arXiv subjects

A. Arnau

Publications and source records attributed to A. Arnau.

At least 19 recordsLinked to original sources

arXiv2026

Chromium chalcohalide Janus monolayer ferromagnets with perpendicular magnetic anisotropy and high Curie temperature

Using density functional theory, we revisit the magnetic properties of a recently proposed family of noncentrosymmetric two-dimensional magnetic materials, chromium chalcohalide monolayers, CrXY (X=S, Se, Te; Y=Cl, Br, I). These systems consist of three atomic planes stacked in the X-Cr-Y sequence, which breaks inversion symmetry, giving rise to their designation as "Janus" monolayers. We consider both 1T and 1H structural polymorphs of CrXY. Among the two polymorphs, the 1T phase is consistently more favorable, with energy gains exceeding 0.55 eV per formula unit. Our total-energy calculations reveal that all dynamically stable CrXY monolayers exhibit ferromagnetic coupling. However, robust out-of-plane magnetic anisotropy is observed only in the CrSI and CrSeI compositions, for both 1T and 1H structures. The perpendicular magnetic anisotropy results from a constructive interplay between single-ion and anisotropic exchange contributions that overcome the dipole-dipole interaction. We further quantify the Dzyaloshinskii-Moriya interaction (DMI) in CrSI and CrSeI for both polymorphs, and reveal a weak-to-moderate DMI strength as compared to the isotropic exchange interaction term. Finally, for systems exhibiting ferromagnetic coupling and perpendicular magnetic anisotropy, the exchange and anisotropy parameters derived from density functional theory calculations are employed as inputs for large-scale atomistic spin dynamics simulations to probe the temperature evolution of real-space magnetic structures. The calculated Curie temperatures are at least 210 K for 1T-CrSI, 235-260 K for 1H-CrSeI, and 370-410 K for 1H-CrSI. In contrast, the sizable DMI in 1T-CrSeI results in a worm-like domain ground state at zero external field and enables the stabilization of skyrmions under a perpendicular magnetic field.

cond-mat.mtrl-sci↗

arXiv2026

Single Atom Magnets on Thermally Stable Adsorption Sites: Dy on NaCl(100)

We report magnetic bistability in single Dy atoms on NaCl(100) thin films. Individual Dy atoms substituting Na at the surface of the NaCl layer are thermally stable up to at least 300 K, display $4f^{9}$ occupancy, out-of-plane easy magnetization axis, and long spin relaxation time $T_1$ of about 10 s at 2.5 K; thereby they are the first single atom magnet on a thermally stable adsorption site. Dy atoms adsorbed onto the Cl and bridge sites display $4f^{10}$ occupancy. Dy on top-Cl exhibit magnetic hysteresis and a $T_1$ of 550 s at 0.3 T and 2.5 K. The observed slow magnetic relaxation of Dy on both adsorption sites introduces NaCl as an effective platform for single atom magnets.

cond-mat.mtrl-sci↗

arXiv2024

Unveiling the inter-layer interaction in a 1H/1T TaS$_2$ van de Waals heterostructure

This study delves into the intriguing properties of 1H/1T-TaS$_2$ van der Waals heterostructure, focusing on the transparency of the 1H layer to the Charge Density Wave of the underlying 1T layer. Despite the sizable interlayer separation and metallic nature of the 1H layer, positive bias voltages result in a pronounced superposition of the 1T charge density wave structure on the 1H layer. The conventional explanation relying on tunneling effects proves insufficient. Through a comprehensive investigation combining lowtemperature scanning tunneling microscopy, scanning tunneling spectroscopy, non-contact atomic force microscopy, and firstprinciples calculations, we propose an alternative interpretation. The transparency effect arises from a weak yet substantial electronic coupling between the 1H and 1T layers, challenging prior understanding of the system. Our results highlight the critical role played by interlayer electronic interactions in van der Waals heterostructures to determine the final ground states of the systems.

cond-mat.str-el↗

arXiv2023

Spin wave excitations in low dimensional systems with large magnetic anisotropy

The low energy excitation spectrum of a two-dimensional ferromagnetic material is dominated by single-magnon excitations that show a gapless parabolic dispersion relation with the spin wave vector. This occurs as long as magnetic anisotropy and anisotropic exchange are negligible compared to isotropic exchange. However, to maintain magnetic order at finite temperatures, it is necessary to have sizable anisotropy to open a gap in the spin wave excitation spectrum. We consider four real two-dimensional systems for which ferromagnetic order at finite temperature has been observed or predicted. Density functional theory calculations of the total energy differences for different spin configurations permit us to extract the relevant parameters and connect them with a spin Hamiltonian. The corresponding values of the Curie temperature are estimated using a simple model and found to be mostly determined by the value of the isotropic exchange. The exchange and anisotropy parameters are used in a toy model of finite-size periodic chains to study the low-energy excitation spectrum, including single-magnon and two-magnon excitations. At low energies we find that single-magnon excitations appear in the spectrum together with two-magnon excitations. These excitations present a gap that grows particularly for large values of the magnetic anisotropy or anisotropic exchange, relative to the isotropic exchange.

