arXiv subjects

Giovanni Borghi

Publications and source records attributed to Giovanni Borghi.

12 recordsLinked to original sources

arXiv2023

koopmans: an open-source package for accurately and efficiently predicting spectral properties with Koopmans functionals

Over the past decade we have developed Koopmans functionals, a computationally efficient approach for predicting spectral properties with an orbital-density-dependent functional framework. These functionals impose a generalized piecewise linearity condition to the entire electronic manifold, ensuring that orbital energies match the corresponding electron removal/addition energy differences (in contrast to semi-local DFT, where a mismatch between the two lies at the heart of the band gap problem and, more generally, the unreliability of Kohn-Sham orbital energies). This strategy has proven to be very powerful, yielding molecular orbital energies and solid-state band structures with comparable accuracy to many-body perturbation theory but at greatly reduced computational cost while preserving a functional formulation. This paper reviews the theory of Koopmans functionals, discusses the algorithms necessary for their implementation, and introduces koopmans, an open-source package that contains all of the code and workflows needed to perform Koopmans functional calculations and obtain reliable spectral properties of molecules and materials.

cond-mat.mtrl-sci↗

arXiv2016

First-principles photoemission spectroscopy of DNA and RNA nucleobases from Koopmans-compliant functionals

The need to interpret ultraviolet photoemission data strongly motivates the refinement of first-principles techniques able to accurately predict spectral properties. In this work we employ Koopmans-compliant functionals, constructed to enforce piecewise linearity in approximate density functionals, to calculate the structural and electronic properties of DNA and RNA nucleobases. Our results show that not only ionization potentials and electron affinities are accurately predicted with mean absolute errors < 0.1 eV, but also that calculated photoemission spectra are in excellent agreement with experimental ultraviolet photoemission spectra. In particular, the role and contribution of different tautomers to the photoemission spectra are highlighted and discussed in detail. The structural properties of nucleobases are also investigated, showing an improved description with respect to local and semilocal density-functional theory. Methodologically, our results further consolidate the role of Koopmans-compliant functionals in providing, through orbital-density-dependent potentials, accurate electronic and spectral properties.

physics.chem-ph↗

arXiv2014

Photoemission Spectroscopy and Orbital Imaging from Koopmans-Compliant Functionals

The determination of spectral properties from first principles can provide powerful connections between microscopic theoretical predictions and experimental data, but requires complex electronic-structure formulations that fall outside the domain of applicability of common approaches, such as density-functional theory. We show here that Koopmans-compliant functionals, constructed to enforce piecewise linearity in energy functionals with respect to fractional occupations - i.e. with respect to charged excitations - provide molecular photoemission spectra and momentum maps of Dyson orbitals that are in excellent agreement with experimental ultraviolet photoemission spectroscopy and orbital tomography data. These results highlight the role of Koopmans-compliant functionals as accurate and inexpensive quasiparticle approximations to the spectral potential.

cond-mat.mtrl-sci↗

arXiv2014

Koopmans-compliant functionals and their performance against reference molecular data

Koopmans-compliant functionals emerge naturally from extending the constraint of piecewise linearity of the total energy as a function of the number of electrons to each fractional orbital occupation. When applied to approximate density-functional theory, these corrections give rise to orbital-density-dependent functionals and potentials. We show that the simplest implementations of Koopmans' compliance provide accurate estimates for the quasiparticle excitations and leave the total energy functional almost or exactly intact, i.e., they describe correctly electron removals or additions, but do not necessarily alter the electronic charge density distribution within the system. Additional functionals can then be constructed that modify the potential energy surface, including e.g. Perdew-Zunger corrections. These functionals become exactly one-electron self-interaction free and, as all Koopmans-compliant functionals, are approximately many-electron self-interaction free. We discuss in detail these different formulations, and provide extensive benchmarks for the 55 molecules in the reference G2-1 set, using Koopmans-compliant functionals constructed from local-density or generalized-gradient approximations. In all cases we find excellent performance in the electronic properties, comparable or improved with respect to that of many-body perturbation theories, such as G$_0$W$_0$ and self-consistent GW, at a fraction of the cost and in a variational framework that also delivers energy derivatives. Structural properties and atomization energies preserve or slightly improve the accuracy of the underlying density-functional approximations (Note: Supplemental Material is included in the source).

cond-mat.mtrl-sci↗

arXiv2013

Self-consistent Gutzwiller study of bcc Fe: interplay of ferromagnetic order and kinetic energy

The Gutzwiller technique has long been known as a method to include correlations in electronic structure calculations. Here we implement an ab-initio Gutzwiller+LDA calculation, and apply it to a classic problem, the ferromagnetism of bulk bcc Fe, whose nature has attracted recent interest. In the conventional Stoner-Wohlfarth model, the ferromagnetic ordering of iron sets in so that the electrons can reduce their mutual Coulomb repulsion at the extra cost of some increase of electron kinetic energy. Density functional theory within the spin polarized local density approximation (LDA) has long supported that picture, showing that ferromagnetic alignment causes band narrowing and a corresponding wavefunction localization, whence a kinetic energy increase. However, because of its inadequate treatment of strong intra-site correlations for localized d orbitals, LDA cannot be relied upon, particularly when it comes to separately describing fine potential and kinetic energy imbalances. With ab-initio Gutzwiller+LDA, we indeed find that the effect of correlations is to dramatically reverse the balance, the ferromagnetic ordering of Fe in fact causing a decrease of kinetic energy, at the cost of some increase of potential energy. The underlying physical mechanism, foreshadowed long ago by Goodenough and others, and more recently supported by LDA+DMFT calculations, is that correlations cause eg and t2g 3d orbitals to behave very differently. Weakly dispersive eg states are spin-polarized and almost localized, while, more than half filled, the t2g are broad band, fully delocalized states. Owing to intra-atomic Hund's rule exchange which aligns eg and t2g spins, the propagation of itinerant t2g holes is only allowed when different atomic spins are ferromagnetically aligned. We thus conclude that double exchange is at work already in the most popular ferromagnetic metal.

