arXiv subjects

Elisa Molinari

Publications and source records attributed to Elisa Molinari.

At least 19 recordsLinked to original sources

arXiv2026

Tracking the Lithiation State of Li$_x$Si from Machine-Learned XPS Binding Energies

X-ray Photoelectron Spectroscopy (XPS) is a powerful technique to probe chemical states and interfacial processes in battery materials, but a quantitative interpretation is often hindered by the complex, heterogeneous microstructures that form during operation and dominate electrochemical cycling. Silicon based anodes represent a paradigmatic example in Li batteries, as (de)lithiation proceeds through the formation of strongly disordered Li$_x$Si phases and crystal-amorphous transformations that are hard to characterize. Here, we introduce a computational framework that combines machine-learning (ML) prediction of core-level binding energies to large-scale atomistic simulations -- Grand Canonical Monte Carlo (GCMC) complemented with molecular dynamics (MD), driven by a ML potential -- for a systematic sampling of lithiation states and local atomic environments. This approach yields stoichiometry maps that match the characteristic experimental trends observed in operando and ex situ XPS measurements, including the distinctive Si $2p$ spectroscopic signatures associated with the crystal-to-amorphous disordering driving early delithiation.

cond-mat.mtrl-sci↗

arXiv2025

Stabilization of sliding ferroelectricity through exciton condensation

Sliding ferroelectricity is a phenomenon that arises from the insurgence of spontaneous electronic polarization perpendicular to the layers of two-dimensional (2D) systems upon the relative sliding of the atomic layer constituents. Because of the weak van der Waals (vdW) interactions between layers, sliding and the associated symmetry breaking can occur at low energy cost in materials such as transition-metal dichalcogenides. Here we discuss theoretically the origin and quantitative understanding of the phenomenon by focusing on a prototype structure, the WTe2 bilayer, where sliding ferroelectricity was first experimentally observed. We show that excitonic effects induce relevant energy band renormalizations in the ground state, and exciton condensation contributes significantly to stabilizing ferroelectricity upon sliding, beyond previous predictions that disregard electron-hole interaction effects. Enhanced excitonic effects in 2D and vdW sliding are general phenomena that point to sliding ferroelectricity as relevant for a broad class of important materials, where the intrinsic electric dipole can couple with other quantum phenomena and, in turn, an external electric field can control the quantum phases through ferroelectricity in unexplored ways.

cond-mat.mtrl-sci↗

arXiv2025

Making atomistic materials calculations accessible with the AiiDAlab Quantum ESPRESSO app

Despite the wide availability of density functional theory (DFT) codes, their adoption by the broader materials science community remains limited due to challenges such as software installation, input preparation, high-performance computing setup, and output analysis. To overcome these barriers, we introduce the Quantum ESPRESSO app, an intuitive, web-based platform built on AiiDAlab that integrates user-friendly graphical interfaces with automated DFT workflows. The app employs a modular Input-Process-Output model and a plugin-based architecture, providing predefined computational protocols, automated error handling, and interactive results visualization. We demonstrate the app's capabilities through plugins for electronic band structures, projected density of states, phonon, infrared/Raman, X-ray and muon spectroscopies, Hubbard parameters (DFT+$U$+$V$), Wannier functions, and post-processing tools. By extending the FAIR principles to simulations, workflows, and analyses, the app enhances the accessibility and reproducibility of advanced DFT calculations and provides a general template to interface with other first-principles calculation codes.

