Showing 1–41 of 41 results for author: Gatti, M

Designing explicit functionals for the charge density in terms of a potential

Authors: Muhammed H. Güneş, Ayoub Aouina, Vitaly Gorelov, Matteo Gatti, Lucia Reining

Abstract: One of the most powerful strategies to address properties of real many-body systems is to incorporate data obtained for models, for example, to use data of the homogeneous electron gas in order to build the Local Density Approximation for the Kohn-Sham exchange-correlation potential. In the present work, we examine to what extent we can use model data to design functionals directly for observables… ▽ More One of the most powerful strategies to address properties of real many-body systems is to incorporate data obtained for models, for example, to use data of the homogeneous electron gas in order to build the Local Density Approximation for the Kohn-Sham exchange-correlation potential. In the present work, we examine to what extent we can use model data to design functionals directly for observables of materials. In particular, we study different approximations for the charge density of real inhomogeneous materials expressed as a simple, explicit functional of a given Kohn-Sham potential, using as central building block the Lindhard density-density response function of the homogeneous electron gas. Our increasingly realistic set of approximations includes a fully nearsighted expression equivalent to the Thomas-Fermi approximation, functional Taylor expansions, and different approximations to the Connector Theory developed in [Aouina \textit{et al.}, npj Computational Materials {\bf 11}, 242 (2025)]. In all cases, the charge density is obtained without ever solving the Kohn-Sham Schrödinger equation. Results for cubic helium, a prototypical strongly inhomogeneous material, as well as the covalent semiconductor silicon and metallic aluminum, systematically improve with higher levels of approximation. At the present stage, the results may be used for qualitative discussions or as optimized starting point for a self-consistent Kohn-Sham cycle. More generally, their quality indicates that this is a promising route to obtain functional expressions for observables that are relatively simple to calculate and to analyze. △ Less

Submitted 31 July, 2026; v1 submitted 4 May, 2026; originally announced May 2026.

Direct measurement of the longitudinal exciton dispersion in hBN by resonant inelastic x-ray scattering

Authors: Alessandro Nicolaou, Kari Ruotsalainen, Laura Susana, Victor Porée, Luiz Galvao Tizei, Jaakko Koskelo, Takashi Taniguchi, Kenji Watanabe, Alberto Zobelli, Matteo Gatti

Abstract: We report resonant inelastic X-ray scattering (RIXS) measurements on the prototypical hexagonal boron nitride hBN layered compound. The RIXS results at the B and N K edges have been combined with electron energy loss spectroscopy (EELS) experiments and ab initio calculations within the framework of the Bethe-Salpeter equation of many-body perturbation theory. By means of this tight interplay of di… ▽ More We report resonant inelastic X-ray scattering (RIXS) measurements on the prototypical hexagonal boron nitride hBN layered compound. The RIXS results at the B and N K edges have been combined with electron energy loss spectroscopy (EELS) experiments and ab initio calculations within the framework of the Bethe-Salpeter equation of many-body perturbation theory. By means of this tight interplay of different spectroscopies, the lowest longitudinal exciton of hBN has been identified. Moreover, its qualitatively different dispersions along the $Γ$K and the $Γ$M directions of the Brillouin zone have been determined. Our study advocates soft X-ray RIXS and EELS to be a promising combination to investigate electronic excitations in materials. △ Less

Submitted 13 January, 2025; v1 submitted 8 January, 2025; originally announced January 2025.

Industrial-scale Prediction of Cement Clinker Phases using Machine Learning

Authors: Sheikh Junaid Fayaz, Nestor Montiel-Bohorquez, Shashank Bishnoi, Matteo Romano, Manuele Gatti, N. M. Anoop Krishnan

Abstract: Cement production, exceeding 4.1 billion tonnes and contributing 2.4 tonnes of CO2 annually, faces critical challenges in quality control and process optimization. While traditional process models for cement manufacturing are confined to steady-state conditions with limited predictive capability for mineralogical phases, modern plants operate under dynamic conditions that demand real-time quality… ▽ More Cement production, exceeding 4.1 billion tonnes and contributing 2.4 tonnes of CO2 annually, faces critical challenges in quality control and process optimization. While traditional process models for cement manufacturing are confined to steady-state conditions with limited predictive capability for mineralogical phases, modern plants operate under dynamic conditions that demand real-time quality assessment. Here, exploiting a comprehensive two-year operational dataset from an industrial cement plant, we present a machine learning framework that accurately predicts clinker mineralogy from process data. Our model achieves unprecedented prediction accuracy for major clinker phases while requiring minimal input parameters, demonstrating robust performance under varying operating conditions. Through post-hoc explainable algorithms, we interpret the hierarchical relationships between clinker oxides and phase formation, providing insights into the functioning of an otherwise black-box model. This digital twin framework can potentially enable real-time optimization of cement production, thereby providing a route toward reducing material waste and ensuring quality while reducing the associated emissions under real plant conditions. Our approach represents a significant advancement in industrial process control, offering a scalable solution for sustainable cement manufacturing. △ Less

Submitted 28 December, 2024; v1 submitted 16 December, 2024; originally announced December 2024.

K-edge XANES of octahedral aluminum compounds: similarities and differences via the analysis of excitonic properties

Authors: Newman Amoyaw, Abezu Agegnehu, Francesco Sottile, Matteo Gatti, M. Laura Urquiza

Abstract: This study presents an \textit{ab initio} investigation of the XANES spectra at the aluminum K edge for three compounds: Al$_2$O$_3$, AlF$_3$ and AlCl$_3$, where the Al atoms share the same oxidation state~(III) and are coordinated in an octahedral symmetry. The XANES spectra calculated within the independent-particle approximation reveal significant differences, including shifts in the spectrum o… ▽ More This study presents an \textit{ab initio} investigation of the XANES spectra at the aluminum K edge for three compounds: Al$_2$O$_3$, AlF$_3$ and AlCl$_3$, where the Al atoms share the same oxidation state~(III) and are coordinated in an octahedral symmetry. The XANES spectra calculated within the independent-particle approximation reveal significant differences, including shifts in the spectrum onset, variations in the spectral shapes, and the presence of a pre-peak in the case of AlCl$_3$, all in correspondence with the behavior of the PDOS of the absorbing atom in the different materials. The origin of the features stems from the specific band structure of each compound. When electron--hole interactions are taken into account through the solution of the Bethe-Salpeter equation, a series of dark and bright excitons with large binding energies and Frenkel character is obtained. The strong excitonic effects lead to the suppression of the pre-peak in AlCl$_3$ and further accentuate the differences among the three Al K-edge spectra. △ Less

Submitted 13 November, 2024; originally announced November 2024.

Connections between resonant inelastic x-ray scattering and complementary x-ray spectroscopies: probing excitons at Al K and L$_1$ edges of $α$-Al$_2$O$_3$

Authors: M. Laura Urquiza, Matteo Gatti, Francesco Sottile

Abstract: We present an ab initio study of neutral core and valence electronic excitations in α-Al2O3 by solving the Bethe-Salpeter equation (BSE) of many-body perturbation theory within an all-electron framework. Calculated spectra at the Al K and L1 edges are in remarkable agreement with available experiments from X-ray absorption (XAS) and X-ray Raman spectroscopy once excitonic effects are taken into ac… ▽ More We present an ab initio study of neutral core and valence electronic excitations in α-Al2O3 by solving the Bethe-Salpeter equation (BSE) of many-body perturbation theory within an all-electron framework. Calculated spectra at the Al K and L1 edges are in remarkable agreement with available experiments from X-ray absorption (XAS) and X-ray Raman spectroscopy once excitonic effects are taken into account. The combination of the BSE spectra for the two techniques confirms the dipole-forbidden nature of the exciton prepeak as suggested by recent calculations based on density-functional theory. Moreover, we make predictions for resonant inelastic X-ray scattering (RIXS) spectra at K and L1 edges, which strikingly fully overlap also beyond an independent-particle picture. The RIXS calculations reveal two distinct regimes as a function of incoming photon energy. Below and at the XAS threshold, we observe Raman-like features, characterised by strong excitonic effects, which we directly compare to peaks in the loss function. Above the XAS threshold, instead, fluorescence features become predominant: RIXS spectra can be well described and analyzed within an independent-particle approximation showing similarity with the X-ray emission spectrum. △ Less

Submitted 25 January, 2024; v1 submitted 11 September, 2023; originally announced September 2023.

