Staff directory Aitor Mugarza Ezpeleta

Aitor Mugarza Ezpeleta

ICREA Research Professor and Group Leader
aitor.mugarza(ELIMINAR)@icn2.cat
Atomic Manipulation and Spectroscopy

Publications

2020

  • Band Depopulation of Graphene Nanoribbons Induced by Chemical Gating with Amino Groups

    Li J., Brandimarte P., Vilas-Varela M., Merino-Díez N., Moreno C., Mugarza A., Mollejo J.S., Sánchez-Portal D., Garcia De Oteyza D., Corso M., Garcia-Lekue A., Peña D., Pascual J.I. ACS Nano; 14 (2): 1895 - 1901. 2020. 10.1021/acsnano.9b08162. IF: 14.588

    The electronic properties of graphene nanoribbons (GNRs) can be precisely tuned by chemical doping. Here we demonstrate that amino (NH2) functional groups attached at the edges of chiral GNRs (chGNRs) can efficiently gate the chGNRs and lead to the valence band (VB) depopulation on a metallic surface. The NH2-doped chGNRs are grown by on-surface synthesis on Au(111) using functionalized bianthracene precursors. Scanning tunneling spectroscopy resolves that the NH2 groups significantly upshift the bands of chGNRs, causing the Fermi level crossing of the VB onset of chGNRs. Through density functional theory simulations we confirm that the hole-doping behavior is due to an upward shift of the bands induced by the edge NH2 groups. © 2020 American Chemical Society.


  • Enhanced Magnetism through Oxygenation of FePc/Ag(110) Monolayer Phases

    Bartolomé E., Bartolomé J., Sedona F., Lobo-Checa J., Forrer D., Herrero-Albillos J., Piantek M., Herrero-Martín J., Betto D., Velez-Fort E., García L.M., Panighel M., Mugarza A., Sambi M., Bartolomé F. Journal of Physical Chemistry C; 124 (25): 13993 - 14006. 2020. 10.1021/acs.jpcc.0c01988. IF: 4.189

    Iron phthalocyanines (FePc) adsorbed onto a Ag(110) substrate self-assemble into different monolayer phases going from rectangular to different oblique phases, with increasing molecular density. We have investigated the oxygen uptake capability of the different phases and their associated magneto-structural changes. Our study combines scanning tunneling microscopy and spectroscopy (STM/STS), X-ray magnetic circular dichroism (XMCD), and density functional theory (DFT) calculations. STM measurements reveal that the oxygenation reaction of the FePc/Ag(110) generally involves a displacement and a rotation of the molecules, which affects the electronic state of the Fe centers. The oxygen intercalation between FePc and the substrate is greatly obstructed by the steric hindrance in the high-density phases, to the point that a fraction of oblique phase molecules cannot change their position after oxidizing. Depending on the oxidation state and adsoption geometry, the STS spectra show clear differences in the Fe local density of states, which are mirrored in the XAS and XMCD experiments. Particularly, XMCD spectra of the oxidized phases reflect the distribution of FePc species (nonoxygenated, oxygenated-rotated, and oxygenated-unrotated) in the different cases. Sum rule analysis yields the effective spin (mseff) and orbital (mL) magnetic moments of Fe in the different FePc species. Upon oxygenation, the magnetic moment of FePc molecules increases about an order of magnitude, reaching mTOT ∼2.2 μB per Fe atom. © 2020 American Chemical Society.


