arXiv subjects

G. Stefanucci

Publications and source records attributed to G. Stefanucci.

At least 19 recordsLinked to original sources

arXiv2025

Time-linear quantum transport simulations with correlated nonequilibrium Green's functions

We present a time-linear scaling method to simulate open and correlated quantum systems out of equilibrium. The method inherits from many-body perturbation theory the possibility to choose selectively the most relevant scattering processes in the dynamics, thereby paving the way to the real-time characterization of correlated ultrafast phenomena in quantum transport. The open system dynamics is described in terms of an embedding correlator from which the time-dependent current can be calculated using the Meir-Wingreen formula. We show how to efficiently implement our approach through a simple grafting into recently proposed time-linear Green's function methods for closed systems. Electron-electron and electron-phonon interactions can be treated on equal footing while preserving all fundametal conservation laws.

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arXiv2023

Cheers: a linear-scaling KBE+GKBA code

The interaction of electrons with quantized phonons and photons underlies the ultrafast dynamics of systems ranging from molecules to solids, giving rise to a plethora of physical phenomena experimentally accessible using time-resolved techniques. Green's function methods offer an invaluable interpretation tool since scattering mechanisms of growing complexity can be selectively incorporated in the theory. cheers is a general-purpose nonequilibrium Green's function code that implements virtually all known many-body approximations and is designed for first principles studies of ultrafast processes in molecular and model solid state systems. The aims of generality, extensibility, efficiency, and user friendliness of the code are achieved through the underlying theory development and the use of modern software design practices. Here, we motivate the necessity for the creation of such a code and overview its design and capabilities.

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arXiv2021

Real-time GW: Toward an ab initio description of the ultrafast carrier and exciton dynamics in two-dimensional materials

We demonstrate the feasibility of the time-linear scaling formulation of the $GW$ method [Phys. Rev. Lett. {\bf 124}, 076601 (2020)] for {\it ab initio} simulations of optically driven two-dimensional materials. The time-dependent $GW$ equations are derived and solved numerically in the basis of Bloch states. We address carrier multiplication and relaxation in photo-excited graphene and find deviations from the typical exponential behavior predicted by the Markovian Boltzmann approach. For resonantly pumped semiconductor we discover a self-sustained screening cascade leading to the Mott transition of coherent excitons. Our results draw attention to the importance of non-Markovian and dynamical screening effects in out-of-equilibrium phenomena.

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arXiv2021

From carriers and virtual excitons to exciton populations: Insights into time-resolved ARPES spectra from an exactly solvable model

We calculate the {\em exact} time-resolved ARPES spectrum of a two-band model semiconductor driven out of equilibrium by resonant and nonresonant laser pulses, highlighting the effects of phonon-induced decoherence and relaxation. {\em Resonant} excitations initially yield a replica of the {\em valence} band shifted upward by the energy of the exciton peak in photoabsorption. This phase is eventually destroyed by phonon-induced decoherence: the valence-band replica lowers in energy by the Stokes shift, locating at the energy of the exciton peak in photoluminescence, and its width grows due to phonon dressing. {\em Nonresonant} excitations initially yield a map of the conduction band. Then electrons transfer their excess energy to the lattice and bind with the holes left behind to form excitons. In this relaxed regime a replica of the {\em conduction} band appears inside the gap. At fixed momentum the lineshape of the conduction-band replica versus the photoelectron energy is proportional to the exciton wavefunction in "energy space" and it is highly asymmetric. Although the two-band model represents an oversimplified description of real materials the highlighted features are qualitative in nature; hence they provide useful insights into time-resolved ARPES spectra and their physical interpretation.

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arXiv2020

Ultrafast creation and melting of nonequilibrium excitonic condensates in bulk WSe$_{2}$

We study the screened dynamics of the nonequilibrium excitonic consensate forming in a bulk WSe$_{2}$ when illuminated by coherent light resonant with the lowest-energy exciton. Intervalley scattering causes electron migration from the optically populated K valley to the conduction band minimum at $Σ$. Due to the electron-hole unbalance at the K point a plasma of quasi-free holes develops, which efficiently screens the interaction of the remaining excitons. We show that this plasma screening causes an ultrafast melting of the nonequilibrium consensate and that during melting coherent excitons and quasi-free electron-hole pairs coexist. The time-resolved spectral function does exhibit a conduction and excitonic sidebands of opposite convexity and relative spectral weight that changes in time. Both the dependence of the time-dependent conduction density on the laser intensity and the time-resolved spectral function agree with recent experiments.

