arXiv subjects

Nicola Lanatà

Publications and source records attributed to Nicola Lanatà.

36 records · Page 2Linked to original sources

arXiv2021

Ultrafast electronic line width broadening in the C 1s core level of graphene

Core level binding energies and absorption edges are at the heart of many experimental techniques concerned with element-specific structure, electronic structure, chemical reactivity, elementary excitations and magnetism. X-ray photoemission spectroscopy (XPS) in particular, can provide information about the electronic and vibrational many-body interactions in a solid as these are reflected in the detailed energy distribution of the photoelectrons. Ultrafast pump-probe techniques add a new dimension to such studies, introducing the ability to probe a transient state of the many-body system. Here we use a free electron laser to investigate the effect of a transiently excited electron gas on the core level spectrum of graphene, showing that it leads to a large broadening of the C 1s peak. Confirming a decade-old prediction, the broadening is found to be caused by an exchange of energy and momentum between the photoemitted core electron and the hot electron system, rather than by vibrational excitations. This interpretation is supported by a line shape analysis that accounts for the presence of the excited electrons. Fitting the spectra to this model directly yields the electronic temperature of the system, in agreement with electronic temperature values obtained from valence band data. Furthermore, making use of time- and momentum-resolved C 1s spectra, we illustrate how the momentum change of the outgoing core electrons leads to a small but detectable change in the time-resolved photoelectron diffraction pattern and to a nearly complete elimination of the core level binding energy variation associated with the narrow $σ$-band in the C 1s state. The results demonstrate that the XPS line shape can be used as an element-specific and local probe of the excited electron system and that X-ray photoelectron diffraction investigations remain feasible at very high electronic temperatures.

cond-mat.str-el↗

arXiv2021

Electronic Properties of Single-Layer CoO$_2$/Au(111)

We report direct measurements via angle-resolved photoemission spectroscopy (ARPES) of the electronic dispersion of single-layer CoO$_2$. The Fermi contour consists of a large hole pocket centered at the $\overlineΓ$ point. To interpret the ARPES results, we use density functional theory (DFT) in combination with the multi-orbital Gutzwiller Approximation (DFT+GA), basing our calculations on crystalline structure parameters derived from x-ray photoelectron diffraction and low-energy electron diffraction. Our calculations are in good agreement with the measured dispersion. We conclude that the material is a moderately correlated metal. We also discuss substrate effects, and the influence of hydroxylation on the CoO$_2$ single-layer electronic structure.

cond-mat.mtrl-sci↗

arXiv2021

Moiré induced electronic structure modifications in monolayer V$_{2}$S$_{3}$ on Au(111)

There is immense interest in how the local environment influences the electronic structure of materials at the single layer limit. We characterize moiré induced spatial variations in the electronic structure of in-situ grown monolayer V2S3 on Au(111) by means of low temperature scanning tunneling microscopy and spectroscopy. We observe a long-range modulation of the integrated local density of states (LDOS), and quantify this modulation with respect to the moiré superstructure for multiple orientations of the monolayer with respect to the substrate. Scanning tunneling spectroscopy reveals a prominent peak in the LDOS, which is shifted in energy at different points of the moiré superstructure. Comparing ab initio calculations with angle-resolved photoemission, we are able to attribute this peak to bands that exhibit a large out-of-plane d-orbital character. This suggests that the moiré driven variations in the measured density of states is driven by a periodic modulation of the monolayer-substrate hybridization.

cond-mat.mtrl-sci↗

arXiv2020

Local bottom-up effective theory of non-local electronic interactions

A cardinal obstacle to understanding and predicting quantitatively the properties of solids and large molecules is that, for these systems, it is very challenging to describe beyond the mean-field level the quantum-mechanical interactions between electrons belonging to different atoms. Here we show that there exists an exact dual equivalence relationship between the seemingly-distinct physical problems of describing local and non-local interactions in many-electron systems. This is accomplished using a theoretical construction analogue to the quantum link approach in lattice gauge theories, featuring the non-local electron-electron interactions as if they were mediated by auxiliary high-energy fermionic particles interacting in a purely-local fashion. Besides providing an alternative theoretical direction of interpretation, this result may allow us to study both local and non-local interactions on the same footing, utilizing the powerful state-of-the-art theoretical and computational frameworks already available.

