arXiv subjects

Fabio Caruso

Publications and source records attributed to Fabio Caruso.

At least 19 recordsLinked to original sources

arXiv2026

X-ray driven displacive excitation of coherent phonons

Modulating electron-phonon coupling offers a route to control structural displacements and tune material functionality. Valence-to-conduction band transitions, however, provide limited leverage over the driving force. Here, we demonstrate coherent lattice dynamics in trigonal tellurium using free-electron laser pulses tuned to the Te N4,5-edge. Over a broad fluence range, the oscillation amplitude obeys the displacive excitation of coherent phonons framework, extended to core resonance with twice the driving efficiency of a visible pump. Ab initio calculations decompose the force into competing multiband contributions, inaccessible to optical excitation, whose balance shifts as carriers relax. Tunable extreme-ultraviolet and X-ray pulses thus open a regime in which the displacive response is set by band-dependent coupling to the lattice, not by the number and temperature of the photocarriers alone.

cond-mat.mtrl-sci↗

arXiv2026

Nonlinear electron-phonon coupling drives light-induced symmetry switching in charge-density waves

Ultrafast optical excitation in charge-density wave (CDW) crystals can transiently suppress long-range order, driving the lattice toward higher symmetry on femtosecond timescales. Here, we formulate and implement a first-principles theory of light-induced melting of CDW order. The approach is based on the structural dynamics in the Heisenberg picture, and it explicitly accounts for quartic lattice anharmonicities, nonlinear electron-phonon interactions, and photoexcitation-induced modifications of the potential energy surface. We illustrate these concepts through first-principles calculations of the ultrafast melting of CDW order in monolayer TiSe$_2$ - a prototypical CDW crystal with a 2$\times$2 structural reconstruction. The simulations are in good agreement with existing experiments, and they capture the defining features of CDW melting, such as the damped coherent structural motion, the transient renormalization of the soft mode, and the restoration of CDW order over timescales of a few picoseconds. Besides identifying nonlinear electron-phonon interactions as the primary mechanism driving symmetry switching in CDW systems, our work establishes a generally applicable theoretical framework to treat quartic anharmonicities and light-induced phase transitions in first-principles ultrafast dynamics simulations.

cond-mat.mtrl-sci↗

arXiv2026

Coherent Phonon-Driven Band Renormalizations in 1T$'$-MoTe$_2$

Here, we investigate phonon mode- and electron band-selective electron-phonon couplings in centrosymmetric 1T$'$-MoTe$_2$ using time- and angle-resolved photoemission spectroscopy combined with frequency-domain analysis. Femtosecond near-infrared pulses excite coherent $A_g$-symmetric phonon modes at 2.34 THz, 3.34 THz, and 3.86 THz, which manifest as oscillatory modulations in photoemission intensity and binding energy across the valence bands. Pixel-wise Fourier analysis using recently developed methodologies reveals pronounced band selectivity with distinct coupling strengths for different electronic states and phonon modes, enabling the evaluation of band-renormalization amplitudes in the range of few meV. Ab initio calculations qualitatively reproduce the experimentally observed coupling patterns and relative trends, demonstrating the capability of combined experimental and theoretical approaches to resolve ultrafast electron-phonon interactions in quantum materials.

cond-mat.mtrl-sci↗

arXiv2026

Watching Polarons Form in Real Time

Polaron formation in pump-probe experiments is an inherently non-equilibrium phenomenon, driven by the ultrafast coupled dynamics of electrons and phonons, and culminating in the emergence of a localized quasiparticle state. In this work, we present a first-principles quantum-kinetic theory of polaron formation that captures the real-time evolution of electronic and lattice degrees of freedom in presence of electron-phonon coupling. We implement this framework to investigate the ultrafast polaron formation in the prototypical polar insulator MgO. This approach allows us to determine the characteristic timescales of polaron localization and to identify its distinctive dynamical fingerprint. Our results establish clear and experimentally accessible criteria for identifying polaron formation in pump-probe experiments.

cond-mat.mtrl-sci↗

arXiv2025

Ultrafast Strongly Anisotropic Valleytronics in SnSe

Valleytronics aims to control electrons in a valley-specific manner for quantum information manipulation. Due to their strong in-plane anisotropy, which enables polarization-controlled optical transitions to distinct nondegenerate valleys, group-IV monochalcogenides have been recently proposed as promising candidates for next-generation valleytronic materials. However, ultrafast nonequilibrium dynamics following optical preparation of valley-polarized states remain completely unexplored in these systems. Combining time- and angle-resolved extreme-ultraviolet photoemission spectroscopy with time-dependent Boltzmann equation simulations, we investigate ultrafast valley polarization dynamics following polarization-controlled photoexcitation in SnSe. We show that selective excitation to valleys at global conduction minima yields nearly unity and time-independent valley polarization. In contrast, photoexcitation to the other valley channel leads to ultrafast decay and reversal of valley polarization on sub-picosecond timescales due to intervalley scattering mediated by strong electron-phonon coupling with an optical phonon mode. Our findings reveal strongly anisotropic and radically different nonequilibrium valley physics than in most common two-dimensional valleytronics materials.