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arXiv2020

Self-consistent screening enhances stability of the nonequilibrium excitonic insulator phase

The nonequilibrium excitonic insulator (NEQ-EI) is an excited state of matter characterized by a finite density of coherent excitons and a time-dependent macroscopic polarization. The stability of this exciton superfluid as the density grows is jeopardized by the increased screening efficiency of the looser excitons. In this work we put forward a Hartree plus Screened Exchange HSEX scheme to predict the critical density at which the transition toward a free electron-hole plasma occurs. The dielectric function is calculated self-consistently using the NEQ-EI polarization and found to vanish in the long-wavelength limit. This property makes the exciton superfluid stable up to relatively high densities. Numerical results for the MoS$_{2}$ monolayers indicate that the NEQ-EI phase survives up to densities of the order of $10^{12}\mathrm{cm}^{-2}$.

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arXiv2020

Dynamically screened vertex correction to $GW$

Diagrammatic perturbation theory is a powerful tool for the investigation of interacting many-body systems, the self-energy operator $Σ$ encoding all the variety of scattering processes. In the simplest scenario of correlated electrons described by the $GW$ approximation for the electron self-energy, a particle transfers a part of its energy to neutral excitations. Higher-order (in screened Coulomb interaction $W$) self-energy diagrams lead to improved electron spectral functions (SF) by taking more complicated scattering channels into account and by adding corrections to lower order self-energy terms. However, they also may lead to unphysical negative spectral function. The resolution of this difficulty has been demonstrated in our previous works. The main idea is to represent the self-energy operator in a Fermi Golden rule form which leads to the manifestly positive definite SF and allows for a very efficient numerical algorithm. So far, the method has only been applied to 3D electron gas, which is a paradigmatic system, but a rather simple one. Here, we systematically extend the method to 2D including realistic systems such as mono and bilayer graphene. We focus on one of the most important vertex function effects involving the exchange of two particles in the final state. We demonstrate that it should be evaluated with the proper screening and discuss its influence on the quasiparticle properties.

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arXiv2020

Floquet topological phase of nondriven $p$-wave nonequilibrium excitonic insulators

The nontrivial topology of {\it p}-wave superfluids make these systems attractive candidate in information technology. In this work we report on the topological state of a {\it p}-wave nonequilibrium excitonic insulator (NEQ-EI) and show how to steer a nontopological band-insulator with bright $p$-excitons toward this state by a suitable laser pulse, thus achieving a dynamical topological phase transition. The underlying mechanism behind the transition is the broken gauge-symmetry of the NEQ-EI which causes self-sustained persistent oscillations of the excitonic condensate and hence a Floquet topological state for high enough exciton densities. We show the formation of Floquet Majorana modes at the boundaries of the open system and discuss topological spectral signatures for ARPES experiments. We emphasize that the topological properties of a {\it p}-wave NEQ-EI arise exclusively from the electron-hole Coulomb interaction as the system is not driven by external fields.

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arXiv2019

Pump driven normal-to-excitonic insulator transition: Josephson oscillations and signatures of BEC-BCS crossover in time-resolved ARPES

We consider a ground-state wide-gap band insulator turning into a nonequilibrium excitonic insulator (NEQ-EI) upon visiting properly selected and physically relevant highly excited states. The NEQ-EI phase, characterized by self-sustained oscillations of the complex order parameter, neatly follows from a Nonequilibrium Green's Function treatment on the Konstantinov-Perel' contour. We present the first {\em ab initio} band structure of LiF, a ground-state bulk insulator, in different NEQ-EI states and show that these states can be generated by currently available pump pulses. We highlight two general features of time-resolved ARPES spectra: (1) during the pump-driving the excitonic spectral structure undergoes a convex-to-concave shape transition and {\em concomitantly} the state of the system goes through a BEC-BCS crossover; (2) attosecond pulses shone after the pump-driving at different times $t_{\rm delay}$ generate a photocurrent which {\em oscillates} in $t_{\rm delay}$ with a pump-tunable frequency -- we show that this phenomenon is similar to the AC response of an exotic Josephson junction.

