elena cannuccia | "Tor Vergata" University of Rome (original) (raw)

Papers by elena cannuccia

Nano Letters, 2012

Strong in-plane bonding and weak Van der Waals inter-planar interactions characterize a large num... more Strong in-plane bonding and weak Van der Waals inter-planar interactions characterize a large number of layered materials, as epitomized by graphite. The advent of graphene (G), individual layers from graphite, and atomic layers isolated from a few other van der Waals bonded layered compounds has enabled the ability to pick, place and stack atomic layers of arbitrary compositions and build unique layered materials, which would be otherwise impossible to synthesize via other known techniques. Here we demonstrate this concept for solids consisting of randomly stacked layers of graphene and hexagonal boron nitride (h-BN). Dispersions of exfoliated h-BN layers and graphene have been prepared by liquid phase exfoliation methods and mixed, in various concentrations, to create artificially stacked h-BN/G solids. These van der Waals stacked hybrid solid materials show interesting electrical, mechanical and optical properties distinctly different from their starting parent layers. From extensive first principle calculations we identify i) a novel approach to control the dipole at the h-BN/G interface by properly sandwiching or sliding layers of h-BN and graphene, ii) a way to inject carriers in graphene upon UV excitations of the Frenkel-like exciton of the h-BN layer(s). Our combined approach could be used to create artificial materials, made from the van der Waals stacking of robust atomic layers of different layered solids with vastly different properties.

Physical Review B, Apr 8, 2015

The accuracy of the many-body perturbation theory GW formalism to calculate electron-phonon coupl... more The accuracy of the many-body perturbation theory GW formalism to calculate electron-phonon coupling matrix elements has been recently demonstrated in the case of a few important systems. However, the related computational costs are high and thus represent strong limitations to its widespread application. In the present study, we explore two less demanding alternatives for the calculation of electron-phonon coupling matrix elements on the many-body perturbation theory level. Namely, we test the accuracy of the static Coulomb-hole plus screened-exchange (COHSEX) approximation and further of the constant screening approach, where variations of the screened Coulomb potential W upon small changes of the atomic positions along the vibrational eigenmodes are neglected. We find this latter approximation to be the most reliable, whereas the static COHSEX ansatz leads to substantial errors. Our conclusions are validated in a few paradigmatic cases: diamond, graphene and the C60 fullerene. These findings open the way for combining the present many-body perturbation approach with efficient linear-response theories.

Second Harmonic Generation (SHG) of single-layer monochalcogenides, such as GaSe and InSe, has be... more Second Harmonic Generation (SHG) of single-layer monochalcogenides, such as GaSe and InSe, has been recently reported [2D Mater. 5 (2018) 025019; J. Am. Chem. Soc. 2015, 137, 79947997] to be extremely strong with respect to bulk and multilayer forms. To clarify the origin of this strong SHG signal, we perform first-principles real-time simulations of linear and non-linear optical properties of these two-dimensional semiconducting materials. The simulations, based on ab-initio many-body theory, accurately treat the electron-hole correlation and capture excitonic effects that are deemed important to correctly predict the optical properties of such systems. We find indeed that, as observed for other 2D systems, the SHG intensity is redistributed at excitonic resonances. The obtained theoretical SHG intensity is an order of magnitude smaller than that reported at the experimental level. This result is in substantial agreement with previously published simulations which neglected the ele...

arXiv: Materials Science, 2013

In this paper we investigate from first principles the effect of the electron-phonon interaction ... more In this paper we investigate from first principles the effect of the electron-phonon interaction in two paradigmatic nanostructures: trans-polyacetylene and polyethylene. We found that the strong electron-phonon interaction leads to the appearance of complex structures in the frequency dependent electronic self-energy. Those structures rule out any quasi-particle picture, and make the adiabatic and static approximations commonly used in the well-established Heine Allen Cardona (HAC) approach inadequate. We propose, instead, a fully ab-initio dynamical formulation of the problem within the Many Body Perturbation Theory framework. The present dynamical theory reveals that the structures appearing in the electronic self-energy are connected to the existence of packets of correlated electron/phonon states. These states appear in the spectral functions even at T=0,KT=0\,KT=0,K, revealing the key role played by the zero point motion effect. We give a physical interpretation of these states by dis...

