1st TIMES Workshop joint with ETSF Real-Time collaboration-team meeting
The joint TIMES-ETSF workshop will be organized as follows:
7-8 April: Introductory talks tailored for doctoral students and early stage postdoctoral researchers. Poster session open to all PhD students and early stage post-docs.
9-10 April: Advanced talks focused on non-equilibrium simulations, plus invited experimental talks on TR-ARPES and pump & probe spectroscopy.
Talks on ongoing developments are welcome and we plan open discussions in the spirit of ETSF collaboration teams meetings.
Some of the topics covered during the event.
Theory: Ehrenfest (T. Todorov), nonlinear optics (M. Gruning), machine learning (D. Wilkins), HPC and first principles (D. Sangalli), defects (S. Achilli), magnons (A. Molina-Sanchez), finite systems TDDFT (P. Mai Dinh), non-equilibrium Green's function theory (E. Perfetto), exciton-phonon (C. Attaccalite).
Experimental talks: Time-Resolved ARPES (Y. Zhang), pump & probe techniques and transient absorption (S. Dal Conte).
Free registration and abstract submission. Deadline 1st March 2025
Ehrenfest dynamics (ED) is the simplest form of non-adiabatic electron-nuclear dynamics, that is, it is based on solving explicit coupled equations of motion for the electrons and nuclei as opposed to making the Born-Oppenheimer approximation (BOA). ED correctly reduces to BOA in the limit of slow nuclei, while capturing electronic excitations and the resulting departures from BOA in the opposite limit. ED is highly intuitive and computationally tractable, as well as being fully formally compatible with TDDFT. This often makes it the method of choice in modern atomistic simulations which require a dynamical coupling between the two subsystems. However, ED misses a crucial physical process: the electronic force noise responsible for spontaneous phonon emission. This in turn results in an inability to capture thermal equilibration, Joule heating and inelastic I-V spectral features in transport. What ED does capture is the so-called electronic friction responsible for the energy loss from high-velocity ions to electrons. These limitations make ED suitable for some problems and completely inappropriate for others. This talk will first discuss ED from a physical standpoint, then derive it formally, discuss its strengths and weaknesses, and finally point to ways of going beyond ED.
After introducing nonlinear optics and a few nonlinear-optics-based spectroscopic techniques, I will argue why it is important to have theory and simulations to interpret, support and guide these experiments. I will then discuss how to predict nonlinear optical response of materials from simulations and conclude with the challenges posed by these simulations.
In recent years, machine learning (ML) approaches to circumvent quantum-mechanical calculations have become increasingly popular, allowing atomistic simulations with the accuracy of electronic structure theory to be run in a fraction of the time needed for ab inition molecular dynamics (AIMD).
I will describe the background and basics of these approaches, explaining the theory behind ML predictions of the interatomic forces and how to get started on training an ML potential.
I discuss the evolution of computer facilities, the present status of available HPC machines and how scientific codes needs to be adapted to run on these machines. I'll show the example of the Yambo code, and show how the code evolved in recent years to be able to exploit MPI, OpenMP and more recently GPUs.
Let´s go to the pub
S2.1 (30’+5’) Non linear optics
S2.2 (30’+5’) Magnons and magnetic materials
S2.3 (30’+5’) Finite Systems
I will give an overview of the possible applications of point defects in quantum technologies, showing what are the quantity of interest that can be calculated and predicted, and I will discuss the main approaches that are currently exploited to achieve them.
In this talk I present an ab inito approach to calculate spin waves (magnons) based in the Bethe-Salpeter Equation.
The irradiation of a finite electronic system by an ultrafast XUV pulse can lead to the ultrafast creation of core-holes with strong subsequent electronic rearrangements. A standard theoretical tool to investigate such excitations that avoids the cost of the simulation of the XUV irradiation is to start the dynamics with an instantaneous core-hole excitation. In a recent series of papers aiming at investigating in some detail the excitation processes induced by a short XUV pulse, we identified an unexpected scenario characterized by the development of large amplitude dipole oscillations [1,2,3]. Once irradiation by a far-off resonance laser pulse is over, the dipole signal normally shrinks down to zero. Total ionization rapidly levels off and stays constant. For a certain range of laser frequencies, we observe that after some time the dipole increases again to reach sizable values, thus generating extra ionization. We presently call this phenomenon a dipole instability as it cannot be mapped to well-known deexcitation mechanisms such as Auger. A dipole instability can be measured on time-resolved photo-electron spectra with the delayed appearance of an extra low energy peak, which could be used for experimental identification of the process. It has been shown by comparing various numerical approaches that this instability is not a numerical problem. But it is not yet excluded that it is a defect of the level of theory used to describe electrons, actually time-dependent density functional theory at the local density approximation level.
