Publications de Alfredo Pasquarello

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Using a combined experimental and theoretical approach, we address C 1⁢𝑠 core-level shifts in amorphous carbon. Experimental results are obtained by x-ray photoelectron spectroscopy (XPS) and electron-energy-loss spectroscopy (EELS) on thin-film samples of different atomic density, obtained by a pulsed-laser deposition growth process. The XPS spectra are deconvoluted into two contributions, which are attributed to sp2- and sp3-hybridized atoms, respectively, separated by 0.9 eV, independent of atomic densit

We investigate the interaction of Ti atoms with thin films made of 3,4,9,10 perylenetetracarboxylic dianhydride (PTCDA) molecules by means of self-consistent electronic structure calculations within a generalized gradient approximated density-functional theory framework. Following experimental suggestions, we model the thin films in terms of the bulk crystallographic structure of PTCDA. We fully optimize the atomic PTCDA structures in the presence of Ti impurities by local minimizations of the electronic to

We propose monolayer epitaxial graphene and hexagonal boron nitride (h-BN) as ultimate thickness covalent spacers for magnetoresistive junctions. Using a first-principles approach, we investigate the structural, magnetic, and spin transport properties of s ...

Models of amorphous (HfO2)x(SiO2)(1), for varying hafnium, content x are generated by ab initio molecular dynamics. The structural properties are analyzed in terms of pair distribution functions from which typical bond lengths and coordination numbers are extracted. Electronic band gaps are calculated with a hybrid density functional and are found to decrease in a nonlinear way with increasing x, in accord with experimental observations. Static dielectric constants are evaluated through the application of a

The electron self-interaction problem in density functional theory affects the accurate modeling of polarons, particularly their localization and formation energy. Charged and neutral density functional formulations have been developed to address this issue, yet their relationship remains unclear. Here, we demonstrate their equivalence in treating the many-body self-interaction of the polaron state. In particular, we connect with each other piecewise-linear functionals based on adding an extra charge to the

We carry out ion scattering simulations to investigate the nature of the transition region at the Si(100)-SiO2 interface. Ion scattering experiments performed in the channeling geometry provide us with a genuine interfacial property, the excess Si yield, resulting from distortions in the Si substrate and from Si atoms in intermediate oxidation states. To interpret the ion scattering data, we first generate a series of model structures for the interface by applying sequentially classical molecular dynamics a

We introduce a scheme for the calculation of band offsets and defect energy levels at semiconductor-oxide interfaces. Our scheme is based on the use of realistic atomistic models of the interface structure and of hybrid functionals for the evaluation of th ...

We investigate structural, electronic, dynamical, and dielectric properties of zircon (ZrSiO4) within density-functional theory. The atomic structure is fully relaxed and the structural parameters are found to differ by less than 1.5% from the experimental data. The associated electronic band structure and density of states are also presented. Using density-functional perturbation theory, we obtain the phonon frequencies at the center of the Brillouin zone, the Born effective charge tensors, and the dielect

We perform first-principles molecular dynamics of liquid oxygen in which the magnetic structure evolves according to a generalized density-functional scheme allowing for noncollinear spin configurations. We investigate both structural correlations between the orientations of the molecular axes and magnetic correlations between the orientations of the molecular magnetic moments, demonstrating a clear relation between the local molecular configuration and the relative magnetic arrangement. The nuclear structu

We address the rate of O-2 diffusion through the oxide layer at Si-SiO2 interfaces using an atomic-scale approach. In particular, we investigate the combined effect of a percolative diffusion mechanism and of a dense oxide layer located close to the silicon substrate. We first extend our atomic-scale description of O-2 diffusion in amorphous SiO2 to the case of a densified oxide. This yields an activation energy which compares well with the experimental result. Next, we investigate the dependence of the O-2

The structural and electronic properties of Ga-As suboxide representative of the transition region at the GaAs/oxide interface are studied through density functional calculations. Two amorphous models generated by quenches from the melt are taken under con ...

We study the reactivity and the energy level associated with the reduction of the H2O+ radical cation in liquid water by combining ab initio molecular dynamics simulations at the hybrid functional level, a grand-canonical formulation of solutes in aqueous ...