cond-mat.mtrl-sci↗

arXiv2025

Impact of electron-phonon interaction on the electronic structure of interfaces between organic molecules and a MoS$_2$ monolayer

By means of first-principles calculations, we investigate the role of electron-phonon interaction in the electronic structure of hybrid interfaces, formed by MoS$_2$ and monolayers of the organic molecules pyrene and pyridine, respectively. Quasiparticle energies are initially obtained within the $G_0W_0$ approximation and subsequently used to evaluate the electron-phonon self-energy and momentum-resolved spectral functions to assess the temperature renormalization of the band structure. We find that the band-gap renormalization by zero-point vibrations of both hybrid systems is comparable to that of pristine MoS$_2$, with a value of approximately 80 meV. Pronounced features of molecular origin emerge in the spectral function of the valence region, which we attribute to satellites arising from out-of-plane vibrational modes of the organic monolayers. For pyrene, this satellite exhibits a predominantly molecular character, while for pyridine, it has a hybrid nature, originating from the coupling of molecular vibrations to the MoS$_2$ valence band.

cond-mat.mtrl-sci↗

arXiv2025

Coherent phonon control beyond amplitude saturation in a sliding ferroelectric

The breakdown of Hooke's law marks the onset of nonlinear behaviour: when displacements become large, restoring forces weaken and conventional proportionality fails. In quantum materials, intense optical excitation can drive the crystal lattice into a similar regime, where established linear relations between light, electrons, and phonons no longer hold. Sliding ferroelectrics are particularly susceptible, as controlling their polarization requires large interlayer shifts. Displacive excitation of coherent phonons, the principal mechanism for launching structural motion, typically assumes that lattice-driving forces scale linearly with the photo-excited carrier density. Whether this linearity survives at high excitation, however, remains largely unexplored, and its breakdown can fundamentally limit accessible lattice displacements. Here we show that such nonlinear limitations can be surpassed in a sliding ferroelectric by timing, rather than strengthening the optical drive. Time-resolved second-harmonic generation reveals that the interlayer sliding phonon governing ferroelectricity saturates and even diminishes under single-pulse excitation. First-principles calculations attribute this nonlinearity to band-specific electron-phonon coupling that induces competing forces on the lattice. By splitting the optical energy into two well-timed pulses that avoid populating counteracting states, we achieve markedly larger phonon amplitudes at fixed total fluence. The resulting enhanced sliding motion exposes a regime of anharmonic phonon coupling that emerges only far from equilibrium. Our findings show that nonlinear limits in driven solids can be overcome, opening new pathways for steering lattice motion in quantum materials.

physics.optics↗

arXiv2025

Electronic correlation effects in the response of graphene and MoS2 monolayers to the impact of highly-charged ions

The interaction of highly-charged ions with monolayers of graphene and MoS2 is theoretically investigated based on nonequilibrium Green Functions (NEGF). In a recent paper [Niggas et al., Phys. Rev. Lett. 129, 086802 (2022)] dramatic differences in the response of the two materials to an impacting slow ion were reported. Here, this analysis is extended, focusing on the effect of electron-electron correlations in the monolayer on the electronic response to the ion. We apply the recently developed time-linear G1-G2 scheme [Schluenzen et al., Phys. Rev. Lett. 124, 076601 (2020)] combined with an embedding approach [Balzer et al., Phys. Rev. B 107, 155141 (2023)]. We demonstrate that, while electronic correlations have a minor effect in graphene, they significantly influence the electron dynamics in the case of MoS2. Our key results are the ultrafast dynamics of the charge density and induced electrostatic potential in the vicinity of the impact point of the ion.

cond-mat.mes-hall↗

arXiv2025

Roadmap on Advancements of the FHI-aims Software Package

Electronic-structure theory is the foundation of the description of materials including multiscale modeling of their properties and functions. Obviously, without sufficient accuracy at the base, reliable predictions are unlikely at any level that follows. The software package FHI-aims has proven to be a game changer for accurate free-energy calculations because of its scalability, numerical precision, and its efficient handling of density functional theory (DFT) with hybrid functionals and van der Waals interactions. It treats molecules, clusters, and extended systems (solids and liquids) on an equal footing. Besides DFT, FHI-aims also includes quantum-chemistry methods, descriptions for excited states and vibrations, and calculations of various types of transport. Recent advancements address the integration of FHI-aims into an increasing number of workflows and various artificial intelligence (AI) methods. This Roadmap describes the state-of-the-art of FHI-aims and advancements that are currently ongoing or planned.

