arXiv subjects

Samuel Beaulieu

Publications and source records attributed to Samuel Beaulieu.

At least 19 recordsLinked to original sources

arXiv2026

Phonon-induced pseudogap phase in TiSe$_2$

To comprehend quantum ordered states, such as charge density waves (CDW), in layered transition metal dichalcogenides (TMDCs), it is essential to uncover their underlying normal states. Here, we use time- and angle-resolved extreme ultraviolet photoemission spectroscopy and ab initio electron-phonon calculations to perform excited state band mapping of three prototypical 1T TMDCs, i.e., TiSe$_2$, HfTe$_2$, and ZrTe$_2$, at room temperature. The results reveal the profound impact of strong electron-phonon-induced thermal fluctuations on the normal-phase electronic structure. Specifically, in the moderate electron-phonon coupling regime, as in HfTe$_2$ and ZrTe$_2$, thermal fluctuations only lead to small spectral broadening and band renormalization. In the strongly coupled case, exemplified by TiSe$_2$, we observe soft-phonon-induced, momentum-dependent suppression of spectral weight, i.e., pseudogaps - extending up to 1 eV above the Fermi level. Our work establishes the normal phase of TiSe$_2$ as a phonon-induced pseudogap phase governed by strong CDW fluctuations, thereby uncovering previously missing aspects of the TiSe$_2$ phase diagram, with broader implications for other TMDCs in the strong electron-phonon coupling regime.

cond-mat.str-el↗

arXiv2026

Enhanced Valley Polarization via Nonlinear Cascaded Quantum-Geometric Selection Rules

The quantum geometric properties of Bloch electrons fundamentally govern light-matter interactions and optical selection rules in solids. In semiconducting transition-metal dichalcogenides, circularly polarized excitation near the band edge enables valley-selective interband transitions, providing the basis for valleytronics. While nonlinear optical protocols are being developed to manipulate and probe valley selection rules, they largely rely on band-edge transitions that proceed via virtual intermediate states. Here, we demonstrate a doubly resonant cascaded nonlinear pathway from the valence band to high-lying states, mediated by a real intermediate state whose participation substantially reshapes the valley optical selection rules. Using time- and angle-resolved extreme-ultraviolet photoemission spectroscopy in combination with a time-dependent Lindblad master-equation formalism, we show that this cascaded nonlinear photoexcitation produces a substantially enhanced high-lying valley polarization compared to the conventional linear optical response near the band edge. The extension of the quantum-geometry-based selection rules to the nonlinear regime and high-lying bands offers new perspectives for ultrafast valleytronics and should play a determinant role in strong-field-driven phenomena in quantum materials.

cond-mat.mtrl-sci↗

arXiv2026

Ultrafast nonlinear Hall effect in black phosphorus

The nonlinear Hall effect (NHE) is a recently discovered member of the Hall effect family in which the Hall voltage shows a nonlinear behavior when a transverse electric field is applied. While the NHE does not require broken time-reversal symmetry, such as that induced by a magnetic field, it requires broken inversion symmetry, which limits the range of suitable systems and potential applications. Here, we demonstrate an ultrafast NHE in centrosymmetric black phosphorus through dynamical symmetry breaking using femtosecond light pulses. We provide a detailed microscopic picture of excited carrier dynamics and induced fields using momentum-resolved photoemission spectroscopy combined with \textit{ab-initio} calculations. The ultrafast NHE is observed exclusively for the light polarization aligned with the armchair high-symmetry direction and persists over 300 fs, which opens new possibilities for selective and ultrafast light-to-current conversions.

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↗

arXiv2026

Dielectric Screening in Electromagnetic Dressing of Semiconductors

Nonequilibrium manipulation of quantum materials via electromagnetic dressing provides an on-demand route to tailoring electronic band structures through Floquet engineering. Time- and angle-resolved photoemission spectroscopy offers a direct means to probe these light-dressed electronic states. In such photoemission experiments, dressing can also occur for quasi-free electrons outside the material, giving rise to Volkov states. In certain cases, strong surface screening reduces the penetration of the driving field into the solid, resulting in Volkov contributions that dominate over Floquet ones. In this work, we systematically investigate the influence of materials' dielectric properties on Floquet-Volkov dressing of semiconductors, focusing on bulk layered van der Waals materials GeS, SnS, and 2H-WSe$_2$. First, by combining a simple model based on Fresnel equations with an electron-scattering description of Volkov amplitudes, we use polarization-dependent Volkov sideband intensities to extract a lower bound for the real part of the materials' dielectric functions, which typically lie between the reported dielectric constants for monolayer and bulk crystals. We demonstrate that increasing the fluence of the pump laser enables the generation of high-order Volkov sidebands which exhibit clear signatures of nonlinear light-matter interactions. Finally, we show that for our experimental geometry, the quasi-transparent nature of semiconductors in below-band-gap driving regime allows the optical pump to propagate within the sample and undergo multiple total internal reflections, producing temporally delayed Volkov replicas in pump-probe measurements via dressing of photoelectrons by evanescent fields. These systematic studies uncover previously unexplored aspects of Floquet-Volkov dressing in solids, highlighting the role of dielectric screening of the driving field.