Exciton band structure of V$_2$O$_5$

Authors: Vitaly Gorelov, Lucia Reining, Matteo Gatti

Abstract: Excitonic effects due to the correlation of electrons and holes in excited states of matter dominate the optical spectra of many interesting materials. They are usually studied in the long-wavelength limit. Here we investigate excitons at non-vanishing momentum transfer, corresponding to shorter wavelengths. We calculate the exciton dispersion in the prototypical layered oxide V$_2$O$_5$ by solvin… ▽ More Excitonic effects due to the correlation of electrons and holes in excited states of matter dominate the optical spectra of many interesting materials. They are usually studied in the long-wavelength limit. Here we investigate excitons at non-vanishing momentum transfer, corresponding to shorter wavelengths. We calculate the exciton dispersion in the prototypical layered oxide V$_2$O$_5$ by solving the Bethe-Salpeter equation of many-body perturbation theory. We discuss the change of excitation energy and intensity as a function of wavevector for bright and dark excitons, respectively, and we analyze the origin of the excitons along their dispersion. We highlight the important role of the electron-hole exchange with its impact on the exciton dispersion, the singlet-triplet splitting and the difference between the imaginary part of the macroscopic dielectric function and the loss function. △ Less

Submitted 17 April, 2023; originally announced April 2023.

Pseudopotential Bethe-Salpeter calculations for shallow-core x-ray absorption near-edge structures: excitonic effects in Al2O3

Authors: M. Laura Urquiza, Matteo Gatti, Francesco Sottile

Abstract: We present an ab initio description of optical and shallow-core x-ray absorption spectroscopies in a unified formalism based on the pseudopotential plane-wave method at the level of the Bethe-Salpeter equation (BSE) within Green's functions theory. We show that norm-conserving pseudopotentials are reliable and accurate not only for valence, but also for semicore electron excitations. In order to v… ▽ More We present an ab initio description of optical and shallow-core x-ray absorption spectroscopies in a unified formalism based on the pseudopotential plane-wave method at the level of the Bethe-Salpeter equation (BSE) within Green's functions theory. We show that norm-conserving pseudopotentials are reliable and accurate not only for valence, but also for semicore electron excitations. In order to validate our approach, we compare BSE absorption spectra obtained with two different codes: the pseudopotential-based code EXC and the all-electron full-potential code Exciting. We take corundum $α$-Al$_2$O$_3$ as an example, being a prototypical material that presents strong electron-hole interactions for both valence and core electron excitations. We analyze in detail the optical absorption spectrum as well as the Al L$_1$ and L$_{2,3}$ edges in terms of anisotropy, crystal local fields, interference and excitonic effects. We perform a thorough inspection of the origin and localization of the lowest-energy excitons, and conclude highlighting the purely electronic character off the pre-edge of L$_1$ and the dichroic nature of the optical and L$_{23}$ spectra. △ Less

Submitted 24 April, 2023; v1 submitted 10 January, 2023; originally announced January 2023.

Comments: 16 pages, 11 figures

Short-range excitonic phenomena in low-density metals

Authors: Jaakko Koskelo, Lucia Reining, Matteo Gatti

Abstract: Excitonic effects in metals are commonly supposed to be weak, because the Coulomb interaction is strongly screened. We investigate the low-density regime of the homogeneous electron gas, where low-energy collective excitations and ghost modes were anticipated. Using the Bethe-Salpeter equation (BSE), we show that both phenomena exist thanks to reduced screening at short distances. This is not capt… ▽ More Excitonic effects in metals are commonly supposed to be weak, because the Coulomb interaction is strongly screened. We investigate the low-density regime of the homogeneous electron gas, where low-energy collective excitations and ghost modes were anticipated. Using the Bethe-Salpeter equation (BSE), we show that both phenomena exist thanks to reduced screening at short distances. This is not captured by common approximations used in ab initio BSE calculations, but requires vertex corrections that take the fermionic nature of charges into account. The electron-hole wavefunction of the low-energy modes shows strong and very anisotropic electron-hole correlation, which speaks for an excitonic character of these modes. The fact that short-range physics is at the origin of these phenomena explains why, on the other hand, also the simple adiabatic local density approximation to time-dependent density functional theory can capture these effects. △ Less

Submitted 1 January, 2023; originally announced January 2023.

Robustness of electronic screening effects in electron spectroscopies: example of V$_2$O$_5$

Authors: Vitaly Gorelov, Lucia Reining, Walter R. L. Lambrecht, Matteo Gatti

Abstract: In bulk and low-dimensional extended systems, the screening of excitations by the electron cloud is a key feature governing spectroscopic properties. Widely used computational approaches, especially in the framework of many-body perturbation theory, such as the GW approximation and the resulting approximate Bethe-Salpeter equation, are explicitly formulated in terms of the screened Coulomb interac… ▽ More In bulk and low-dimensional extended systems, the screening of excitations by the electron cloud is a key feature governing spectroscopic properties. Widely used computational approaches, especially in the framework of many-body perturbation theory, such as the GW approximation and the resulting approximate Bethe-Salpeter equation, are explicitly formulated in terms of the screened Coulomb interaction. In the present work we explore the effect of screening in absorption and electron energy loss spectroscopy, concentrating on the effect of local distortions on the screening and elucidating the resulting changes in the various spectra. Using the layered bulk oxide V$_2$O$_5$ as prototype material, we show in which way local distortions affect the screening, and in which way changes in the screening impact electron energy loss and absorption spectra including excitons. We highlight cancellations that make many-body effects in the spectra very robust with respect to structural modifications, while the band structure undergoes significant changes and the nature of the excitations may also be affected. This yields insight concerning the structure-properties relations that are crucial for the use of V$_2$O$_5$ as energy storage material, and more generally, that may be used to optimize the analysis and the calculation of electronic spectra in complex materials. △ Less

Submitted 25 January, 2023; v1 submitted 11 November, 2022; originally announced November 2022.

Accurate Kohn-Sham auxiliary system from the ground state density of solids

Authors: Ayoub Aouina, Matteo Gatti, Siyuan Chen, Shiwei Zhang, Lucia Reining

Abstract: The Kohn-Sham (KS) system is an auxiliary system whose effective potential is unknown in most cases. It is in principle determined by the ground state density, and it has been found numerically for some low-dimensional systems by inverting the KS equations starting from a given accurate density. For solids, only approximate results are available. In this work, we determine accurate exchange correl… ▽ More The Kohn-Sham (KS) system is an auxiliary system whose effective potential is unknown in most cases. It is in principle determined by the ground state density, and it has been found numerically for some low-dimensional systems by inverting the KS equations starting from a given accurate density. For solids, only approximate results are available. In this work, we determine accurate exchange correlation (xc) potentials for Si and NaCl using the ground state densities obtained from Auxiliary Field Quantum Monte Carlo calculations. We show that these xc potentials can be rationalized as an ensemble of environment-adapted functions of the local density. The KS band structure can be obtained with high accuracy. The true KS band gap turns out to be larger than the prediction of the local density approximation, but significantly smaller than the measurable photoemission gap, which confirms previous estimates. Finally, our findings show that the conjecture that very different xc potentials can lead to very similar densities and other KS observables is true also in solids, which questions the meaning of details of the potentials and, at the same time, confirms the stability of the KS system. △ Less

Submitted 24 April, 2023; v1 submitted 8 July, 2022; originally announced July 2022.