cond-mat.mtrl-sci↗

arXiv2025

Binding and spontaneous condensation of excitons in narrow-gap carbon nanotubes

Ultraclean, undoped carbon nanotubes are observed to be always insulating, even when the gap predicted by band theory is zero: the residual band gap is then thought to have a many-body origin. Here we theoretically show that the correlated insulator is excitonic in $all stable$ narrow-gap tubes irrespective of their size, thus extending our previous claim, limited to gapless (armchair) tubes [D.~Varsano, S.~Sorella, D.~Sangalli, M.~Barborini, S.~Corni, E.~Molinari, M.~Rontani, Nature Communications $\mathbf{8}$, 1461 (2017)]. We derive the scaling law of the exciton binding energy with the tube radius and chirality, and compute self-consistently the fundamental transport gap of the excitonic insulator, by enhancing the two-band model with an accurate treatment of screening validated from first principles. Our findings point to the broader connection between the exciton length scale, dictated by structure, and the stability of the excitonic phase.

cond-mat.mes-hall↗

arXiv2024

Understanding the irreversible lithium loss in silicon anodes using multi-edge X-ray scattering analysis

During the first charge-discharge cycle, silicon-based batteries show an important capacity loss because of the formation of the solid electrolyte interphase (SEI) and morphological changes due to expansion-contraction sequence upon alloying. To understand this first-cycle irreversibility, quantitative methods are needed to characterize the chemical environment of silicon and lithium in the bulk of the cycled electrodes. Here we report a methodology based on multi-edge X-ray Raman Scattering performed on model silicon electrodes prepared in fully lithiated and fully delithiated states after the first cycle. The spectra were recorded at the C, O, F and Li K edges, as well as Si L2,3 edge. They were analysed using linear combinations of both experimental and computed reference spectra. We used prototypical SEI compounds as Li2CO3, LiF and LiPF6, as well as electrode constituents as binder and conductive carbon, cristalline Si, native SiO2,LixSi phases (x being the lithiation index) to identify the main species, isolate their relative contributions, and quantitatively evaluate the proportions of organic and inorganic products. We find that 30% of the carbonates formed in the SEI during the lithiation are dissolved on delithiation, and that part of the Li15Si4 alloys remain present after delithiation. By combining electrochemical analysis and XRS results, we identify that 17% of the lithium lost in the first cycle is trapped in disconnected silicon particles, while 30% form a fluorine-rich stable SEI and 53% a carbonate-rich partially-dissolvable SEI. These results pave the way to systematic, reference data-informed, and modelling assisted studies of SEI characteristics in the bulk of electrodes prepared under controlled state-of-charge and state-of-health conditions.

cond-mat.mtrl-sci↗

arXiv2023

Distinguishing different stackings in layered materials via luminescence spectroscopy

Despite its simple crystal structure, layered boron nitride features a surprisingly complex variety of phonon-assisted luminescence peaks. We present a combined experimental and theoretical study on ultraviolet-light emission in hexagonal and rhombohedral bulk boron nitride crystals. Emission spectra of high-quality samples are measured via cathodoluminescence spectroscopy, displaying characteristic differences between the two polytypes. These differences are explained using a fully first-principles computational technique that takes into account radiative emission from ``indirect'', finite-momentum, excitons via coupling to finite-momentum phonons. We show that the differences in peak positions, number of peaks and relative intensities can be qualitatively and quantitatively explained, once a full integration over all relevant momenta of excitons and phonons is performed.

cond-mat.mtrl-sci↗

arXiv2023

Magnetic tuning of the tunnel coupling in an optically active quantum dot molecule

Self-assembled optically active quantum dot molecules (QDMs) allow the creation of protected qubits via singlet-triplet spin states. The qubit energy splitting of these states is defined by the tunnel coupling strength and is, therefore, determined by the potential landscape and thus fixed during growth. Applying an in-plane magnetic field increases the confinement of the hybridized wave functions within the quantum dots, leading to a decrease of the tunnel coupling strength. We achieve a tuning of the coupling strength by $(53.4\pm1.7)$ %. The ability to fine-tune this coupling is essential for quantum network and computing applications that require quantum systems with near identical performance.

