Daniel Farias

Universidad Autónoma de Madrid | UAM · Department of Condensed Matter Physics

Nonlinear Hall Effect As the heart and with the tremendous applications of terahertz technology, photodetectors suffer from considerable drawbacks imposed by weak optical absorption, and inefficient charge‐separation mechanisms. In article number 2209557, Antonio Politano, Lin Wang, and co‐workers report the nonlinear Hall effect operating at terah...

The zero magnetic-field nonlinear Hall effect (NLHE) refers to the second-order transverse current induced by an applied alternating electric field; it indicates the topological properties of inversion-breaking crystals. Despite several studies on the NLHE induced by the Berry-curvature dipole in Weyl semimetals, the direct current conversion by re...

The fabrication of atomically smooth metal films on oxide substrates is a considerable challenge because most deposition methods used on metals do not work properly on supporting oxides. Here we report an alternative procedure based on thermal laser epitaxy of materials. We present results for 15-nm-thick ruthenium films grown on c-plane sapphire a...

The control of heat at the nanoscale via the excitation of localized surface plasmons in nanoparticles (NPs) irradiated with light holds great potential in several fields (cancer therapy, catalysis, desalination). To date, most thermoplasmonic applications are based on Ag and Au NPs, whose cost of raw materials inevitably limits the scalability for...

The knowledge of the thermal expansion coefficient is of crucial importance to prevent the poor performance of devices, especially when these are made up of several layers of different materials, as in the case of 2D heterostructures. Helium atom scattering is a suitable tool for the direct measurement of the surface thermal expansion coefficient o...

Here, we discuss the key features of electrocatalysis with mitrofanovite (Pt3Te4), a recently discovered mineral with superb performances in hydrogen evolution reaction. Mitrofanovite is a layered topological metal with spin-polarized topological surface states with potential applications for spintronics. However, mitrofanovite is also an exception...

Tin diselenide (SnSe2) is a layered semiconductor with broad application capabilities in the fields of energy storage, photocatalysis, and photodetection. Here, we correlate the physicochemical properties of this van der Waals semiconductor to sensing applications for detecting chemical species (chemosensors) and millimeter waves (Terahertz photode...

Helium Atom Scattering (HAS) and Helium Spin-Echo scattering (HeSE), together helium scattering, are well established, but non-commercial surface science techniques. They are characterised by the beam inertness and very low beam energy (<0.1 eV) which allows essentially all materials and adsorbates, including fragile and/or insulating materials and...

We have determined the electron–phonon interaction in type II Dirac semimetallic 1T-PdTe2 by means of helium atom scattering. While 1T-PdTe2 is isostructural with 1T-PtTe2, only the former is superconductor. The difference can be traced to the substantially larger value of the electron–phonon coupling in 1T-PdTe2, λ = 0.58, obtained from the Debye-...

Closed-shell atoms scattered from a metal surface exchange energy and momentum with surface phonons mostly via the interposed surface valence electrons, i.e., via the creation of virtual electron-hole pairs. The latter can then decay into surface phonons via electron-phonon interaction, as well as into acoustic surface plasmons (ASPs). While the fi...

We have measured high-resolution time-of-flight (TOF) spectra of methane scattered from an Ir(111) surface at an incident energy of 81 meV. The angular distributions of scattered CH4 reveal the presence of a sharp and intense specular peak in addition to sharp features corresponding to rotationally inelastic diffraction (RID) peaks along the two ma...

Tin diselenide (SnSe2) is a van der Waals semiconductor, which spontaneously forms a sub-nanometric SnO2 skin once exposed to air. Here, by means of surface-science spectroscopies and density functional theory, we have investigated the charge redistribution at the SnO2-SnSe2 heterojunction in both oxidative and humid environments. Explicitly, we fi...

We report an extensive structural characterization of the surface of 1T-PtTe2 crystals by means of Helium Atom Scattering (HAS) and ultra-high resolution scanning tunnelling microscopy (STM). HAS reveals that this bi-dimensional surface presents a large structural corrugation (∼ 0.33 ˚A) in ΓM direction. The surface lattice constant of PtTe2 derive...

