arXiv subjects

Mauro Fanciulli

Publications and source records attributed to Mauro Fanciulli.

18 recordsLinked to original sources

arXiv2025

Soft x-rays with Orbital Angular Momentum for resonant scattering experiments at the SOLEIL synchrotron

The paper presents a comprehensive description of a new setup implemented and commissioned at the SEXTANTS beamline of the SOLEIL synchrotron for absorption and scattering experiments with x-ray beams carrying an orbital angular momentum, also known as twisted x-ray beams. Two alternative methods have been implemented, based on the use of either spiral zone plates or fork gratings devices, and we show how they can be used for both defining and assessing the orbital angular momentum of an x-ray beam. We show also how multiple devices can be used in sequence to define an integer arithmetic of the orbital angular momentum of the final x-ray beam. Finally, we report the results of the first resonant scattering pilot experiments in transmission and reflection mode, intended to assess the feasibility of future users measurements. The availability of twisted soft x-rays complements the range of experimental techniques in elastic, resonant and coherent scattering available at the SEXTANTS beamline of the SOLEIL synchrotron.

cond-mat.mtrl-sci↗

arXiv2025

Optical spin-orbit interaction induced by magnetic textures

Contrary to the optical spin angular momentum (SAM), the role played by the orbital angular momentum (OAM) of light in magneto-optics remains largely unexplored. However, the SAM and OAM are known to be coupled when light interacts with non-homogeneous and non-isotropic materials. Here we predict that the OAM carried by each photon in a light beam is modified upon reflection on magnetic textures like skyrmions, and that the sign of this variation is governed by the SAM of the incident field. Our predictions can be readily tested by performing circular dichroism measurements, and may provide new routes to shape light's angular momentum with magnetism.

physics.optics↗

arXiv2025

Dependency of quantum time scales on symmetry

Although used extensively in everyday life, time is one of the least understood quantities in physics, especially on the level of quantum mechanics. Here we use an experimental method based on spin- and angle-resolved photoemission spectroscopy from spin-degenerate dispersive states to determine the Eisenbud-Wigner-Smith (EWS) time delay of photoemission. This time scale of the quantum transition is measured for materials with different dimensionality and correlation strength. A direct link between the dimensionality, or rather the symmetry of the system, and the attosecond photoionisation time scale is found. The quasi 2-dimensional transition metal dichalcogenides 1T-TiSe$_2$ and 1T-TiTe$_2$ show time scales around 150 as, whereas in quasi 1-dimensional CuTe the photoionisation takes more than 200 as. This is in stark contrast with the 26 as found for 3-dimensional pure Cu. These results provide new insights into the role of symmetry in quantum time scales and may provide a route to understanding the role of time in quantum mechanics.

cond-mat.mes-hall↗

arXiv2025

ARPES studies of Hf(0001) surface: flat bands formation in the dice lattice

We present the first electronic structure measurements of the Hf(0001) single-crystal surface using angle-resolved photoemission spectroscopy (ARPES). The ARPES results are supported by theoretical calculations performed using the full-potential linearized augmented plane wave (FLAPW) method and the Korringa-Kohn-Rostoker (KKR) Green function method. In addition to insight into the electronic structure of Hf(0001), our results reveal the impact of surface contamination, particularly oxygen and carbon, on the predicted surface state. Moreover, we observe a flat band induced by both, the presence of oxygen and the dice structure of the surface. The orbital texture of Hf bands is confirmed by linear dichroism studies

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

Collective topological spin dynamics in a correlated spin glass

The interplay between spin-orbit interaction (SOI) and magnetic order is currently one of the most active research fields in condensed matter physics and leading the search for materials with novel and tunable magnetic and spin properties. Here we report on a variety of unexpected and unique observations in thin multiferroic \Ge$_{1-x}$Mn$_x$Te films. The ferrimagnetic order in this ferroelectric semiconductor is found to reverse with current pulses six orders of magnitude lower as for typical spin-orbit torque systems. Upon a switching event, the magnetic order spreads coherently and collectively over macroscopic distances through a correlated spin-glass state. Lastly, we present a novel methodology to controllably harness this stochastic magnetization dynamics, allowing us to detect spatiotemporal nucleation of topological spin textures we term ``skyrmiverres''.

cond-mat.mtrl-sci↗

arXiv2021

Observation of magnetic helicoidal dichroism with extreme ultraviolet light vortices

We report on the experimental evidence of magnetic helicoidal dichroism, observed in the interaction of an extreme ultraviolet vortex beam carrying orbital angular momentum with a magnetic vortex. Numerical simulations based on classical electromagnetic theory show that this dichroism is based on the interference of light modes with different orbital angular momenta, which are populated after the interaction between the light phase chirality and the magnetic topology. This observation gives insight into the interplay between orbital angular momentum and magnetism, and sets the framework for the development of new analytical tools to investigate ultrafast magnetization dynamics.

