Alfredo Pasquarello

QUANTUM ESPRESSO: a modular and open-source software project for quantum simulations of materials

QUANTUM ESPRESSO is an integrated suite of computer codes for electronic-structure calculations and materials modeling, based on density-functional theory, plane waves, and pseudopotentials (norm-conserving, ultrasoft, and projector-augmented wave). The acronym ESPRESSO stands for opEn Source Package for Research in Electronic Structure, Simulation, and Optimization. It is freely available to researchers around the world under the terms of the GNU General Public License. QUANTUM ESPRESSO builds upon newly-restructured electronic-structure codes that have been developed and tested by some of the original authors of novel electronic-structure algorithms and applied in the last twenty years by some of the leading materials modeling groups worldwide. Innovation and efficiency are still its main focus, with special attention paid to massively parallel architectures, and a great effort being devoted to user friendliness. QUANTUM ESPRESSO is evolving towards a distribution of independent and interoperable codes in the spirit of an open-source project, where researchers active in the field of electronic-structure calculations are encouraged to participate in the project by contributing their own codes or by implementing their own ideas into existing codes.

Oxygen vacancy in monoclinic HfO2: A consistent interpretation of trap assisted conduction, direct electron injection, and optical absorption experiments

The authors calculate energy levels associated with the oxygen vacancy in monoclinic HfO2 using a hybrid density functional which accurately reproduces the experimental band gap. The most stable charge states are obtained for varying Fermi level in the HfO2 band gap. To compare with measured defect levels, they determine total energy differences specific to the considered experiment. Their results show that the oxygen vacancy can consistently account for the defect levels observed in (Poole-Frenkel-type) trap assisted conduction, direct electron injection, and optical absorption experiments. (c) 2006 American Institute of Physics.

Interface structure between silicon and its oxide by first-principles molecular dynamics

The requirement for increasingly thin (<50 Å) insulating oxide layers in silicon-based electronic devices highlights the importance of characterizing the Si–SiO2 interface structure at the atomic scale. Such a characterization relies to a large extent on an understanding of the atomic-scale mechanisms that govern the oxidation process. The widely used Deal–Grove model invokes a two-step process in which oxygen first diffuses through the amorphous oxide network before attacking the silicon substrate, resulting in the formation of new oxide at the buried interface. But it remains unclear how such a process can yield the observed near-perfect interface. Here we use first-principles molecular dynamics to generate a model interface structure by simulating the oxidation of three silicon layers. The resulting structure reveals an unexpected excess of silicon atoms at the interface, yet shows no bonding defects. Changes in the bonding network near the interface occur during the simulation via transient exchange events wherein oxygen atoms are momentarily bonded to three silicon atoms — this mechanism enables the interface to evolve without leaving dangling bonds.

Band offsets at semiconductor-oxide interfaces from hybrid density-functional calculations.

Band offsets at semiconductor-oxide interfaces are determined through a scheme based on hybrid density functionals, which incorporate a fraction alpha of Hartree-Fock exchange. For each bulk component, the fraction alpha is tuned to reproduce the experimental band gap, and the conduction and valence band edges are then located with respect to a reference level. The lineup of the bulk reference levels is determined through an interface calculation, and shown to be almost independent of the fraction alpha. Application of this scheme to the Si-SiO2, SiC-SiO2, and Si-HfO2 interfaces yields excellent agreement with experiment.

Defect energy levels in density functional calculations: alignment and band gap problem.

For materials of varying band gap, we compare energy levels of atomically localized defects calculated within a semilocal and a hybrid density-functional scheme. Since the latter scheme partially relieves the band gap problem, our study describes how calculated defect levels shift when the band gap approaches the experimental value. When suitably aligned, defect levels obtained from total-energy differences correspond closely, showing average shifts of at most 0.2 eV irrespective of band gap. Systematic deviations from ideal alignment increase with the extent of the defect wave function. A guideline for comparing calculated and experimental defect levels is provided.

Migration of oxygen vacancy in HfO2 and across the HfO2∕SiO2 interface: A first-principles investigation

Oxygen vacancy migration is studied in monoclinic HfO2 and across its interface with SiO2 through density functional calculations. In HfO2, long-range diffusion shows activation barriers of 2.4 and 0.7eV for the neutral and doubly positively charged vacancy, respectively. In the latter case, the migration preferentially occurs along one-dimensional pathways. A HfO2∕SiO2 interface model is constructed to address O vacancy migration across high-κ gate stacks. The vacancy is shown to stabilize in its neutral charge state upon entering the SiO2 layer.

Ring Currents in Icosahedral C60

A new formulation of the current within the London approximation allows the calculation of ring currents in topologically complex molecules. Application of this theory to C60 demonstrates the existence of remarkable π electron ring currents. Paramagnetic currents, in size comparable to the ones in benzene, flow within the pentagons, whereas weaker diamagnetic currents flow all around the C.60 molecule. The overall vanishing ring-current magnetic susceptibility results from a cancellation of diamagnetic and paramagnetic contributions. The presence of ring currents significantly affects chemical shifts as measured in nuclear magnetic resonance experiments. In contrast to the magnetic susceptibility, which is a property of the molecule as a whole, chemical shifts are sensitive to the local magnetic field and the effect of ring currents does not vanish.

First-principles codes for computational crystallography in the Quantum-ESPRESSO package

Abstract The Quantum-ESPRESSO package is a multi-purpose and multi-platform software for ab-initio calculations of condensed matter (periodic and disordered) systems. Codes in the package are based on density functional theory and on a plane wave/pseudopotential description of the electronic ground state and are ideally suited for structural optimizations (both at zero and at finite temperature), linear response calculations (phonons, elastic constants, dielectric and Raman tensors, etc.) and high-temperature molecular dynamics. Examples of applications of the codes included in the package are briefly discussed.

Structurally relaxed models of the Si(001)-SiO2 interface

We present a first‐principles investigation of the structural properties of two models for the Si(001)–SiO2 interface. The models derive from attaching tridymite, a crystalline form of SiO2, to Si(001), and then allowing for full relaxation. These models do not show electronic states in the silicon gap, as required by electrical experiments. They contain the three intermediate oxidation states of silicon, consistent with photoemission experiments. We study bond length and bond angle distributions and measures of local strain. The strain is localized to a transition region at the interface. Strain does not persist in the full oxide.

Magnetoresistive junctions based on epitaxial graphene and hexagonal boron nitride

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 such junctions based on structurally well-defined interfaces with (111) fcc or (0001) hcp ferromagnetic transition metals. We find low resistance area products, strong exchange couplings across the interface, and magnetoresistance ratios exceeding 100% for certain chemical compositions. These properties can be fine tuned, making the proposed junctions attractive for nanoscale spintronics applications.