cond-mat.str-el↗

arXiv2020

Bypassing the computational bottleneck of quantum-embedding theories for strong electron correlations with machine learning

A cardinal obstacle to performing quantum-mechanical simulations of strongly-correlated matter is that, with the theoretical tools presently available, sufficiently-accurate computations are often too expensive to be ever feasible. Here we design a computational framework combining quantum-embedding (QE) methods with machine learning. This allows us to bypass altogether the most computationally-expensive components of QE algorithms, making their overall cost comparable to bare Density Functional Theory (DFT). We perform benchmark calculations of a series of actinide systems, where our method describes accurately the correlation effects, reducing by orders of magnitude the computational cost. We argue that, by producing a larger-scale set of training data, it will be possible to apply our method to systems with arbitrary stoichiometries and crystal structures, paving the way to virtually infinite applications in condensed matter physics, chemistry and materials science.

cond-mat.str-el↗

arXiv2018

Critical role of electronic correlations in determining crystal structure of transition metal compounds

The choice that a solid system "makes" when adopting a crystal structure (stable or metastable) is ultimately governed by the interactions between electrons forming chemical bonds. By analyzing 6 prototypical binary transition-metal compounds we demonstrate here that the orbitally-selective strong $d$-electron correlations influence dramatically the behavior of the energy as a function of the spatial arrangements of the atoms. Remarkably, we find that the main qualitative features of this complex behavior can be traced back to simple electrostatics, i.e., to the fact that the strong $d$-electron correlations influence substantially the charge transfer mechanism, which, in turn, controls the electrostatic interactions. This result advances our understanding of the influence of strong correlations on the crystal structure, opens a new avenue for extending structure prediction methodologies to strongly correlated materials, and paves the way for predicting and studying metastability and polymorphism in these systems.

cond-mat.str-el↗

arXiv2017

Emergent Bloch Excitations in Mott Matter

We develop a unified theoretical picture for excitations in Mott systems, portraying both the heavy quasiparticle excitations and the Hubbard bands as features of an emergent Fermi liquid state formed in an extended Hilbert space, which is non-perturbatively connected to the physical system. This observation sheds light on the fact that even the incoherent excitations in strongly correlated matter often display a well defined Bloch character, with pronounced momentum dispersion. Furthermore, it indicates that the Mott point can be viewed as a topological transition, where the number of distinct dispersing bands displays a sudden change at the critical point. Our results, obtained from an appropriate variational principle, display also remarkable quantitative accuracy. This opens an exciting avenue for fast realistic modeling of strongly correlated materials.

cond-mat.str-el↗

arXiv2017

Slave Boson Theory of Orbital Differentiation with Crystal Field Effects: Application to UO$_2$

We derive an exact operatorial reformulation of the rotational invariant slave boson method and we apply it to describe the orbital differentiation in strongly correlated electron systems starting from first principles. The approach enables us to treat strong electron correlations, spin-orbit coupling and crystal field splittings on the same footing by exploiting the gauge invariance of the mean-field equations. We apply our theory to the archetypical nuclear fuel UO$_2$, and show that the ground state of this system displays a pronounced orbital differention within the $5f$ manifold, with Mott localized $Γ_8$ and extended $Γ_7$ electrons.

