Understanding the ultrafast demagnetization of transition metals requires pump-probe experiments sensitive to the time evolution of the electronic, spin, and lattice thermodynamic baths. By means of time-resolved photoelectron energy and spin-polarization measurements in the low-pump-fluence regime on iron, we disentangle the different dynamics of hot electrons and demagnetization in the subpicosecond and picosecond time range. We observe a broadening of the Fermi-Dirac distribution, following the excitation of nonthermal electrons at specific region of the iron valence band. The corresponding reduction of the spin polarization is remarkably delayed with respect to the dynamics of electronic temperature. The experimental results are corroborated with a microscopic 3-temperature model highlighting the role of thermal disorder in the quenching of the average spin magnetic moment, and indicating Elliot-Yafet type spin-flip scattering as the main mediation mechanism, with a spin-flip probability of 0.1 and a rate of energy exchange between electrons and lattice of 2.5Kfs−1.
van der Waals materials provide a versatile toolbox for the emergence of new quantum phenomena and fabrication of functional heterostructures. Among them, the trihalide VI3 stands out for its unique magnetic and structural landscape. Here we investigate the spin and orbital magnetic degrees of freedom in the layered ferromagnet VI3 by means of temperature-dependent X-ray absorption spectroscopy and X-ray magnetic circular and linear dichroism. We detect localized electronic states and reduced magnetic dimensionality, due to electronic correlations. We furthermore provide experimental evidence of (a) an unquenched orbital magnetic moment (up to 0.66(7) μB/V atom) in the ferromagnetic state and (b) an instability of the orbital moment in the proximity of the spin reorientation transition. Our results support a coherent picture where electronic correlations give rise to a strong magnetic anisotropy and a large orbital moment and establish VI3 as a prime candidate for the study of orbital quantum effects.
In the search of low cost and more efficient electronic devices, here the properties of SrVO3 transparent conductor oxide (TCO) thin film are investigated, both visible-range optically transparent and highly conductive, it stands as a promising candidate to substitute the standard indium-tin-oxide (ITO) in applications. Its surface stability under water (both liquid and vapor) and other gaseous atmospheres is especially addressed. Through the use of spectroscopy characterizations, X-ray photoemission and operando X-ray absorption measurements, the formation of a thin Sr-rich V5+ layer located at the surface of the polycrystalline SrVO3 film with aging is observed, and for the first time how it can be removed from the surface by solvating in water atmosphere. The surface recovery is associated to an etching process, here spectroscopically characterized in operando conditions, allowing to follow the stoichiometric modification under reaction. Once exposed in oxygen atmosphere, the Sr-rich V5+ layer forms again. The findings improve the understanding of aging effects in perovskite oxides, allowing for the development of functionalized films in which it is possible to control or to avoid an insulating surface layer. This constitutes an important step towards the large-scale use of V-based TCOs, with possible implementations in oxide-based electronics.
Hydrogen production from methanol decomposition to syngas (H2 + CO) is a promising alternative route for clean energy transition. One major challenge is related to the quest for stable, cost-effective, and selective catalysts operating below 400 °C. We illustrate an investigation of the surface reactivity of a Ni3Sn4 catalyst working at 250 °C, by combining density functional theory, operando X-ray absorption spectroscopy, and high-resolution transmission electron microscopy. We discovered that the catalytic reaction is driven by surface tin-oxide phases, which protects the underlying Ni atoms from irreversible chemical modifications, increasing the catalyst durability. Moreover, we found that Sn content plays a key role in enhancing the H2 selectivity, with respect to secondary products such as CO2. These findings open new perspectives for the engineering of scalable and low-cost catalysts for hydrogen production.