[1] P.-G. Reinhard, P.M. Dinh, D. Dundas, E. Suraud, M. Vincendon, On the stability of hole states in molecules and clusters, Eur. Phys. J. Spec. Top. 232 (2023) 2095
[2] P.-G. Reinhard, D. Dundas, P.M. Dinh, M. Vincendon, E. Suraud, Unexpected dipole instabilities in small molecules after ultrafast XUV irradiation, Phys. Rev. A 107 (2023) L020801
[3] D. Hughes, D. Dundas, P.M. Dinh, M. Vincendon, P.-G. Reinhard, E. Suraud, Dipole instability in molecules irradiated by XUV pulses, Eur. Phys. J. D 77 (2023) 177
We developed an algorithm to study ultrafast electronic relaxation and decoherence process in solid-state systems using real-time,real-space time-dependent density functional theory (RT-TDDFT) that is currently being implemented in the Octopus software package. The methodology will employ a natural orbital representation to monitor the time evolution of the electronic density matrix, enabling the study of coherence loss following electronic excitation. The system states will be then expanded in terms of adiabatic eigenstates, allowing for a detailed examination of population transfer and decoherence mechanisms within an orbital-based framework.
The approach is intended to be applied to crystalline materials, where we aim to explore how exchange and correlation effects influence relaxation dynamics.This work will go beyond linear response theory, targeting a deeper understanding of non-equilibrium electronic
processes in periodic systems. By developing and applying this framework, the project aspires to contribute to the advancement of
first-principles modeling of ultrafast phenomena in condensed matter.
This project explores the process of light-induced structural symmetry breaking in polar semiconductors and insulators. When photoinduced electrons and holes interact with the lattice through electron-phonon interaction (EPI), they can lead to the formation of symmetry breaking structural distortions. The goal of this project is to investigate the ultrafast dynamics of electrons and phonons in presence of EPI and light induced structural changes. Our study will begin by considering the polar material, lithium fluoride (LiF). Subsequently, we will extend our investigation to more complex systems, including halide perovskites. This work aims to move the first steps towards the predictive computational modelling of light-induced symmetry breaking within the framework of ab initio calculations.
1st TIMES Workshop joint with ETSF Real-Time collaboration-team meeting
Torsten Geirsson
Supervisor: Alejandro Molina-Sánchez
The CrSBr van der Waals layered material is a magnetic semiconductor which has gained attention due to its unique magnetic and optical properties. It is air-stable and allows for manipulation of its magnetic order with a small external field, positioning it as a promising candidate for magnonics applications. In this context, CrSBr can enable energy-efficient information transfer through coherent magnons. Information about these low-energy magnetic excitations could potentially be accessed optically through their coupling with excitons, which possess higher excitation energies, making the exciton-magnon interaction an active area of research.
As part of the first workshop of the TIMES network, we will present our latest work on first-principle studies of this material in both bulk and monolayer forms. This includes computational studies of the ground state properties using density functional theory, as well as excited state properties using many-body perturbation theory. Out of the excited state properties, we investigate the excitons using the Bethe-Salpeter equation and real-time simulations. Future research will explore ultrafast excitonic dynamics and magnon-exciton coupling in this material.
The interaction of a finite electronic system, such as a molecule or cluster, with an intense ultrafast laser pulse induces complex electronic and ionic dynamics. A key theoretical approach to studying these dynamics is to initiate the system with an instantaneous core-hole excitation, bypassing the need for direct simulation of the laser interaction. Recently, an unexpected phenomenon has been identified, termed dipole instability, where large-amplitude dipole oscillations emerge after irradiation by an ultrafast extreme ultraviolet (XUV) pulse[1,2,3]. This instability manifests as a delayed resurgence of the dipole signal, leading to additional ionization at later times. The dipole instability in TDDFT simulations at the LDA level questions whether it's a real effect or a flaw in the theory.