Delocalization of dark and bright excitons in flat-band materials and the optical properties of V$_2$O$_5$

Authors: Vitaly Gorelov, Lucia Reining, Martin Feneberg, Rüdiger Goldhahn, André Schleife, Walter R. L. Lambrecht, Matteo Gatti

Abstract: The simplest picture of excitons in materials with atomic-like localization of electrons is that of Frenkel excitons, where electrons and holes stay close together, which is associated with a large binding energy. Here, using the example of the layered oxide V$_2$O$_5$ , we show how localized charge-transfer excitations combine to form excitons that also have a huge binding energy but, at the same… ▽ More The simplest picture of excitons in materials with atomic-like localization of electrons is that of Frenkel excitons, where electrons and holes stay close together, which is associated with a large binding energy. Here, using the example of the layered oxide V$_2$O$_5$ , we show how localized charge-transfer excitations combine to form excitons that also have a huge binding energy but, at the same time, a large electron-hole distance, and we explain this seemingly contradictory finding. The anisotropy of the exciton delocalization is determined by the local anisotropy of the structure, whereas the exciton extends orthogonally to the chains formed by the crystal structure. Moreover, we show that the bright exciton goes together with a dark exciton of even larger binding energy and more pronounced anisotropy. These findings are obtained by combining first principles many-body perturbation theory calculations, ellipsometry experiments, and tight binding modelling, leading to very good agreement and a consistent picture. Our explanation is general and can be extended to other materials. △ Less

Submitted 29 April, 2022; v1 submitted 28 April, 2022; originally announced April 2022.

Journal ref: npj Comput Mater 8, 94 (2022)

Dynamical screening in SrVO$_3$: Inelastic x-ray scattering experiments and ab initio calculations

Authors: Kari Ruotsalainen, Alessandro Nicolaou, Christoph J. Sahle, Anna Efimenko, James M. Ablett, Jean-Pascal Rueff, Dharmalingam Prabhakaran, Matteo Gatti

Abstract: We characterize experimentally and theoretically the high-energy dielectric screening properties of the prototypical correlated metal SrVO$_3$. The dynamical structure factor measured by inelastic x-ray scattering spectroscopy as a function of momentum transfer is in very good agreement with first-principles calculations in the adiabatic local density approximation to time-dependent density-functi… ▽ More We characterize experimentally and theoretically the high-energy dielectric screening properties of the prototypical correlated metal SrVO$_3$. The dynamical structure factor measured by inelastic x-ray scattering spectroscopy as a function of momentum transfer is in very good agreement with first-principles calculations in the adiabatic local density approximation to time-dependent density-functional theory. Our results reveal the crucial importance of crystal local fields in the charge response function of correlated materials: They lead to depolarization effects for localised excitations and couple spectra from different Brillouin zones. △ Less

Submitted 10 June, 2021; originally announced June 2021.

Subpicosecond metamagnetic phase transition driven by non-equilibrium electron dynamics

Authors: Federico Pressacco, Davide Sangalli, Vojtěch Uhlíř, Dmytro Kutnyakhov, Jon Ander Arregi, Steinn Ymir Agustsson, Günter Brenner, Harald Redlin, Michael Heber, Dmitry Vasilyev, Jure Demsar, Gerd Schönhense, Matteo Gatti, Andrea Marini, Wilfried Wurth, Fausto Sirotti

Abstract: Femtosecond light-induced phase transitions between different macroscopic orders provide the possibility to tune the functional properties of condensed matter on ultrafast timescales. In first-order phase transitions, transient non-equilibrium phases and inherent phase coexistence often preclude non-ambiguous detection of transition precursors and their temporal onset. Here, we present a study com… ▽ More Femtosecond light-induced phase transitions between different macroscopic orders provide the possibility to tune the functional properties of condensed matter on ultrafast timescales. In first-order phase transitions, transient non-equilibrium phases and inherent phase coexistence often preclude non-ambiguous detection of transition precursors and their temporal onset. Here, we present a study combining time-resolved photoelectron spectroscopy and ab-initio electron dynamics calculations elucidating the transient subpicosecond processes governing the photoinduced generation of ferromagnetic order in antiferromagnetic FeRh. The transient photoemission spectra are accounted for by assuming that not only the occupation of electronic states is modified during the photoexcitation process. Instead, the photo-generated non-thermal distribution of electrons modifies the electronic band structure. The ferromagnetic phase of FeRh, characterized by a minority band near the Fermi energy, is established 350+- 30 fs after the laser excitation. Ab-initio calculations indicate that the phase transition is initiated by a photoinduced Rh-to-Fe charge transfer. △ Less

Submitted 18 February, 2021; originally announced February 2021.

Journal ref: Nature Communications 12, 5088 (2021)

First-principles study of excitons in the optical spectra of silver chloride

Authors: Arnaud Lorin, Matteo Gatti, Lucia Reining, Francesco Sottile

Abstract: Silver chloride is a material that has been investigated and used for many decades. Of particular interest are its optical properties, but only few fundamental theoretical studies exist. We present first-principles results for the optical properties of AgCl, obtained using time-dependent density functional theory and many-body perturbation theory. We show that optical properties exhibit strong exc… ▽ More Silver chloride is a material that has been investigated and used for many decades. Of particular interest are its optical properties, but only few fundamental theoretical studies exist. We present first-principles results for the optical properties of AgCl, obtained using time-dependent density functional theory and many-body perturbation theory. We show that optical properties exhibit strong excitonic effects, which are correctly captured only by solving the Bethe-Salpeter equation starting from quasiparticle self-consistent GW results. Numerical simulations are made feasible by using a model screening for the electron-hole interaction in a way that avoids the calculation of the static dielectric constant. A thorough analysis permits us to discuss localization in bright and dark excitons of silver chloride. △ Less

Submitted 18 September, 2020; originally announced September 2020.

Strategies to build functionals of the density, or functionals of Green's functions: what can we learn?

Authors: Ayoub Aouina, Matteo Gatti, Lucia Reining

Abstract: The many-body problem can in general not be solved exactly, and one of the most prominent approximations is to build perturbation expansions. A huge variety of expansions is possible, which differ by the quantity to be expanded, the expansion variable, the starting point, and ideas how to resum or terminate the series. Although much has been discussed and much has been done, some choices were made… ▽ More The many-body problem can in general not be solved exactly, and one of the most prominent approximations is to build perturbation expansions. A huge variety of expansions is possible, which differ by the quantity to be expanded, the expansion variable, the starting point, and ideas how to resum or terminate the series. Although much has been discussed and much has been done, some choices were made for historical reasons, in particular, limited computation or storage capacities. The present work aims at examining the justifications for different choices made in different contexts, by comparing ingredients of functionals based on \GF s on one side, and on the charge density on the other side. Of particular interest will be the question of how to build an optimal starting point for the approximation of non-local quantities, making use of near- or far-sightedness, and daring to consider models beyond the homogeneous electron gas. This will include the use of connector approximations. We will also discuss why it is a good idea to build functionals of the density. △ Less

Submitted 23 June, 2020; originally announced June 2020.