cond-mat.mtrl-sci↗

arXiv2026

Accurate and efficient simulation of photoemission spectroscopy via Kohn-Sham scattering states

We introduce an efficient first-principles framework for simulating angle-resolved photoemission spectroscopy (ARPES) based on the direct computation of photoelectron states as solutions of the Kohn-Sham equation with scattering boundary conditions. While the one-step theory of photoemission has a long and successful history, existing implementations are often tied to specialized electronic-structure formalisms. Our approach is formally equivalent to the Lippmann-Schwinger formulation, and it is directly compatible with standard plane-wave and real-space density functional theory codes, enabling seamless integration with advanced exchange-correlation functionals and modern electronic-structure workflows. By providing explicit photoelectron wave functions, the method allows for a transparent analysis of matrix-element effects, multiple scattering, and experimental geometry. We demonstrate the accuracy and predictive power of the framework through circular-dichroism ARPES simulations for monolayer graphene and bulk $2H$-WSe$_2$, achieving excellent agreement with experimental data over a wide photon-energy range. Our results establish a robust and accessible route toward quantitative ARPES modeling, opening the door to systematic studies of orbital textures, many-body effects, and nonequilibrium phenomena within widely used ab initio platforms.

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

Ultrafast Nonequilibrium Enhancement of Electron-Phonon Interaction in 2H-MoTe$_2$

Understanding nonequilibrium electron-phonon interactions at the microscopic level and on ultrafast timescales is a central goal of modern condensed matter physics. Combining time- and angle-resolved extreme ultraviolet photoemission spectroscopy with constrained density functional perturbation theory, we demonstrate that photoexcited carrier density can serve as a tuning knob to enhance electron-phonon interactions in nonequilibrium conditions. Specifically, nonequilibrium band structure mapping and valley-resolved ultrafast population dynamics in semiconducting transition-metal dichalcogenide 2H-MoTe$_2$ reveal band-gap renormalizations and reduced population lifetimes as photoexcited carrier densities increase. Through theoretical analysis of photoinduced electron and phonon energy and linewidth renormalizations, we attribute these transient features to nonequilibrium modifications of electron-phonon coupling matrix elements. The present study advances our understanding of microscopic coupling mechanisms enabling control over relaxation pathways in driven solids.

cond-mat.mtrl-sci↗

arXiv2025

Electron-phonon-dominated charge-density-wave fluctuations in TiSe$_2$ accessed by ultrafast nonequilibrium dynamics

The complex phase diagram of 1T-TiSe2 consists of a charge density wave (CDW) below 200 K, and CDW fluctuations of still unknown origin at higher temperatures. Here, we use time-resolved extreme ultraviolet momentum microscopy and density functional perturbation theory to uncover the formation mechanism of CDW fluctuations and their spectral features at 295 K. We investigated the transient dynamics of fluctuations upon nonresonant ultrafast photoexcitation, and directly correlate it with the CDW soft-phonon hardening. Surprisingly, our results show that the coherent amplitude mode modulating ultrafast CDW recovery persists above TCDW, and reveal that CDW fluctuations are dominated by the electron-phonon interaction rather than excitonic correlations as commonly believed. Our findings on these microscopic CDW fluctuations clarify the complex interplay between electronic and lattice degrees of freedom at elevated temperatures and, therefore, could be useful in understanding the nature of the CDW phase transition in 1T-TiSe2 and similar quantum materials.

