arXiv subjects

D. Sangalli

Publications and source records attributed to D. Sangalli.

17 recordsLinked to original sources

arXiv2026

Extreme-ultraviolet optical response of atomically-thin molybdenum disulfide

We report multi-angle reflectivity measurements in the extreme-ultraviolet (XUV) range for mono- and bilayer MoS$_2$ on a Si$_3$N$_4$ substrate. Using a single-sheet 2D conductivity model, we extract the complex optical response of the MoS$_2$ bilayer between 25 and 90 eV and derive an effective refractive index by introducing a thickness equal to the interlayer spacing. The MoS$_2$ monolayer response is consistently reproduced either by halving the 2D conductivity or the effective thickness, indicating a robust scaling with layer number. The resulting optical constants display a broad resonance at the Mo N$_{2,3}$ edge with no signatures of sharp core-exciton features despite the reduced dimensionality. First-principles calculations reproduce the experimental results and show that local-field (Hartree) effects dominate the XUV response, while screened-exchange (SEX) contributions remain weak and mainly induce spectral shifts. Our analysis demonstrates that excitonic effects play a minor role in the XUV optical response of atomically thin MoS$_2$, highlighting key differences with respect to the visible and infrared regimes, and calling for a reassessment of the use of Mo-based transition metal dichalcogenides in attosecond spectroscopy and XUV excitonics.

physics.optics↗

arXiv2024

Excitonic response in TMD heterostructures from first-principles: impact of stacking, twisting, and interlayer distance

Van der Waals heterostructures of two-dimensional transition metal dichalcogenides provide a unique platform to engineer optoelectronic devices tuning their optical properties via stacking, twisting, or straining. Using ab initio Many-Body Perturbation Theory, we predict the electronic and optical (absorption and photoluminescence spectra) properties of MoS$_2$/WS$_2$ and MoSe$_2$/WSe$_2$ hetero-bilayers with different stacking and twisting. We analyse the valley splitting and optical transitions, and explain the enhancement or quenching of the inter- and intra-layer exciton states. Contrary to established models, that focus on transitions near the high-symmetry point K, our results include all possible transitions across the Brillouin Zone. This result, for a twisted Se-based heterostructures, in an interlayer exciton with significant electron density in both layers and a mixed intralayer exciton distributed over both MoSe$_2$ and WSe$_2$. We propose that it should be possible to produce an inverted order of the excitonic states in some MoSe$_2$/WSe$_2$ heterostructures, where the energy of the intralayer WSe$_2$ exciton is lower than that in MoSe$_2$. We predict the variability of the exciton peak positions ($\sim$100 meV) and the exciton radiative lifetimes, from pico- to nano-seconds, and even micro-seconds in twisted bilayers. The control of exciton energies and lifetimes paves the way towards applications in quantum information technologies and optical sensing.

cond-mat.mes-hall↗

arXiv2023

Exciton-Exciton transitions involving strongly bound excitons: an ab initio approach

In pump-probe spectroscopy, two laser pulses are employed to garner dynamical information from the sample of interest. The pump initiates the optical process by exciting a portion of the sample from the electronic ground state to an accessible electronic excited state, an exciton. Thereafter, the probe interacts with the already excited sample. The change in the absorbance after pump provides information on transitions between the excited states and their dynamics. In this work we study these exciton-exciton transitions by means of an ab initio real time propagation scheme based on dynamical Berry phase formulation. The results are then analyzed taking advantage of a Fermi-golden rule approach formulated in the excitonic basis-set and in terms of the symmetries of the excitonic states. Using bulk LiF and 2D hBN as two prototype materials, we discuss the selection rules for transitions involving strongly bound excitons, for which the hydrogen model cannot be used.

cond-mat.mtrl-sci↗

arXiv2022

Real-time ab initio description of the photon-echo mechanisms in extended systems: the case study of bulk GaAs

In this paper we present an ab initio real-time analysis of free polarization decay and photon echo in extended systems. As a prototype material, we study bulk GaAs driven by ultra-short laser pulses of 10 fs (energy spread of 0.4 eV), with frequency tuned in the continuum of the optical spectrum. We compute the electronic polarization P(t), and define a computational procedure to extract the echo signal in the dipole approximation. Results are obtained in both the low and high field regime, and compared with a two-levels system (TLS) model, with parameters extracted from the ab initio simulations. ab initio results are in optimal agreement with the TLS in the low-field case, whereas some differences are observed in the high-field regime where the multi-band nature of GaAs becomes relevant. In the high field regime we compute the pulse area, and look for fluences with pulse area close to π. We highlight that such fluences are well below the damage threshold of GaAs. However a unique value of the area cannot be defined, due to the strong dependence of the transition dipoles in the energy window excited by the laser pulse.

cond-mat.mtrl-sci↗

arXiv2022

Excitons and carriers in transient absorption and time-resolved ARPES spectroscopy: an abinitio approach

I present a fully abinitio scheme to model transient spectroscopy signals in presence of strongly bound excitons. Using LiF as a prototype material, I show that the scheme is able to capture the exciton signature both in time-resolved ARPES and transient absorption experiments. The approach is completely general and can become the reference scheme for modelling pump and probe experiment in a wide range of materials.