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arXiv2018

The dissection algorithm for the second-Born self-energy

We describe an algorithm to efficiently compute the second-Born self-energy of many-body perurbation theory. The core idea consists in dissecting the set of all four-index Coulomb integrals into properly chosen subsets, thus avoiding to loop over those indices for which the Coulomb integrals are zero or negligible. The scaling properties of the algorithm with the number of basis functions is discussed. The computational gain is demonstrated in the case of one-particle Kohn-Sham basis for organic molecules.

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arXiv2018

CHEERS: A tool for Correlated Hole-Electron Evolution from Real-time Simulations

We put forward a practical nonequilibrium Green's function (NEGF) scheme to perform real-time evolutions of many-body interacting systems driven out of equilibrium by external fields. CHEERS is a computational tool to solve the NEGF equation of motion in the so called generalized Kadanoff-Baym ansatz and it can be used for model systems as well as first-principles Hamiltonians. Dynamical correlation (or memory) effects are added to the Hartree-Fock dynamics through a many-body self-energy. Applications to time-dependent quantum transport, time-resolved photoabsorption and other ultrafast phenomena are discussed.

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arXiv2018

Benchmarking Nonequilibrium Green's Functions against Configuration Interaction for time-dependent Auger decay processes

We have recently proposed a Nonequilibrium Green's Function (NEGF) approach to include Auger decay processes in the ultrafast charge dynamics of photoionized molecules. Within the so called Generalized Kadanoff-Baym Ansatz the fundamental unknowns of the NEGF equations are the reduced one-particle density matrix of bound electrons and the occupations of the continuum states. Both unknowns are one-time functions like the density in Time-Dependent Functional Theory (TDDFT). In this work we assess the accuracy of the approach against Configuration Interaction (CI) calculations in one-dimensional model systems. Our results show that NEGF correctly captures qualitative and quantitative features of the relaxation dynamics provided that the energy of the Auger electron is much larger than the Coulomb repulsion between two holes in the valence shells. For the accuracy of the results dynamical electron-electron correlations or, equivalently, memory effects play a pivotal role. The combination of our NEGF approach with the Sham-Schlüter equation may provide useful insights for the development of TDDFT exchange-correlation potentials with a history dependence.

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arXiv2018

Real-time dynamics of Auger wavepackets and decays in ultrafast charge migration processes

The Auger decay is a relevant recombination channel during the first few femtoseconds of molecular targets impinged by attosecond XUV or soft X-ray pulses. Including this mechanism in time--dependent simulations of charge--migration processes is a difficult task, and Auger scatterings are often ignored altogether. In this work we present an advance of the current state-of-the-art by putting forward a real--time approach based on nonequilibrium Green's functions suitable for first-principles calculations of molecules with tens of active electrons. To demonstrate the accuracy of the method we report comparisons against accurate grid simulations of one-dimensional systems. We also predict a highly asymmetric profile of the Auger wavepacket, with a long tail exhibiting ripples temporally spaced by the inverse of the Auger energy.

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arXiv2018

Ultrafast Charge Migration in XUV Photoexcited Phenylalanine: a First-Principles Study Based on Real-Time Nonequilibrium Green's Functions

The early stage density oscillations of the electronic charge in molecules irradiated by an attosecond XUV pulse takes place on femto- or subfemtosecond timescales. This ultrafast charge migration process is a central topic in attoscience as it dictates the relaxation pathways of the molecular structure. A predictive quantum theory of ultrafast charge migration should incorporate the atomistic details of the molecule, electronic correlations and the multitude of ionization channels activated by the broad-bandwidth XUV pulse. In this work we propose a first-principles Non Equilibrium Green's Function method fulfilling all three requirements, and apply it to a recent experiment on the photoexcited phenylalanine aminoacid. Our results show that dynamical correlations are necessary for a quantitative overall agreement with the experimental data. In particular, we are able to capture the transient oscillations at frequencies 0.15PHz and 0.30PHz in the hole density of the amine group, as well as their suppression and the concomitant development of a new oscillation at frequency 0.25PHz after about 14 femtoseconds.