cond-mat.mes-hall↗

arXiv2023

Quenching of low-energy optical absorption in bilayer C$_3$N polytypes

In this work we provide a first principles description of the electronic and optical properties of bilayers C$_3$N, with different stacking motifs AB, AB$'$ and AA$'$. Starting from quasi-particle electronic band-structures, we solve the Bethe Salpeter Equation (BSE) to access the excitonic properties of these bilayers. For all stacking sequences, we see strong optical absorption at energies lower than but close to that of the monolayer. Most relevant, we predict a strong quenching of the low-energy optical absorption, with negligible oscillator strength of low-lying bound excitons. This is a unique phenomenology that does not arise in the monolayer case, nor in other common homo-bilayers. We explain these findings in terms of the small interband dipole matrix elements associated to the valence-conduction transitions involved in these excitons, and discuss them in view of the different stacking motifs.

cond-mat.mtrl-sci↗

arXiv2023

Towards high-throughput many-body perturbation theory: efficient algorithms and automated workflows

The automation of ab initio simulations is essential in view of performing high-throughput (HT) computational screenings oriented to the discovery of novel materials with desired physical properties. In this work, we propose algorithms and implementations that are relevant to extend this approach beyond density functional theory (DFT), in order to automate many-body perturbation theory (MBPT) calculations. Notably, a novel algorithm pursuing the goal of an efficient and robust convergence procedure for GW and BSE simulations is provided, together with its implementation in a fully automated framework. This is accompanied by an automatic GW band interpolation scheme based on maximally-localized Wannier functions, aiming at a reduction of the computational burden of quasiparticle band structures while preserving high accuracy. The proposed developments are validated on a set of representative semiconductor and metallic systems.

cond-mat.mtrl-sci↗

arXiv2023

Efficient full frequency GW for metals using a multipole approach for the dielectric screening

The properties of metallic systems with important and structured excitations at low energies, such as Cu, are challenging to describe with simple models like the plasmon pole approximation (PPA), and more accurate and sometimes prohibitive full frequency approaches are usually required. In this paper we propose a numerical approach to $GW$ calculations on metals that takes into account the frequency dependence of the screening via the multipole approximation (MPA), an accurate and efficient alternative to current full-frequency methods that was recently developed and validated for semiconductors and overcomes several limitations of PPA. We now demonstrate that MPA can be successfully extended to metallic systems by optimizing the frequency sampling for this class of materials and introducing a simple method to include the $\mathbf{q}\to 0$ limit of the intra-band contributions. The good agreement between MPA and full frequency results for the calculations of quasi-particle energies, polarizability, self-energy and spectral functions in different metallic systems confirms the accuracy and computational efficiency of the method. Finally, we discuss the physical interpretation of the MPA poles through a comparison with experimental electron energy loss spectra for Cu.

cond-mat.mtrl-sci↗

arXiv2022

Graphene decoupling through oxygen intercalation on Gr/Co and Gr/Co/Ir interfaces

We perform a density functional theory study of the effects of oxygen adsorption on the structural and electronic properties of Gr/Co(0001) and Gr/Co/Ir(111) interfaces. In both interfaces, the graphene-Co distance increases with increasing O concentration. The oxygen intercalation effectively decreases the electronic interaction, preventing the hybridization of graphene states with Co $d$-orbitals, hence (partly) restoring the typical Dirac cone of pristine graphene. In the case of graphene/Co 1ML/Ir(111), which presents a moiré pattern, the interplay between the O distribution and the continuous change of the graphene-Co registry can be used to tune graphene corrugation and electronic properties. The computed electronic properties are in very good agreement with previously reported angle resolved photoemission spectroscopy and photoemission electron microscopy measurements for Gr/Co(0001).