Insights into one-body density matrices using deep learning

Authors: Jack Wetherell, Andrea Costamagna, Matteo Gatti, Lucia Reining

Abstract: The one-body reduced density matrix (1-RDM) of a many-body system at zero temperature gives direct access to many observables, such as the charge density, kinetic energy and occupation numbers. It would be desirable to express it as a simple functional of the density or of other local observables, but to date satisfactory approximations have not yet been found. Deep learning is the state-of the ar… ▽ More The one-body reduced density matrix (1-RDM) of a many-body system at zero temperature gives direct access to many observables, such as the charge density, kinetic energy and occupation numbers. It would be desirable to express it as a simple functional of the density or of other local observables, but to date satisfactory approximations have not yet been found. Deep learning is the state-of the art approach to perform high dimensional regressions and classification tasks, and is becoming widely used in the condensed matter community to develop increasingly accurate density functionals. Autoencoders are deep learning models that perform efficient dimensionality reduction, allowing the distillation of data to its fundamental features needed to represent it. By training autoencoders on a large data-set of 1-RDMs from exactly solvable real-space model systems, and performing principal component analysis, the machine learns to what extent the data can be compressed and hence how it is constrained. We gain insight into these machine learned constraints and employ them to inform approximations to the 1-RDM as a functional of the charge density. We exploit known physical properties of the 1-RDM in the simplest possible cases to perform feature engineering, where we inform the structure of the models from known mathematical relations, allowing us to integrate existing understanding into the machine learning methods. By comparing various deep learning approaches we gain insight into what physical features of the density matrix are most amenable to machine learning, utilising both known and learned characteristics. △ Less

Submitted 21 May, 2020; originally announced May 2020.

Comments: 15 pages, 11 figures

Resonant inelastic x-ray scattering study of doping and temperature dependence of low-energy excitations in La$_{1-x}$Sr$_x$VO$_3$

Authors: Kari Ruotsalainen, Matteo Gatti, James M. Ablett, Flora Yakhou-Harris, Jean-Pascal Rueff, Adrian David, Wilfrid Prellier, Alessandro Nicolaou

Abstract: We present a temperature and doping dependent resonant inelastic X-ray scattering experiment at the V L$_{2,3}$ and O K edges in La$_{1-x}$Sr$_x$VO$_3$ for $x=0$ and $x=0.1$. This material is a canonical example of a compound that exhibits a filling control metal-insulator transition and undergoes orbital ordering and antiferromagnetic transitions at low temperature. Temperature dependent measurem… ▽ More We present a temperature and doping dependent resonant inelastic X-ray scattering experiment at the V L$_{2,3}$ and O K edges in La$_{1-x}$Sr$_x$VO$_3$ for $x=0$ and $x=0.1$. This material is a canonical example of a compound that exhibits a filling control metal-insulator transition and undergoes orbital ordering and antiferromagnetic transitions at low temperature. Temperature dependent measurements at the V L$_{3}$ edge reveal an intra-t$_{2g}$ excitation that blueshifts by 40 meV from room temperature to 30 K at a rate that differs between the para- and antiferromagnetic phases. The lineshape can be partially explained by a purely local model using crystal field theory calculations. At $x=0.1$ the doping is shown to affect the local electronic structure primarily on the O sites, which is in disagreement with a simple Mott-Hubbard picture. We reveal the presence of phonon overtone features at the O K edge, which evidences that the low energy part of the spectrum is dominated by phonon response. △ Less

Submitted 17 May, 2021; v1 submitted 23 April, 2020; originally announced April 2020.

Journal ref: Phys. Rev. B 103, 235158 (2021)

Re-using model results to determine materials properties: connector theory approach

Authors: Marco Vanzini, Ayoub Aouina, Martin Panholzer, Matteo Gatti, Lucia Reining

Abstract: Computational materials design often profits from the fact that some complicated contributions are not calculated for the real material, but replaced by results of models. We turn this approximation into a very general and in principle exact theory by introducing the concept of a connector, which is a prescription of how to use the results of a model system in order to simulate a real system. We s… ▽ More Computational materials design often profits from the fact that some complicated contributions are not calculated for the real material, but replaced by results of models. We turn this approximation into a very general and in principle exact theory by introducing the concept of a connector, which is a prescription of how to use the results of a model system in order to simulate a real system. We set the conditions that must be fulfilled for the existence of an exact connector. We demonstrate that, and why, this approach is a very convenient starting point for approximations. We also show that the connector theory can be used to design new functionals, for example for density functional theory. We illustrate our purposes with simple but realistic examples. △ Less

Submitted 12 March, 2021; v1 submitted 19 March, 2019; originally announced March 2019.

Dispersing and non-dispersing satellites in the photoemission spectra of aluminum

Authors: Jianqiang Sky Zhou, Lucia Reining, Alessandro Nicolaou, Azzedine Bendounan, Kari Ruotsalainen, Marco Vanzini, J. J. Kas, J. J. Rehr, Matthias Muntwiler, Vladimir N. Strocov, Fausto Sirotti, Matteo Gatti

Abstract: Satellites in electronic spectra are pure many-body effects, and their study has been of increasing interest in both experiment and theory. The presence of satellites due to plasmon excitations can be understood with simple models of electron-boson coupling. It is far from obvious how to match such a model to real spectra, where more than one kind of quasi-particle and of satellite excitation coex… ▽ More Satellites in electronic spectra are pure many-body effects, and their study has been of increasing interest in both experiment and theory. The presence of satellites due to plasmon excitations can be understood with simple models of electron-boson coupling. It is far from obvious how to match such a model to real spectra, where more than one kind of quasi-particle and of satellite excitation coexist. Our joint experimental and theoretical study shows that satellites in the angle-resolved photoemission spectra of the prototype simple metal aluminum consist of a superposition of dispersing and non-dispersing features. Both are due to electron-electron interaction, but the non-dispersing satellites also reflect the thermal motion of the atoms. Moreover, besides their energy dispersion, we also show and explain a strong shape dispersion of the satellites. By taking into account these effects, our first principles calculations using the GW+C approach of many-body perturbation theory reproduce and explain the experimental spectra to an unprecedented extent. △ Less

Submitted 29 November, 2018; originally announced November 2018.

Spectroscopy of the Hubbard dimer: the spectral potential

Authors: Marco Vanzini, Lucia Reining, Matteo Gatti

Abstract: The spectral potential is the dynamical generalization of the Kohn-Sham potential. It targets, in principle exactly, the spectral function in addition to the electronic density. Here we examine the spectral potential in one of the simplest solvable models exhibiting a non-trivial interplay between electron-electron interaction and inhomogeneity, namely the asymmetric Hubbard dimer. We discuss a ge… ▽ More The spectral potential is the dynamical generalization of the Kohn-Sham potential. It targets, in principle exactly, the spectral function in addition to the electronic density. Here we examine the spectral potential in one of the simplest solvable models exhibiting a non-trivial interplay between electron-electron interaction and inhomogeneity, namely the asymmetric Hubbard dimer. We discuss a general strategy to introduce approximations, which consists in calculating the spectral potential in the homogeneous limit (here represented by the symmetric Hubbard dimer) and importing it in the real inhomogeneous system through a suitable "connector". The comparison of different levels of approximation to the spectral potential with the exact solution of the asymmetric Hubbard dimer gives insights about the advantages and the difficulties of this connector strategy for applications in real materials. △ Less

Submitted 21 September, 2018; originally announced September 2018.