The possibility of modifying the ferromagnetic response of a multiferroic heterostructure via fully optical means exploiting the photovoltaic/photostrictive properties of the ferroelectric component is an effective method for tuning the interfacial properties. In this study, the effects of 405 nm visible-light illumination on the ferroelectric and ferromagnetic responses of (001) Pb(Mg1/3Nb2/3)O3-0.4PbTiO3 (PMN-PT)/Ni heterostructures are presented. By combining electrical, structural, magnetic, and spectroscopic measurements, how light illumination above the ferroelectric bandgap energy induces a photovoltaic current and the photostrictive effect reduces the coercive field of the interfacial magnetostrictive Ni layer are shown. Firstly, a light-induced variation in the Ni orbital moment as a result of sum-rule analysis of x-ray magnetic circular dichroic measurements is reported. The reduction of orbital moment reveals a photogenerated strain field. The observed effect is strongly reduced when polarizing out-of-plane the PMN-PT substrate, showing a highly anisotropic photostrictive contribution from the in-plane ferroelectric domains. These results shed light on the delicate energy balance that leads to sizeable light-induced effects in multiferroic heterostructures, while confirming the need of spectroscopy for identifying the physical origin of interface behavior.
The generation and control of surface acoustic waves (SAWs) in a magnetic material are objects of an intense research effort focused on magnetoelastic properties, with fruitful ramifications in spin-wave-based quantum logic and magnonics. We implement a transient grating setup to optically generate SAWs also seeding coherent spin waves via magnetoelastic coupling in ferromagnetic media. In this work we report on SAW-driven ferromagnetic resonance (FMR) experiments performed on polycrystalline Ni thin films in combination with time-resolved Faraday polarimetry, which allows extraction of the value of the effective magnetization and of the Gilbert damping. The results are in full agreement with measurements on the very same samples from standard FMR. Higher-order effects due to parametric modulation of the magnetization dynamics, such as down-conversion, up-conversion, and frequency mixing, are observed, testifying the high sensitivity of this technique.
Machine-learning techniques are revolutionizing the way to perform efficient materials modeling. We here propose a combinatorial machine-learning approach to obtain physical formulas based on simple and easily accessible ingredients, such as atomic properties. The latter are used to build materials features that are finally employed, through linear regression, to predict the energetic stability of semiconducting binary compounds with respect to zinc blende and rocksalt crystal structures. The adopted models are trained using a dataset built from first-principles calculations. Our results show that already one-dimensional (1D) formulas well describe the energetics; a simple grid-search optimization of the automatically obtained 1D-formulas enhances the prediction performance at a very small computational cost. In addition, our approach allows one to highlight the role of the different atomic properties involved in the formulas. The computed formulas clearly indicate that “spatial” atomic properties (i.e., radii indicating maximum probability densities for 𝑠,𝑝,𝑑 electronic shells) drive the stabilization of one crystal structure with respect to the other, suggesting the major relevance of the radius associated with the 𝑝-shell of the cation species.
We unravel the interplay of topological properties and the layered (anti)ferromagnetic ordering in EuSn2P2, using spin and chemical selective electron and X-ray spectroscopies supported by first-principle calculations. We reveal the presence of in-plane long-range ferromagnetic order triggering topological invariants and resulting in the multiple protection of topological Dirac states. We provide clear evidence that layer-dependent spin-momentum locking coexists with ferromagnetism in this material, a cohabitation that promotes EuSn2P2 as a prime candidate axion insulator for topological antiferromagnetic spintronics applications.
In this work, we apply for the first time ambient pressure operando soft X-ray absorption spectroscopy (XAS) to investigate the location, structural properties, and reactivity of the defective sites present in the prototypical metal–organic framework HKUST-1. We obtained direct evidence that Cu+ defective sites form upon temperature treatment of the powdered form of HKUST-1 at 160 °C and that they are largely distributed on the material surface. Further, a thorough structural characterization of the Cu+/Cu2+ dimeric complexes arising from the temperature-induced dehydration/decarboxylation of the pristine Cu2+/Cu2+ paddlewheel units is reported. In addition to characterizing the surface defects, we demonstrate that CO2 may be reversibly adsorbed and desorbed from the surface defective Cu+/Cu2+ sites. These findings show that ambient pressure soft-XAS, combined with state-of-the-art theoretical calculations, allowed us to shed light on the mechanism involving the decarboxylation of the paddlewheel units on the surface to yield Cu+/Cu2+ complexes and their reversible restoration upon exposure to gaseous CO2.