cond-mat.mes-hall↗

arXiv2022

Multiplet Effects in the Electronic Correlation of One-Dimensional Magnetic Transition-Metal Oxides on Metals

We use the constrained random phase approximation (cRPA) method to calculate the Hubbard $U$ parameter in four one-dimensional magnetic transition metal atom oxides of composition XO$_2$ (X = Mn, Fe, Co, Ni) on Ir(100). In addition to the expected screening of the oxide, i.e., a significant reduction of the $U$ value by the presence of the metal substrate, we find a strong dependence on the electronic configuration (multiplet) of the X($d$) orbital. Each particular electronic configuration attained by atom X is dictated by the O ligands, as well as by the charge transfer and hybridization with the Ir(100) substrate. We find that MnO$_2$ and NiO$_2$ chains exhibit two different screening regimes, while the case of CoO$_2$ is somewhere in between. The electronic structure of the MnO$_2$ chain remains almost unchanged upon adsorption. Therefore, in this regime, the additional screening is predominantly generated by the electrons of the neighboring metal surface atoms. The screening strength for NiO$_2$/Ir(100) is found to depend on the Ni($d$) configuration in the adsorbed state. The case of FeO$_2$ shows an exceptional behavior, as it is the only insulating system in the absence of metallic substrate and, thus, it has the largest $U$ value. However, this value is significantly reduced by the two mentioned screening effects after adsorption.

cond-mat.mtrl-sci↗

arXiv2021

Nature of Interfacial Dzyaloshinskii-Moriya Interactions in Graphene/Co/Pt(111) Multilayer Heterostructures

DFT calculations within the generalized Bloch theorem approach show that interfacial Dzyaloshinskii-Moriya interactions (DMI) at both interfaces of Graphene/Co$_n$/Pt(111) multilayer heterostructures are decoupled for $n \geq 3$. Unlike the property of magnetocrystalline anisotropy for this system, DMI is not affected by stacking defects in the Co layer. The effect of Graphene (Gr) is to invert the chirality of the vaccum/Co interfacial DMI, overall reducing the DMI of the heterostructure, which is nevertheless dominated by the strong spin-orbit coupling (SOC) of Pt. A spectral analysis in the reciprocal space shows that DMI at both the Gr/Co and Co/Pt interfaces have the same nature, namely SOC-split hybrid bands of $d$-orbital character. This proves that a DMI model based on a single band, such the Rashba DMI model, is insuficient to describe the behaviour of this family of Gr-capped $3d/5d$ metal heterostructures.

cond-mat.mtrl-sci↗

arXiv2021

Native point defects and their implications for the Dirac point gap at MnBi$_2$Te$_4$(0001)

The Dirac point gap at the surface of the antiferromagnetic topological insulator MnBi$_2$Te$_4$ is a highly debated issue. While the early photoemission measurements reported on large gaps in agreement with theoretical predictions, other experiments found vanishingly small splitting of the MnBi$_2$Te$_4$ Dirac cone. Here, we study the crystalline and electronic structure of MnBi$_2$Te$_4$(0001) using scanning tunneling microscopy/spectroscopy (STM/S), micro($μ$)-laser angle resolved photoemission spectroscopy (ARPES), and density functional theory (DFT) calculations. Our topographic STM images clearly reveal features corresponding to point defects in the surface Te and subsurface Bi layers that we identify with the aid of STM simulations as Bi$_\text{Te}$ antisites (Bi atoms at the Te sites) and Mn$_\text{Bi}$ substitutions (Mn atoms at the Bi sites), respectively. X-ray diffraction (XRD) experiments further evidence the presence of cation (Mn-Bi) intermixing. Altogether, this affects the distribution of the Mn atoms, which, inevitably, leads to a deviation of the MnBi$_2$Te$_4$ magnetic structure from that predicted for the ideal crystal structure. Our transport measurements suggest that the degree of this deviation varies from sample to sample. Consistently, the ARPES/STS experiments reveal that the Dirac point gap of the topological surface state is different for different samples/sample cleavages. Our DFT surface electronic structure calculations show that, due to the predominant localization of the topological surface state near the Bi layers, Mn$_\text{Bi}$ defects can cause a strong reduction of the MnBi$_2$Te$_4$ Dirac point gap, given the recently proved antiparallel alignment of the Mn$_\text{Bi}$ moments with respect to those of the Mn layer. Our results provide a key to puzzle out the MnBi$_2$Te$_4$ Dirac point gap mystery.