Though it is well recognized that the space between graphene cover and the metal substrate can act as a 2D nanoreactor, several issues are still unresolved, including the role of the metal substrate, the mechanisms ruling water intercalation and the identification of sites at which water is decomposed. Here, we solve these issues by means of densit...

Diffraction with matter waves has been reported since the beginning of quantum mechanics. In free space, diffraction effects have been observed even with objects as large as C60 molecules. However, in scattering from a solid surface, pure elastic diffraction features have never been observed with molecules larger than D2. Here we report the observa...

We have studied the (0001) surface of 2H-MoS2 by means of helium atom scattering (HAS). The electron-phonon coupling constant lambda of this system has been determined by measuring the thermal attenuation of the specular peak at surface temperatures between 100 and 500 K. HAS diffraction also reveals a 3% planar dilation of the surface layer, while...

The ability of the different approaches proposed to date to include the effects of van der Waals (vdW) dispersion forces in density functional theory (DFT) is currently under debate. Here, we used the diffraction of He on a Ru(0 0 0 1) surface as a challenging benchmark system to analyze the suitability of several representative approaches, from th...

Ice formation on surfaces is a fascinating phenomenon with a major impact on a huge range of human activities from airplanes and shipping to wind mill parks, power transmission and telecommunication. A significant amount of work has been dedicated to the development of icephobic surfaces, often with the basis in superhydrophobicity. However, most s...

We have measured the temperature dependence of angular distributions of CH4 from Pt(111) at an incident energy of 109 meV. A broad angular distribution has been observed along the two main symmetry directions, whereby the peak center shifts from the supra-specular position to the sub-specular position when the surface temperature increases from 120...

Chemical vapor deposition (CVD) is one of the best ways to scalably grow low cost, high quality graphene on metal substrates; unfortunately, it not ideal for producing graphene on dielectric substrates. Here, we demontrate production of a high quality graphene layer on Sapphire using CVD with a copper catalyst. The catalyst consists of a thin coppe...

We show that neutral atom scattering is suitable to determine the coupling strength between a two-dimensional (2D) material and the underlying substrate. This information can be obtained from the thermal attenuation of the specular intensity, as well as from angular distributions of He and Ne atoms in the low incident energy (~50 meV) regime. For g...

We report helium diffraction from natural MoS2 single crystal. The high quality of the samples studied lead to the appearance of sharp and intense in-plane and out-of-plane diffraction peaks, with unusually low background. The pronounced out-of-plane features observed confirm the high corrugation of the 2D surface unit cell. The observation of diff...

Low energy (few meV) acoustic phonon modes of graphene/Ir (111) have been measured using inelastic helium atom scattering (HAS). The data clearly show two vibrational modes with energies 4.6 and 7.2 meV at Γ¯ with dispersionless branches due to Umklapp scattering. The phonons observed are not the ones expected for a free-standing graphene monolayer...

The structure of graphene on Ru(0001) has been studied with neon atom scattering. Angular distributions of Ne scattered in the quantum regime have been recorded for a variety of incident conditions. For both in-plane and out-of-plane spectra, the intensity of the measured diffraction beams is comparable to that of the specular peak. This is in cont...

Measurements of Ne atom scattering from a clean and ordered Ir(111) surface are shown under conditions in which the scattering is nearly classical. The only quantum mechanical manifestation is a small elastic specular diffraction intensity. The energy-resolved intensity spectra exhibit only a single broad peak attributed to multiple phonon excitati...

The excitation spectrum of graphene on Cu(111), probed by momentum-resolved electron energy loss spectroscopy, exhibits multiple acoustic surface plasmons (ASP), arising from both the graphene overlayer and the Cu(111) substrate. We have investigated the mutual interaction between the ASP of the graphene overlayer and that of the underlying Cu(111)...

The fabrication of atomically smooth metal films on supporting oxides is a quite demanding task, since most physical vapor deposition methods used on metals do not work properly on oxide substrates. Here, we report an alternative procedure, based on performing laser polishing of a fused silica substrate before depositing the metallic thin film. Thi...

We report Ne atom scattering from clean Ru(0001) and from Ru(0001) covered with a single layer of graphene. For both systems, the quantum and classical regimes are observed and analyzed. The quantum to classical boundary is explored by varying both the surface temperature and the incident Ne kinetic energy. The classical smooth surface model is use...