A key aspect of this study is examining the numerical reproducibility of dipole instability to determine its reliability and physical significance. Numerical reproducibility can be influenced by various factors, including input parameters, mathematical libraries, compilers, and numerical precision settings. Different libraries, such as FFTW and MKL, employ distinct algorithms and numerical approximations for Fourier transforms and matrix operations, introducing variations in key observables[4]. Specifically, we analyze how different compilers and libraries used in the QDD code may introduce numerical discrepancies that affect the observed dynamics. Further investigation is necessary to assess the robustness of dipole instability across different computational environments and laser field strengths, determining whether these discrepancies remain within acceptable tolerances or significantly impact the physical interpretation of results. Ensuring reproducibility across different computational environments is essential for building confidence in theoretical predictions of dipole instability phenomena. Our findings will provide insights into the reliability of numerical simulations in ultrafast electron dynamics research.
[1] P.-G. Reinhard, P.M. Dinh, D. Dundas, E. Suraud, M. Vincendon, On the stability of hole states in molecules and clusters, Eur. Phys. J. Spec. Top. 232 (2023) 2095
[2] P.-G. Reinhard, D. Dundas, P.M. Dinh, M. Vincendon, E. Suraud, Unexpected dipole instabilities in small molecules after ultrafast XUV irradiation, Phys. Rev. A 107 (2023) L020801
[3] D. Hughes, D. Dundas, P.M. Dinh, M. Vincendon, P.-G. Reinhard, E. Suraud, Dipole instability in molecules irradiated by XUV pulses, Eur. Phys. J. D 77 (2023) 177
[4] Benjamin A. Antunes, Claude Mazel, David R.C. Hill. Performance and reproducibility assessment of quantum dissipative dynamics framework: a comparative study of Fortran compilers, MKL, and FFTW. 2024. _x005F_xffff_hal-04684180v1
The present work focuses on coupling semi-classical Boltzmann dynamics with the non-equilibrium Bethe-Salpeter equation in order to describe bound state absorption/emission and intraband relaxation processes occurring in a WS2 monolayer.
The appeal stems from the direct access to the electronic/hole populations and the real-time evolution of the non-equilibrium thermal density matrix including electron-phonon coupling effects and scattering, allowing for the calculation of time-dependent excitonic production/decay rates.
Furthermore, the excitonic spectra are computed at each time-step using density matrix response theory from first principles. It allows for calculation of renormalised quasielectron bandgaps and pronounced screening effects at higher carrier concentrations in the material.
Electron-electron interactions and electron-hole recombination will not be considered, as their dynamics lie out of the femtosecond timescale of electron-phonon interactions.
The prediction of polarization and optical responses are crucial for anticipating charge transfer properties and identifying functional materials involved in defect engineering and single-photon emitters.
To do so, Nickel Iodide magnetic cell is simplified to reproduce the antiferromagnetic behavior raised from the helicoidal incommensurate magnetic periodicity within the layers that switches to opposite direction between layers. Various magnetic unit cells are to insight to attempt to define the one better reproducing the experimental features.
By DFT (and DFT+U) we obtain the optimized basis set describing the ground state.
To go beyond the linear response theory, we use the ab initio approach provided in Yambo. It allows to quantitatively describe the optical properties of out-of-equilibrium systems that are involved in pump-probe experiments.
By real-time integration, Schrodinger equation is solved for the Hamiltonian corrected by adding many-body effects obtained by many-body perturbation theory. First, we account for many-body correlations by quasiparticle corrections at the GW level. Furthermore, excitonic effects are also included by solving the Bethe-Saltpeter equation (BSE) for the electron-hole propagator.
Density Functional Theory (DFT) is a widely-used computational approach for investigating electronic properties of materials, relying heavily on the choice of appropriate exchange-correlation functionals. Among these functionals, the generalized gradient approximation (GGA) is widely used due to its effectiveness in capturing electron exchange-correlation effects beyond local approximations. However, calculating numerical functional derivatives directly from their definition is impractical, as it involves introducing separate delta-like perturbations and evaluating derivatives individually at every point in real space. In this poster, we present an efficient alternative approach to compute the functional derivatives of the spin-polarized exchange-correlation potential ($V_{xc}$) by taking advantage of already existing subroutines in the Yambo software. We demonstrate that accurate numerical functional derivatives can be obtained through numerical partial derivatives of $V_{xc}$. By appropriately combining those, we achieve high-accuracy functional derivatives of exchange-correlation potential suitable for spectral analyses and study of magnon interactions using GGA functionals.