The European Physical Journal B, 2020

By means of ab initio band structure methods and model Hamiltonians we investigate the electronic... more By means of ab initio band structure methods and model Hamiltonians we investigate the electronic, spin and topological properties of four monopnictides crystallizing in bct structure. We show that the Weyl bands around a WP W1 or W2 possess a strong anisotropy and tilt of the accompanying Dirac cones. These effects are larger for W2 nodes than for W1 ones. The node tilts and positions in energy space significantly influence the DOS of single-particle Weyl excitations. The node anisotropies destroy the conventional picture of (anti)parallel spin and wave vector of a Weyl fermion. This also holds for the Berry curvature around a node, while the monopole charges are independent as integrated quantities. The pairing of the nodes strongly modifies the spin texture and the Berry curvature for wave vectors in between the two nodes. Spin components may change their orientation. Integrals over planes perpendicular to the connection line yield finite Zak phases and winding numbers for planes...

Physical Review Materials, 2020

Direct observation of temperature dependence of individual bands of semiconductors for a wide tem... more Direct observation of temperature dependence of individual bands of semiconductors for a wide temperature region is not straightforward, in particular. However, this fundamental property is a prerequisite in understanding the electron-phonon coupling of semiconductors. Here we apply ab initio many body perturbation theory to the electron-phonon coupling on hexagonal silicon carbide (SiC) crystals and determine the temperature dependence of the bands. We find a significant electron-phonon renormalization of the band gap at 0 K. Both the conduction and valence bands shift at elevated temperatures exhibiting a different behavior. We compare our theoretical results with the observed thermal evolution of SiC band edges, and discuss our findings in the light of high temperature SiC electronics and defect qubits operation.

Physical Review Materials, 2019

Electronic properties and lattice dynamics of bulk ε-GaSe and one, two and three tetralayers GaSe... more Electronic properties and lattice dynamics of bulk ε-GaSe and one, two and three tetralayers GaSe are investigated by means of density functional and density functional perturbation theory. The fewtetralayers systems are semiconductors with an indirect nature of the fundamental band gap and a Mexican-hat-shape is observed at the top of the valence band. The phonon branches analysis reveals the dynamical stability for all systems considered together with the LO-TO splitting breakdown in two-dimensional systems. In-plane (E) and out-of-plane (A) zone-center lattice vibrations dominate the Raman and IR spectra.

Physical Review Materials, 2019

Journal of Physics: Condensed Matter, 2019

yambo is an open source project aimed at studying excited state properties of condensed matter sy... more yambo is an open source project aimed at studying excited state properties of condensed matter systems from first principles using many-body methods. As input, yambo requires ground state electronic structure data as computed by density functional theory codes such as Quantum ESPRESSO and Abinit. yambo’s capabilities include the calculation of linear response quantities (both independent-particle and including electron–hole interactions), quasi-particle corrections based on the GW formalism, optical absorption, and other spectroscopic quantities. Here we describe recent developments ranging from the inclusion of important but oft-neglected physical effects such as electron–phonon interactions to the implementation of a real-time propagation scheme for simulating linear and non-linear optical properties. Improvements to numerical algorithms and the user interface are outlined. Particular emphasis is given to the new and efficient parallel structure that makes it possible to exploit m...