We have measured high-resolution angular distributions of methane scattered from a Ni(111) surface at incident energies between 68.9 meV and 108.6 meV. A sharp and intense specular peak has been observed, in addition to sharp features corresponding to rotationally inelastic diffraction (RID) peaks along the two main symmetry directions of Ni(111)....

The reactivity of a Pd monolayer epitaxially grown on Ru(0001) toward O2 has been investigated by molecular beam techniques. O2 initial sticking coefficients were determined using the King and Wells method in the incident energy range of 40–450 meV and for sample temperatures of 100 K and 300 K, and compared to the corresponding values measured on...

The development of novel reflective optical elements is essential to improve the focusing of neutral atomic beams. The recent availability of commercial thin crystals led to a renewed interest in curved mirrors as reflective elements for He microscopy. We have investigated the reflectivity to incoming He atoms of Cu(111), Ni(111), and Ru(0001) crys...

We present angle- and energy-resolved analysis of Ne scattering from a Ni(111) surface at moderate beam energy and surface temperature. Variation of the surface temperature allows the transition from a pure quantum regime, where single-phonon events are observed, to the classical regime, where multiphonon excitations are observed. The latter can be...

By means of a combination of surface-science spectroscopies and theory, we investigate the mechanisms ruling the catalytic role of epitaxial graphene (Gr) grown on transition-metal substrates for the production of hydrogen from water. Water decomposition at the Gr/metal interface at room temperature provides a hydrogenated Gr sheet, which is buckle...

The study of surface phonon dispersion curves is motivated by the quest for a detailed understanding of the forces between the atoms at the surface and in the bulk. In the case of graphene, additional motivation comes from the fact that thermal conductivity is dominated by contributions from acoustic phonons, while optical phonon properties are ess...

We report a new method to produce high--quality, transparent graphene/sapphire samples, using Cu as a catalyst. The starting point is a high-quality graphene layer prepared by CVD on Cu(111)/Al_2O_3. Graphene on sapphire is obtained in-situ by evaporation of the Cu film in UHV. He-diffraction, atomic force microscopy (AFM), Raman spectroscopy and o...

Inelastic Helium Atom Scattering (HAS) is suitable to determine low-energy (few meV) vibrations spatially localized on structures in the nanometer range. This is illustrated for the nanodomes that appear often on graphene (Gr) epitaxially grown on single crystal metal surfaces. The nature of the inelastic losses observed in Gr/Ru(0001) and Gr/Cu/Ru...

We report helium diffraction from graphene grown by chemical vapour deposition (CVD) using copper foil. This method reveals acoustic phonons, which are physically important to thermal conductance as well as a sensitive probe of graphene's interactions with the underlying substrate. Helium diffraction is made possible by the high quality of graphene...

Helium atom scattering and low-energy electron diffraction have been used for studying the adsorption of TCNQ molecules on Cu(100). By optimizing the TCNQ growth procedure through the analysis of He-diffraction spectra, a large-scale (over 2000 Å), well-ordered overlayer has been attained by TCNQ deposition at 110 K followed by thermal annealing at...

We present an experimental study of the initial dissociative sticking probability of H2 on the Pd- Cu(111) surface alloy at Pd coverages between 1-10%. The measurements have been performed using a supersonic molecular beam with an incident energy range Ei = 75-163meV. In agreement with a recent STM study, our results confirm that small amounts of P...

The dissociative adsorption of molecular hydrogen on PdxRu1-x/Ru(0001) (0 ≤ x ≤ 1) has been investigated by means of He atom scattering, Density Functional Theory and quasi-classical trajectory calculations. Regardless of their surroundings, Pd atoms in the alloy are always less reactive than Ru ones. However, the reactivity of Ru atoms is enhanced...

This chapter describes studies of diffractive phenomena from solid surfaces using He, Ne and H2 beams at incident energies between 20 and 200 meV. We summarize recent H2 diffraction experiments and calculations based on ab initio determined six–dimensional PESs performed on several single–crystal metal surfaces. The general trends observed in exper...