The effects of competing magnetic interactions in stabilizing different spin configurations are drawing renewed attention in order to unveil emerging topological spin textures and to highlight microscopic mechanisms leading to their stabilization. The possible key role of the two-site exchange anisotropy in selecting specific helicity and vorticity of skyrmionic lattices has only recently been proposed. In this work, we explore the phase diagram of a frustrated localized magnet characterized by a two-dimensional centrosymmetric triangular lattice, focusing on the interplay between the two-ion anisotropy and the single-ion anisotropy. The effects of an external magnetic field applied perpendicularly to the magnetic layer, are also investigated. By means of Monte Carlo simulations, we find an abundance of different spin configurations, going from trivial to high-order Q skyrmionic and meronic lattices. In closer detail, we find that a dominant role is played by the two-ion over the single-ion anisotropy in determining the planar spin texture; the strength and the sign of single ion anisotropy, together with the magnitude of the magnetic field, tune the perpendicular spin components, mostly affecting the polarity (and, in turn, the topology) of the spin texture. Our analysis confirms the crucial role of the anisotropic symmetric exchange in systems with dominant short-range interactions; at the same time, we predict a rich variety of complex magnetic textures, which may arise from a fine tuning of competing anisotropic mechanisms.
We investigated the relationship between ferromagnetism and metallicity in strained La0.67Ca0.33MnO3 films grown on lattice-mismatched NdGaO3 (001) by means of spectroscopic techniques directly sensitive to the ferromagnetic state, to the band structure, and to the chemical state of the atoms. In this system, the ferromagnetic metallic (FMM) phase spatially coexists with an insulating one in most of the phase diagram. First, the observation of an almost 100% spin polarization of the photoelectrons at the Fermi level in the fundamental state provides direct evidence of the half-metallicity of the FMM phase, a result that has been previously observed through direct probing of the valence band only on unstrained, phase-homogeneous La0.67Sr0.33MnO3. Second, the spin polarization results to be correlated with the occupancy at the Fermi level for all the investigated temperature regimes. These outcomes show that the half-metallic behavior predicted by a double-exchange model persists even in phase-separated manganites. Moreover, the correlation between metallicity and ferromagnetic alignment is confirmed by X-ray magnetic circular dichroism, a more bulk-sensitive technique, allowing one to explain transport properties in terms of the conduction through aligned FMM domains.
The role of X-ray based electron spectroscopies in determining chemical, electronic, and magnetic properties of solids has been well-known for several decades. A powerful approach is angle-resolved photoelectron spectroscopy, whereby the kinetic energy and angle of photoelectrons emitted from a sample surface are measured. This provides a direct measurement of the electronic band structure of crystalline solids. Moreover, it yields powerful insights into the electronic interactions at play within a material and into the control of spin, charge, and orbital degrees of freedom, central pillars of future solid state science. With strong recent focus on research of lower-dimensional materials and modified electronic behavior at surfaces and interfaces, angle-resolved photoelectron spectroscopy has become a core technique in the study of quantum materials. In this review, we provide an introduction to the technique. Through examples from several topical materials systems, including topological insulators, transition metal dichalcogenides, and transition metal oxides, we highlight the types of information which can be obtained. We show how the combination of angle, spin, time, and depth-resolved experiments are able to reveal “hidden” spectral features, connected to semiconducting, metallic and magnetic properties of solids, as well as underlining the importance of dimensional effects in quantum materials.
