arXiv subjects

Umberto De Giovannini

Publications and source records attributed to Umberto De Giovannini.

At least 19 recordsLinked to original sources

arXiv2026

First-principles electron-phonon scattering in real-time TDDFT

Real-time time-dependent density functional theory provides a first-principles description of coherent electron dynamics in laser-driven solids, but its unitary formulation cannot capture the irreversible scattering, relaxation, and decoherence processes that drive excited carriers toward equilibrium. Here, we develop a dissipative rt-TDDFT framework in which first-principles electron-phonon interactions enter the evolution of the reduced one-body density matrix through self-energy-derived collision integrals within the Born-Markov approximation. The approach retains the quantum-coherent real-time propagation of the electronic system while introducing phonon-mediated transitions that redistribute carriers in energy and crystal momentum, thereby incorporating the microscopic momentum-transfer processes responsible for relaxation in real materials. The resulting framework provides a practical first-principles route to simulate relaxation, decoherence, and time-resolved spectroscopic signatures in realistic crystalline materials.

cond-mat.mtrl-sci↗

arXiv2026

First-principles Floquet analysis from real-time propagation

We present a real-time Floquet analysis method for extracting quasi-energies and Floquet states directly from propagated wavefunctions. By reconstructing the one-period evolution operator from overlaps between time-evolved states, the method avoids the explicit construction of the enlarged Floquet Hamiltonian and adds negligible computational overhead to time-dependent simulations. To resolve the ambiguity inherent in the reduced-zone representation, we introduce an unfolding procedure based on the harmonic decomposition of Floquet states, which recovers their underlying equilibrium band character beyond the reduced Floquet Brillouin zone. The reconstructed wavefunctions further provide access to the symmetry properties of individual light-induced sidebands. We demonstrate the applicability and generality of the method in first-principles time-dependent simulations of real materials, ranging from two-dimensional monolayers to a three-dimensional bulk semiconductor, and including finite pulses without strict time periodicity. This framework directly connects real-time simulations with Floquet observables, enabling practical analysis of light-driven electronic structure in materials.

cond-mat.mtrl-sci↗

arXiv2026

Circular dichroism in the photoelectron angular distribution of achiral molecules

Circular dichroism in the angular distribution (CDAD) is the effect that the angular intensity distribution of photoemitted electrons depends on the handedness of the incident circularly polarized light. A CDAD may arise from intrinsic material properties like chirality, spin-orbit interaction, or quantum-geometrical effects on the electronic structure. In addition, CDAD has also been reported for achiral organic molecules at the interface to metallic substrates. For this latter case, we investigate two prototypical $π$-conjugated molecules, namely tetracene and pentacene, whose frontier orbitals have a similar shape but exhibit distinctly different symmetries. By comparing experimental CDAD momentum maps with simulations within time-dependent density functional theory, we show how the final state of the photoelectron must be regarded as the source of the CDAD in such otherwise achiral systems. We gain additional insight into the mechanism by employing a simple scattering model for the final state, which allows us to decompose the CDAD signal into partial wave contributions.

cond-mat.mtrl-sci↗

arXiv2026

Symmetry-Driven Floquet Engineering in Multivalley SnS

Coherent interactions between time-periodic electromagnetic fields and materials offer a powerful platform for engineering light-matter hybrid Floquet states with tailored functionalities. In particular, the ability to manipulate the wavefunction symmetry of such Floquet states has recently emerged as a new frontier in the field of nonequilibrium control of quantum materials. Here, we investigate symmetry-driven Floquet engineering in bulk multivalley semiconductor tin sulfide (SnS) using time-, polarization-, and angle-resolved extreme ultraviolet photoemission spectroscopy, group-theory analysis, and time-dependent density functional theory. We demonstrate that the material's inherent symmetry gives rise to pronounced symmetry-driven photoemission selection rules for both equilibrium bands and light-induced Floquet states, which we probed through nonequilibrium linear dichroism in extreme ultraviolet photoemission. By leveraging the interplay between crystal and driving-light symmetry, we establish deterministic control over the parity of Floquet--Bloch states. Indeed, we show that the symmetry of Floquet--Bloch states can be fully controlled by the relative alignment between the drive polarization and the crystal axes, enabling selective parity inversion with respect to the equilibrium valence and conduction bands. Furthermore, we show that symmetry-driven parity engineering allows for polarization- and valley-selective band renormalization. These findings advance the understanding of guiding principles for wavefunction symmetry engineering, providing pathways for selectively controlling both the parity and renormalization of electronic states in quantum materials using tailored electromagnetic fields.