cond-mat.str-el↗

arXiv2015

Gutzwiller Renormalization Group

We develop a variational scheme called "Gutzwiller renormalization group" (GRG), which enables us to calculate the ground state of Anderson impurity models (AIM) with arbitrary numerical precision. Our method can exploit the low-entanglement property of the ground state in combination with the framework of the Gutzwiller wavefunction, and suggests that the ground state of the AIM has a very simple structure, which can be represented very accurately in terms of a surprisingly small number of variational parameters. We perform benchmark calculations of the single-band AIM that validate our theory and indicate that the GRG might enable us to study complex systems beyond the reach of the other methods presently available and pave the way to interesting generalizations, e.g., to nonequilibrium transport in nanostructures.

cond-mat.str-el↗

arXiv2015

Electronic structure and energetics of praseodymium and $α$-plutonium under pressure

We develop a new implementation of the Gutzwiller approximation (GA) and interface it with the local density approximation (LDA). This formulation enables us to study complex $4f$ and $5f$ systems. We perform calculations of praseodymium and $α$-plutonium under pressure, which compare very well with the experiments. Our study of praseodymium indicates that both structure change and $f$-delocalization are important to obtain the correct phase diagram and, in particular, the pressure-induced volume-collapse transition. Our calculations of $α$-plutonium indicate that, even though the $f$ electrons are delocalized in this phase, the electron-correlations affect substantially its electronic structure and thermodynamical properties.

cond-mat.str-el↗

arXiv2015

Finite-Temperature Gutzwiller Approximation from Time-Dependent Variational Principle

We develop an extension of the Gutzwiller approximation to finite temperatures based on the Dirac-Frenkel variational principle. Our method does not rely on any entropy inequality, and is substantially more accurate than the approaches proposed in previous works. We apply our theory to the single-band Hubbard model at different fillings, and show that our results compare quantitatively well with dynamical mean field theory in the metallic phase. We discuss potential applications of our technique within the framework of first principle calculations.

cond-mat.str-el↗

arXiv2014

Interplay of spin-orbit and entropic effects in Cerium

We perform first-principle calculations of elemental cerium and compute its pressure-temperature phase diagram, finding good quantitative agreement with the experiments. Our calculations indicate that, while a signature of the volume-collapse transition appears in the free energy already at low temperatures, at larger temperatures this signature is enhanced because of the entropic effects, and originates an actual thermodynamical instability. Furthermore, we find that the catalyst determining this feature is --- in all temperature regimes --- a pressure-induced effective reduction of the $f$-level degeneracy due to the spin-orbit coupling. Our analysis suggests also that the lattice vibrations might be crucial in order to capture the behavior of the pressure-temperature transition line at large temperatures.

cond-mat.str-el↗

arXiv2014

Energetics and Electronic Structure of Plutonium

Plutonium is the most exotic and mysterious element in the periodic table. It has 6 metallic phases and peculiar physical properties not yet understood. One of the most intriguing properties of Pu is that relatively small changes of temperature can induce transitions between different structures, that are accompanied by very large changes of equilibrium volumes. This fact has stimulated extensive theoretical and experimental studies. In spite of this, a convincing explanation of the metallurgic properties of Pu based on fundamental principles is still lacking, and none of the previous theories has been able to describe simultaneously the energetics and the $f$ electronic structure of all of the phases of Pu on the same footing. Here we provide a bird's eye view of Pu by studying the zero-temperature pressure-volume phase diagram and the $f$ electronic structure of all of its crystalline phases from first principles. In particular, we clarify the way in which the $f$-electron correlations determine its unusual energetics. Our theoretical energetics and ground-state $f$ electronic structure are both in good quantitative agreement with the experiments.

cond-mat.str-el↗

arXiv2013

Principle of Maximum Entanglement Entropy and Local Physics of Correlated many-body Electron-Systems

We argue that, because of the quantum-entanglement, the local physics of the strongly-correlated materials at zero temperature is described in very good approximation by a simple generalized Gibbs distribution, which depends on a relatively small number local quantum thermodynamical potentials. We demonstrate that our statement is exact in certain limits, and we perform numerical calculations of the iron compounds FeSe and FeTe and of the elemental cerium by employing the Gutzwiller Approximation (GA) that strongly support our theory in general.