cond-mat.mtrl-sci↗

arXiv2022

Excitonic effects in graphene-like C$_3$N

Monolayer C$_3$N is an emerging two-dimensional indirect band gap semiconductor with interesting mechanical, thermal, and electronic properties. In this work we present a description of C$_3$N electronic and dielectric properties, focusing on the so-called momentum-resolved exciton band structure. Excitation energies and oscillator strengths are computed in order to characterize bright and dark states, and discussed also with respect to the crystal symmetry. Activation of excitonic states is observed for finite transferred momenta: Indeed, we find an active indirect exciton at $\sim$ 0.9 eV, significantly lower than the direct optical gap of 1.96 eV, with excitonic binding energies in the range 0.6-0.9 eV for the lowest states. As for other 2D materials, we find a quasi-linear excitonic dispersion close to $Γ$, which however shows a downward convexity related to the indirect band gap of C$_3$N as well as to the dark nature of the involved excitons.

cond-mat.mes-hall↗

arXiv2022

Anomalous screening in narrow-gap carbon nanotubes

The screening of Coulomb interaction controls many-body physics in carbon nanotubes, as it tunes the range and strength of the force that acts on charge carriers and binds electron-hole pairs into excitons. In doped tubes, the effective Coulomb interaction drives the competition between Luttinger liquid and Wigner crystal, whereas in undoped narrow-gap tubes it dictates the Mott or excitonic nature of the correlated insulator observed at low temperature. Here, by computing the dielectric function of selected narrow- and zero-gap tubes from first principles, we show that the standard effective-mass model of screening systematically underestimates the interaction strength at long wavelength, hence missing the binding of low-energy excitons. The reason is that the model critically lacks the full three-dimensional topology of the tube, being adapted from graphene theory. As ab inito calculations are limited to small tubes, we develop a two-band model dielectric function based on the plane-wave expansion of Bloch states and the exact truncated Coulomb cutoff technique. We demonstrate that our -- computationally cheap -- approach provides the correct screening for narrow-gap tubes of any size and chirality. A striking result is that the screened interaction remains long-ranged even in gapless tubes, as an effect of the microscopic local fields generated by the electrons moving on the curved tube surface. As an application, we show that the effective electron-electron force that is felt at distances relevant to quantum transport experiments is super Coulombic.

cond-mat.mes-hall↗

arXiv2021

Non-adiabatic suppression of 3D excitonic screening in black phosphorus by mid-infrared pulses

The competition between the electron-hole Coulomb attraction and the three-dimensional dielectric screening dictates the optical properties of layered semiconductors. In low-dimensional materials, the equilibrium dielectric environment can be significantly altered by the ultrafast excitation of photo-carriers, leading to renormalized band gap and exciton binding energies. Recently, black phosphorus emerged as a 2D material with strongly layer-dependent electronic properties. Here, we resolve the coherent response of screening to sub-gap photo-excitation in bulk black phosphorus and find that mid-infrared pulses tuned across the band gap drive a transient non-thermal suppression of the dielectric screening, which is revealed by the emergence of the single-layer exciton resonance. Our work exposes the role of interlayer interactions in determining the electronic properties of 2D materials and discloses the possibility of optically manipulate them, which is of great relevance for the engineering of versatile van der Waals low-dimensional materials.

cond-mat.mtrl-sci↗

arXiv2021

Evidence for equilibrium excitons and exciton condensation in monolayer WTe2

A single monolayer of the layered semimetal WTe2 behaves as a two-dimensional topological insulator, with helical conducting edge modes surrounding a bulk state that becomes insulating at low temperatures. Here we present evidence that the bulk state has a very unusual nature, containing electrons and holes bound by Coulomb attraction (excitons) that spontaneously form in thermal equilibrium. On cooling from room temperature to 100 K the conductivity develops a V-shaped dependence on electrostatic doping, while the chemical potential develops a ~43 meV step at the neutral point. These features are much sharper than is possible in an independent-electron picture, but they can be largely accounted for by positing that some of the electrons and holes are paired in equilibrium. Our calculations from first principles show that the exciton binding energy is larger than 100 meV and the radius as small as 4 nm, explaining their formation at high temperature and doping levels. Below 100 K more strongly insulating behavior is seen, suggesting that a charge-ordered state forms. The observed absence of charge density waves in this state appears surprising within an excitonic insulator picture, but we show that it can be explained by the symmetries of the exciton wave function. Monolayer WTe2 therefore presents an exceptional combination of topological properties and strong correlations over a wide temperature range.