cond-mat.str-el↗

arXiv2025

Time- and Polarization-Resolved Extreme Ultraviolet Momentum Microscopy

We report the development of an instrument combining an ultrafast, high-repetition-rate, polarization-tunable monochromatic extreme ultraviolet (XUV, 21.6 eV) beamline and a next-generation momentum microscope endstation. This setup enables time- and angle-resolved photoemission spectroscopy of quantum materials, offering multimodal photoemission dichroism capabilities. The momentum microscope simultaneously detects the full surface Brillouin zone over an extended binding energy range. It is equipped with advanced electron optics, including a new type of front lens that supports multiple operational modes. Enhanced spatial resolution is achieved by combining the small XUV beam footprint (33 $\mu$m by 45 $\mu$m) with the selection of small regions of interest using apertures positioned in the Gaussian plane of the momentum microscope. This instrument achieves an energy resolution of 44 meV and a temporal resolution of 144 fs. We demonstrate the capability to perform linear, Fourier, and circular dichroism in photoelectron angular distributions from photoexcited 2D materials. This functionality paves the way for time-, energy-, and momentum-resolved investigations of orbital and quantum geometrical properties underlying electronic structures of quantum materials driven out of equilibrium.

cond-mat.mtrl-sci↗

arXiv2024

Frontier orbitals control dynamical disorder in molecular semiconductors

Charge transport in organic semiconductors is limited by dynamical disorder. Design rules for new high-mobility materials have therefore focused on limiting its two foundations: structural fluctuations and the transfer integral gradient. However, it has remained unclear how these goals should be translated into molecular structures. Here we show that a specific shape of the frontier orbital, with a lack of nodes along the long molecular axis, reduces the transfer integral gradient and therefore the dynamical disorder. We investigated single crystals of the prototypical molecular semiconductors pentacene and picene by angle-resolved photoemission spectroscopy and dynamical disorder calculations. We found that picene exhibits a remarkably low dynamical disorder. By separating in- and out-of-plane components of dynamical disorder, we identify the reason as a reduced out-of-plane disorder from a small transfer integral derivative. Our results demonstrate that molecules with an armchair $\pi$-electron topology and same-phase frontier orbitals like picene are promising molecular building blocks for the next generation of organic semiconductors.

cond-mat.mtrl-sci↗

arXiv2024

Floquet-Bloch Valleytronics

Driving quantum materials out-of-equilibrium makes it possible to generate states of matter inaccessible through standard equilibrium tuning methods. Upon time-periodic coherent driving of electrons using electromagnetic fields, the emergence of Floquet-Bloch states enables the creation and control of exotic quantum phases. In transition metal dichalcogenides, broken inversion symmetry within each monolayer results in a non-zero Berry curvature at the K and K$^{\prime}$ valley extrema, giving rise to chiroptical selection rules that are fundamental to valleytronics. Here, we bridge the gap between these two concepts and introduce Floquet-Bloch valleytronics. Using time- and polarization-resolved extreme ultraviolet momentum microscopy combined with state-of-the-art ab initio theory, we demonstrate the formation of valley-polarized Floquet-Bloch states in 2H-WSe$_2$ upon below-bandgap coherent electron driving with chiral light pulses. We investigate quantum path interference between Floquet-Bloch and Volkov states, showing that this interferometric process depends on the valley pseudospin and light polarization-state. Conducting extreme ultraviolet photoemission circular dichroism in these nonequilibrium settings reveals the potential for controlling the orbital character of Floquet-engineered states. These findings link Floquet engineering and quantum geometric light-matter coupling in two-dimensional materials. They can serve as a guideline for reaching novel out-of-equilibrium phases of matter by dynamically breaking symmetries through coherent dressing of winding Bloch electrons with tailored light pulses.

cond-mat.mes-hall↗

arXiv2024

Excited States Band Mapping and Ultrafast Nonequilibrium Dynamics in Topological Dirac Semimetal 1T-ZrTe$_2$

We performed time- and polarization-resolved extreme ultraviolet momentum microscopy on topological Dirac semimetal candidate 1T-ZrTe$_2$. Excited states band mapping uncovers the previously inaccessible linear dispersion of the Dirac cone above the Fermi level. We study the orbital texture of bands using linear dichroism in photoelectron angular distributions. These observations provide hints on the topological character of 1T-ZrTe$_2$. Time-, energy- and momentum-resolved nonequilibrium carrier dynamics reveal that intra- and inter-band scattering processes play a capital role in the relaxation mechanism, leading to multivalley electron-hole accumulation near the Fermi level. We also show that electrons' inverse lifetime has a linear dependence on their binding energy. Our time- and polarization-resolved XUV photoemission results shed light on the excited state electronic structure of 1T-ZrTe$_2$ and provide valuable insights into the relatively unexplored field of quantum-state-resolved ultrafast dynamics in 3D topological Dirac semimetals.

cond-mat.mtrl-sci↗

arXiv2024

Strong-field ionization of chiral molecules with bicircular laser fields : sub-barrier dynamics, interference, and vortices