We investigate the electronic and nuclear dynamics of molecules upon photoexcitation in real time. To that end, single- and multi-trajectory Ehrenfest dynamics is employed. Different sampling techniques for the initial nuclear conditions, such as harmonic sampling or sampling with a quantum thermostat, are compared.
In recent years, two-dimensional (2D) materials have gained significant attention as promising platforms for next-generation quantum technologies[1] offering a wide range of electronic properties, optical and mechanical properties. Specifically, engineering 2D materials provides insights into the mechanisms underlying properties such as magnetism, which is of great interest for the development of 2D-based devices in spintronics and quantum information processing[2,3]. Exploiting point defects, both intrinsic or generated by atom implantation, to achieve desired functionalities represents nowadays one feasible strategy.
The aim of this work is to explore the properties of different defects in 2D materials, namely graphene and hexagonal boron nitride (hBN), as representative of a semimetal and a wide-gap 2D material. One of the aspect we are considering is the possibility to accurately describe defects states in the wide gap of hBN by using the DFT-1/2[4] approach, which rectifies the lack of self-energy of the band gap by adding or subtracting a fraction of electron to the pseudopotentials based on a trimming function, limiting the computational cost with respect to many-body techniques. This method allows us to reproduce the experimental material’s band gap but, in some cases, fails to describe defect states accurately. To address this issue, we are trying to apply the decoupled DFT-1/2[5] method to offer a GW precision while maintaining the computational cost of DFT for studying defective systems in dilute limit.
Second, we are exploring different configurations of substitutional Mn atoms and dimers on graphene/Cu(111) in order to support experimental measurements available with theoretical calculations. We are characterizing the structural conformation of the defect by considering different possible models aiming to clarify the local structure that can reproduce the available STM images. We analyse defect formation energies while incorporating van der Waals corrections to improve the description of interatomic interactions. Additionally, we’re adding Hubbard U correction for studying the dependence of the properties mentioned above on the spin of the defect.
References
[1] Montblanch, A. R., Barbone, M., Aharonovich, I., Atatüre, M., & Ferrari, A. C. (2023). Layered materials as a platform for quantum technologies. Nature Nanotechnology, 18(6), 555–571.
[2] Lin, P., Villarreal, R., Achilli, S., Bana, H., Nair, M. N., Tejeda, A., Verguts, K., De Gendt, S., Auge, M., Hofsäss, H., De Feyter, S., Di Santo, G., Petaccia, L., Brems, S., Fratesi, G., & Pereira, L. M. C. (2021). Doping Graphene with Substitutional Mn. ACS Nano, 15(3), 5449–5458.
[3] Qiu, Z., Vaklinova, K., Huang, P., Grzeszczyk, M., Watanabe, K., Taniguchi, T., Novoselov, K. S., Lu, J., & Koperski, M. (2024). Atomic and Electronic structure of defects in HBN: Enhancing Single-Defect functionalities. ACS Nano, 18(35), 24035–24043.
[4] Ferreira, L. G., Marques, M., & Teles, L. K. (2008). Approximation to density functional theory for the calculation of band gaps of semiconductors. Physical Review B, 78(12).
[5] Claes, J., Partoens, B., & Lamoen, D. (2023). Decoupled DFT- 12 method for defect excitation energies. Physical Review. B./Physical Review. B, 108(12).
Summary of the PhD project.
Social activity: meeting and round table with drinks & catering
Monolayers of Transition Metal Dichalcogenides (TMDs) have become a central focus in condensed matter physics research due to their exceptional optoelectronic properties, which arise from strong light-matter interactions. Quantum confinement and reduced Coulomb screening make TMDs an ideal system for studying excitons (i.e. ground state and excited state excitons) and their many-body complexes (i.e. trions, biexcitons, and other multiparticle states), as well as exploring their tunability in response to external factors like electric and magnetic fields, or lattice strain. Furthermore, the ability to optically control and manipulate the spin/valley degree of freedom has significantly amplified interest in these materials. The possibility to stack two or more TMD layers and form vertical 2D heterostructures hosting long-lived interlayer excitons further enrich the excitons physics of these materials.