Physical Review B, 2017

Linear and nonlinear optical properties of low dimensional nanostructures have attracted a large ... more Linear and nonlinear optical properties of low dimensional nanostructures have attracted a large interest in the scientific community as tools to probe the strong confinement of the electrons and for possible applications in optoelectronic devices. In particular it has been shown that the linear optical response of carbon nanotubes [Science 308, 838 (2005)] and graphene nanoribbons [Nat. Comm. 5, 4253 (2014)] is dominated by bounded electron-hole pairs, the excitons. The role of excitons in linear response has been widely studied, but still little is known on their effect on nonlinear susceptibilities. Using a recently developed methodology [Phys. Rev. B 88, 235113 (2013)] based on well-established ab-initio many-body perturbation theory approaches, we find that quasiparticle shifts and excitonic effects significantly modify the third-harmonic generation in carbon nanotubes and graphene nanoribbons. For both systems the net effect of many-body effects is to reduce the intensity of the main peak in the independent particle spectrum and redistribute the spectral weight among several excitonic resonances.

Acta Biophysica Romana 2008, Apr 10, 2008

The optical properties of flavin mononucleotide (FMN) in gas phase, in the oxidized, reduced and ... more The optical properties of flavin mononucleotide (FMN) in gas phase, in the oxidized, reduced and anionic form, are studied by ab-initio methods. The optical spectra were obtained within the density functional theory (DFT) using BLYP functionals and within the time dependent density functional theory (TDDFT) using both the local-density (LDA) and the generalized gradient (GGA) approximation. At the DFT level we verified that only the cromophore is optically active, at least within the range of energy of our interest. At TDDFT level the optical spectra showed a qualitative good agreement with the experimental ones. An analysis of states involved in the low energy structure of the spectra was carried out within Casida linear response.

Nature communications, Jan 22, 2016

The development of theories and methods devoted to the accurate calculation of the electronic qua... more The development of theories and methods devoted to the accurate calculation of the electronic quasi-particle states and levels of molecules, clusters and solids is of prime importance to interpret the experimental data. These quantum systems are often modelled by using the Born-Oppenheimer approximation where the coupling between the electrons and vibrational modes is not fully taken into account, and the electrons are treated as pure quasi-particles. Here, we show that in small diamond cages, called diamondoids, the electron-vibration coupling leads to the breakdown of the electron quasi-particle picture. More importantly, we demonstrate that the strong electron-vibration coupling is essential to properly describe the overall lineshape of the experimental photoemission spectrum. This cannot be obtained by methods within Born-Oppenheimer approximation. Moreover, we deduce a link between the vibronic states found by our many-body perturbation theory approach and the well-known Jahn-T...

Physical Chemistry Chemical Physics, 2015

By using a real-time approach based on Green's function theory we predict a strong second-har... more By using a real-time approach based on Green's function theory we predict a strong second-harmonic generation (SHG) for frequencies at which Ti:sapphire laser operates and for which the materials are transparent.

First-principles electronic-structure codes are in constant development and evolution to adapt wi... more First-principles electronic-structure codes are in constant development and evolution to adapt with increasing computational capabilities and simulation sizes. Hence, verification and validation of codes, as well as new theoretical methods, are of utmost importance if one wants to provide reliable results. In this work we present a rigorous and careful study of all the quantities that enters into the calculation of the zero point motion renormalization of the direct band gap of diamond due to electron-phonon coupling. This study has been done within the Allen-Heine-Cardona (AHC) formalism [1] as implemented into Abinit [2] and Yambo [3] on top of Quantum Espresso [4]. In this work we aim at quantifying the agreement between the codes for the different quantities of interest. This study shows that one can get less than 10−7Ha/at10^{-7}Ha/at10−7Ha/at differences on the total energy, 0.01 cm$^{-1}$ on the phonon frequencies, 0.005 on the electron-phonon matrix elements and less than 1 meV on the zero-point-motion renormalization. At the LDA level, the converged direct band gap renormalization in diamond due to electron-phonon coupling in the AHC formalism is -408 meV (reduction of the band gap). [1] P. B. Allen and M. Cardona, Phys. Rev. B 24, 7479 (1981). [2] X. Gonze et al, Computer Physics Communications 180, 2582 (2009). [3] E. Cannuccia and A. Marini, Phys. Rev. Lett. 107, 255501 (2011). [4] P. Giannozzi et al., Journal of Physics: Condensed Matter 21, 395502 (2009).