cond-mat.mtrl-sci↗

arXiv2025

Momentum-Resolved Signatures of Carrier Screening Effects on Electron-Phonon Coupling in MoS$_2$

Electron-phonon coupling is central to many condensed matter phenomena. Harnessing these effects for novel material functionality in materials always involves non-equilibrium electronic states, which in turn alter quasi-free-carrier density and screening. Thus, gaining a fundamental understanding of the interplay of carrier screening and electron-phonon coupling is essential for advancing ultrafast science. Prior works have mainly focused on the impact of carrier screening on electronic structure properties. Here we investigate the non-equilibrium lattice dynamics of MoS2 after a photoinduced Mott transition. The experimental data are closely reproduced by ab-initio ultrafast dynamics simulations. We find that the non-thermal diffuse scattering signals in the vicinity of the Bragg peaks, originating from long-wavelength phonon emission, can only be reproduced upon explicitly accounting for the screening of electron-phonon interaction introduced by the Mott transition. These results indicate the screening influences electron-phonon coupling, leading to a suppression of intravalley phonon-assisted carrier relaxation. Overall, the combined experimental and computational approach introduced here offers new prospects for exploring the influence of screening of the electron-phonon interactions and relaxation pathways in driven solids.

cond-mat.mtrl-sci↗

arXiv2025

Origin of phonon decoherence

Phonon decoherence determines the characteristic timescales over which coherent lattice vibrations decay, making it a crucial process for understanding the non-equilibrium dynamics of crystal lattices after excitation by a pump pulse. Here, we report a theoretical and computational investigation of the origin of phonon decoherence within a first-principles many-body framework. We derive quantum kinetic equations for the dynamics of coherent phonons by explicitly accounting for dissipation processes induced by electron-phonon and phonon-phonon interactions. The decoherence rate and frequency renormalization are formulated in terms of the non-equilibrium phonon self energy, providing a framework amenable for ab initio calculations. To validate this approach, we conduct a first-principles study of phonon decoherence for the elemental semimetals antimony and bismuth. The robust agreement with available temperature- and fluence-dependent experimental data confirms the accuracy of our theoretical and computational framework. More generally, our findings reveal that either electron-phonon and phonon-phonon coupling can prevail in determining the decoherence time, depending on the temperature and driving conditions. Overall, this work fills a critical gap in the theoretical understanding of phonon decoherence, providing a predictive framework for determining the timescales of light-induced structural dynamics in driven solids.

cond-mat.str-el↗

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

Phonon-mediated renormalization of exciton energies and absorption spectra in polar semiconductors

We investigate the influence of vibrational screening on the excitonic and optical properties of solids based on first-principles electronic-structure calculations. We solve the Bethe-Salpeter equation - the state-of-the-art description of excitons - by explicitly accounting for phonon-assisted screening effects in the screened Coulomb interaction. In the examples of the polar semiconductors ZnS, MgO, and GaN, the exciton binding energies at the absorption onset are found to be renormalized by a few tens of meV. Similar effects are also found for higher-lying unbound electron-hole pairs, leading to red-shifts of the absorption peaks by up to 50 meV. Our analysis reveals that vibrational screening is dictated by long-range Fröhlich coupling involving polar longitudinal optical phonons, whereas the remaining vibrational degrees of freedom are negligible. Overall, by elucidating the influence of phonon screening on the excitonic states and absorption spectra of these selected ionic semiconductors, this study contributes to advancing the ab initio methodology and the fundamental understanding of exciton-phonon coupling in solids.

cond-mat.mtrl-sci↗

arXiv2024

Strain-induced activation of chiral-phonon emission in monolayer WS$_2$

We report a theoretical investigation of the ultrafast dynamics of electrons and phonons in strained monolayer WS$_2$ following photoexcitation. We show that strain substantially modifies the phase space for electron-phonon scattering, unlocking new relaxation pathways that are unavailable in the pristine monolayer. In particular, strain triggers a transition between distinct dynamical regimes of the non-equilibrium lattice dynamics characterized by the emission of chiral phonons under high strain and linearly-polarized phonons under low strain. For valley-polarized electronic excitations, this mechanism can be exploited to selectively activate the emission of chiral phonons -- phonons carrying a net angular momentum. Our simulations are based on state-of-the-art ab-initio methods and focus exclusively on realistic excitation and strain conditions that have already been achieved in recent experimental studies. Overall, strain emerges as a powerful tool for controlling chiral phonons emission and relaxation pathways in multivalley quantum materials.