cond-mat.str-el↗

arXiv2013

The $γ$-$α$ iso-structural Transition in Cerium, a Critical Element

Below the critical temperature $T_c\simeq 600 K$, an iso-structural transition, named $γ$-$α$ transition,can be induced in Cerium by applying pressure. This transition is first-order, and is accompanied by a sizable volume collapse. A conclusive theoretical explanation of this intriguing phenomenon has still not been achieved, and the physical pictures proposed so far are still under debate. In this work, we illustrate zero-temperature first-principle calculations which clearly demonstrate that the $γ$-$α$ transition is induced by the interplay between the electron-electron Coulomb interaction and the spin-orbit coupling. We address the still unresolved problem on the existence of a second low-$T$ critical point, i.e., whether the energetic effects alone are sufficient or not to induce the $γ$-$α$ transition at zero temperature.

cond-mat.str-el↗

arXiv2012

Time-Dependent and Steady-State Gutzwiller approach for nonequilibrium transport in nanostructures

We extend the time-dependent Gutzwiller variational approach, recently introduced by Schirò and Fabrizio, Phys. Rev. Lett. 105 076401 (2010), to impurity problems. Furthermore, we derive a consistent theory for the steady state, and show its equivalence with the previously introduced nonequilibrium steady-state extension of the Gutzwiller approach. The method is shown to be able to capture dissipation in the leads, so that a steady state is reached after a sufficiently long relaxation time. The time-dependent method is applied to the single orbital Anderson impurity model at half-filling, modeling a quantum dot coupled to two leads. In these first exploratory calculations the Gutzwiller projector is limited to act only on the impurity. The strengths and the limitations of this approximation are assessed via comparison with state of the art continuous time quantum Monte Carlo results. Finally, we discuss how the method can be systematically improved by extending the region of action of the Gutzwiller projector.

cond-mat.mes-hall↗

arXiv2011

Efficient implementation of the Gutzwiller variational method

We present a self-consistent numerical approach to solve the Gutzwiller variational problem for general multi-band models with arbitrary on-site interaction. The proposed method generalizes and improves the procedure derived by Deng et al., Phys. Rev. B. 79 075114 (2009), overcoming the restriction to density-density interaction without increasing the complexity of the computational algorithm. Our approach drastically reduces the problem of the high-dimensional Gutzwiller minimization by mapping it to a minimization only in the variational density matrix, in the spirit of the Levy and Lieb formulation of DFT. For fixed density the Gutzwiller renormalization matrix is determined as a fixpoint of a proper functional, whose evaluation only requires ground-state calculations of matrices defined in the Gutzwiller variational space. Furthermore, the proposed method is able to account for the symmetries of the variational function in a controlled way, reducing the number of variational parameters. After a detailed description of the method we present calculations for multi-band Hubbard models with full (rotationally invariant) Hund's rule on-site interaction. Our analysis shows that the numerical algorithm is very efficient, stable and easy to implement. For these reasons this method is particularly suitable for first principle studies -- e.g., in combination with DFT -- of many complex real materials, where the full intra-atomic interaction is important to obtain correct results.

cond-mat.str-el↗

arXiv2011

Variational approach to transport in quantum dots

We have derived a variational principle that defines the nonequilibrium steady-state transport across a correlated impurity mimicking, e.g., a quantum dot coupled to biased leads. This variational principle has been specialized to a Gutzwiller's variational space, and applied to the study of the simple single-orbital Anderson impurity model at half filling, finding a good qualitative accord with the observed behavior in quantum dots for the expected regime of values of the bias. Beyond the purely theoretical interest in the formal definition of a variational principle in a nonequilibrium problem, the particular methods proposed have the important advantage to be simple and flexible enough to deal with more complicated systems and variational spaces.

cond-mat.str-el↗