Verification and validation of codes, as well as new theoretical methods, are of utmost importanc... more Verification and validation of codes, as well as new theoretical methods, are of utmost importance if one wants to provide reliable results. In this work we present a rigorous and careful study of all the quantities that enters into the calculation of the zero-point motion renormalization of the direct band gap of diamond due to electron-phonon coupling. This study has been done within the Allen-Heine-Cardona (AHC) formalism as implemented into Abinit and Yambo on top of Quantum Espresso. In this work we aim at quantifying the agreement between the codes for the different quantities of interest. This study shows that one can get less than 10−5Ha/at10^{-5}Ha/at10−5Ha/at differences on the total energy, 0.07 cm$^{-1}$ on the phonon frequencies,0.5% on the electron-phonon matrix elements and less than 4 meV on the zero-point motion renormalization. At the LDA level, the converged direct bandgap renormalization in diamond due to electron-phonon coupling in the AHC formalism is -409 meV (reduction of the bandgap)[1]. [1] S. Ponce et al., arXiv:1309.0729 [cond-mat.mtrl-sci] and submitted for publication in Comput. Mat. Science (2013).

Physical Review Letters, 2011

Physical Review B, 2014

We combine the effect of the electron-electron and electron-phonon interactions to study the elec... more We combine the effect of the electron-electron and electron-phonon interactions to study the electronic and optical properties of zb-GaN. We show that only by treating the two effects at the same time it is possible to obtain an unprecedented agreement of the zero and finite-temperature electronic gaps and absorption spectra with the experimental results. Compared to the state-ofthe-art results our calculations predict a large effect on the main absorption peak position and width as well as on the overall absorption lineshape. These important modifications are traced back to the combined electron-phonon damping mechanism and non uniform GW level corrections. Our results demonstrate the importance of treating on equal footing the electron and phonon mediated correlation effects to obtain an accurate description of the III-nitrides group physical properties.

physica status solidi (c), 2008

ABSTRACT

Nano Letters, 2012

Strong in-plane bonding and weak Van der Waals inter-planar interactions characterize a large num... more Strong in-plane bonding and weak Van der Waals inter-planar interactions characterize a large number of layered materials, as epitomized by graphite. The advent of graphene (G), individual layers from graphite, and atomic layers isolated from a few other van der Waals bonded layered compounds has enabled the ability to pick, place and stack atomic layers of arbitrary compositions and build unique layered materials, which would be otherwise impossible to synthesize via other known techniques. Here we demonstrate this concept for solids consisting of randomly stacked layers of graphene and hexagonal boron nitride (h-BN). Dispersions of exfoliated h-BN layers and graphene have been prepared by liquid phase exfoliation methods and mixed, in various concentrations, to create artificially stacked h-BN/G solids. These van der Waals stacked hybrid solid materials show interesting electrical, mechanical and optical properties distinctly different from their starting parent layers. From extensive first principle calculations we identify i) a novel approach to control the dipole at the h-BN/G interface by properly sandwiching or sliding layers of h-BN and graphene, ii) a way to inject carriers in graphene upon UV excitations of the Frenkel-like exciton of the h-BN layer(s). Our combined approach could be used to create artificial materials, made from the van der Waals stacking of robust atomic layers of different layered solids with vastly different properties.

Physical Review B, Apr 8, 2015

The accuracy of the many-body perturbation theory GW formalism to calculate electron-phonon coupl... more The accuracy of the many-body perturbation theory GW formalism to calculate electron-phonon coupling matrix elements has been recently demonstrated in the case of a few important systems. However, the related computational costs are high and thus represent strong limitations to its widespread application. In the present study, we explore two less demanding alternatives for the calculation of electron-phonon coupling matrix elements on the many-body perturbation theory level. Namely, we test the accuracy of the static Coulomb-hole plus screened-exchange (COHSEX) approximation and further of the constant screening approach, where variations of the screened Coulomb potential W upon small changes of the atomic positions along the vibrational eigenmodes are neglected. We find this latter approximation to be the most reliable, whereas the static COHSEX ansatz leads to substantial errors. Our conclusions are validated in a few paradigmatic cases: diamond, graphene and the C60 fullerene. These findings open the way for combining the present many-body perturbation approach with efficient linear-response theories.