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arXiv2018

Time-dependent i-DFT exchange-correlation potentials with memory: Applications to the out-of-equilibrium Anderson model

We have recently put forward a steady-state density functional theory (i-DFT) to calculate the transport coefficients of quantum junctions. Within i-DFT it is possible to obtain the steady density on and the steady current through an interacting junction using a fictitious noninteracting junction subject to an effective gate and bias potential. In this work we extend i-DFT to the time domain for the single-impurity Anderson model. By a reverse engineering procedure we extract the exchange-correlation (xc) potential and xc bias at temperatures above the Kondo temperature $T_{\rm K}$. The derivation is based on a generalization of a recent paper by Dittmann et al. [arXiv:1706.04547]. Interestingly the time-dependent (TD) i-DFT potentials depend on the system's history only through the first time-derivative of the density. We perform numerical simulations of the early transient current and investigate the role of the history dependence. We also empirically extend the history-dependent TD i-DFT potentials to temperatures below $T_{\rm K}$. For this purpose we use a recently proposed parametrization of the i-DFT potentials which yields highly accurate results in the steady state.

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arXiv2018

Charge separation in donor-C60 complexes with real-time Green's functions: The importance of nonlocal correlations

We use the Nonequilibrium Green's Function (NEGF) method to perform real-time simulations of the ultrafast electron dynamics of photoexcited donor-C60 complexes modeled by a Pariser-Parr-Pople Hamiltonian. The NEGF results are compared to mean-field Hartree-Fock (HF) calculations to disentangle the role of correlations. Initial benchmarking against numerically highly accurate time-dependent Density Matrix Renormalization Group calculations verifies the accuracy of NEGF. We then find that charge-transfer (CT) excitons partially decay into charge separated (CS) states if dynamical non-local correlation corrections are included. This CS process occurs in ~10 fs after photoexcitation. In contrast, the probability of exciton recombination is almost 100% in HF simulations. These results are largely unaffected by nuclear vibrations; the latter become however essential whenever level misalignment hinders the CT process. The robust nature of our findings indicate that ultrafast CS driven by correlation-induced decoherence may occur in many organic nanoscale systems, but it will only be correctly predicted by theoretical treatments that include time-nonlocal correlations.

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arXiv2017

AC transport in Correlated Quantum Dots: From Kondo to Coulomb blockade regime

We explore the finite bias DC differential conductance of a correlated quantum dot under the influence of an AC field, from the low-temperature Kondo to the finite temperature Coulomb blockade regime. Real-time simulations are performed using a time-dependent generalization of the steady-state density functional theory (i-DFT) [Nano Lett. {\bf 15}, 8020 (2015)]. The numerical simplicity of i-DFT allows for unprecedented long time evolutions. Accurate values of average current and density are obtained by integrating over several periods of the AC field. We find that (i) the zero-temperature Kondo plateau is suppressed, (ii) the photon-assisted conductance peaks are shifted due to correlations and (iii) the Coulomb blockade is lifted with a concomitant smoothening of the sharp diamond edges.

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arXiv2017

Transport through correlated systems with density functional theory

We present recent advances in Density Functional Theory (DFT) for applications to the field of quantum transport, with particular emphasis on transport through strongly correlated systems. We review the foundations of the popular Landauer-Büttiker(LB)+DFT approach. This formalism, when using approximations to the exchange-correlation (xc) potential with steps at integer occupation, correctly captures the Kondo plateau in the zero bias conductance at zero temperature but completely fails to capture the transition to the Coulomb blockade (CB) regime as temperature increases. To overcome the limitations of LB+DFT the quantum transport problem is treated from a time-dependent (TD) perspective using TDDFT, an exact framework to deal with nonequilibrium situations. The steady-state limit of TDDFT shows that in addition to an xc potential in the junction, there also exists an xc correction to the applied bias. Open shell molecules in the CB regime provide the most striking examples of the importance of the xc bias correction. Using the Anderson model as guidance we estimate these corrections in the limit of zero bias. For the general case we put forward a steady-state DFT which is based on the one-to-one correspondence between the pair of basic variables steady density on and steady current across the junction and the pair local potential on and bias across the junction. Like TDDFT, this framework also leads to both an xc potential in the junction and an xc correction to the bias. Unlike in TDDFT, these potentials are independent of history. We highlight the universal features of both xc potential and xc bias corrections for junctions in the CB regime and provide an accurate parametrization for the Anderson model at arbitrary temperatures and interaction strengths thus providing a unified DFT description for both Kondo and CB regimes and the transition between them.

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