Strong-field ionization by counter-rotating two-color laser fields produces quantum interference between photoelectrons emitted on the leading and trailing edges of the laser field oscillations. We show that in chiral molecules, this interference is asymmetric along the light propagation direction and strongly enhances the sensitivity of the attoclock scheme to molecular chirality. Calculations in a toy-model molecule with a short-range chiral potential show that this enhanced sensitivity already emerges at the exit of the tunnel. We investigate the possible sources of chiral sensitivity in the tunneling process, and find that the interference between electron vortices plays a crucial role in the chiral response.

physics.atom-ph↗

arXiv2023

Berry Curvature Signatures in Chiroptical Excitonic Transitions

The topology of the electronic band structure of solids can be described by its Berry curvature distribution across the Brillouin zone. We theoretically introduce and experimentally demonstrate a general methodology based on the measurement of energy- and momentum-resolved optical transition rates, allowing to reveal signatures of Berry curvature texture in reciprocal space. By performing time- and angle-resolved photoemission spectroscopy of atomically thin WSe$_2$ using polarization-modulated excitations, we demonstrate that excitons become an asset in extracting the quantum geometrical properties of solids. We also investigate the resilience of our measurement protocol against ultrafast scattering processes following direct chiroptical transitions.

cond-mat.mtrl-sci↗

arXiv2023

Ultrafast Hidden Spin Polarization Dynamics of Bright and Dark Excitons in 2H-WSe$_2$

We performed spin-, time- and angle-resolved extreme ultraviolet photoemission spectroscopy (STARPES) of excitons prepared by photoexcitation of inversion-symmetric 2H-WSe$_2$ with circularly polarized light. The very short probing depth of XUV photoemission permits selective measurement of photoelectrons originating from the top-most WSe$_2$ layer, allowing for direct measurement of hidden spin polarization of bright and momentum-forbidden dark excitons. Our results reveal efficient chiroptical control of bright excitons' hidden spin polarization. Following optical photoexcitation, intervalley scattering between nonequivalent K-K' valleys leads to a decay of bright excitons' hidden spin polarization. Conversely, the ultrafast formation of momentum-forbidden dark excitons acts as a local spin polarization reservoir, which could be used for spin injection in van der Waals heterostructures involving multilayer transition metal dichalcogenides.

cond-mat.mes-hall↗

arXiv2022

Observation of multi-directional energy transfer in a hybrid plasmonic-excitonic nanostructure

Hybrid plasmonic devices involve a nanostructured metal supporting localized surface plasmons to amplify light-matter interaction, and a non-plasmonic material to functionalize charge excitations. Application-relevant epitaxial heterostructures, however, give rise to ballistic ultrafast dynamics that challenge the conventional semiclassical understanding of unidirectional nanometal-to-substrate energy transfer. We study epitaxial Au nanoislands on WSe$_2$ with time- and angle-resolved photoemission spectroscopy and femtosecond electron diffraction: this combination of techniques resolves material, energy and momentum of charge-carriers and phonons excited in the heterostructure. We observe a strong non-linear plasmon-exciton interaction that transfers the energy of sub-bandgap photons very efficiently to the semiconductor, leaving the metal cold until non-radiative exciton recombination heats the nanoparticles on hundreds of femtoseconds timescales. Our results resolve a multi-directional energy exchange on timescales shorter than the electronic thermalization of the nanometal. Electron-phonon coupling and diffusive charge-transfer determine the subsequent energy flow. This complex dynamics opens perspectives for optoelectronic and photocatalytic applications, while providing a constraining experimental testbed for state-of-the-art modelling.

cond-mat.mes-hall↗

arXiv2022

Ultrafast Polarization-Tunable Monochromatic Extreme Ultraviolet Source at High-Repetition-Rate

We report on the development of a high-order harmonic generation (HHG)-based ultrafast high-repetition-rate (250 kHz) monochromatic extreme ultraviolet (21.6 eV) source with polarization tunability, specifically designed for multi-modal dichroism in time- and angle-resolved photoemission spectroscopy. Driving HHG using an annular beam allows us to spatially separate the high-power 515 nm driving laser from the XUV beamlet while preserving the linear polarization axis angle tunability. This enables controlling the polarization state of the XUV radiation with a fixed all-reflective phase-shifter. This scheme leads to a source brightness of 5.7$\times$10$^{12}$ photons/s at 21.6 eV (up to 4$\times$10$^{11}$ photons/s on target) with ellipticities as high as 90$\%$.

physics.optics↗