Comments: Contribution to the Topical Issue "Special issue in honor of Hardy Gross", edited by C.A. Ullrich, F.M.S. Nogueira, A. Rubio, and M.A.L. Marques

Journal ref: Vanzini, M., Reining, L. & Gatti, M. Eur. Phys. J. B (2018) 91: 192. https://doi.org/10.1140/epjb/e2018-90277-3

Cubic BN optical gap and intragap optically active defects

Authors: Anna Tararan, Stefano di Sabatino, Matteo Gatti, Takashi Taniguchi, Kenji Watanabe, Lucia Reining, Luiz H. G. Tizei, Mathieu Kociak, Alberto Zobelli

Abstract: We report a comprehensive study on the optical properties of cubic boron nitride (c-BN) and its optically active defects. Using electron energy-loss spectroscopy (EELS) within a monochromated scanning transmission electron microscope (STEM) on the highest-quality crystals available, we demonstrate unequivocally that the optical-gap energy of c-BN slightly exceeds 10 eV. Further theoretical analysi… ▽ More We report a comprehensive study on the optical properties of cubic boron nitride (c-BN) and its optically active defects. Using electron energy-loss spectroscopy (EELS) within a monochromated scanning transmission electron microscope (STEM) on the highest-quality crystals available, we demonstrate unequivocally that the optical-gap energy of c-BN slightly exceeds 10 eV. Further theoretical analysis in the framework of the Bethe-Salpeter equation of many-body perturbation theory supports this result. The spatial localization of defect-related emissions has been investigated using nanometric resolved cathodoluminescence (nano-CL) in a STEM. By high-temperature annealing a c-BN powder, we have promoted phase transitions in nanometric domains which have been detected by the appearance of specific hexagonal-phase signatures in both EELS and CL spectra. A high number of intragap optically active centers are known in c-BN, but the literature is rather scattered and hence has been summarized here. For several emission lines we have obtained nano-CL maps which show emission spot sizes as small as few tens of nanometers. Finally, by coupling nano-CL to a Hanbury-Brown-Twiss intensity interferometer, we have addressed individual spots in order to identify the possible presence of single-photon sources. The observed CL bunching effect is compatible with a limited set of single-photon emitters and it permits obtaining emission lifetimes of the order of the nanosecond. △ Less

Submitted 29 June, 2018; originally announced June 2018.

Journal ref: Phys. Rev. B 98, 094106 (2018)

Laser induced phase transition in epitaxial FeRh layers studied by pump-probe valence band photoemission

Authors: Federico Pressacco, Vojtěch Uhlíř, Matteo Gatti, Alessandro Nicolaou, Azzedine Bendounan, Jon Ander Arregi, Sheena K. K. Patel, Eric E. Fullerton, Damjan Krizmancic, Fausto Sirotti

Abstract: We use time-resolved X-ray photoelectron spectroscopy to probe the electronic and magnetization dynamics in FeRh films after ultrafast laser excitations. We present experimental and theoretical results which investigate the electronic structure of the FeRh during the first-order phase transition identifying a clear signature of the magnetic phase. We find that a spin polarized feature at the Fermi… ▽ More We use time-resolved X-ray photoelectron spectroscopy to probe the electronic and magnetization dynamics in FeRh films after ultrafast laser excitations. We present experimental and theoretical results which investigate the electronic structure of the FeRh during the first-order phase transition identifying a clear signature of the magnetic phase. We find that a spin polarized feature at the Fermi edge is a fingerprint of the magnetic status of the system that is independent of the long-range ferromagnetic alignment of the magnetic domains. We use this feature to follow the phase transition induced by a laser pulse in a pump-probe experiment and find that the magnetic transition occurs in less than 50 ps, and reaches its maximum in 100 ps. △ Less

Submitted 2 March, 2018; originally announced March 2018.

Comments: 16 pages, 5 figures

Journal ref: Struct. Dyn. 5, 034501 (2018)

Revisiting the origin of satellites in core level photoemission of transparent conducting oxides: the case of $n$-doped SnO$_2$

Authors: F. Borgatti, J. A. Berger, D. Céolin, J. S. Zhou, J. J. Kas, M. Guzzo, C. F. McConville, F. Offi, G. Panaccione, A. Regoutz, D. J. Payne, J. -P. Rueff, O. Bierwagen, M. E. White, J. S. Speck, M. Gatti, R. G. Egdell

Abstract: The longstanding problem of interpretation of satellite structures in core level photoemission spectra of metallic systems with a low density of conduction electrons is addressed using the specific example of Sb-doped SnO$_2$. Comparison of {\it ab initio} many-body calculations with experimental hard X-ray photoemission spectra of the Sn 4$d$ states shows that strong satellites are produced by co… ▽ More The longstanding problem of interpretation of satellite structures in core level photoemission spectra of metallic systems with a low density of conduction electrons is addressed using the specific example of Sb-doped SnO$_2$. Comparison of {\it ab initio} many-body calculations with experimental hard X-ray photoemission spectra of the Sn 4$d$ states shows that strong satellites are produced by coupling of the Sn core hole to the plasma oscillations of the free electrons introduced by doping. Within the same theoretical framework, spectral changes of the valence band spectra are also related to dynamical screening effects. These results demonstrate that, for the interpretation of electron correlation features in the core level photoelectron spectra of such narrow-band materials, going beyond the homogeneous electron gas electron-plasmon coupling model is essential. △ Less

Submitted 24 October, 2017; originally announced October 2017.

Comments: 6 PAGES, 4 FIGURES

Journal ref: Phys. Rev. B 97, 155102 (2018)

Cumulant Green's function calculations of plasmon satellites in bulk sodium: influence of screening and the crystal environment

Authors: Jianqiang Sky Zhou, Matteo Gatti, J. J. Kas, J. J. Rehr, Lucia Reining

Abstract: We present ab initio calculations of the photoemission spectra of bulk sodium using different flavors of the cumulant expansion approximation for the Green's function. In particular, we study the dispersion and intensity of the plasmon satellites. We show that the satellite spectrum is much more sensitive to many details than the quasi-particle (QP) spectrum, which suggests that the experimental i… ▽ More We present ab initio calculations of the photoemission spectra of bulk sodium using different flavors of the cumulant expansion approximation for the Green's function. In particular, we study the dispersion and intensity of the plasmon satellites. We show that the satellite spectrum is much more sensitive to many details than the quasi-particle (QP) spectrum, which suggests that the experimental investigation of satellites could yield additional information beyond the usual studies of the band structure. In particular, a comparison to the homogeneous electron gas (HEG) shows that the satellites are influenced by the crystal environment, although the crystal potential in sodium is weak. Moreover, the temperature dependence of the lattice constant is reflected in the position of the satellites. Details of the screening also play an important role; in particular, the contribution of transitions from 2s and 2p semi-core levels influence the satellites, but not the QP. Moreover, inclusion of contributions to the screening beyond the RPA has an effect on the satellites. Finally, we elucidate the importance of the coupling of electrons and holes by comparing the results of the time-ordered (TOC) and the retarded (RC) cumulant expansion approximations. Again, we find small but noticeable differences. Since all the small effects add up, our most advanced calculation yields a satellite position which is improved with respect to previous calculations by almost one eV. This stresses the fact that the calculation of satellites is much more delicate than the calculation of a QP band structure. △ Less

Submitted 14 August, 2017; originally announced August 2017.

Journal ref: Phys. Rev. B 97, 035137 (2018)

Non-local and non-adiabatic effects in the charge-density response of solids: a time-dependent density functional approach

Authors: Martin Panholzer, Matteo Gatti, Lucia Reining

Abstract: The charge-density response of extended materials is usually dominated by the collective oscillation of electrons, the plasmons. Beyond this feature, however, intriguing many-body effects are observed. They cannot be described by one of the most widely used approaches for the calculation of dielectric functions, which is time-dependent density functional theory (TDDFT) in the adiabatic local densi… ▽ More The charge-density response of extended materials is usually dominated by the collective oscillation of electrons, the plasmons. Beyond this feature, however, intriguing many-body effects are observed. They cannot be described by one of the most widely used approaches for the calculation of dielectric functions, which is time-dependent density functional theory (TDDFT) in the adiabatic local density approximation (ALDA). Here we propose an approximation to the TDDFT exchange-correlation kernel which is non-adiabatic and non-local. It is derived in the homogeneous electron gas and implemented in the real system in a simple mean density approximation. This kernel contains effects that are completely absent in the ALDA; in particular, it correctly describes the double plasmon in the dynamic structure factor of sodium, and it shows the characteristic low-energy peak that appears in systems with low electronic density. It also leads to an overall quantitative improvement of spectra. △ Less

Submitted 19 December, 2018; v1 submitted 3 August, 2017; originally announced August 2017.