Second Harmonic Generation (SHG) of single-layer monochalcogenides, such as GaSe and InSe, has be... more Second Harmonic Generation (SHG) of single-layer monochalcogenides, such as GaSe and InSe, has been recently reported [2D Mater. 5 (2018) 025019; J. Am. Chem. Soc. 2015, 137, 79947997] to be extremely strong with respect to bulk and multilayer forms. To clarify the origin of this strong SHG signal, we perform first-principles real-time simulations of linear and non-linear optical properties of these two-dimensional semiconducting materials. The simulations, based on ab-initio many-body theory, accurately treat the electron-hole correlation and capture excitonic effects that are deemed important to correctly predict the optical properties of such systems. We find indeed that, as observed for other 2D systems, the SHG intensity is redistributed at excitonic resonances. The obtained theoretical SHG intensity is an order of magnitude smaller than that reported at the experimental level. This result is in substantial agreement with previously published simulations which neglected the ele...

arXiv: Materials Science, 2013

In this paper we investigate from first principles the effect of the electron-phonon interaction ... more In this paper we investigate from first principles the effect of the electron-phonon interaction in two paradigmatic nanostructures: trans-polyacetylene and polyethylene. We found that the strong electron-phonon interaction leads to the appearance of complex structures in the frequency dependent electronic self-energy. Those structures rule out any quasi-particle picture, and make the adiabatic and static approximations commonly used in the well-established Heine Allen Cardona (HAC) approach inadequate. We propose, instead, a fully ab-initio dynamical formulation of the problem within the Many Body Perturbation Theory framework. The present dynamical theory reveals that the structures appearing in the electronic self-energy are connected to the existence of packets of correlated electron/phonon states. These states appear in the spectral functions even at T=0,KT=0\,KT=0,K, revealing the key role played by the zero point motion effect. We give a physical interpretation of these states by dis...

The European Physical Journal B, 2020

By means of ab initio band structure methods and model Hamiltonians we investigate the electronic... more By means of ab initio band structure methods and model Hamiltonians we investigate the electronic, spin and topological properties of four monopnictides crystallizing in bct structure. We show that the Weyl bands around a WP W1 or W2 possess a strong anisotropy and tilt of the accompanying Dirac cones. These effects are larger for W2 nodes than for W1 ones. The node tilts and positions in energy space significantly influence the DOS of single-particle Weyl excitations. The node anisotropies destroy the conventional picture of (anti)parallel spin and wave vector of a Weyl fermion. This also holds for the Berry curvature around a node, while the monopole charges are independent as integrated quantities. The pairing of the nodes strongly modifies the spin texture and the Berry curvature for wave vectors in between the two nodes. Spin components may change their orientation. Integrals over planes perpendicular to the connection line yield finite Zak phases and winding numbers for planes...

Physical Review Materials, 2020

Direct observation of temperature dependence of individual bands of semiconductors for a wide tem... more Direct observation of temperature dependence of individual bands of semiconductors for a wide temperature region is not straightforward, in particular. However, this fundamental property is a prerequisite in understanding the electron-phonon coupling of semiconductors. Here we apply ab initio many body perturbation theory to the electron-phonon coupling on hexagonal silicon carbide (SiC) crystals and determine the temperature dependence of the bands. We find a significant electron-phonon renormalization of the band gap at 0 K. Both the conduction and valence bands shift at elevated temperatures exhibiting a different behavior. We compare our theoretical results with the observed thermal evolution of SiC band edges, and discuss our findings in the light of high temperature SiC electronics and defect qubits operation.

