principles, incorporating both beyond-quasiparticle effects and vertex corrections. Using the recently developed first-principles scGD0 method, we calculate spectral functions with nonperturbative effects such as broadening, satellites, and energy-dependent renormalization. We show that the scGD0 spectral functions outperform one-shot G0D0 and cumulant approximations in model Hamiltonians and real materials, eliminating unphysical spectral kinks and correctly predicting the phonon emission continuum. Building on this, we introduce the self-consistent ladder formalism for transport, which captures vertex corrections due to electron-phonon interactions. This approach unifies and improves upon the two state-of-the-art approaches for first-principles phonon-limited transport: the bubble approximation and the Boltzmann transport equation. Moreover, as a charge-conserving approximation, it enables consistent calculations of the optical conductivity and dielectric function. We validate the developed method against numerically exact results for model Hamiltonians in the dilute polaronic limit and apply it to real materials. Our results show quantitative agreement with the experimental dc conductivities in intrinsic semiconductors Si and ZnO and the SrVO3 metal, as well as excellent agreement with the experimental THz optical and dielectric properties of Si and ZnO. This work unifies first-principles and many-body approaches for studying transport, opening new directions for applying many-body theory to materials with strong electron-phonon interactions.

cond-mat.mtrl-sci↗
arXiv2026

In search of the electron-phonon contribution to total energy

The total energy is a fundamental characteristic of solids, molecules, and nanostructures. In most first-principles calculations of the total energy, the nuclear kinetic operator is decoupled from the many-body electronic Hamiltonian and nuclear potential, and the dynamics of the nuclei is reintroduced afterward. This two-step procedure introduced by Born and Oppenheimer (BO) is approximate. Energies beyond the electronic and vibrational (or phononic) main contributions might be relevant when small energy differences are important, such as when predicting stable polymorphs or describing magnetic energy landscape. We clarify the different flavors of BO decoupling and give an exact formulation for the total energy in the basis of BO electronic wavefunctions. Then, we list contributions, beyond the main ones, that appear in a perturbative expansion in powers of $M_0^{-1/4}$, where $M_0$ is a typical nuclear mass, up to sixth order. Some of these might be grouped and denoted the electron-phonon contribution to total energy, $E^{\textrm{elph}}$, that first appears at fourth order. The electronic inertial mass contributes at sixth order. We clarify that the sum of the Allen-Heine-Cardona zero-point renormalization of eigenvalues over occupied states is not the electron-phonon contribution to the total energy but a part of the phononic contribution. The computation of the lowest-order $E^{\textrm{elph}}$ is implemented and shown to be small but non-negligible (3.8~meV per atom) in the case of diamond and its hexagonal polymorph. We also estimate the electronic inertial mass contribution and the quasi-harmonic one. We confirm the size consistency of all computed terms.

cond-mat.mtrl-sci↗
arXiv2026

Opposite impact of thermal expansion and phonon anharmonicity on the phonon-limited resistivity of elemental metals from first principles

Understanding electrical resistivity in metals remains a central challenge in quantifying charge transport at finite temperature. Current first-principles calculations based on the Boltzmann transport equation often match experiments, yet they almost always neglect the effect of thermal expansion and phonon anharmonicity. We show that both effects exert an opposite impact on electron-phonon coupling and on electrical resistivity. Thermal expansion enhances the coupling and leads to overestimation of resistivity, whereas anharmonic effects reduce it. By explicitly incorporating both effects, we establish a more complete description of resistivity in elemental metals, demonstrated here for Pb, Nb, and Al.

cond-mat.mtrl-sci↗
arXiv2025

Impact of electronic correlations on the superconductivity of high-pressure CeH$_9$

Rare-earth superhydrides have attracted considerable attention because of their high critical superconducting temperature under extreme pressures. They are known to have localized valence electrons, implying strong electronic correlations. However, such many-body effects are rarely included in first-principles studies of rare-earth superhydrides because of the complexity of their high-pressure phases. In this work, we use a combined density functional theory and dynamical mean-field theory approach to study both electrons and phonons in the prototypical rare-earth superhydride CeH$_9$, shedding light on the impact of electronic correlations on its critical temperature for phonon-mediated superconductivity. Our findings indicate that electronic correlations result in a larger electronic density at the Fermi level, a bigger superconducting gap, and softer vibrational modes associated with hydrogen atoms. Together, the inclusion of these correlation signatures increases the Migdal-Eliashberg superconducting critical temperature from 47 K to 96 K, close to the measured 95 K. Our results reconcile experimental observations and theoretical predictions for CeH$_9$ and herald a path towards the quantitative modeling of phonon-mediated superconductivity for interacting electron systems.