Topological spin structures, such as magnetic skyrmions, hold great promises for data storage applications, thanks to their inherent stability. In most cases, skyrmions are stabilized by magnetic fields in non-centrosymmetric systems displaying the chiral Dzyaloshinskii-Moriya exchange interaction, while spontaneous skyrmion lattices have been reported in centrosymmetric itinerant magnets with long-range interactions. Here, a spontaneous anti-biskyrmion lattice with unique topology and chirality is predicted in the monolayer of a semiconducting and centrosymmetric metal halide, NiI2. Our first-principles and Monte Carlo simulations reveal that the anisotropies of the short-range symmetric exchange, when combined with magnetic frustration, can lead to an emergent chiral interaction that is responsible for the predicted topological spin structures. The proposed mechanism finds a prototypical manifestation in two-dimensional magnets, thus broadening the class of materials that can host spontaneous skyrmionic states. Skyrmions, topological spin textures, are typically stabilized by the Dzyaloshinskii-Moriya interaction and an applied magnetic field. In this theoretical study, by analysing monolayer NiI2, the authors suggest that two-site anisotropy with magnetic frustration can stabilize a skyrmion lattice.
Here, we present an integrated ultra-high vacuum apparatus—named MBE-Cluster —dedicated to the growth and in situ structural, spectroscopic, and magnetic characterization of complex materials. Molecular Beam Epitaxy (MBE) growth of metal oxides, e.g., manganites, and deposition of the patterned metallic layers can be fabricated and in situ characterized by reflection high-energy electron diffraction, low-energy electron diffraction, Auger electron spectroscopy, x-ray photoemission spectroscopy, and azimuthal longitudinal magneto-optic Kerr effect. The temperature can be controlled in the range from 5 K to 580 K, with the possibility of application of magnetic fields H up to ±7 kOe and electric fields E for voltages up to ±500 V. The MBE-Cluster operates for in-house research as well as user facility in combination with the APE beamlines at Sincrotrone-Trieste and the high harmonic generator facility for time-resolved spectroscopy.
Two-dimensional (2D) metallic states induced by oxygen vacancies (VOs) at oxide surfaces and interfaces provide opportunities for the development of advanced applications, but the ability to control the behavior of these states is still limited. We used angle resolved photoelectron spectroscopy combined with density-functional theory (DFT) to study the reactivity of VO-induced states at the (001) surface of anatase TiO2, where both 2D metallic and deeper lying in-gap states (IGs) are observed. The 2D and IG states exhibit remarkably different evolutions when the surface is exposed to molecular O2: while IGs are almost completely quenched, the metallic states are only weakly affected. DFT calculations indeed show that the IGs originate from surface VOs and remain localized at the surface, where they can promptly react with O2. In contrast, the metallic states originate from subsurface vacancies whose migration to the surface for recombination with O2 is kinetically hindered on anatase TiO2 (001), thus making them much less sensitive to oxygen dosing.
We present the results of a photon energy and polarization dependent angle-resolved photoemission spectroscopy (ARPES) study on high quality, epitaxial SrNbO3 thin films prepared in situ by pulsed laser deposition (PLD). We show that the Fermi surface is composed of three bands mainly due to t(2g) orbitals of Nb 4d, in analogy with the 3d-based perovskite systems. The bulk band dispersion for the conduction and valence states obtained by density functional theory (DFT) is generally consistent with the ARPES data. The small discrepancy in the bandwidth close to the Fermi level seems to result from the interplay of correlation effects and the presence of vacancies. The ARPES results are complemented by soft x-ray photoemission spectroscopy measurements in order to provide indications on the chemical states and the stoichiometry of the material.