physics.optics↗

arXiv2020

Triple point fermions in ferroelectric GeTe

Ferroelectric GeTe is unveiled to exhibit an intriguing multiple non-trivial topology of the electronic band structure due to the existence of triple-point and type-II Weyl fermions, which goes well beyond the giant Rashba spin splitting controlled by external fields as previously reported. Using spin- and angle-resolved photoemission spectroscopy combined with ab initio density functional theory, the unique spin texture around the triple point caused by the crossing of one spin degenerate and two spin-split bands along the ferroelectric crystal axis is derived. This consistently reveals spin winding numbers that are coupled with time reversal symmetry and Lorentz invariance, which are found to be equal for both triple-point pairs in the Brillouin zone. The rich manifold of effects opens up promising perspectives for studying non-trivial phenomena and multi-component fermions in condensed matter systems.

cond-mat.mtrl-sci↗

arXiv2020

Bulk and surface electronic states in the dosed semimetallic HfTe$\boldsymbol{_2}$

The dosing of layered materials with alkali metals has become a commonly used strategy in ARPES experiments. However, precisely what occurs under such conditions, both structurally and electronically, has remained a matter of debate. Here we perform a systematic study of 1T-HfTe$_2$, a prototypical semimetal of the transition metal dichalcogenide family. By utilizing photon energy-dependent angle-resolved photoemission spectroscopy (ARPES), we have investigated the electronic structure of this material as a function of Potassium (K) deposition. From the k$_z$ maps, we observe the appearance of 2D dispersive bands after electron dosing, with an increasing sharpness of the bands, consistent with the wavefunction confinement at the topmost layer. In our highest-dosing cases, a monolayer-like electronic structure emerges, presumably as a result of intercalation of the alkali metal. Here, by bringing the topmost valence band below $E_F$, we can directly measure a band overlap of $\sim$ 0.2 eV. However, 3D bulk-like states still contribute to the spectra even after considerable dosing. Our work provides a reference point for the increasingly popular studies of the alkali metal dosing of semimetals using ARPES.

cond-mat.mtrl-sci↗

arXiv2020

Magnetic helicoidal dichroism in reflection by magnetic vortices

Identifying and imaging spin textures of ever more complex magnetic structures has become a major challenge in the past decade, especially at ultrashort timescales. Most of current approaches are based on the analysis of their polarization and magnetization-dependent reflectivities. Based on our joint publication XXX XX XXXXXX, we introduce a different concept, centered on the coupling of magnetic structures with light beams carrying orbital angular momentum (OAM). Upon reflection by a magnetic vortex, an incoming beam with a unique value $\ell$ of OAM gets enriched in the neighboring OAM modes $\ell\pm 1$. It results in anisotropic far-field images, which are identified as a Magnetic Helicoidal Dichroism (MHD) signal. Their analysis allow to retrieve the complex magneto-optical constants with excellent precision. This method, which does not require any polarization-resolved analysis, is promising for a quick identification of spin textures, including with attosecond to femtosecond time resolutions.

physics.optics↗

arXiv2020

Electromagnetic theory of Helicoidal Dichroism in reflection from magnetic structures

We present the classical electromagnetic theory framework of reflection of a light beam carrying Orbital Angular Momentum (OAM) by an in-plane magnetic structure with generic symmetry. Depending on the magnetization symmetry, we find a change in the OAM content of the reflected beam due to magneto-optic interaction and an asymmetric far-field intensity profile. This leads to three types of Magnetic Helicoidal Dichroism (MHD), observed when switching the OAM of the incoming beam, the magnetization sign, or both. In cases of sufficient symmetries, we establish analytical formulas which link an experimentally accessible MHD signal up to $10\%$ to the Magneto-Optical Kerr Effect (MOKE) constants. Magnetic vortices are particularly enlightening and promising targets, for which we explore the implications of our theory in the joint publication XX XXX XXXXXX.

physics.optics↗

arXiv2020

Single spin-polarised Fermi surface in SrTiO$_3$ thin films

The 2D electron gas (2DEG) formed at the surface of SrTiO$_3$(001) has attracted great interest because of its fascinating physical properties and potential as a novel electronic platform, but up to now has eluded a comprehensible way to tune its properties. Using angle-resolved photoemission spectroscopy with and without spin detection we here show that the band filling can be controlled by growing thin SrTiO$_3$ films on Nb doped SrTiO$_3$(001) substrates. This results in a single spin-polarised 2D Fermi surface, which bears potential as platform for Majorana physics. Based on our results it can furthermore be concluded that the 2DEG does not extend more than 2 unit cells into the film and that its properties depend on the amount of SrO$_x$ at the surface and possibly the dielectric response of the system.