cond-mat.str-el↗

arXiv2010

Compressibility of bilayer graphene

Bilayer graphene is a recently isolated and intriguing class of many-body systems with massive chiral quasiparticles. We present theoretical results for the electronic compressibility of bilayer graphene that are based on a four-band continuum band structure model combined with a random phase approximation treatment of electronic correlations. We find that the compressibility is strongly suppressed by electron-electron interactions at low carrier densities. Correlations do not lead to any qualitative new features, but are crucially important for a quantitative understanding of this fundamental thermodynamic property of graphene bilayers.

cond-mat.mes-hall↗

arXiv2009

Strongly correlated metal interfaces in the Gutzwiller approximation

We study the effect of spatial inhomogeneity on the physics of a strongly correlated electron system exhibiting a metallic phase and a Mott insulating phase, represented by the simple Hubbard model. In three dimensions, we consider various geometries, including vacuum-metal-vacuum, a junction between a weakly and a strongly correlated metal, and finally the double junctions metal-Mott insulator-metal and metal-strongly correlated metal- metal. We applied to these problems the self-consistent Gutzwiller technique recently developed in our group, whose approximate nature is compensated by an extreme flexibility,ability to treat very large systems, and physical transparency. The main general result is a clear characterization of the position dependent metallic quasiparticle spectral weight. Its behavior at interfaces reveals the ubiquitous presence of exponential decays and crossovers, with decay lengths of clear physical significance. The decay length of metallic strength in a weakly-strongly correlated metal interface is due to poor screening in the strongly correlated side. The decay length of metallic strength from a metal into a Mott insulator (or into vacuum) is due to tunneling. In both cases, the decay length is a bulk property, and diverges with a critical exponent ($\sim 1/2$ in the present approximation, mean field in character) as the (continuous, paramagnetic) Mott transition is approached.

cond-mat.str-el↗

arXiv2009

Dynamical response functions and collective modes of bilayer graphene

Bilayer graphene (BLG) has recently attracted a great deal of attention because of its electrically tunable energy gaps and its unusual electronic structure. In this Letter we present analytical and semi-analytical expressions, based on the four-band continuum model, for the layer-sum and layer-difference density response functions of neutral and doped BLG. These results demonstrate that BLG density-fluctuations can exhibit either single-component massive-chiral character or standard two-layer character, depending on energy and doping.

cond-mat.str-el↗

arXiv2009

Fermi Velocity Enhancement in Monolayer and Bilayer Graphene

In single-layer graphene sheets non-local interband exchange leads to a renormalized Fermi-surface effective mass which vanishes in the low carrier-density limit. We report on a comparative study of Fermi surface effective mass renormalization in single-layer and AB-stacked bilayer graphene. We explain why the mass does not approach zero in the bilayer case, although its value is still strongly suppressed.

cond-mat.str-el↗

arXiv2009

Surface dead layer for quasiparticles near a Mott transition

Electron quasiparticles are progressively weakened by correlations upon approaching a continuos Mott metal insulator transition in a bulk solid. We show that corresponding to the bulk weakening, a dead layer forms below the surface of the solid, where quasiparticles are exponentially suppressed. The surface dead layer depth is a bulk property, and diverges when the Mott transition is approached. We describe this phenomenon in a Hubbard model within a self-consistent Gutzwiller approximation. Photoemission data of Rodolakis et al. in V2O3 appear to be in accord with this physical picture.

cond-mat.str-el↗

arXiv2008

Pseudospin Magnetism in Graphene

We predict that neutral graphene bilayers are pseudospin magnets in which the charge density-contribution from each valley and spin spontaneously shifts to one of the two layers. The band structure of this system is characterized by a momentum-space vortex which is responsible for unusual competition between band and kinetic energies leading to symmetry breaking in the vortex core. We discuss the possibility of realizing a pseudospin version of ferromagnetic metal spintronics in graphene bilayers based on hysteresis associated with this broken symmetry.

cond-mat.str-el↗

arXiv2007

The Role of Electron-electron Interactions in Graphene ARPES Spectra

We report on a theoretical study of the influence of electron-electron interactions on ARPES spectra in graphene that is based on the random-phase-approximation and on graphene's massless Dirac equation continuum model. We find that level repulsion between quasiparticle and plasmaron resonances gives rise to a gap-like feature at small k. ARPES spectra are sensitive to the electron-electron interaction coupling strength $α_{\rm gr}$ and might enable an experimental determination of this material parameter.

cond-mat.mes-hall↗