Since the advent of 2D materials, out-of-equilibrium spectroscopies have been extensively used to study the mechanisms of formation, relaxation, and recombination of excitons occurring on different timescales, ranging from tens of femtoseconds to nanoseconds.
Here, I will exploit helicity resolved broadband pump-probe optical spectroscopy to study valley-resolved exciton dynamics in monolayers TMD focusing on intervalley coupling dynamics between excitons, valley depolarization mechanisms and inter/intravalley biexcitons. The second part of the talk will be devoted to the study of exciton/carrier dynamics in TMD heterobilayers and hybrid TMD/halide perovskite heterostructures.
In this lecture I will present a general introduction to the exciton-phonon coupling problem with some application to phonon-assisted absorption/emission.
Exciton dynamics, encompassing ultrafast photogeneration, diffusion, and thermalization, plays a crucial role in optoelectronic, photovoltaic, and photocatalytic processes.
In this talk we discuss a novel many-body approach to describe exciton dynamics from first-principles. We show that the introduction of an auxiliary exciton species, termed “irreducible exciton”, is crucial to formulate a theory free from overscreening of the electron-phonon interaction. The resulting Excitonic Bloch Equations, while having comparable computational cost as the well-known Excitonic Boltzmann Equations, enable a comprehensive description of the temporal evolution of coherent, irreducible, and incoherent excitons during and after the optical excitation.
Within this framework, we explore the real-time dynamics of exciton formation, elucidating the mechanism by which quasi-free electron-hole pairs, generated by above-gap photoexcitation, are dynamically converted into bound excitons.
Two-dimensional (2D) materials have the potential to revolutionize the quantum technology industry by enabling the development of quantum information devices [1]. Defect centers in wide-bandgap semiconductors, such as transition metal dichalcogenides (TMDs), have been proposed as quantum bits (qubits) [2] and have been shown to behave as single photon emitters (SPE) [3]. Advances in this field require a microscopic understanding of electron-electron and electron-ion interactions in TMDs with defects, which can be achieved through first-principles simulations. However, the computational cost associated with state-of-the-art ab initio codes for such large systems is prohibitive, leaving significant gaps in the understanding of the interaction between the exciton and the defect site.
We leverage the reduced electronic Wannierized space to build a tight-binding-like two-particle Hamiltonian where the Coulomb interaction can be taken either from first-principles (from codes such as Yambo [4,5]) or from a model potential [6]. This approach allows us to compute optical responses including excitonic effects of large systems [7], bypassing computational bottlenecks related to memory and time. We validate the methodology against a fully first-principles approach by computing the optical absorption of a WS₂ monolayer and exciton dispersion of LiF bulk. We demonstrate its scalability through a study of defective WS₂ systems. Future works include the theoretical development and implementation of a Wannierized electron-electron kernel for the BSE equation, enabling the study of excitonic interactions beyond the Tamm-Dancoff approximation [8].
References
[1] X. Liu and M. Hersam., “2D materials for quantum information science” Nature Reviews Materials, volume 4, pages 669–684 (2019).
[2] A. Srivastava et al., ”Optically active quantum dots in monolayer WSe2” Nature Nanotech, volume 10, pages 491–496 (2015).
[3] He, YM. et al., “Single quantum emitters in monolayer semiconductors”. Nature Nanotech, volume 10, pages 497–502 (2015).
[4] A. Marini et al., “An ab initio tool for excited state calculations” Computer Physics Commmunications, volume 180 (8), pages 1392-1403 (2009).
[5] D. Sangalli et al., “Many-body perturbation theory calculation using the Yambo code” Journal of Physics: Condensed Matter, volume 31, 325902 (2019).
[6] D. Alexandre C et al., “Wantibexos: A Wannier based tight binding code for electronic band structure, excitonic and optoelectronic properties of solids”. Computer Physics Communications, volume 285, (2023).
[7] H. Bethe and E. Salpeter, "A Relativistic Equation for Bound-State Problems". Physical review, volume 84 (6), pages 1232-1242 (1951).
[8] S. Hirata, M. Head-Gordon, “Time-dependent density functional theory within the Tamm–Dancoff approximation”, Chemical Physics Letters, Volume 314, Pages 291-299, (1999).