cond-mat.mtrl-sci↗

arXiv2020

Origin of the Large Perpendicular Magnetic Anisotropy in Nanometer-thick Epitaxial Graphene/Co/Heavy Metal Heterostructures

A combination of theoretical modelling and experiments reveals the origin of the large perpendicular magnetic anisotropy (PMA) that appears in nanometer-thick epitaxial Co films intercalated between graphene (Gr) and a heavy metal (HM) substrate, as a function of the Co thickness. High quality epitaxial Gr/Co\n/HM(111) (HM=Pt,Ir) heterostructures are grown by intercalation below graphene, which acts as a surfactant that kinetically stabilizes the pseudomorphic growth of highly perfect Co face-centered tetragonal ($fct$) films, with a reduced number of stacking faults as the only structural defect observable by high resolution scanning transmission electron microscopy (HR-STEM). Magneto-optic Kerr effect (MOKE) measurements show that such heterostructures present PMA up to large Co critical thicknesses of about 4~nm (20~ML) and 2~nm (10~ML) for Pt and Ir substrates, respectively, while X-ray magnetic circular dichroism (XMCD) measurements show an inverse power law of the anistropy of the orbital moment with Co thickness, reflecting its interfacial nature, that changes sign at about the same critical values. First principles calculations show that, regardless of the presence of graphene, ideal Co $fct$ films on HM buffers do not sustain PMAs beyond around 6~MLs due to the in-plane contribution of the inner bulk-like Co layers. The large experimental critical thicknesses sustaining PMA can only be retrieved by the inclusion of structural defects that promote a local $hcp$ stacking such as twin boundaries or stacking faults. Remarkably, a layer resolved analysis of the orbital momentum anisotropy reproduces its interfacial nature, and reveals that the Gr/Co interface contribution is comparable to that of the Co/Pt(Ir).

cond-mat.mtrl-sci↗

arXiv2018

Magnetocrystalline Anisotropy of Fe-based $L1_0$ Alloys: Validity of Approximate Methods to Treat the Spin-Orbit Interaction

First-principles calculations are used to gauge different levels of approximation to calculate the magnetocrystalline anisotropy energies (MAE) of five $L1_0$ FeMe alloys (Me=Co, Cu, Pd, Pt, Au). We find that a second-order perturbation (2PT) treatment of the spin-orbit interaction (SOI) breaks down for the alloys containing heavier ions, while it provides a very accurate description of the MAE behaviour of FeCo, FeCu, and FePd. Moreover, the robustness of the 2PT approximation is such that in these cases it accounts for the MAE of highly-non-neutral alloys and, thus, it can be used to predict their performance when dopants are present or when they are subject to applied gate bias, which are typical conditions in working magnetoelectric devices. We also observe that switching of the easy axis direction can be induced in some of these alloys by addition or removal of, at least, one electron per cell. In all cases, the details of the bandstructure are responsible for the finally observed MAE value and, therefore, suggest a limited predicting power of models based on the expected orbital moment values and bandwidths. Finally, we have confirmed the importance of various calculation parameters to obtain converged MAE values, in particular, those related to the accuracy of the Fermi level determination.

cond-mat.mtrl-sci↗

arXiv2014

On the Mechanical and Electronic Properties of Thiolated Gold Nanocrystals

We present a quantitative exploration, combining experiment and simulation, of the mechanical and electronic properties, as well as the modifications induced by an alkylthiolated coating, at the single NP level. We determine the response of the NPs to external pressure in a controlled manner by using an atomic force microscope tip. We find a strong reduction of their Young modulus, as compared to bulk gold, and a significant influence of strain in the electronic properties of the alkylthiolated NPs. Electron transport measurements of tiny molecular junctions (NP/alkylthiol/CAFM tip) show that the effective tunnelling barrier through the adsorbed monolayer strongly decreases with increasing the applied load, which translates in a remarkable and unprecedented increase of the tunnel current. These observations are successfully explained using simulations based on finite element analysis (FEA) and first-principles calculations that permit to consider the coupling between the mechanical response of the system and the electric dipole variations at the interface.

cond-mat.mes-hall↗

arXiv2012

Electronic stopping power in gold: The role of d electrons and the H/He anomaly

The electronic stopping power of H and He moving through gold is obtained to high accuracy using time-evolving density-functional theory, thereby bringing usual first-principles accuracies into this kind of strongly coupled, continuum non-adiabatic processes in condensed matter. The two key unexplained features of what observed experimentally have been reproduced and understood: (i) The non-linear behaviour of stopping power versus velocity is a gradual crossover as excitations tail into the d-electron spectrum; and (ii) the low-velocity H/He anomaly (the relative stopping powers are contrary to established theory) is explained by the substantial involvement of the d electrons in the screening of the projectile even at the lowest velocities where the energy loss is generated by s-like electron-hole pair formation only.