cond-mat.mtrl-sci↗

arXiv2025

Polarized Houston State Framework for Nonequilibrium Driven Open Quantum Systems

We introduce a new theoretical framework -- the polarized Houston basis -- to model nonequilibrium dynamics in driven open quantum systems, formulated for use within the quantum master equation. This basis extends conventional Houston states by incorporating field-induced polarization effects, enabling a more accurate description of excitation dynamics under external driving. Using a one-dimensional dimer-chain model, we examine band population dynamics through projections onto polarized Houston states, original Houston states, and naive Bloch states. We find that the polarized Houston basis significantly suppresses spurious Bloch-state excitations and virtual transitions present in standard Houston approaches, allowing for a cleaner extraction of real excitations. When implemented in the relaxation time approximation of the quantum master equation, this formalism also yields a substantial reduction of unphysical DC currents in insulating systems. Our results highlight the polarized Houston basis as a powerful tool for simulating nonequilibrium phenomena in light-driven open quantum materials.

quant-ph↗

arXiv2025

Floquet optical selection rules in black phosphorus

Optical selection rules endorsed by symmetry are crucial for understanding the optical properties of quantum materials and the associated ultrafast spectral phenomena. Here, we introduce momentum-resolved Floquet optical selection rules using group theory to elucidate the pump-probe photoemission spectral distributions of monolayer black phosphorus (BP), which are governed by the symmetries of both the material and the lasers. Using time-dependent density functional theory (TDDFT), we further investigate the dynamical evolution of Floquet(-Volkov) states in the photoemission spectra of monolayer BP, revealing their spectral weights at specific momenta for each sideband. These observations are comprehensively explained by the proposed Floquet optical selection rules. Our framework not only clarifies experimental photoemission spectra but also uncovers unexplored characteristics under different pump-probe configurations. Our results are expected to deepen the understanding of light-induced ultrafast spectra in BP and can be extended to other Floquet systems.

cond-mat.mtrl-sci↗

arXiv2025

The 2025 Roadmap to Ultrafast Dynamics: Frontiers of Theoretical and Computational Modelling

The exploration of ultrafast phenomena is a frontier of condensed matter research, where the interplay of theory, computation, and experiment is unveiling new opportunities for understanding and engineering quantum materials. With the advent of advanced experimental techniques and computational tools, it has become possible to probe and manipulate nonequilibrium processes at unprecedented temporal and spatial resolutions, providing insights into the dynamical behavior of matter under extreme conditions. These capabilities have the potential to revolutionize fields ranging from optoelectronics and quantum information to catalysis and energy storage. This Roadmap captures the collective progress and vision of leading researchers, addressing challenges and opportunities across key areas of ultrafast science. Contributions in this Roadmap span the development of ab initio methods for time-resolved spectroscopy, the dynamics of driven correlated systems, the engineering of materials in optical cavities, and the adoption of FAIR principles for data sharing and analysis. Together, these efforts highlight the interdisciplinary nature of ultrafast research and its reliance on cutting-edge methodologies, including quantum electrodynamical density-functional theory, correlated electronic structure methods, nonequilibrium Green's function approaches, quantum and ab initio simulations.

cond-mat.mtrl-sci↗

arXiv2024

Attosecond All-Optical Retrieval of Valley Polarization via Circular Dichroism in Transient Absorption