Time-resolved angle-resolved photoemission spectroscopy (tr-ARPES) provides direct access to the non-equilibrium electronic structure of solids, combining sub-femtosecond temporal resolution with momentum-resolved spectral information. On the theoretical side, time-dependent density functional theory (TDDFT) has become a key ab-initio approach for modeling ultrafast processes in real materials, capturing the evolution of electronic states under time-dependent external fields. The combination of tr-ARPES and TDDFT opened the door to a first-principles understanding of photoinduced phenomena across a broad range of dynamical regimes, ranging from Floquet-engineered band structures to transient carrier populations and non-thermal electronic states. We discuss recent applications of TDDFT-based simulations of tr-ARPES, highlighting methodological advances, challenges related to spectral interpretation, and emerging opportunities for linking theory and experiment in strongly driven systems.
Using an ultrashort pulsed extreme ultraviolet laser as the photon source, angle-resolved photoemission spectroscopy (ARPES) has expanded the study of electronic structure beyond the 3D momentum space and into the temporal domain with femtosecond resolution. This advancement in studying electron dynamics greatly enhances the characterization and identification of exotic phase transitions in novel materials, particularly in charge-ordered low-dimensional materials.
In this talk, I will introduce the time-resolved ARPES implemented at the Artemis facility, along with recent results from the beamline dedicated to this technique. I will also discuss the ongoing upgrade of the materials science end-station, which will include both a momentum microscope and a hemispherical analyzer. This upgrade will maximize the capability and performance of time-resolved ARPES and will be available to UK academics and their collaborators through Laserlab-Europe starting in 2026.
This study investigates the application of response theory to describe excited-state electron charge dynamics, focusing on the phenomena of charge separation. We examine the strengths and limitations of linear and quadratic response theories, using a model that mimics the behavior of photovoltaic systems. While linear response theory effectively describes small perturbations, it cannot account for charge separation, which is essential in photovoltaic and photocatalytic applications. Quadratic response theory, however, enables a qualitatively more accurate modeling of charge dynamics under stronger fields of various forms, providing close agreement with exact time propagation methods. Additionally, we explore the role of Coulomb interactions and identify optimal conditions under which they enhance charge separation, demonstrating the broader potential of response theory in improving the design and performance of optoelectronic devices.
Magnetic skyrmions are vortex-like spin textures of potential interest for racetrack memories, nanoengines, but also quantum and neuromorphic computing. We consider magnetic skyrmions from localised-spin textures coupled to itinerant electrons. The electron dynamics is described either via Green's functions (NEGF-GKBA) or exact diagonalization, and the localised spins classically or quantum mechanically. We start with skyrmion textures made of classical spins, treated via the Landau–Lifshitz–Gilbert equation, to investigate how electronic spin currents and dilute spin disorder affect skyrmion transport. We show that the skyrmion dynamics is sensitive to the specific form of the spin disorder due to the local spin dynamics around the magnetic impurities. We then move to a quantum-mechanical treatment of the spins, to address the case of quantum nano-skyrmions. The spin-electron exchange is treated at the mean-field level, while Tensor Networks are used for the localised spins. We motivate this approach via exact benchmarks and show by examples that itinerant electrons distinctly affect the properties, including entanglement, of quantum nanoskyrmions. Finally, we discuss ongoing work where the description is extended to include optical cavity photons. Preliminary results are presented and discussed, according to the progress made before the workshop.
The Auger process is well known as a spectroscopic fingerprint for elemental identification, but it also provides a unique platform for studying electron entanglement. To gain theoretical insight into this latter aspect, we address entanglement formation in a model molecule following electronic transitions induced by a light-pulse, and its subsequent disappearance due to decoherence effects arising from the molecule nuclear dynamics and electronic de-excitation. In this pilot study, we consider a simplified molecular system coupled to a discretised continuum, allowing for a qualitative yet approximation-free exploration of entanglement dynamics. Our model is based on parameters similar to those of the Beryllium dimer, whose low dissociation energy allows us to examine possible connections between entanglement and molecular dissociation.We will also briefly report on work in progress, where we are exploring how additional fields or pulses can manipulate entanglement between the photoelectron and the Auger electron.
Optical femtosecond transient absorption and reflectivity spectroscopies are the most common experimental techniques used to study the ultrafast dynamics of photoexcited carriers and excitons in materials. However, unfortunately, the interpretation of the data is far from straightforward and often requires support from theoretical studies to be able give a reliable interpretation of the raw data.