Here, we report on a novel narrowband High Harmonic Generation (HHG) light source designed for ultrafast photoelectron spectroscopy (PES) on solids. Notably, at 16.9 eV photon energy, the harmonics bandwidth equals 19 meV. This result has been obtained by seeding the HHG process with 230 fs pulses at 515 nm. The ultimate energy resolution achieved on a polycrystalline Au sample at 40 K is ∼22 meV at 16.9 eV. These parameters set a new benchmark for narrowband HHG sources and have been obtained by varying the repetition rate up to 200 kHz and, consequently, mitigating the space charge, operating with ≈3×107 electrons/s and ≈5×108 photons/s. By comparing the harmonics bandwidth and the ultimate energy resolution with a pulse duration of ∼105 fs (as retrieved from time-resolved experiments on bismuth selenide), we demonstrate a new route for ultrafast space-charge-free PES experiments on solids close to transform-limit conditions.
The redox process of pretreated Co3O4 thin film coatings has been studied by ambient pressure soft X-ray absorption spectroscopy. The Co3O4 coatings were composed of nanoparticles of about 10 nm in size as prepared by pulsed laser deposition. The thin film coatings were pretreated in He or in H2 up to 150 °C prior to exposure to the reactive gases. The reactivity toward carbon monoxide and oxygen was monitored by near-edge X-ray absorption fine structure (NEXAFS) spectroscopy during gas exposures. The results indicate that the samples pretreated in He show reactivity only at high temperature, while the samples pretreated in H2 are reactive also at room temperature. X-ray photoemission spectroscopy measurements in ultra-high vacuum and NEXAFS simulations with the CTM4XAS code further specify the results.
We combine time-resolved pump-probe magneto-optical Kerr effect and photoelectron spectroscopy experiments supported by theoretical analysis to determine the relaxation dynamics of delocalized electrons in half-metallic ferromagnetic manganite La1−xSrxMnO3. We observe that the half-metallic character of La1−xSrxMnO3 determines the timescale of both the electronic phase transition and the quenching of magnetization, revealing a quantum isolation of the spin system in double-exchange ferromagnets extending up to hundreds of picoseconds. We demonstrate the use of time-resolved hard x-ray photoelectron spectroscopy as a unique tool to single out the evolution of strongly correlated electronic states across a second-order phase transition in a complex material.
Polar lacunar spinels, such as GaV4S8 and GaV4Se8, were proposed to host skyrmion phases under magnetic field. In this work, we put forward, as a candidate for Néel-type skyrmion lattice, the isostructural GaMo4S8, which is systematically studied via both first-principles calculations and Monte Carlo simulations of a model Hamiltonian. Electric polarization, driven by the Jahn-Teller distortion, is predicted to arise in GaMo4S8, showing a comparable size but an opposite sign with respect to that evaluated in V-based counterparts and explained in terms of different electron counting arguments and resulting distortions. Interestingly, a larger spin-orbit coupling of 4d orbitals with respect to 3d orbitals in vanadium spinels leads to stronger Dzyaloshinskii-Moriya interactions, which are beneficial to stabilize a cycloidal spin texture, as well as smaller-sized skyrmions (radius<10nm). Furthermore, the possibly large exchange anisotropy of GaMo4S8 may lead to a ferroelectric-ferromagnetic ground state as an alternative to the ferroelectric-skyrmionic one, thus calling for further experimental verification.
Currently, there is a flurry of research interest on materials with an unconventional electronic structure, and we have already seen significant progress in their understanding and engineering towards real-life applications. The interest erupted with the discovery of graphene and topological insulators in the previous decade. The electrons in graphene simulate massless Dirac Fermions with a linearly dispersing Dirac cone in their band structure, while in topological insulators, the electronic bands wind non-trivially in momentum space giving rise to gapless surface states and bulk bandgap. Weyl semimetals in condensed matter systems are the latest addition to this growing family of topological materials. Weyl Fermions are known in the context of high energy physics since almost the beginning of quantum mechanics. They apparently violate charge conservation rules, displaying the 'chiral anomaly', with such remarkable properties recently theoretically predicted and experimentally verified to exist as low energy quasiparticle states in certain condensed matter systems. Not only are these new materials extremely important for our fundamental understanding of quantum phenomena, but also they exhibit completely different transport phenomena. For example, massless Fermions are susceptible to scattering from non-magnetic impurities. Dirac semimetals exhibit non-saturating extremely large magnetoresistance as a consequence of their robust electronic bands being protected by time reversal symmetry. These open up whole new possibilities for materials engineering and applications including quantum computing. In this review, we recapitulate some of the outstanding properties of WTe2, namely, its non-saturating titanic magnetoresistance due to perfect electron and hole carrier balance up to a very high magnetic field observed for the very first time. It also indicative of hosting Lorentz violating type-II Weyl Fermions in its bandstructure, again first predicted candidate material to host such a remarkable phase. We primarily focus on the findings of our ARPES, spin-ARPES, and time-resolved ARPES studies complemented by first-principles calculations.