cond-mat.str-el↗

arXiv2021

Frequency dependence in GW made simple using a multi-pole approximation

In the $GW$ approximation, the screened interaction $W$ is a non-local and dynamical potential that usually has a complex frequency dependence. A full description of such dependence is possible but often computationally demanding. For this reason, it is still common practice to approximate $W(ω)$ using a plasmon pole (PP) model. Such approach, however, may deliver an accuracy limited by its simplistic description of the frequency dependence of the polarizability, i.e. of $W$. In this work we explore a multi-pole approach (MPA) and develop an effective representation of the frequency dependence of $W$. We show that an appropriate sampling of the polarizability in the frequency complex plane and a multi-pole interpolation can lead to a level of accuracy comparable with full-frequency methods at much lower computational cost. Moreover, both accuracy and cost are controllable by the number of poles used in MPA. Eventually we validate the MPA approach in selected prototype systems, showing that full-frequency quality results can be obtained with a limited number of poles.

cond-mat.mtrl-sci↗

arXiv2020

Evidence of ideal excitonic insulator in bulk MoS2 under pressure

Spontaneous condensation of excitons is a long sought phenomenon analogous to the condensation of Cooper pairs in a superconductor. It is expected to occur in a semiconductor at thermodynamic equilibrium if the binding energy of the excitons---electron (e) and hole (h) pairs interacting by Coulomb force---overcomes the band gap, giving rise to a new phase: the 'excitonic insulator' (EI). Transition metal dichalcogenides are excellent candidates for the EI realization because of reduced Coulomb screening, and indeed a structural phase transition was observed in few-layer systems. However, previous work could not disentangle to which extent the origin of the transition was in the formation of bound excitons or in the softening of a phonon. Here we focus on bulk MoS2 and demonstrate theoretically that at high pressure it is prone to the condensation of genuine excitons of finite momentum, whereas the phonon dispersion remains regular. Starting from first-principles many-body perturbation theory, we also predict that the self-consistent electronic charge density of the EI sustains an out-of-plane permanent electric dipole moment with an antiferroelectric texture in the layer plane: At the onset of the EI phase, those optical phonons that share the exciton momentum provide a unique Raman fingerprint for the EI formation. Finally, we identify such fingerprint in a Raman feature that was previously observed experimentally, thus providing direct spectroscopic confirmation of an ideal excitonic insulator phase in bulk MoS2 above 30 GPa.

cond-mat.str-el↗

arXiv2019

Electronic and optical properties of doped TiO2 by many-body perturbation theory

Doping is one of the most common strategies for improving the photocatalytic and solar energy conversion properties of TiO$_2$, hence an accurate theoretical description of the electronic and optical properties of doped TiO$_2$ is of both scientific and practical interest. In this work we use many-body perturbation theory techniques to investigate two typical n-type dopants, Niobium and Hydrogen, in TiO$_2$ rutile. Using the GW approximation to determine band edges and defect energy levels, and the Bethe Salpeter equation for the calculation of the absorption spectra, we find that the defect energy levels form non-dispersive bands %associated with localized states lying $\simeq 2.2 eV$ above the top of the corresponding valence bands ($\simeq 0.9 eV$ below the conduction bands of the {\it pristine} material). The defect states are also responsible for the appearance of low energy absorption peaks that enhance the solar spectrum absorption of rutile. The spatial distributions of the excitonic wavefunctions associated with these low energy excitations are very different for the two dopants, suggesting a larger mobility of photoexcited electrons in Nb-TiO$_2$.

cond-mat.mtrl-sci↗