In this presentation, we will discuss some of the limitations of the optical FTAS measurements and variations on the traditional FTAS and FTRS which give complementary information on the dynamics of ex-cited states in materials. In particular, we will discuss the use of polarization of the pump and probe sources to investigate how long the material “remembers” the polarization of the exciting light. Some results on the photoexcitation of WS2 and FAPbBr3 will be described.
Oscillations are sometimes observed superimposed on the signals of FTAS and FTRS signals. Depending on the frequency of these oscillations they are either assigned to emission of coherent optical or acoustic phonons. We have recently observed the emission of optical phonons in a thin film of NbO2 [1] and these results will be described. Also, acoustic phonons were observed in the case of single crystals of inorganic Cs3Bi2I9 and the hybrid MA3Bi2I9 and MA3Sb2I9 perovskites [2]. The mechanism behind the emission of acoustic phonons is quite different in comparison to that which generates optical phonons.
Finally, some alternative pump-probe techniques involving sources with wavelengths outside the visible/IR range such High Harmonic Generations sources and X-ray Free Electron Laser sources will be examined. Some preliminary results will be presented [3].
Keywords
Transient Absorption, Carrier Dynamics, Exciton Dynamics, Polarization, Coherent Phonons
References
[1] "Niobium oxide films with variable stoichiometry: structure, morphology and ultrafast dynamics"
Pelatti, S.; Benedetti, S.; Ammirati, G.; O'Keeffe, P.; Catone, D.; Turchini, S.; Huang, X.; Uemura, Y.; Alves Lima, F.; Milne, C.; Luches, P. accepted for publication in J. Chem. Phys. C
[2] “The speed of sound and elastic properties of single crystals of inorganic and hybrid lead-free iodide perovskites obtained by femtosecond transient optical reflectivity” G. Ammirati, P. O’Keeffe, S. Turchini, D. Catone, A. Paladini, F. Toschi, S. Gavranovic, J. Pospisil, G. Mannino, S. Valastro, and F. Martelli submitted to Phys. Rev. Mat.
[3] Ultrafast dynamics of photoinduced polaron formation in cerium oxide
S. Pelatti, E. Spurio D. Catone, P. O’Keeffe, S. Turchini, G. Ammirati, F. Paleari, D. Varsano, S. Benedetti, A. di Bona, S. D’Addato, Y. Jiang, P. Zalden, Y. Uemura, H. Wang, D. Vinci, X. Huang, F. Lima, M. Biednov, D. Khakhulin, C. Milne, F. Boscherini, P. Luches in preparation
In this talk, I will present a theoretical study on the Ehrenfest time-dependent Hartree-Screened Exchange (TD-HSEX) equations of motion, focusing on the role of core electrons and electron-phonon (e-ph) interactions in near-equilibrium regimes. TD-HSEX, derived from non-equilibrium Green’s functions (NEGF) theory[1], accurately captures coherent excitonic dynamics beyond the limitations of time-dependent density functional theory (TD-DFT)[2-5]. However, the coupling to nuclear motion introduces subtle effects. I will show that core electron screening partially cancels the bare nuclear potential, but this cancellation breaks down in Ehrenfest dynamics, where the valence density no longer follows the nuclei adiabatically, leading to a residual correction term. I will also show that, in the small displacement limit, Ehrenfest dynamics reproduces equilibrium phonon frequencies but introduces a renormalization of both frequencies and lifetimes. I will discuss how bare e-ph couplings, typically used in Ehrenfest, should be replaced by partially screened ones, accounting for electrons excluded from real-time propagation (e.g., core states in pseudopotential frameworks). These findings refine the theoretical foundation of Ehrenfest TD-HSEX and provide insights into modeling ultrafast structural dynamics and light-induced phenomena in complex materials.
Bibliography
[1] E. Perfetto, Y. Pavlyukh, and G. Stefanucci, Phys. Rev. Lett. 128, 016801 (2022).
[2] M. Gruning and C. Attaccalite, Phys. Rev. B 89, 081102 (2014).
[3] C. Attaccalite, E. Cannuccia, and M. Gruning, Phys. Rev. B 95, 125403 (2017).