cond-mat.mtrl-sci↗

arXiv2019

Strongly coupled coherent phonons in single-layer MoS$_2$

We present a transient absorption setup combining broadband detection over the visible-UV range with high temporal resolution ($\sim$20fs) which is ideally suited to trigger and detect vibrational coherences in different classes of materials. We generate and detect coherent phonons (CPs) in single layer (1L) MoS$_2$, as a representative semiconducting 1L-transition metal dichalcogenide (TMD), where the confined dynamical interaction between excitons and phonons is unexplored. The coherent oscillatory motion of the out-of-plane $A'_{1}$ phonons, triggered by the ultrashort laser pulses, dynamically modulates the excitonic resonances on a timescale of few tens fs. We observe an enhancement by almost two orders of magnitude of the CP amplitude when detected in resonance with the C exciton peak, combined with a resonant enhancement of CP generation efficiency. Ab initio calculations of the change in 1L-MoS$_2$ band structure induced by the $A'_{1}$ phonon displacement confirm a strong coupling with the C exciton. The resonant behavior of the CP amplitude follows the same spectral profile of the calculated Raman susceptibility tensor. This demonstrates that CP excitation in 1L-MoS$_2$ can be described as a Raman-like scattering process. These results explain the CP generation process in 1L-TMDs, paving the way for coherent all-optical control of excitons in layered materials in the THz frequency range.

cond-mat.mtrl-sci↗

arXiv2019

Magneto-optical response of chromium trihalide monolayers: chemical trends

Chromium trihalides (CrI$_3$, CrBr$_3$ and CrCl$_3$) form a prominent family of isostructural insulating layered materials in which ferromagnetic order has been observed down to the monolayer. Here we provide a comprehensive computational study of magneto-optical properties that are used as probes for the monolayer ferromagnetic order: magnetic circular dichroism and magneto-optic Kerr effect. Using a combination of density functional and Bethe-Salpeter theories, we calculate both the optical absorption and the magneto-optical Kerr angle spectra, including both excitonic effects and spinorial wave functions. We compare the magneto-optical response of the chromium trihalides series and we find that its strength is governed by the spin-orbit coupling of the ligand atoms (I, Br, Cl).

cond-mat.mtrl-sci↗

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.

cond-mat.mes-hall↗

arXiv2019

Many-body perturbation theory calculations using the yambo code

yambo is an open source project aimed at studying excited state properties of condensed matter systems from first principles using many-body methods. As input, yambo requires ground state electronic structure data as computed by density functional theory codes such as quantum-espresso and abinit. yambo's capabilities include the calculation of linear response quantities (both independent-particle and including electron-hole interactions), quasi-particle corrections based on the GW formalism, optical absorption, and other spectroscopic quantities. Here we describe recent developments ranging from the inclusion of important but oft-neglected physical effects such as electron-phonon interactions to the implementation of a real-time propagation scheme for simulating linear and non-linear optical properties. Improvements to numerical algorithms and the user interface are outlined. Particular emphasis is given to the new and efficient parallel structure that makes it possible to exploit modern high performance computing architectures. Finally, we demonstrate the possibility to automate workflows by interfacing with the yambopy and AiiDA software tools.

cond-mat.mtrl-sci↗

arXiv2018

Direct observation of intravalley spin relaxation in single-layer WS$_2$

In monolayer Transition Metal Dichalcogenides (TMDs) the valence and conduction bands are spin split because of the strong spin-orbit interaction. In tungsten-based TMDs the spin-ordering of the conduction band is such that the so-called dark exciton, consisting of an electron and a hole with opposite spin orientation, has lower energy than the A exciton. A possible mechanism leading to the transition from bright to dark excitons involves the scattering of the electrons from the upper to the lower conduction band state in K. Here we exploit the valley selective optical selection rules and use two-color helicity-resolved pump-probe spectroscopy to directly measure the intravalley spin-flip relaxation dynamics of electrons in the conduction band of single-layer WS$_2$. This process occurs on a sub-ps time scale and it is significantly dependent on the temperature, indicative of a phonon-assisted relaxation. These experimental results are supported by time-dependent ab-initio calculations which show that the intra-valley spin-flip scattering occurs on significantly longer time scales only exactly at the K point. In a realistic situation the occupation of states away from the minimum of the conduction band leads to a dramatic reduction of the scattering time.

cond-mat.mtrl-sci↗

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.

cond-mat.mes-hall↗

arXiv2017

Optical properties of periodic systems within the current-current response framework: pitfalls and remedies

We compare the optical absorption of extended systems using the density-density and current-current linear response functions calculated within many-body perturbation theory. The two approaches are formally equivalent for a finite momentum $\mathbf{q}$ of the external perturbation. At $\mathbf{q}=\mathbf{0}$, however, the equivalence is maintained only if a small $q$ expansion of the density-density response function is used. Moreover, in practical calculations this equivalence can be lost if one naively extends the strategies usually employed in the density-based approach to the current-based approach. Specifically we discuss the use of a smearing parameter or of the quasiparticle lifetimes to describe the finite width of the spectral peaks and the inclusion of electron-hole interaction. In those instances we show that the incorrect definition of the velocity operator and the violation of the conductivity sum rule introduce unphysical features in the optical absorption spectra of three paradigmatic systems: silicon (semiconductor), copper (metal) and lithium fluoride (insulator). We then demonstrate how to correctly introduce lifetime effects and electron-hole interactions within the current-based approach.