By performing density functional theory and Green's functions calculations, complemented by x-ray photoemission spectroscopy, we investigate the electronic structure of Fe/GeTe(111), a prototypical ferromagnetic/Rashba-ferroelectric interface. We reveal that such a system exhibits several intriguing properties resulting from the complex interplay of exchange interaction, electric polarization, and spin-orbit coupling. Despite a rather strong interfacial hybridization between Fe and GeTe bands, resulting in a complete suppression of the surface states of the latter, the bulk Rashba bands are hardly altered by the ferromagnetic overlayer. This could have a deep impact on spin-dependent phenomena observed at this interface, such as spin-to-charge interconversion, which are likely to involve bulk rather than surface Rashba states.
The delicate interplay of electronic charge, spin, and orbital degrees of freedom is in the heart of many novel phenomena across the transition metal oxide family. Here, by combining high-resolution angle-resolved photoemission spectroscopy and first principles calculations (with and without spin-orbit coupling), the electronic structure of the rutile binary iridate,
IrO2, is investigated. The detailed study of electronic bands measured on a high-quality single crystalline sample and use of a wide range of photon energy provide a huge improvement over the previous studies. The excellent agreement between theory and experimental results shows that the single-particle DFT description of IrO2 band structure is adequate, without the need of invoking any treatment of correlation effects. Although many observed features point to a 3D nature of the electronic structure, clear surface effects are revealed. The discussion of the orbital character of the relevant bands crossing the Fermi level sheds light on spin-orbit-coupling-driven phenomena in this material, unveiling a spin-orbit-induced avoided crossing, a property likely to play a key role in its large spin Hall effect.
We present a new experimental setup for performing X-ray Absorption Spectroscopy (XAS) in the soft X-ray range at ambient pressure. The ambient pressure XAS setup is fully compatible with the ultra high vacuum environment of a synchrotron radiation spectroscopy beamline end station by means of ultrathin Si3N4 membranes acting as windows for the X-ray beam and seal of the atmospheric sample environment. The XAS detection is performed in total electron yield (TEY) mode by probing the drain current from the sample with a picoammeter. The high signal/noise ratio achievable in the TEY mode, combined with a continuous scanning of the X-ray energies, makes it possible recording XAS spectra in a few seconds. The first results show the performance of this setup to record fast XAS spectra from sample surfaces exposed at atmospheric pressure, even in the case of highly insulating samples. The use of a permanent magnet inside the reaction cell enables the measurement of X-ray magnetic circular dichroism at ambient pressure.
In the rapidly growing field of spintronics, simultaneous control of electronic and magnetic properties is essential, and the perspective of building novel phases is directly linked to the control of tuning parameters, for example, thickness and doping. Looking at the relevant effects in interface-driven spintronics, the reduced symmetry at a surface and interface corresponds to a severe modification of the overlap of electron orbitals, that is, to a change of electron hybridization. Here we report a chemically and magnetically sensitive depth-dependent analysis of two paradigmatic systems, namely La1−xSrxMnO3 and (Ga,Mn)As. Supported by cluster calculations, we find a crossover between surface and bulk in the electron hybridization/correlation and we identify a spectroscopic fingerprint of bulk metallic character and ferromagnetism versus depth. The critical thickness and the gradient of hybridization are measured, setting an intrinsic limit of 3 and 10 unit cells from the surface, respectively, for (Ga,Mn)As and La1−xSrxMnO3, for fully restoring bulk properties.