cond-mat.str-el↗

arXiv2019

Spin-, time- and angle-resolved photoemission spectroscopy on WTe$_2$

We combined a spin-resolved photoemission spectrometer with a high-harmonic generation (HHG) laser source in order to perform spin-, time- and angle-resolved photoemission spectroscopy (STARPES) experiments on the transition metal dichalcogenide bulk WTe$_2$, a possible Weyl type-II semimetal. Measurements at different femtosecond pump-probe delays and comparison with spin-resolved one-step photoemission calculations provide insight into the spin polarization of electrons above the Fermi level in the region where Weyl points of WTe$_2$ are expected. We observe a spin accumulation above the Weyl points region, that is consistent with a spin-selective bottleneck effect due to the presence of spin polarized cone-like electronic structure. Our results support the feasibility of STARPES with HHG, which despite being experimentally challenging provides a unique way to study spin dynamics in photoemission.

cond-mat.mtrl-sci↗

arXiv2018

Determination of the time scale of photoemission from the measurement of spin polarization

The Eisenbud-Wigner-Smith (EWS) time delay of photoemission depends on the phase term of the matrix element describing the transition. Because of an interference process between partial channels, the photoelectrons acquire a spin polarization which is also related to the phase term. The analytical model for estimating the time delay by measuring the spin polarization is reviewed in this manuscript. In particular, the distinction between scattering EWS and interfering EWS time delay will be introduced, providing an insight in the chronoscopy of photoemission. The method is applied to the recent experimental data for Cu(111) presented in M. Fanciulli et al., PRL 118, 067402 (2017), allowing to give better upper and lower bounds and estimates for the EWS time delays.

cond-mat.mes-hall↗

arXiv2018

Spin-resolved electronic response to the phase transition in MoTe$_2$

The semimetal MoTe$_2$ is studied by spin- and angle- resolved photoemission spectroscopy to probe the detailed electronic structure underlying its broad range of response behavior. A novel spin-texture is uncovered in the bulk Fermi surface of the non-centrosymmetric structural phase that is consistent with first-principles calculations. The spin-texture is three-dimensional, both in terms of momentum dependence and spin-orientation, and is not completely suppressed above the centrosymmetry-breaking transition temperature. Two types of surface Fermi arc are found to persist well above the transition temperature. The appearance of a large Fermi arc depends strongly on thermal history, and the electron quasiparticle lifetimes are greatly enhanced in the initial cooling. The results indicate that polar instability with strong electron-lattice interactions exists near the surface when the bulk is largely in a centrosymmetric phase.

cond-mat.mtrl-sci↗

arXiv2017

Observation of Wannier-Stark localization at the surface of BaTiO$_3$ films by photoemission

Observation of Bloch oscillations and Wannier-Stark localization of charge carriers is typically impossible in single-crystals, because an electric field higher than the breakdown voltage is required. In BaTiO$_3$ however, high intrinsic electric fields are present due to its ferroelectric properties. With angle-resolved photoemission we directly probe the Wannier-Stark localized surface states of the BaTiO$_3$ film-vacuum interface and show that this effect extends to thin SrTiO$_3$ overlayers. The electrons are found to be localized along the in-plane polarization direction of the BaTiO$_3$ film.

cond-mat.mes-hall↗

arXiv2017

Observation of a two-dimensional electron gas at CaTiO$_3$ film surfaces

The two-dimensional electron gas at the surface of titanates gathered attention due to its potential to replace conventional silicon based semiconductors in the future. In this study, we investigated films of the parent perovskite CaTiO$_3$, grown by pulsed laser deposition, by means of angular-resolved photoelectron spectroscopy. The films show a c(4x2) surface reconstruction after the growth that is reduced to a p(2x2) reconstruction under UV-light. At the CaTiO$_3$ film surface, a two-dimensional electron gas (2DEG) is found with an occupied band width of 400 meV. With our findings CaTiO$_3$ is added to the group of oxides with a 2DEG at their surface. Our study widens the phase space to investigate strontium and barium doped CaTiO$_3$ and the interplay of ferroelectric properties with the 2DEG at oxide surfaces. This could open up new paths to tailor two-dimensional transport properties of these systems towards possible applications.

cond-mat.mes-hall↗

arXiv2016

Spin polarization and attosecond time delay in photoemission from spin degenerate states of solids

After photon absorption, electrons from a dispersive band of a solid require a finite time in the photoemission process before being photoemitted as free particles, in line with recent attosecondresolved photoemission experiments. According to the Eisenbud-Wigner-Smith model, the time delay is due to a phase shift of different transitions that occur in the process. Such a phase shift is also at the origin of the angular dependent spin polarization of the photoelectron beam, observable in spin degenerate systems without angular momentum transfer by the incident photon. We propose a semi-quantitative model which permits to relate spin and time scales in photoemission from condensed matter targets and to better understand spin- and angle-resolved photoemission spectroscopy (SARPES) experiments on spin degenerate systems. We also present the first experimental determination by SARPES of this time delay in a dispersive band, which is found to be greater than 26 as for electrons emitted from the sp-bulk band of the model system Cu(111).

cond-mat.mes-hall↗