cond-mat.mtrl-sci↗

arXiv2010

Substrate-Induced Cooperative Effects in Water Adsorption from Density Functional Calculations

Density Functional Theory calculations are used to investigate the role of substrate-induced cooperative effects on the adsorption of water on a partially oxidized transition metal surface, O(2x2)/Ru(0001). Focussing particularly on the dimer configuration, we analyze the different contributions to its binding energy. A significant reinforcement of the intermolecular hydrogen-bond (H-bond), also supported by the observed frequency shifts of the vibration modes, is attributed to the polarization of the donor molecule when bonded to the Ru atoms in the substrate. This result is further confirmed by our calculations for a water dimer interacting with a small Ru cluster, which clearly show that the observed effect does not depend critically on fine structural details and/or the presence of co-adsorbates. Interestingly, the cooperative reinforcement of the H-bond is suppressed when the acceptor molecule, instead of the donor, is bonded to the surface. This simple observation can be used to rationalize the relative stability of different condensed structures of water on metallic substrates.

cond-mat.mtrl-sci↗

arXiv2010

Potential Energy Landscape for hot electrons in periodically nanostructured graphene

We explore the spatial variations of the unoccupied electronic states of graphene epitaxially grown on Ru(0001) and observed three unexpected features: the first graphene image state is split in energy, unlike all other image states, the split state does not follow the local work function modulation, and a new interfacial state at +3 eV appears on some areas of the surface. These results show the system behaves as a self-organized periodic array of quantum dots.

cond-mat.mes-hall↗

arXiv2009

Study of the induced potential produced by ultrashort pulses on metal surfaces

The influence of the induced potential on photoelectron emission from metal surfaces is studied for grazing incidence of ultrashort laser pulses. To describe this process we introduce a distorted wave method, the Surface Jellium Volkov approach, which includes the perturbation on the emitted electron produced by both the laser and the induced fields. The method is applied to an Al(111) surfaces, contrasting the results with the numerical solutions to the time-dependent Schrodinger equation (TDSE). We found that SJV approach reproduces well the main features of emission spectra, accounting properly for effects originated by the induced potential.

cond-mat.other↗

arXiv2008

Dynamic Jahn-Teller effect in electron transport through single C60 molecules

Scanning tunneling spectra on single C60 molecules that are sufficiently decoupled from the substrate exhibit a characteristic fine structure, which is explained as due to the dynamic Jahn-Teller effect. Using electron-phonon couplings extracted from density functional theory we calculate the tunneling spectrum through the C60- anionic state and find excellent agreement with measured data.

cond-mat.mes-hall↗

arXiv2008

Formation of dispersive hybrid bands at an organic-metal interface

An electronic band with quasi-one dimensional dispersion is found at the interface between a monolayer of a charge-transfer complex (TTF-TCNQ) and a Au(111) surface. Combined local spectroscopy and numerical calculations show that the band results from a complex mixing of metal and molecular states. The molecular layer folds the underlying metal states and mixes with them selectively, through the TTF component, giving rise to anisotropic hybrid bands. Our results suggest that, by tuning the components of such molecular layers, the dimensionality and dispersion of organic-metal interface states can be engineered.

cond-mat.mtrl-sci↗

arXiv2007

Electronic stopping power in insulators from first principles

Using time-dependent density-functional theory we calculate from first principles the rate of energy transfer from a moving proton or antiproton to the electrons of an insulating material, LiF. The behavior of the electronic stopping power versus projectile velocity displays an effective threshold velocity of ~0.2 a.u. for the proton, consistent with recent experimental observations, and also for the antiproton. The calculated proton/antiproton stopping-power ratio is ~2.4 at velocities slightly above the threshold (v~0.4 a.u.), as compared to the experimental value of 2.1. The projectile energy loss mechanism is observed to be stationary and extremely local.

cond-mat.mtrl-sci↗

arXiv2007

Role of the electric field in surface electron dynamics above the vacuum level

Scanning tunneling spectroscopy (STS) is used to study the dynamics of hot electrons trapped on a Cu(100) surface in field emission resonances (FER) above the vacuum level. Differential conductance maps show isotropic electron interference wave patterns around defects whenever their energy lies within a surface projected band gap. Their Fourier analysis reveals a broad wave vector distribution, interpreted as due to the lateral acceleration of hot electrons in the inhomogeneous tip-induced potential. A line-shape analysis of the characteristic constant-current conductance spectra permits to establish the relation between apparent width of peaks and intrinsic line-width of FERs, as well as the identification of the different broadening mechanisms.

cond-mat.mtrl-sci↗