By combining bulk sensitive soft-x-ray angular-resolved photoemission spectroscopy and first-principles calculations we explored the bulk electron states of WTe2, a candidate type-II Weyl semimetal featuring a large nonsaturating magnetoresistance. Despite the layered geometry suggesting a two-dimensional electronic structure, we directly observe a three-dimensional electronic dispersion. We report a band dispersion in the reciprocal direction perpendicular to the layers, implying that electrons can also travel coherently when crossing from one layer to the other. The measured Fermi surface is characterized by two well-separated electron and hole pockets at either side of the Γ point, differently from previous more surface sensitive angle-resolved photoemission spectroscopy experiments that additionally found a pronounced quasiparticle weight at the zone center. Moreover, we observe a significant sensitivity of the bulk electronic structure of WTe2 around the Fermi level to electronic correlations and renormalizations due to self-energy effects, previously neglected in first-principles descriptions.
Complete photoemission experiments, enabling measurement of the full quantum set of the photoelectron final state, are in high demand for studying materials and nanostructures whose properties are determined by strong electron and spin correlations. Here the implementation of the new spin polarimeter VESPA (Very Efficient Spin Polarization Analysis) at the APE-NFFA beamline at Elettra is reported, which is based on the exchange coupling between the photoelectron spin and a ferromagnetic surface in a reflectometry setup. The system was designed to be integrated with a dedicated Scienta-Omicron DA30 electron energy analyzer allowing for two simultaneous reflectometry measurements, along perpendicular axes, that, after magnetization switching of the two targets, allow the three-dimensional vectorial reconstruction of the spin polarization to be performed while operating the DA30 in high-resolution mode. VESPA represents the very first installation for spin-resolved ARPES (SPARPES) at the Elettra synchrotron in Trieste, and is being heavily exploited by SPARPES users since autumn 2015.
We report the study of anatase TiO2(001)-oriented thin films grown by pulsed laser deposition on LaAlO3(001). A combination of in situ and ex situ methods has been used to address both the origin of the Ti3+-localized states and their relationship with the structural and electronic properties on the surface and the subsurface. Localized in-gap states are analyzed using resonant X-ray photoelectron spectroscopy and are related to the Ti3+ electronic configuration, homogeneously distributed over the entire film thickness. We find that an increase in the oxygen pressure corresponds to an increase in Ti3+ only in a well-defined range of deposition pressure; outside this range, Ti3+ and the strength of the in-gap states are reduced.
In this work the experimental uncertainties concerning electron spin polarization (SP) under various realistic measurement conditions are theoretically derived. The accuracy of the evaluation of the SP of the photoelectron current is analysed as a function of the detector parameters and specifications, as well as of the characteristics of the photoexcitation sources. In particular, the different behaviour of single counter or twin counter detectors when the intensity fluctuations of the source are considered have been addressed, leading to a new definition of the SP detector performance. The widely used parameter called the figure of merit is shown to be inadequate for describing the efficiency of SP polarimeters, especially when they are operated with time-structured excitation sources such as free-electron lasers. Numerical simulations have been performed and yield strong implications in the choice of the detecting instruments in spin-polarization experiments, that are constrained in a limited measurement time. Our results are therefore applied to the characteristics of a wide set of state-of-the-art spectroscopy facilities all over the world, and an efficiency diagram for SP experiments is derived. These results also define new mathematical instruments for handling the correct statistics of SP measurements in the presence of source intensity fluctuations.