Through first-principles simulation methods, we assign the origin of Fermi-level pinning at GaAs surfaces and interfaces to the bistability between the As-As dimer and two As dangling bonds, which transform into each other upon charge trapping. This defect ...

We introduce a density-functional perturbational scheme based on ultrasoft pseudopotentials for calculating dielectric tensors of periodic systems. We obtain a variational functional for the second-order derivative of the energy with respect to an electric field. Our scheme makes use of the correspondence between all-electron and pseudo-wave-functions introduced in the projector augmented wave method. While we here specifically focus on ultrasoft pseudopotentials, our scheme also covers the case of norm-con

We investigate how various treatments of exact exchange affect defect charge transition levels and band edges in hybrid functional schemes for a variety of systems. We distinguish the effects of long-range vs short-range exchange and of local vs nonlocal e ...

We use piecewise-linear functionals to study the polaron energy landscape and hopping rates in beta-Ga2O3, which we adopt as an example of an anisotropic material hosting multiple polaronic states. We illustrate various functionals for polaron localization ...

Using a first-principles density functional approach, we calculate the first-order Raman intensities of alpha -quartz. The dynamical charge tensors, vibrational frequencies and eigenmodes, and polarizability tensors are obtained within a perturbational approach. We calculate Raman intensities by evaluating the variation of the polarizability tensors for finite displacements of the atoms. Calculated intensities agree well with experimental data, showing an average error of 13% for relative intensities. Using

We investigate the synergy occurring in photocatalytic water splitting between the metal-organic framework MIL-125-NH(2)and two co-catalysts, namely NiO and Ni2P, by calculating their band edge alignment with respect to the redox levels of liquid water. Fo ...

The electronic properties of the oxygen vacancy and interstitial in amorphous Al2O3 are studied via ab initio molecular dynamics simulations and hybrid functional calculations. Our results indicate that these defects do not occur in amorphous Al2O3, due to ...

Using density-functional theory, we compute the optical and static dielectric constants for a set of Zr silicates modeled by various SiO2 crystals, with Zr atoms substitutional to Si, and by an amorphous structure. We then derive a microscopic scheme that relates the dielectric constants to structural units centered on Si and Zr atoms through the definition of characteristic parameters. Applied to amorphous (ZrO2)(x)(SiO2)(1-x), these schemes describe the observed dependence of the dielectric constants on t

Alfredo Pasquarello

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𝑠⁢𝑝²⁢/𝑠⁢𝑝³ hybridization ratio in amorphous carbon from C 1⁢𝑠 core-level shifts: X-ray photoelectron spectroscopy and first-principles calculation

First-principles electronic structure study of Ti-PTCDA contacts

Magnetoresistive junctions based on epitaxial graphene and hexagonal boron nitride

Amorphous hafnium silicates: structural, electronic and dielectric properties

Equivalence of charged and neutral density functional formulations for correcting the many-body self-interaction of polarons

Ion scattering simulations of the Si(100)-SiO2 interface

First principles investigation of defect energy levels at semiconductor-oxide interfaces: Oxygen vacancies and hydrogen interstitials in the Si-SiO2-HfO2 stack

First-principles study of structural, electronic, dynamical, and dielectric properties of zircon

Noncollinear magnetism in liquid oxygen: A first-principles molecular dynamics study

Dependence of the O-2 diffusion rate on oxide thickness during silicon oxidation

Interfacial Ga-As suboxide: Structural and electronic properties

Reactivity and energy level of a localized hole in liquid water

Origin of Fermi-level pinning at GaAs surfaces and interfaces

Density-functional perturbational theory for dielectric tensors in the ultrasoft pseudopotential scheme

Alignment of defect levels and band edges through hybrid functionals: Effect of screening in the exchange term

Polaron hopping through piecewise-linear functionals

Raman scattering intensities in alpha-quartz: A first-principles investigation

Unraveling the synergy between metal-organic frameworks and co-catalysts in photocatalytic water splitting

Oxygen defects in amorphous Al2O3: A hybrid functional study

Dielectric constants of Zr silicates: A first-principles study