[4] E. Perfetto, D. Sangalli, A. Marini, and G. Stefanucci, Phys. Rev. Materials 3, 124601 (2019).
[5] D. Sangalli, M. D’Alessandro, and C. Attaccalite, Phys. Rev. B 107, 205203 (2023).
Coherent phonons offer a powerful means to manipulate structural and electronic properties of materials on ultrafast timescales, enabling control of light-induced phase transitions and non-equilibrium dynamical phenomena.
We develop an $\textit{ab initio}$ framework to describe the excitation of coherent phonons in semimetals by combining electron-phonon coupling calculations and the time-dependent Boltzmann equation [1]. Our approach enables us to accurately model light-induced structural changes and transient band-structure renormalization following photo-excitation [2]. To illustrate its predictive capability, we investigate the fingerprints of coherent phonons in antimony (Sb) and tungsten ditelluride (WTe$\mathrm{_2}$) through a combination of tr-ARPES measurements and simulations. The robust agreement between the photoemission experiments and theory validates our methodology and highlights new opportunities to control structural and electronic degrees of freedom in semimetals via coherent phonon excitation.
[1] F. Caruso and D. Novko. Adv. Phys. X 7, 2095925 (2022)
[2] C. Emeis, et al. arXiv:2407.17118 (2024)
Approaching the attosecond time scale is crucial for advancing petahertz opto-electronics, that is the capability to coherently control and manipulate the optical properties of solid-state materials when excited by PHz fields, offering processing speed much beyond current limits.
With this respect, Attosecond Transient Reflection Spectroscopy (ATRS) in solids has emerged as a reliable tool to access the attosecond electron dynamics driven by few-femtosecond infrared (IR) pulses. Based on a pump-probe scheme, it relies on high-harmonic generation (HHG) to produce attosecond extreme-ultraviolet (XUV) pulses that serve as broad-band probing radiation, enabling a sub-femtosecond temporal resolution. The reflection geometry used in ATRS offers several advantages over more conventional absorption-based methods, including enhanced surface sensitivity and improved heat dissipation. In addition, it allows for high-intensity IR excitation (up to 10¹⁴ W/cm²), unveiling highly nonlinear processes that have remained so far unexplored.
In my discussion, I will first present the attosecond beamline at the Attosecond Research Centre at Politecnico di Milano, specifically developed to study the attosecond charge dynamics in solids. This setup features a unique two-foci geometry that allows for the measurement of transient reflectivity changes with an absolute pump-probe delay calibration. I will then present key results obtained using this scheme, including the investigation of attosecond photo-injection dynamics in germanium, the motion of virtual charges in diamond, and the ultrafast optical response of core excitons in magnesium fluoride. This will lead me to discuss our recent findings in lithium fluoride, where we observed a much more intricate interplay that results from the simultaneous excitation of virtual charge dynamics and a never-observed hybrid exciton state, arising from the interaction between a hole in the valence band and an electron in the deep conduction band.
In conclusion, ATRS allows us to study the non-linear processes that stem from coherent light-matter interaction when a material is strongly driven out of equilibrium by a few-fs IR pulse. The attosecond resolution reveals transient reflectivity changes that give direct access to the complex electron dynamics occurring in the medium, offering a deeper understanding of the strengths and limitations of such materials in the framework of petahertz opto-electronics and expanding our knowledge of fundamental solid-state physics.
Sum frequency generation (SFG) and difference frequency generation (DFG) are second order nonlinear processes where two lasers with frequencies ω_1 and ω_2 combine to produce a response at frequency ω = ω_1 ± ω_2 . Compared with other nonlinear responses, such as second harmonic generation, SFG and DFG allow for tunability over a larger range and by selecting the two laser frequencies, one can enhance the response through resonance with specific electron-hole transitions. We put forward a
first-principles framework based on the real-time solution of an effective Schrödinger equation to calculate the SFG and DFG in bulk materials. Within this framework, we can select from various levels of theory for the effective one-particle Hamiltonian to account for local-field effects and electron-hole interactions. To assess the approach, we calculate the SFG and DFG of two-dimensional crystals, h-BN and MoS2 monolayers, both within the independent-particle picture and including many-body effects.
Additionally, we demonstrate that our approach can also extract higher-order response functions, such as field-induced second harmonic generation. We provide an example using bilayer hBN.
Principal researchers and doctoral students