cond-mat.mes-hall↗

arXiv2024

A momentum-resolved view of polaron formation in materials

A combined experimental and computational methodology for interrogating the phonon contribution to polaron formation in real materials is developed. Using LiF as an example, we show that the recent ab-initio theory of Sio et. al [PRL 122, 246403 (2019)] makes predictions of the momentum- and branch dependent phonon amplitides in polaron quasiparticles that are testable using ultrafast electron diffuse scattering (UEDS) and related techniques. The large electron polaron in LiF has UEDS signatures that are qualitatively similar to those expected from a simple point-defect model, but the small hole polaron exhibits a profoundly anisotropic UEDS pattern that is in poor agreement with a point-defect model. We also show that these polaron diffuse scattering signatures are directly emblematic of the underlying polaron wavefunction. The combination of new time and momentum resolved experimental probes of nonequilibrium phonons with novel computational methods promises to complement the qualitative results obtained via model Hamiltonians with a first principles, material-specific quantiative understanding of polarons and their properties.

cond-mat.mtrl-sci↗

arXiv2023

Ultrafast nonadiabatic phonon renormalization in photoexcited single-layer MoS$_2$

Comprehending nonequilibrium electron-phonon dynamics at the microscopic level and at the short time scales is one of the main goals in condensed matter physics. Effective temperature models and time-dependent Boltzmann equations are standard techniques for exploring and understanding nonequilibrium state and the corresponding scattering channels. However, these methods consider only the time evolution of carrier occupation function, while the self-consistent phonon dressing in each time instant coming from the nonequilibrium population is ignored, which makes them less suitable for studying ultrafast phenomena where softening of the phonon modes plays an active role. Here, we combine ab-initio time-dependent Boltzmann equations and many-body phonon self-energy calculations to investigate the full momentum- and mode-resolved nonadiabatic phonon renormalization picture in the MoS$_2$ monolayer under nonequilibrium conditions. Our results show that the nonequilibrium state of photoexcited MoS$_2$ is governed by multi-valley topology of valence and conduction bands that brings about characteristic anisotropic electron-phonon thermalization paths and the corresponding phonon renormalization of strongly-coupled modes around high-symmetry points of the Brillouin zone. As the carrier population is thermalized towards its equilibrium state, we track in time the evolution of the remarkable phonon anomalies induced by nonequilibrium and the overall enhancement of the phonon relaxation rates. This work shows potential guidelines to tailor the electron-phonon relaxation channels and control the phonon dynamics under extreme photoexcited conditions.

cond-mat.mtrl-sci↗

arXiv2023

Vibrational dichroism of chiral valley phonons

Valley degrees of freedom in transition-metal dichalcogenides influence thoroughly electron-phonon coupling and its nonequilibrium dynamics. We conducted a first-principles study of the quantum kinetics of chiral phonons following valley-selective carrier excitation with circularly-polarized light. Our numerical investigations treat the ultrafast dynamics of electrons and phonons on equal footing within a parameter-free ab-initio framework. We report the emergence of valley-polarized phonon populations in monolayer MoS$_2$ that can be selectively excited at either the K or K' valleys depending on the light helicity. The resulting vibrational state is characterized by a distinctive chirality, which lifts time-reversal symmetry of the lattice on transient timescales. We show that chiral valley phonons can further lead to fingerprints of vibrational dichroism detectable by ultrafast diffuse scattering and persisting beyond 10 ps. The valley polarization of nonequilibrium phonon populations could be exploited as information carrier, thereby extending the paradigm of valleytronics to the domain of vibrational excitations.

cond-mat.mes-hall↗

arXiv2023

Plasmonic polarons induced by alkali-atom deposition in hafnium disulfide (1$T$-HfS$_2$)

We combine ab-initio calculations based on many-body perturbation theory and the cumulant expansion with angle-resolved photoemission spectroscopy (ARPES) to quantify the electron-plasmon interaction in the highly-doped semiconducting transition metal dichalcogenide 1$T$-HfS$_2$. ARPES reveals the emergence of satellite spectral features in the vicinity of quasiparticle excitations at the bottom of the conduction band, suggesting coupling to bosonic excitations with a characteristic energy of 200 meV. Our first-principles calculations of the photoemission spectral function reveal that these features can be ascribed to electronic coupling to carrier plasmons (doping-induced collective charge-density fluctuations). We further show that reduced screening at the surface enhances the electron-plasmon interaction and is primarily responsible for the emergence of plasmonic polarons.

cond-mat.str-el↗