cond-mat.mtrl-sci↗

arXiv2016

First-principles approach to excitons in time-resolved and angle-resolved photoemission spectra

We show that any {\em quasi-particle} or GW approximation to the self-energy does not capture excitonic features in time-resolved (TR) photoemission spectroscopy. In this work we put forward a first-principles approach and propose a feasible diagrammatic approximation to solve this problem. We also derive an alternative formula for the TR photocurrent which involves a single time-integral of the lesser Green's function. The diagrammatic approximation applies to the {\em relaxed} regime characterized by the presence of quasi-stationary excitons and vanishing polarization. The main distinctive feature of the theory is that the diagrams must be evaluated using {\em excited} Green's functions. As this is not standard the analytic derivation is presented in detail. The final result is an expression for the lesser Green's function in terms of quantities that can all be calculated {\em ab initio}. The validity of the proposed theory is illustrated in a one-dimensional model system with a direct gap. We discuss possible scenarios and highlight some universal features of the exciton peaks. Our results indicate that the exciton dispersion can be observed in TR {\em and} angle-resolved photoemission.

cond-mat.mes-hall↗

arXiv2016

Dielectrics in a time-dependent electric field: a real-time approach based on density-polarization functional theory

In the presence of a (time-dependent) macroscopic electric field the electron dynamics of dielectrics cannot be described by the time-dependent density only. We present a real-time formalism that has the density and the macroscopic polarization P as key quantities. We show that a simple local function of P already captures long-range correlation in linear and nonlinear optical response functions. Specifically, after detailing the numerical implementation, we examine the optical absorption, the second- and third-harmonic generation of bulk Si, GaAs, AlAs and CdTe at different level of approximation. We highlight links with ultranonlocal exchange-correlation functional approximations proposed within linear response time-dependent density functional theory framework.

cond-mat.mtrl-sci↗

arXiv2015

Non-equilibrium Bethe-Salpeter equation for transient photo-absorption spectroscopy

In this work we propose an accurate first-principle approach to calculate the transient photo--absorption spectrum measured in Pump\&\,Probe experiments. We formulate a condition of {\em adiabaticity} and thoroughly analyze the simplifications brought about by the fulfillment of this condition in the non--equilibrium Green's function (NEGF) framework. Starting from the Kadanoff-Baym equations we derive a non--equilibrium Bethe--Salpeter equation (BSE) for the response function that can be implemented in most of the already existing {\em ab--initio} codes. In addition, the {\em adiabatic} approximation is benchmarked against full NEGF simulations in simple model hamiltonians, even under extreme, nonadiabatic conditions where it is expected to fail. We find that the non--equilibrium BSE is very robust and captures important spectral features in a wide range of experimental configurations.

cond-mat.other↗

arXiv2015

Complete collisions approximation to the Kadanoff-Baym equation: a first--principles implementation

We show carriers dynamics on bulk Silicon in the sub pico-second time scale. The results are obtained from a first-principles implementation of the the Kadanoff-Baym equations within the generalized Baym-Kadanoff ansatz and the complete collision approximation. The resulting scattering term is similar to the scattering described within the semi-classical Boltzmann equation.

cond-mat.mtrl-sci↗

arXiv2012

Challenges for first-principles methods in theoretical and computational physics: multiple excitations in many-electrons systems and the Aharonov-Bohm effect in carbon nanotubes

In the first part of the thesis we will describe double excitations in the absorption spectrum. Double excitations are a peculiar effect of interacting systems which does not have a counterpart in non-interacting ones. The optical absorption spectrum of a system is obtained by shining light on it. At the microscopic level photons hit the electrons which sit in the ground state and change their configuration. If the light source is not too intense this can be described in linear response; that is only "one photon" processes are involved, only one electron per time can be influenced. Here is where the interaction comes in. The hit electron is linked to the others and so other process take place, one of these is the appearance of multiple excitations. In the second part of the thesis we focus on the application of more standard techniques to the description of carbon nanotubes (CNTs). In particular we focus on the effects of magnetic fields on CNTs. CNTs are quasi 1D-systems composed by carbon atoms which have been discovered in 1952. Under the effect of a magnetic field electrons delocalized on a cylindrical surface display a peculiar behaviour, known as Aharonov-Bohm effect. The Aharonov-Bohm is a pure quantum mechanical effect which does not have any counterpart in classical physics. In CNTs the Aharonov-Bohm modify the electronic gap and so can be used to tune the electronic properties. Though a model able to account for such process is available in the literature, in the present work we will describe the effect of magnetic fields "ab-initio". In the description of CNTs we will use standard approximations which are by far much more accurate and general than any approximation introduced in phenomenological descriptions based on model systems.

cond-mat.str-el↗