The recent discovery of hidden spin polarization emerging in bulk electronic states of specific nonmagnetic crystals is a fascinating phenomenon, though hardly explored yet. Here, we study from a theoretical perspective nonmagnetic
BaNiS2, recently suggested to exhibit a giant Rashba-like spin-orbit splitting of the bulk bands, despite the absence of heavy elements. We employ density functional theory and Green's functions calculations to reveal the exact spin textures of both bulk and surface. We predict unambiguous signatures of spin-polarized electronic states at the surface, which reflect the bulk Rashba splitting and which could be experimentally measured with sufficient resolution: this would constitute a clear report of a bulk-Rashba-induced spin splitting at the surface of centrosymmetric crystals.
ULTRASPIN is an apparatus devoted to the measurement of the spin polarization (SP) of electrons ejected from solid surfaces in a UHV environment. It is designed to exploit ultrafast light sources (free electron laser or laser high harmonic generation) and to perform (photo)electron spin analysis by an arrangement of Mott scattering polarimeters that measure the full SP vector. The system consists of two interconnected UHV vessels: one for surface science sample cleaning treatments, e-beam deposition of ultrathin films, and low energy electron diffraction/AES characterization. The sample environment in the polarimeter allows for cryogenic cooling and in-operando application of electric and magnetic fields. The photoelectrons are collected by an electrostatic accelerator and transport lens that form a periaxial beam that is subsequently directed by a Y-shaped electrostatic deflector to either one of the two orthogonal Mott polarimeters. The apparatus has been designed to operate in the extreme conditions of ultraintense single-X-ray pulses as originated by free electron lasers (up to 1 kHz), but it allows also for the single electron counting mode suitable when using statistical sources such as synchrotron radiation, cw-laser, or e-gun beams (up to 150 kcps).
The behaviour of electrons and holes in a crystal lattice is a fundamental quantum phenomenon, accounting for a rich variety of material properties. Boosted by the remarkable electronic and physical properties of two-dimensional materials such as graphene and topological insulators, transition metal dichalcogenides have recently received renewed attention. In this context, the anomalous bulk properties of semimetallic WTe2 have attracted considerable interest. Here we report angle- and spin-resolved photoemission spectroscopy of WTe2 single crystals, through which we disentangle the role of W and Te atoms in the formation of the band structure and identify the interplay of charge, spin and orbital degrees of freedom. Supported by first-principles calculations and high-resolution surface topography, we reveal the existence of a layer-dependent behaviour. The balance of electron and hole states is found only when considering at least three Te–W–Te layers, showing that the behaviour of WTe2 is not strictly two dimensional.
The manipulation of ferromagnetic layer magnetization via electrical pulse is driving an intense research due to the important applications that this result will have on memory devices and sensors. In this study we realized a magnetotunnel junction in which one layer is made of Galfenol (Fe1-xGax) which possesses one of the highest magnetostrictive coefficient known. The multilayer stack has been grown by molecular beam epitaxy and e-beam evaporation. Optical lithography and physical etching have been combined to obtain 20x20 micron sized pillars. The obtained structures show tunneling conductivity across the junction and a tunnel magnetoresistance (TMR) effect of up to 11.5% in amplitude.
Research on spintronics and on multiferroics leads now to the possibility of combining the properties of these materials in order to develop new functional devices. Here we report the integration of a layer of magnetostrictive material into a magnetic tunnel junction. A FeGa/MgO/Fe heterostructure has been grown on a GaAs(001) substrate by molecular beam epitaxy (MBE) and studied by X-ray magnetic circular dichroism (XMCD). The comparison between magneto optical Kerr effect (MOKE) measurements and hysteresis performed in total electron yield allowed distinguishing the ferromagnetic hysteresis loop of the FeGa top layer from that of the Fe buried layer, evidencing a different switching field of the two layers. This observation indicates an absence of magnetic coupling between the two ferromagnetic layers despite the thickness of the MgO barrier of only 2.5 nm. The in-plane magnetic anisotropy has also been investigated. Overall results show the good quality of the heterostructure and the general feasibility of such a device using magnetostrictive materials in magnetic tunnel junction.