We propose a scheme for retrieving the ultrafast valley polarization (VP) dynamics in two-dimensional hexagonal materials via attosecond circular dichroism (CD) transient absorption spectroscopy. This approach builds on the CD transition between the first and higher conduction bands induced by the circularly polarized probe pulses. The population imbalance at nonequivalent valleys in the first conduction band is proportionally mapped onto the difference in absorption coefficients of two probe pulses with opposite helicities, supporting an unprecedented quantitative retrieval of the corresponding VP dynamics with subfemtosecond time resolution. We theoretically demonstrate the scheme for h-BN and MoS2 through ab initio calculations, achieving an accurate retrieval of the VP dynamics, particularly the transient VP switching processes, with a time resolution of 250 as.

physics.optics↗

arXiv2024

Cavity engineered phonon-mediated superconductivity in MgB$_2$ from first principles quantum electrodynamics

Strong laser pulses can control superconductivity, inducing non-equilibrium transient pairing by leveraging strong-light matter interaction. Here we demonstrate theoretically that equilibrium ground-state phonon-mediated superconductive pairing can be affected through the vacuum fluctuating electromagnetic field in a cavity. Using the recently developed ab initio quantum electrodynamical density-functional theory approximation, we specifically investigate the phonon-mediated superconductive behavior of MgB$_2$ under different cavity setups and find that in the strong light-matter coupling regime its superconducting transition temperature can be, in principles, enhanced by $\approx 73\%$ ($\approx 40\%$) in an in-plane (out-of-plane) polarized cavity. However, in a realistic cavity, we expect the T$_{\rm{c}}$ of MgB$_2$ can increase, at most, by $5$ K via photon vacuum fluctuations. The results highlight that strong light-matter coupling in extended systems can profoundly alter material properties in a non-perturbative way by modifying their electronic structure and phononic dispersion at the same time. Our findings indicate a pathway to the experimental realization of light-controlled superconductivity in solid-state materials at equilibrium via cavity-material engineering.

cond-mat.supr-con↗

arXiv2024

Direct observation of Floquet-Bloch states in monolayer graphene

Floquet engineering is a novel method of manipulating quantum phases of matter via periodic driving [1, 2]. It has successfully been utilized in different platforms ranging from photonic systems [3] to optical lattice of ultracold atoms [4, 5]. In solids, light can be used as the periodic drive via coherent light-matter interaction. This leads to hybridization of Bloch electrons with photons resulting in replica bands known as Floquet-Bloch states. After the direct observation of Floquet-Bloch states in a topological insulator [6], their manifestations have been seen in a number of other experiments [7-14]. By engineering the electronic band structure using Floquet-Bloch states, various exotic phase transitions have been predicted [15-22] to occur. To realize these phases, it is necessary to better understand the nature of Floquet-Bloch states in different materials. However, direct energy and momentum resolved observation of these states is still limited to only few material systems [6, 10, 14, 23, 24]. Here, we report direct observation of Floquet-Bloch states in monolayer epitaxial graphene which was the first proposed material platform [15] for Floquet engineering. By using time- and angle-resolved photoemission spectroscopy (trARPES) with mid-infrared (mid-IR) pump excitation, we detected replicas of the Dirac cone. Pump polarization dependence of these replica bands unequivocally shows that they originate from the scattering between Floquet-Bloch states and photon-dressed free-electron-like photoemission final states, called Volkov states. Beyond graphene, our method can potentially be used to directly observe Floquet-Bloch states in other systems paving the way for Floquet engineering in a wide range of quantum materials.

cond-mat.mes-hall↗

arXiv2023

Are there universal signatures of topological phases in high harmonic generation? Probably not