Diffraction of molecular hydrogen is a very sensitive technique to characterize the potential energy surface (PES) for hydrogen dissociative chemisorption. In this chapter we summarize recent H2 diffraction experiments and calculations based on ab initio determined six-dimensional PESs performed on several single-crystal metal surfaces, ranging fro...

The reactivity of Cu monolayer (ML) and bilayer films grown on Ru(0001) towards O(2) and H(2) has been investigated. O(2) initial sticking coefficients were determined using the King and Wells method in the incident energy range 40-450 meV, and compared to the corresponding values measured on clean Ru(0001) and Cu(111) surfaces. A relative large O(...

We present an experimental and theoretical study of He, Ne and Ar diffraction from the Ru(0001) surface. Close-coupling calculations were performed to estimate the corrugation function and the potential well depth in the atom-surface interaction in all three cases. DFT (density functional theory) calculations, including van der Waals dispersion for...

Diffraction of H2 from surfaces is considered to be a useful tool to characterize molecule/surface interactions and surface topology. In this work, we have studied diffraction of H2 from a strained pseudomorphic monolayer of Cu deposited on Ru(0001), both experimentally and theoretically. Our experimental measurements show a remarkable diffraction...

High-resolution electron energy-loss spectroscopy has been used to study the π plasmon in monolayer graphene grown on Pt(111). A quadratic dispersion has been observed, in contrast to the linear dispersion reported for monolayer graphene grown on SiC(0001) and in agreement with recent experiments on graphene/Ni(111). Despite the weak interaction of...

He-beam diffraction data evaluated for clean Rh(311) yield a best-fit corrugation, which shows a pronounced corrugation amplitude along the (100)-microfacets and a practically vanishing amplitude along the (111)-microfacets in agreement with expectation, but in contrast to Ni(311) where the reverse situation was found. Hydrogen adsorption leads to...

The adsorption of hydrogen on Pd(311) at 100 K has been investigated by high resolution electron energy loss spectroscopy (HREELS). The observed vibration energies are similar to former values reported on low index Pd surfaces, which allowed relating the vibration modes with the different adsorption sites on the (311) surface. In contrast to Ni(311...

We report a He and H diffraction study of graphene-terminated Ru(0001) thin films grown epitaxially on c-axis sapphire. Even for samples exposed for several weeks to ambient conditions, brief annealing in ultrahigh vacuum restored extraordinarily high specular reflectivities for He and H beams (23% and 7% of the incident beam, respectively). The qu...

The dissociative chemisorption of hydrogen at metal surfaces is the first step in the surface chemistry of heterogeneous catalysis. Up to now, most of our understanding of this process has been obtained from sticking probability measurements. Recent experiments have shown that more detailed information on the potential energy surface (PES) governin...

The dispersion and the damping of the sheet plasmon in a graphene monolayer grown on Pt(111) have been studied by using angle-resolved electron energy loss spectroscopy. We found that the dispersion relation of the plasmon mode confined in the graphene sheet is linear, as a consequence of the screening by the metal substrate. Present results demons...

It is shown that the surface of an epitaxial graphene monolayer grown on Ru(0001) could be used as a quite efficient external mirror for He-atom microscopy, with a specular reflectivity of 20% of the incident beam. Furthermore, the system is stable up to 1150 K, and the He reflectivity remains almost unchanged after exposure to air. Additionally, t...

H(2) diffraction from the Ru(001) surface has been measured for incident energies E(i) = 78-150 meV and incident angles Θ(i) = 22.1-64.1°. In-plane and out-of-plane angular distributions were measured for incidence along [110] and [100] directions. Out-of-plane diffraction channels were found to be predominant for the explored experimental conditio...

We present a combined experimental and theoretical study of the diffraction of H(2) from Ru(0001) in the incident energy range 78-150 meV, and a theoretical study of dissociative chemisorption of H(2) in the same system. Pronounced out-of-plane diffraction was observed in the whole energy range studied. The energy dependence of the elastic diffract...

We produce Lissajous figures by modulating a laser beam along two perpendicular directions by means of two tuning forks. When the ratio of the resonant frequencies of the tuning forks is a small rational number, Lissajous figures are clearly resolved on a screen. Because the ratio of frequencies is never exactly a rational number, a phase drift of...