cond-mat.supr-con↗
arXiv2025

Carrier mobilities and electron-phonon interactions beyond DFT

Electron-phonon coupling is a key interaction that governs diverse physical processes such as carrier transport, superconductivity, and optical absorption. Calculating such interactions from first-principles with methods beyond density-functional theory remains a challenge. We introduce here a finite-difference framework for computing electron-phonon couplings for any electronic structure method that provides eigenvalues and eigenvectors, and showcase applications for hybrid and Koopmans functionals, and $GW$ many-body perturbation theory. Our approach introduces a novel projectability scheme based on eigenvalue differences and bypasses many of the limitations of the direct finite difference methods. It also leverages symmetries to reduce the number of independent atomic displacements, thereby keeping computational costs manageable. This approach enables seamless integration with established first-principles codes for generating displaced supercells, performing Wannier interpolations, and evaluating transport properties. Applications to silicon and gallium arsenide show that advanced electronic-structure functionals predict different electron-phonon couplings and modify band curvatures, resulting in much more accurate estimates of intrinsic carrier drift mobilities and effective masses. In general, our method provides a robust and accessible framework for exploring electron-phonon interactions in complex materials with state-of-the-art electronic structure methods.

cond-mat.mtrl-sci↗
arXiv2025

Charting the landscape of Bardeen-Cooper-Schrieffer superconductors in experimentally known compounds

We perform a high-throughput computational search for novel phonon-mediated superconductors, starting from the Materials Cloud 3-dimensional structure database of experimentally known inorganic stoichiometric compounds. We first compute the Allen-Dynes critical temperature (T$_c$) for 4533 non-magnetic metals using a direct and progressively finer sampling of the electron-phonon couplings. For the candidates with the largest T$_c$, we use automated Wannierizations and electron-phonon interpolations to obtain a high-quality dataset for the most promising 250 dynamically stable structures, for which we calculate spectral functions, superconducting bandgaps, and isotropic Migdal-Eliashberg critical temperatures. For 140 of these, we also provide anisotropic Migdal-Eliashberg superconducting gaps and critical temperatures. The approach is remarkably successful in finding known superconductors, and we find 24 unknown ones with a predicted anisotropic T$_{\rm c}$ above 10~K. Among them, we identify a possible double gap superconductor (p-doped BaB$_2$), a non-magnetic half-Heusler ZrRuSb, and the perovskite TaRu$_3$C, all exhibiting significant T$_{\rm c}$. Finally, we introduce a sensitivity analysis to estimate the robustness of the predictions.

cond-mat.supr-con↗
arXiv2025

Variational first-principles approach to self-trapped polarons

The behavior of charge carriers in polar materials is governed by electron-phonon interactions, which affect their mobilities via phonon scattering and may localize carriers into self-induced deformation fields, forming self-trapped polarons. We present a first-principles study of self-trapped polaron formation in paradigmatic polar semiconductors and insulators using the variational polaron equations framework and self-consistent gradient optimization. Our method incorporates long-range corrections to the electron-phonon interaction, essential for finite-size systems. We demonstrate how the variational approach enables the identification of multiple polaronic states and supports the analysis of polarons with arbitrarily large spatial extent via energy filtering. The potential energy surfaces of the resulting polarons exhibit multiple local minima, reflecting distinct, symmetry-broken polaronic configurations in systems with degenerate band edges. Our findings align with previous theoretical studies and establish a robust foundation for future ab initio studies of polarons, especially those employing variational methods.

cond-mat.mtrl-sci↗
arXiv2025

Importance of Non-Adiabatic Effects on Kohn Anomalies in 1D metals

Kohn anomalies are kinks or dips in phonon dispersions which are pronounced in low-dimensional materials. We investigate the effects of non-adiabatic phonon self-energy on Kohn anomalies in one-dimensional metals by developing a model that analyzes how the adiabatic phonon frequency, electron effective mass, and electron-phonon coupling strength influence phonon mode renormalization. We introduce an electron-phonon coupling strength threshold for low-temperature system instability, providing experimentalists with a tool to predict them. Finally, we validate the predictions of our model against first-principles calculations on a 4 Å-diameter carbon nanotube.

cond-mat.mtrl-sci↗
arXiv2025

Impact of anharmonicity on the carrier mobility of the Pb-free CsSnBr$_3$ perovskite

Charge carrier mobilities are critical parameters in halide perovskite solar cells, governing their average carrier velocity under an applied electric field and overall efficiency. Recent advances in first-principles calculations of electron-phonon interactions and carrier mobilities have enabled predictive computations for perovskite solar cells. However, the flexible octahedral frameworks and cationic displacements in these materials challenge the harmonic approximation, leading to significant difficulties in accurately calculating transport properties. To address these issues, we combine temperature-dependent effective potentials with the ab initio Boltzmann transport equations to compute carrier mobilities in a representative lead-free perovskite, CsSnBr$_3$. At room temperature, the electron/hole Hall mobilities in CsSnBr$_3$ are 106/256 cm$^2$/Vs when neglecting anharmonic effects and 59/145 cm$^2$/Vs when included. This overestimation of the harmonic approximation arises from the neglect of scattering coming from soft modes. We provide a workflow for performing first-principles carrier mobility calculations in anharmonic systems, advancing the predictive modeling of perovskite solar cells.

cond-mat.mtrl-sci↗