Physical Review Materials, 2019

Electronic properties and lattice dynamics of bulk ε-GaSe and one, two and three tetralayers GaSe... more Electronic properties and lattice dynamics of bulk ε-GaSe and one, two and three tetralayers GaSe are investigated by means of density functional and density functional perturbation theory. The fewtetralayers systems are semiconductors with an indirect nature of the fundamental band gap and a Mexican-hat-shape is observed at the top of the valence band. The phonon branches analysis reveals the dynamical stability for all systems considered together with the LO-TO splitting breakdown in two-dimensional systems. In-plane (E) and out-of-plane (A) zone-center lattice vibrations dominate the Raman and IR spectra.

Physical Review Materials, 2019

Journal of Physics: Condensed Matter, 2019

yambo is an open source project aimed at studying excited state properties of condensed matter sy... more yambo is an open source project aimed at studying excited state properties of condensed matter systems from first principles using many-body methods. As input, yambo requires ground state electronic structure data as computed by density functional theory codes such as Quantum ESPRESSO and Abinit. yambo’s capabilities include the calculation of linear response quantities (both independent-particle and including electron–hole interactions), quasi-particle corrections based on the GW formalism, optical absorption, and other spectroscopic quantities. Here we describe recent developments ranging from the inclusion of important but oft-neglected physical effects such as electron–phonon interactions to the implementation of a real-time propagation scheme for simulating linear and non-linear optical properties. Improvements to numerical algorithms and the user interface are outlined. Particular emphasis is given to the new and efficient parallel structure that makes it possible to exploit m...

Physical Review B, 2017

Linear and nonlinear optical properties of low dimensional nanostructures have attracted a large ... more Linear and nonlinear optical properties of low dimensional nanostructures have attracted a large interest in the scientific community as tools to probe the strong confinement of the electrons and for possible applications in optoelectronic devices. In particular it has been shown that the linear optical response of carbon nanotubes [Science 308, 838 (2005)] and graphene nanoribbons [Nat. Comm. 5, 4253 (2014)] is dominated by bounded electron-hole pairs, the excitons. The role of excitons in linear response has been widely studied, but still little is known on their effect on nonlinear susceptibilities. Using a recently developed methodology [Phys. Rev. B 88, 235113 (2013)] based on well-established ab-initio many-body perturbation theory approaches, we find that quasiparticle shifts and excitonic effects significantly modify the third-harmonic generation in carbon nanotubes and graphene nanoribbons. For both systems the net effect of many-body effects is to reduce the intensity of the main peak in the independent particle spectrum and redistribute the spectral weight among several excitonic resonances.

Acta Biophysica Romana 2008, Apr 10, 2008

The optical properties of flavin mononucleotide (FMN) in gas phase, in the oxidized, reduced and ... more The optical properties of flavin mononucleotide (FMN) in gas phase, in the oxidized, reduced and anionic form, are studied by ab-initio methods. The optical spectra were obtained within the density functional theory (DFT) using BLYP functionals and within the time dependent density functional theory (TDDFT) using both the local-density (LDA) and the generalized gradient (GGA) approximation. At the DFT level we verified that only the cromophore is optically active, at least within the range of energy of our interest. At TDDFT level the optical spectra showed a qualitative good agreement with the experimental ones. An analysis of states involved in the low energy structure of the spectra was carried out within Casida linear response.

Nature communications, Jan 22, 2016

The development of theories and methods devoted to the accurate calculation of the electronic qua... more The development of theories and methods devoted to the accurate calculation of the electronic quasi-particle states and levels of molecules, clusters and solids is of prime importance to interpret the experimental data. These quantum systems are often modelled by using the Born-Oppenheimer approximation where the coupling between the electrons and vibrational modes is not fully taken into account, and the electrons are treated as pure quasi-particles. Here, we show that in small diamond cages, called diamondoids, the electron-vibration coupling leads to the breakdown of the electron quasi-particle picture. More importantly, we demonstrate that the strong electron-vibration coupling is essential to properly describe the overall lineshape of the experimental photoemission spectrum. This cannot be obtained by methods within Born-Oppenheimer approximation. Moreover, we deduce a link between the vibronic states found by our many-body perturbation theory approach and the well-known Jahn-T...