Comments: 5 pages, 4 figures, supplementary material: 11 pages, 11 figures

Journal ref: Phys. Rev. Lett. 120, 166402 (2018)

Dynamical local connector approximation for electron addition and removal spectra

Authors: Marco Vanzini, Lucia Reining, Matteo Gatti

Abstract: Realistic calculations of electron addition and removal spectra rely most often on Green's functions and complex, non-local self-energies. We introduce a shortcut to obtain the spectral function directly from a local and frequency-dependent, yet real, potential. We calculate this potential in the homogeneous electron gas (HEG), and we design a connector which prescribes the use of the HEG results… ▽ More Realistic calculations of electron addition and removal spectra rely most often on Green's functions and complex, non-local self-energies. We introduce a shortcut to obtain the spectral function directly from a local and frequency-dependent, yet real, potential. We calculate this potential in the homogeneous electron gas (HEG), and we design a connector which prescribes the use of the HEG results to calculate spectral functions of real materials. Benchmark results for several solids demonstrate the potential of our approach. △ Less

Submitted 9 August, 2017; v1 submitted 8 August, 2017; originally announced August 2017.

Comments: 5 pages, 6 figures, 1 supplemental material

Excitons in van der Waals materials: from monolayer to bulk hexagonal boron nitride

Authors: Jaakko Koskelo, Giorgia Fugallo, Mikko Hakala, Matteo Gatti, Francesco Sottile, Pierluigi Cudazzo

Abstract: We present a general picture of the exciton properties of layered materials in terms of the excitations of their single-layer building blocks. To this end, we derive a model excitonic hamiltonian by drawing an analogy with molecular crystals, which are other prototypical van der Waals materials. We employ this simplified model to analyse in detail the excitation spectrum of hexagonal boron nitride… ▽ More We present a general picture of the exciton properties of layered materials in terms of the excitations of their single-layer building blocks. To this end, we derive a model excitonic hamiltonian by drawing an analogy with molecular crystals, which are other prototypical van der Waals materials. We employ this simplified model to analyse in detail the excitation spectrum of hexagonal boron nitride (hBN) that we have obtained from the {\it ab initio} solution of the many-body Bethe-Salpeter equation as a function of momentum. In this way we identify the character of the lowest-energy excitons in hBN, discuss the effects of the interlayer hopping and the electron-hole exchange interaction on the exciton dispersion, and illustrate the relation between exciton and plasmon excitations in layered materials. △ Less

Submitted 20 December, 2016; originally announced December 2016.

Comments: Accepted for Phys. Rev. B

Journal ref: Phys. Rev. B 95, 035125 (2017)

Electron-energy-loss and time-dependent density functional theory study on the plasmon dispersion in 2H-NbS2

Authors: Pierluigi Cudazzo, Eric Müller, Carsten Habenicht, Matteo Gatti, Helmuth Berger, Martin Knupfer, Angel Rubio, Simo Huotari

Abstract: We examine the experimental and theoretical electron-energy loss spectra in 2$H$-Cu$_{0.2}$NbS$_2$ and find that the 1 eV plasmon in this material does not exhibit the regular positive quadratic plasmon dispersion that would be expected for a normal broad-parabolic-band system. Instead we find a nearly non-dispersing plasmon in the momentum-transfer range $q<0.35$ Å$^{-1}$. We argue that for a sto… ▽ More We examine the experimental and theoretical electron-energy loss spectra in 2$H$-Cu$_{0.2}$NbS$_2$ and find that the 1 eV plasmon in this material does not exhibit the regular positive quadratic plasmon dispersion that would be expected for a normal broad-parabolic-band system. Instead we find a nearly non-dispersing plasmon in the momentum-transfer range $q<0.35$ Å$^{-1}$. We argue that for a stoichiometric pure 2$H$-NbS$_2$ the dispersion relation is expected to have a negative slope as is the case for other transition-metal dichalcogenides. The presence of Cu impurities, required to stabilize the crystal growth, tends to shift the negative plasmon dispersion into a positive one, but the doping level in the current system is small enough to result in a nearly-non-dispersing plasmon. We conclude that a negative-slope plasmon dispersion is not connected with the existence of a charge-density-wave order in transition metal dichalcogenides. △ Less

Submitted 13 January, 2017; v1 submitted 10 March, 2016; originally announced March 2016.

Journal ref: New Journal of Physics 18, 103050 (2016)

Exciton band structure in two-dimensional materials

Authors: Pierluigi Cudazzo, Lorenzo Sponza, Christine Giorgetti, Lucia Reining, Francesco Sottile, Matteo Gatti

Abstract: Low-dimensional materials differ from their bulk counterpart in many respects. In particular, the screening of the Coulomb interaction is strongly reduced, which can have important consequences such as the significant increase of exciton binding energies. In bulk materials the binding energy is used as an indicator in optical spectra to distinguish different kinds of excitons, but this is not poss… ▽ More Low-dimensional materials differ from their bulk counterpart in many respects. In particular, the screening of the Coulomb interaction is strongly reduced, which can have important consequences such as the significant increase of exciton binding energies. In bulk materials the binding energy is used as an indicator in optical spectra to distinguish different kinds of excitons, but this is not possible in low-dimensional materials, where the binding energy is large and comparable in size for excitons of very different localization. Here we demonstrate that the exciton band structure, which can be accessed experimentally, instead provides a powerful way to identify the exciton character. By comparing the ab initio solution of the many-body Bethe-Salpeter equation for graphane and single-layer hexagonal BN, we draw a general picture of the exciton dispersion in two-dimensional materials, highlighting the different role played by the exchange electron-hole interaction and by the electronic band structure. Our interpretation is substantiated by a prediction for phosphorene. △ Less

Submitted 31 December, 2015; originally announced December 2015.

Stable room-temperature ferromagnetic phase at the FeRh(100) surface

Authors: Federico Pressacco, Vojtěch Uhlíř, Matteo Gatti, Azzedine Bendounan, Eric E. Fullerton, Fausto Sirotti

Abstract: Interfaces and low dimensionality are sources of strong modifications of electronic, structural, and magnetic properties of materials. FeRh alloys are an excellent example because of the first-order phase transition taking place at $\sim$400 K from an antiferromagnetic phase at room temperature to a high temperature ferromagnetic one. It is accompanied by a resistance change and volume expansion o… ▽ More Interfaces and low dimensionality are sources of strong modifications of electronic, structural, and magnetic properties of materials. FeRh alloys are an excellent example because of the first-order phase transition taking place at $\sim$400 K from an antiferromagnetic phase at room temperature to a high temperature ferromagnetic one. It is accompanied by a resistance change and volume expansion of about 1\%. We have investigated the electronic and magnetic properties of FeRh(100) epitaxially grown on MgO by combining spectroscopies characterized by different probing depths, namely X-ray magnetic circular dichroism and photoelectron spectroscopy. We thus reveal that the symmetry breaking induced at the Rh-terminated surface stabilizes a surface ferromagnetic layer involving five planes of Fe and Rh atoms in the nominally antiferromagnetic phase at room temperature. First-principles calculations provide a microscopic description of the structural relaxation and the electron spin-density distribution that fully support the experimental findings. △ Less

Submitted 3 June, 2015; originally announced August 2015.