We present a detailed experimental and theoretical study of elastic and rotationally inelastic diffraction of D(2) from NiAl(110) in the energy range 85-150 meV. The experiments were performed using a high-resolution, fixed angle geometry apparatus. Quantum and classical dynamical calculations were performed by using a six-dimensional potential ene...

Graphene epitaxially grown on Ru(0001) displays a remarkably ordered pattern of hills and valleys in scanning tunneling microscopy (STM) images. The extent to which the observed 'ripples' are structural or electronic in origin has been much disputed recently. A combination of ultrahigh-resolution STM images and helium atom diffraction data shows th...

High-resolution angular distributions of D(2) scattered from NiAl(110) have been measured at incident energies between 20 and 150 meV. The measurements were performed along the [110] azimuth using a high sensitivity time-of-flight apparatus, which allows the recording of diffraction channels not previously studied, including out-of-plane rotational...

Manipulation of atomic and molecular beams is essential to atom optics applications including atom lasers, atom lithography, atom interferometry and neutral atom microscopy. The manipulation of charge-neutral beams of limited polarizability, spin or excitation states remains problematic, but may be overcome by the development of novel diffractive o...

It is shown that the Pb(√3×√3)R30/Si(111) ordered layer is an excellent mirror for neutral He atoms. It focuses more than 15% of the incoming He atoms into the specular peak, and is stable up to 450 K. Moreover, the reflectivity remains almost unchanged in a time scale of several weeks in ultrahigh vacuum. As a consequence, this system is a very go...

"The smoothest surface ever made", that is, a mirror for atoms comprising a Pb thin film on Si(111), is stabilized through quantum size effects to be atomically flat. It shows an absolute reflectivity for He atoms of 15%, and we report on how values close to 40% could be obtained. This highly reflective mirror can also be electrostatically bent to...

The inside cover shows “the smoothest surface ever made”, a quantum-stabilized, atomically flat Pb film on Si(111), reflecting a He beam. Rodolfo Miranda, Amadeo L. Vazquez de Parga, and co-workers report on p. 3492 that this helium mirror can also be electrostatically bent to focus the He beam into a sub-micrometer spot, implying that current prot...

Due to the extensive research performed over the past three decades, we have gained considerable insight into the properties of metal and semiconductor surfaces. Much less effort has been devoted to the study of more complex surfaces, like alloy and oxide surfaces, even though they are technologically relevant. In this chapter we will discuss how t...

Nearly two-dimensional (2D) metallic systems formed in charge inversion layers and artificial layered materials permit the existence of low-energy collective excitations, called 2D plasmons, which are not found in a three-dimensional (3D) metal. These excitations have caused considerable interest because their low energy allows them to participate...

We review recent experimental and theoretical work aimed to understand the reaction dynamics of hydrogen molecules at metal surfaces. Diffraction experiments and calculations using ab initio determined six-dimensional potential energy surfaces were carried out on several single-crystal surfaces: NiAl(110), Pt(111), Pd(111) and Pd(110). We discuss t...

The intercalation of copper underneath a graphite monolayer (MG) formed on Ni(111) has been investigated by means of HREELS, Auger spectroscopy and LEED. The softening of the graphite phonon modes in the case of the MG/Ni(111) system, caused by a strong interaction with the Ni substrate, is removed after intercalation of copper. The corresponding p...

AFM, STM and diffraction of He and H2 have been used to assess Si(1 1 1)–H(1 × 1) surfaces for their potential as mirrors for matter-waves. The H-passivated samples are produced by wet-chemical methods and delivered to a different laboratory for diffraction measurements. We show that the surface is flat and homogenous over lateral scales of microns...

Sound-like longitudinal plasma waves where thought to only exist in layered systems where spatially separated 2D electron plasmas are realized. Due to their low energy and linear dispersion such waves were proposed as possible candidates to mediate the attractive interaction leading to the formation of Cooper pairs in high TC superconductors. A new...

We have performed H2(D2) diffraction experiments on a Pd(110) surface using two different high-sensitivity set-ups. We have found that, although the total reflectivity of Pd(110) is comparable to that observed in other reactive systems, the corresponding H2(D2) diffraction patterns are quite different: no diffraction peak, including the specular on...