Physical Chemistry Chemical Physics, 2015

By using a real-time approach based on Green's function theory we predict a strong second-har... more By using a real-time approach based on Green's function theory we predict a strong second-harmonic generation (SHG) for frequencies at which Ti:sapphire laser operates and for which the materials are transparent.

First-principles electronic-structure codes are in constant development and evolution to adapt wi... more First-principles electronic-structure codes are in constant development and evolution to adapt with increasing computational capabilities and simulation sizes. Hence, verification and validation of codes, as well as new theoretical methods, are of utmost importance if one wants to provide reliable results. In this work we present a rigorous and careful study of all the quantities that enters into the calculation of the zero point motion renormalization of the direct band gap of diamond due to electron-phonon coupling. This study has been done within the Allen-Heine-Cardona (AHC) formalism [1] as implemented into Abinit [2] and Yambo [3] on top of Quantum Espresso [4]. In this work we aim at quantifying the agreement between the codes for the different quantities of interest. This study shows that one can get less than 10−7Ha/at10^{-7}Ha/at10−7Ha/at differences on the total energy, 0.01 cm$^{-1}$ on the phonon frequencies, 0.005 on the electron-phonon matrix elements and less than 1 meV on the zero-point-motion renormalization. At the LDA level, the converged direct band gap renormalization in diamond due to electron-phonon coupling in the AHC formalism is -408 meV (reduction of the band gap). [1] P. B. Allen and M. Cardona, Phys. Rev. B 24, 7479 (1981). [2] X. Gonze et al, Computer Physics Communications 180, 2582 (2009). [3] E. Cannuccia and A. Marini, Phys. Rev. Lett. 107, 255501 (2011). [4] P. Giannozzi et al., Journal of Physics: Condensed Matter 21, 395502 (2009).

Verification and validation of codes, as well as new theoretical methods, are of utmost importanc... more Verification and validation of codes, as well as new theoretical methods, are of utmost importance if one wants to provide reliable results. In this work we present a rigorous and careful study of all the quantities that enters into the calculation of the zero-point motion renormalization of the direct band gap of diamond due to electron-phonon coupling. This study has been done within the Allen-Heine-Cardona (AHC) formalism as implemented into Abinit and Yambo on top of Quantum Espresso. In this work we aim at quantifying the agreement between the codes for the different quantities of interest. This study shows that one can get less than 10−5Ha/at10^{-5}Ha/at10−5Ha/at differences on the total energy, 0.07 cm$^{-1}$ on the phonon frequencies,0.5% on the electron-phonon matrix elements and less than 4 meV on the zero-point motion renormalization. At the LDA level, the converged direct bandgap renormalization in diamond due to electron-phonon coupling in the AHC formalism is -409 meV (reduction of the bandgap)[1]. [1] S. Ponce et al., arXiv:1309.0729 [cond-mat.mtrl-sci] and submitted for publication in Comput. Mat. Science (2013).

Physical Review Letters, 2011

Physical Review B, 2014

We combine the effect of the electron-electron and electron-phonon interactions to study the elec... more We combine the effect of the electron-electron and electron-phonon interactions to study the electronic and optical properties of zb-GaN. We show that only by treating the two effects at the same time it is possible to obtain an unprecedented agreement of the zero and finite-temperature electronic gaps and absorption spectra with the experimental results. Compared to the state-ofthe-art results our calculations predict a large effect on the main absorption peak position and width as well as on the overall absorption lineshape. These important modifications are traced back to the combined electron-phonon damping mechanism and non uniform GW level corrections. Our results demonstrate the importance of treating on equal footing the electron and phonon mediated correlation effects to obtain an accurate description of the III-nitrides group physical properties.

physica status solidi (c), 2008

ABSTRACT