High harmonic generation (HHG) has developed in recent years as a promising tool for ultrafast materials spectroscopy. At the forefront of these advancements, several works proposed to use HHG as an all-optical probe for topology of quantum matter by identifying its signatures in the emission spectra. However, it remains unclear if such spectral signatures are indeed a robust and general approach for probing topology. To address this point, we perform here a fully ab-initio study of HHG from prototypical two-dimensional topological insulators in the Kane-Mele quantum spin-Hall and anomalous-Hall phases. We analyze the spectra and previously proposed topological signatures by comparing HHG from the topological and trivial phases. We demonstrate and provide detailed microscopic explanations of why in these systems none of the thus far proposed observables uniquely and universally probe material topology. Specifically, we find that the: (i) HHG helicity, (ii) anomalous HHG ellipticity, (iii) HHG elliptical dichroism and (iv) temporal delays in HHG emission, are all unreliable signatures of topological phases. Our results suggest that extreme care must be taken when interpreting HHG spectra for topological signatures, and that contributions from the crystal symmetries and chemical nature might be dominant over those from topology. They hint that a truly universal topological signature in nonlinear optics is unlikely, and raise important questions regarding possible utilization and detection of topology in systems out-of-equilibrium.

cond-mat.mtrl-sci↗

arXiv2023

Mapping light-dressed Floquet bands by highly nonlinear optical excitations and valley polarization

Ultrafast nonlinear optical phenomena in solids have been attracting major interest as novel methodologies for femtosecond spectroscopy of electron dynamics and control of material properties. Here, we theoretically investigate strong-field nonlinear optical transitions in a prototypical two-dimensional material, hBN, and show that the k-resolved conduction band charge occupation patterns induced by an elliptically-polarized laser can be understood in a multi-photon resonant picture; but remarkably, only if using the Floquet light-dressed states instead of the undressed matter states. Consequently, our work establishes a direct measurable signature for band-dressing in nonlinear optical processes in solids, and opens new paths for ultrafast spectroscopy and valley manipulation.

physics.optics↗

arXiv2022

Eliminating artificial boundary conditions in time-dependent density functional theory using Fourier contour deformation

We present an efficient method for propagating the time-dependent Kohn-Sham equations in free space, based on the recently introduced Fourier contour deformation (FCD) approach. For potentials which are constant outside a bounded domain, FCD yields a high-order accurate numerical solution of the time-dependent Schrodinger equation directly in free space, without the need for artificial boundary conditions. Of the many existing artificial boundary condition schemes, FCD is most similar to an exact nonlocal transparent boundary condition, but it works directly on Cartesian grids in any dimension, and runs on top of the fast Fourier transform rather than fast algorithms for the application of nonlocal history integral operators. We adapt FCD to time-dependent density functional theory (TDDFT), and describe a simple algorithm to smoothly and automatically truncate long-range Coulomb-like potentials to a time-dependent constant outside of a bounded domain of interest, so that FCD can be used. This approach eliminates errors originating from the use of artificial boundary conditions, leaving only the error of the potential truncation, which is controlled and can be systematically reduced. The method enables accurate simulations of ultrastrong nonlinear electronic processes in molecular complexes in which the inteference between bound and continuum states is of paramount importance. We demonstrate results for many-electron TDDFT calculations of absorption and strong field photoelectron spectra for one and two-dimensional models, and observe a significant reduction in the size of the computational domain required to achieve high quality results, as compared with the popular method of complex absorbing potentials.

physics.chem-ph↗

arXiv2022

A time-based Chern number in periodically-driven systems in the adiabatic limit

To define the topology of driven systems, recent works have proposed synthetic dimensions as a way to uncover the underlying parameter space of topological invariants. Using time as a synthetic dimension, together with a momentum dimension, gives access to a synthetic 2D Chern number. It is, however, still unclear how the synthetic 2D Chern number is related to the Chern number that is defined from a parametric variable that evolves with time. Here we show that in periodically driven systems in the adiabatic limit, the synthetic 2D Chern number is a multiple of the Chern number defined from the parametric variable. The synthetic 2D Chern number can thus be engineered via how the parametric variable evolves in its own space. We justify our claims by investigating Thouless pumping in two 1D tight-binding models, a three-site chain model and a two-1D-sliding-chains model. The present findings could be extended to higher dimensions and other periodically driven configurations.