Comments: 5 pages, 3 figures, with supplementary material of 4 pages and 3 figures

Exciton energy-momentum map of hexagonal boron nitride

Authors: Giorgia Fugallo, Matteo Aramini, Jaakko Koskelo, Kenji Watanabe, Takashi Taniguchi, Mikko Hakala, Simo Huotari, Matteo Gatti, Francesco Sottile

Abstract: Understanding and controlling the way excitons propagate in solids is a key for tailoring materials with improved optoelectronic properties. A fundamental step in this direction is the determination of the exciton energy-momentum dispersion. Here, thanks to the solution of the parameter-free Bethe- Salpeter equation (BSE), we draw and explain the exciton energy-momentum map of hexagonal boron nitr… ▽ More Understanding and controlling the way excitons propagate in solids is a key for tailoring materials with improved optoelectronic properties. A fundamental step in this direction is the determination of the exciton energy-momentum dispersion. Here, thanks to the solution of the parameter-free Bethe- Salpeter equation (BSE), we draw and explain the exciton energy-momentum map of hexagonal boron nitride (h-BN) in the first three Brillouin zones. We show that h-BN displays strong excitonic effects not only in the optical spectra at vanishing momentum $\mathbf{q}$, as previously reported, but also at large $\mathbf{q}$. We validate our theoretical predictions by assessing the calculated exciton map by means of an inelastic x-ray scattering (IXS) experiment. Moreover, we solve the discrepancies between previous experimental data and calculations, proving then that the BSE is highly accurate through the whole momentum range. Therefore, these results put forward the combination BSE and IXS as the tool of choice for addressing the exciton dynamics in complex materials. △ Less

Submitted 7 August, 2015; originally announced August 2015.

Journal ref: Phys. Rev. B 92, 165122 (2015)

Symmetry of the Fermi surface and evolution of the electronic structure across the paramagnetic-helimagnetic transition in MnSi/Si(111)

Authors: Alessandro Nicolaou, Matteo Gatti, Elena Magnano, Patrick Le Fèvre, Federica Bondino, François Bertran, Antonio Tejeda, Michèle Sauvage-Simkin, Alina Vlad, Yves Garreau, Alessandro Coati, Nicolas Guérin, Fulvio Parmigiani, Amina Taleb-Ibrahimi

Abstract: MnSi has been extensively studied for five decades, nonetheless detailed information on the Fermi surface (FS) symmetry is still lacking. This missed information prevented from a comprehensive understanding the nature of the magnetic interaction in this material. Here, by performing angle-resolved photoemission spectroscopy on high-quality MnSi films epitaxially grown on Si(111), we unveil the FS… ▽ More MnSi has been extensively studied for five decades, nonetheless detailed information on the Fermi surface (FS) symmetry is still lacking. This missed information prevented from a comprehensive understanding the nature of the magnetic interaction in this material. Here, by performing angle-resolved photoemission spectroscopy on high-quality MnSi films epitaxially grown on Si(111), we unveil the FS symmetry and the evolution of the electronic structure across the paramagnetic-helimagnetic transition at T$_C$ $\sim$ 40 K, along with the appearance of sharp quasiparticle emission below T$_C$. The shape of the resulting FS is found to fulfill robust nesting effects. These effects can be at the origin of strong magnetic fluctuations not accounted for by state-of-art quasiparticle self-consistent GW approximation. From this perspective, the unforeseen quasiparticle damping detected in the paramagnetic phase and relaxing only below T$_C$, along with the persistence of the d-bands splitting well above T$_C$, at odds with a simple Stoner model for itinerant magnetism, open the search for exotic magnetic interactions favored by FS nesting and affecting the quasiparticles lifetime. △ Less

Submitted 20 April, 2015; originally announced April 2015.

Journal ref: Phys. Rev. B 92, 081110 (2015)

Instantaneous band gap collapse in photoexcited monoclinic VO$_2$ due to photocarrier doping

Authors: Daniel Wegkamp, Marc Herzog, Lede Xian, Matteo Gatti, Pierluigi Cudazzo, Christina L. McGahan, Robert E. Marvel, Richard F. Haglund Jr., Angel Rubio, Martin Wolf, Julia Stähler

Abstract: Using femtosecond time-resolved photoelectron spectroscopy we demonstrate that photoexcitation transforms monoclinic VO$_2$ quasi-instantaneously into a metal. Thereby, we exclude an 80 femtosecond structural bottleneck for the photoinduced electronic phase transition of VO$_2$. First-principles many-body perturbation theory calculations reveal a high sensitivity of the VO$_2$ bandgap to variati… ▽ More Using femtosecond time-resolved photoelectron spectroscopy we demonstrate that photoexcitation transforms monoclinic VO$_2$ quasi-instantaneously into a metal. Thereby, we exclude an 80 femtosecond structural bottleneck for the photoinduced electronic phase transition of VO$_2$. First-principles many-body perturbation theory calculations reveal a high sensitivity of the VO$_2$ bandgap to variations of the dynamically screened Coulomb interaction, supporting a fully electronically driven isostructral insulator-to-metal transition. We thus conclude that the ultrafast band structure renormalization is caused by photoexcitation of carriers from localized V 3d valence states, strongly changing the screening \emph{before} significant hot-carrier relaxation or ionic motion has occurred. △ Less

Submitted 10 October, 2014; v1 submitted 14 August, 2014; originally announced August 2014.

Journal ref: Phys. Rev. Lett. 113, 216401 (2014)

Local-field effects on the plasmon dispersion of two-dimensional transition metal dichalcogenides

Authors: Pierluigi Cudazzo, Matteo Gatti, Angel Rubio

Abstract: Two-dimensional transition-metal dichalcogenides (TMDs) are gaining increasing attention as alternative to graphene for their very high potential in optoelectronics applications. Here we consider two prototypical metallic 2D TMDs, NbSe$_2$ and TaS$_2$. Using a first-principles approach, we investigate the properties of the localised intraband $d$ plasmon that cannot be modelled on the basis of t… ▽ More Two-dimensional transition-metal dichalcogenides (TMDs) are gaining increasing attention as alternative to graphene for their very high potential in optoelectronics applications. Here we consider two prototypical metallic 2D TMDs, NbSe$_2$ and TaS$_2$. Using a first-principles approach, we investigate the properties of the localised intraband $d$ plasmon that cannot be modelled on the basis of the homogeneous electron gas. Finally, we discuss the effects of the reduced dimensionality on the plasmon dispersion through the interplay between interband transitions and local-field effects. This result can be exploited to tune the plasmonic properties of these novel 2D materials. △ Less

Submitted 11 December, 2013; originally announced December 2013.

Journal ref: New J. Phys. 15 125005 (2013)

The role of non-local exchange in the electronic structure of correlated oxides

Authors: Federico Iori, Matteo Gatti, Angel Rubio

Abstract: We present a systematic study of the electronic structure of several prototypical correlated transition-metal oxides: VO2, V2O3, Ti2O3, LaTiO3, and YTiO3. In all these materials, in the low-temperature insulating phases the local and semilocal density approximations (LDA and GGA) of density-functional theory yield a metallic Kohn-Sham band structure. Here we show that, without invoking strong-corr… ▽ More We present a systematic study of the electronic structure of several prototypical correlated transition-metal oxides: VO2, V2O3, Ti2O3, LaTiO3, and YTiO3. In all these materials, in the low-temperature insulating phases the local and semilocal density approximations (LDA and GGA) of density-functional theory yield a metallic Kohn-Sham band structure. Here we show that, without invoking strong-correlation effects, the role of non-local exchange is essential to cure the LDA/GGA delocalization error and provide a band-structure description of the electronic properties in qualitative agreement with the experimental photoemission results. To this end, we make use of hybrid functionals that mix a portion of non-local Fock exchange with the local LDA exchange-correlation potential. Finally, we discuss the advantages and the shortcomings of using hybrid functionals for correlated transition-metal oxides. △ Less

Submitted 16 January, 2012; originally announced January 2012.

Comments: submitted

The valence electron photoemission spectrum of semiconductors: ab initio description of multiple satellites

Authors: Matteo Guzzo, Giovanna Lani, Francesco Sottile, Pina Romaniello, Matteo Gatti, Joshua J. Kas, John J. Rehr, Mathieu G. Silly, Fausto Sirotti, Lucia Reining

Abstract: The experimental valence band photoemission spectrum of semiconductors exhibits multiple satellites that cannot be described by the GW approximation for the self-energy in the framework of many-body perturbation theory. Taking silicon as a prototypical example, we compare experimental high energy photoemission spectra with GW calculations and analyze the origin of the GW failure. We then propose a… ▽ More The experimental valence band photoemission spectrum of semiconductors exhibits multiple satellites that cannot be described by the GW approximation for the self-energy in the framework of many-body perturbation theory. Taking silicon as a prototypical example, we compare experimental high energy photoemission spectra with GW calculations and analyze the origin of the GW failure. We then propose an approximation to the functional differential equation that determines the exact one-body Green's function, whose solution has an exponential form. This yields a calculated spectrum, including cross sections, secondary electrons, and an estimate for extrinsic and interference effects, in excellent agreement with experiment. Our result can be recast as a dynamical vertex correction beyond GW, giving hints for further developments. △ Less

Submitted 20 September, 2011; v1 submitted 12 July, 2011; originally announced July 2011.

Comments: 4 pages, 2 figures, accepted for publication on Physical Review Letters on Sept 2, 2011

Design of effective kernels for spectroscopy and molecular transport: time-dependent current-density-functional theory

Authors: Matteo Gatti

Abstract: Time-dependent current-density-functional theory (TDCDFT) provides an in principle exact scheme to calculate efficiently response functions for a very broad range of applications. However, the lack of approximations valid for a range of parameters met in experimental conditions has so far delayed its extensive use in inhomogeneous systems. On the other side, in many-body perturbation theory (MBPT)… ▽ More Time-dependent current-density-functional theory (TDCDFT) provides an in principle exact scheme to calculate efficiently response functions for a very broad range of applications. However, the lack of approximations valid for a range of parameters met in experimental conditions has so far delayed its extensive use in inhomogeneous systems. On the other side, in many-body perturbation theory (MBPT) accurate approximations are available, but at a price of a higher computational cost. In the present work the possibility of combining the advantages of both approaches is exploited. In this way an exact equation for the exchange-correlation kernel of TDCDFT is obtained, which opens the way for a systematic improvement of the approximations adopted in practical applications. Finally, an approximate kernel for an efficient calculation of spectra of solids and molecular conductances is suggested and its validity discussed. △ Less

Submitted 20 December, 2010; originally announced December 2010.

Electronic properties of molecular solids: the peculiar case of solid Picene

Authors: Friedrich Roth, Matteo Gatti, Pierluigi Cudazzo, Mandy Grobosch, Benjamin Mahns, Bernd Büchner, Angel Rubio, Martin Knupfer

Abstract: Recently, a new organic superconductor, K-intercalated Picene with high transition temperatures $T_c$ (up to 18\,K) has been discovered. We have investigated the electronic properties of the undoped relative, solid picene, using a combination of experimental and theoretical methods. Our results provide detailed insight into the occuopied and unoccupied electronic states. Recently, a new organic superconductor, K-intercalated Picene with high transition temperatures $T_c$ (up to 18\,K) has been discovered. We have investigated the electronic properties of the undoped relative, solid picene, using a combination of experimental and theoretical methods. Our results provide detailed insight into the occuopied and unoccupied electronic states. △ Less

Submitted 21 September, 2010; originally announced September 2010.

Journal ref: New J. Phys. 12 (2010) 103036

Sodium: a charge-transfer insulator at high pressures

Authors: Matteo Gatti, Ilya V. Tokatly, Angel Rubio

Abstract: By means of first-principles methods we analyze the optical response of transparent dense sodium as a function of applied pressure. We discover an unusual kind of charge-transfer exciton that proceeds from the interstitial distribution of valence electrons repelled away from the ionic cores by the Coulomb interaction and the Pauli repulsion. The predicted absorption spectrum shows a strong aniso… ▽ More By means of first-principles methods we analyze the optical response of transparent dense sodium as a function of applied pressure. We discover an unusual kind of charge-transfer exciton that proceeds from the interstitial distribution of valence electrons repelled away from the ionic cores by the Coulomb interaction and the Pauli repulsion. The predicted absorption spectrum shows a strong anisotropy with light polarization that just at pressures above the metal-insulator transition manifests as sodium being optically transparent in one direction but reflective in the other. This result provides a key information about the crystal structure of transparent sodium, a new unconventional inorganic electride. △ Less

Submitted 2 March, 2010; originally announced March 2010.

Comments: revtex4, 5+8 pages

Valence band electronic structure of V2O3: identification of V and O bands

Authors: E. Papalazarou, Matteo Gatti, M. Marsi, V. Brouet, F. Iori, Lucia Reining, E. Annese, I. Vobornik, F. Offi, A. Fondacaro, S. Huotari, P. Lacovig, O. Tjernberg, N. B. Brookes, M. Sacchi, P. Metcalf, G. Panaccione

Abstract: We present a comprehensive study of the photon energy dependence of the valence band photoemission yield in the prototype Mott-Hubbard oxide V2O3. The analysis of our experimental results, covering an extended photon energy range (20-6000 eV) and combined with GW calculations, allow us to identify the nature of the orbitals contributing to the total spectral weight at different binding energies,… ▽ More We present a comprehensive study of the photon energy dependence of the valence band photoemission yield in the prototype Mott-Hubbard oxide V2O3. The analysis of our experimental results, covering an extended photon energy range (20-6000 eV) and combined with GW calculations, allow us to identify the nature of the orbitals contributing to the total spectral weight at different binding energies, and in particular to locate the V 4s at about 8 eV binding energy. From this comparative analysis we can conclude that the intensity of the quasiparticle photoemission peak, observed close to the Fermi level in the paramagnetic metallic phase upon increasing photon energy, does not have a significant correlation with the intensity variation of the O 2p and V 3d yield, thus confirming that bulk sensitivity is an essential requirement for the detection of this coherent low energy excitation. △ Less

Submitted 25 March, 2009; originally announced March 2009.

Transforming nonlocality into frequency dependence: a shortcut to spectroscopy

Authors: Matteo Gatti, Valerio Olevano, Lucia Reining, Ilya V. Tokatly

Abstract: Measurable spectra are theoretically very often derived from complicated many-body Green's functions. In this way, one calculates much more information than actually needed. Here we present an in principle exact approach to construct effective potentials and kernels for the direct calculation of electronic spectra. In particular, the potential that yields the spectral function needed to describe… ▽ More Measurable spectra are theoretically very often derived from complicated many-body Green's functions. In this way, one calculates much more information than actually needed. Here we present an in principle exact approach to construct effective potentials and kernels for the direct calculation of electronic spectra. In particular, the potential that yields the spectral function needed to describe photoemission turns out to be dynamical but {\it local} and {\it real}. As example we illustrate this ``photoemission potential'' for sodium and aluminium, modelled as homogeneous electron gas, and discuss in particular its frequency dependence stemming from the nonlocality of the corresponding self-energy. We also show that our approach leads to a very short derivation of a kernel that is known to well describe absorption and energy-loss spectra of a wide range of materials. △ Less

Submitted 14 May, 2007; originally announced May 2007.

Journal ref: Phys. Rev. Lett. 99, 057401 (2007)