cond-mat.mtrl-sci↗

arXiv2022

Simulating terahertz field-induced ferroelectricity in quantum paraelectric SrTiO$_3$

Recent experiments have demonstrated that light can induce a transition from the quantum paraelectric to the ferroelectric phase of SrTiO$_3$. Here, we investigate this terahertz field-induced ferroelectric phase transition by solving the time-dependent lattice Schrödinger equation based on first-principles calculations. We find that ferroelectricity originates from a light-induced mixing between ground and first excited lattice states in the quantum paraelectric phase. In agreement with the experimental findings, our study shows that the non-oscillatory second harmonic generation signal can be evidence of ferroelectricity in SrTiO$_3$. We reveal the microscopic details of this exotic phase transition and highlight that this phenomenon is a unique behavior of the quantum paraelectric phase.

cond-mat.mtrl-sci↗

arXiv2022

Attosecond magnetization dynamics in non-magnetic materials driven by intense femtosecond lasers

Irradiating solids with ultrashort laser pulses is known to initiate femtosecond timescale magnetization dynamics. However, sub-femtosecond spin dynamics have not yet been observed or predicted. Here, we explore ultrafast light-driven spin dynamics in a highly non-resonant strong-field regime. Through state-of-the-art ab-initio calculations, we predict that a non-magnetic material can be transiently transformed into a magnetic one via dynamical extremely nonlinear spin-flipping processes, which occur on attosecond timescales and are mediated by a combination of multi-photon and spin-orbit interactions. These are non-perturbative non-resonant analogues to the inverse Faraday effect that build up from cycle-to-cycle as electrons gain angular momentum. Remarkably, we show that even for linearly polarized driving, where one does not intuitively expect any magnetic response, the magnetization transiently oscillates as the system interacts with light. This oscillating response is enabled by transverse anomalous light-driven currents in the solid, and typically occurs on timescales of ~500 attoseconds. We further demonstrate that the speed of magnetization can be controlled by tuning the laser wavelength and intensity. An experimental set-up capable of measuring these dynamics through pump-probe transient absorption spectroscopy is outlined and simulated. Our results pave the way for new regimes of ultrafast manipulation of magnetism.

cond-mat.mes-hall↗

arXiv2022

Infrared Plasmons Propagate through a Hyperbolic Nodal Metal

Metals are canonical plasmonic media at infrared and optical wavelengths, allowing one to guide and manipulate light at the nano-scale. A special form of optical waveguiding is afforded by highly anisotropic crystals revealing the opposite signs of the dielectric functions along orthogonal directions. These media are classified as hyperbolic and include crystalline insulators, semiconductors and artificial metamaterials. Layered anisotropic metals are also anticipated to support hyperbolic waveguiding. Yet this behavior remains elusive, primarily because interband losses arrest the propagation of infrared modes. Here, we report on the observation of propagating hyperbolic waves in a prototypical layered nodal-line semimetal ZrSiSe. The observed waveguiding originates from polaritonic hybridization between near-infrared light and nodal-line plasmons. Unique nodal electronic structures simultaneously suppress interband loss and boost the plasmonic response, ultimately enabling the propagation of infrared modes through the bulk of the crystal.

cond-mat.mes-hall↗

arXiv2022

Controlling Floquet states on ultrashort time scales

The advent of ultrafast laser science offers the unique opportunity to combine Floquet engineering with extreme time resolution, further pushing the optical control of matter into the petahertz domain. However, what is the shortest driving pulse for which Floquet states can be realised remains an unsolved matter, thus limiting the application of Floquet theory to pulses composed by many optical cycles. Here we ionized Ne atoms with few-femtosecond pulses of selected time duration and show that a Floquet state can be established already within 10 cycles of the driving field. For shorter pulses, down to 2 cycles, the finite lifetime of the driven state can still be explained using an analytical model based on Floquet theory. By demonstrating that the population of the Floquet sidebands can be controlled not only with the driving laser pulse intensity and frequency, but also by its duration, our results add a new lever to the toolbox of Floquet engineering.

physics.optics↗