  • Molecular Approach for Engineering Interfacial Interactions in Magnetic/Topological Insulator Heterostructures

    Cuxart M.G., Valbuena M.A., Robles R., Moreno C., Bonell F., Sauthier G., Imaz I., Xu H., Nistor C., Barla A., Gargiani P., Valvidares M., Maspoch D., Gambardella P., Valenzuela S.O., Mugarza A. ACS Nano; 14 (5): 6285 - 6294. 2020. 10.1021/acsnano.0c02498. IF: 14.588

    Controlling interfacial interactions in magnetic/topological insulator heterostructures is a major challenge for the emergence of novel spin-dependent electronic phenomena. As for any rational design of heterostructures that rely on proximity effects, one should ideally retain the overall properties of each component while tuning interactions at the interface. However, in most inorganic interfaces, interactions are too strong, consequently perturbing, and even quenching, both the magnetic moment and the topological surface states at each side of the interface. Here, we show that these properties can be preserved using ligand chemistry to tune the interaction of magnetic ions with the surface states. By depositing Co-based porphyrin and phthalocyanine monolayers on the surface of Bi2Te3 thin films, robust interfaces are formed that preserve undoped topological surface states as well as the pristine magnetic moment of the divalent Co ions. The selected ligands allow us to tune the interfacial hybridization within this weak interaction regime. These results, which are in stark contrast with the observed suppression of the surface state at the first quintuple layer of Bi2Se3 induced by the interaction with Co phthalocyanines, demonstrate the capability of planar metal-organic molecules to span interactions from the strong to the weak limit. © 2020 American Chemical Society.


  • Neutral Organic Radical Formation by Chemisorption on Metal Surfaces

    Ajayakumar M.R., Moreno C., Alcón I., Illas F., Rovira C., Veciana J., Bromley S.T., Mugarza A., Mas-Torrent M. Journal of Physical Chemistry Letters; 11 (10): 3897 - 3904. 2020. 10.1021/acs.jpclett.0c00269. IF: 6.710

    Organic radical monolayers (r-MLs) bonded to metal surfaces are potential materials for the development of molecular (spin)electronics. Typically, stable radicals bearing surface anchoring groups are used to generate r-MLs. Following a recent theoretical proposal based on a model system, we report the first experimental realization of a metal surface-induced r-ML, where a rationally chosen closed-shell precursor 3,5-dichloro-4-[bis(2,4,6-trichlorophenyl)methylen]cyclohexa-2,5-dien-1-one (1) transforms into a stable neutral open-shell species (1) via chemisorption on the Ag(111) surface. X-ray photoelectron spectroscopy reveals that the >C=O group of 1 reacts with the surface, forming a C-O-Ag linkage that induces an electronic rearrangement that transforms 1 to 1. We further show that surface reactivity is an important factor in this process whereby Au(111) is inert towards 1, whereas the Cu(111) surface leads to dehalogenation reactions. The radical nature of the Ag(111)-bound monolayer was further confirmed by angle-resolved photoelectron spectroscopy and electronic structure calculations, which provide evidence of the emergence of the singly occupied molecular orbital (SOMO) of 1. © 2020 American Chemical Society.


  • Stabilizing Edge Fluorination in Graphene Nanoribbons

    Panighel M., Quiroga S., Brandimarte P., Moreno C., Garcia-Lekue A., Vilas-Varela M., Rey D., Sauthier G., Ceballos G., Peña D., Mugarza A. ACS nano; 14 (9): 11120 - 11129. 2020. 10.1021/acsnano.0c01837. IF: 14.588

    The on-surface synthesis of edge-functionalized graphene nanoribbons (GNRs) is challenged by the stability of the functional groups throughout the thermal reaction steps of the synthetic pathway. Edge fluorination is a particularly critical case in which the interaction with the catalytic substrate and intermediate products can induce the complete cleavage of the otherwise strong C-F bonds before the formation of the GNR. Here, we demonstrate how a rational design of the precursor can stabilize the functional group, enabling the synthesis of edge-fluorinated GNRs. The survival of the functionalization is demonstrated by tracking the structural and chemical transformations occurring at each reaction step with complementary X-ray photoelectron spectroscopy and scanning tunneling microscopy measurements. In contrast to previous attempts, we find that the C-F bond survives the cyclodehydrogenation of the intermediate polymers, leaving a thermal window where GNRs withhold more than 80% of the fluorine atoms. We attribute this enhanced stability of the C-F bond to the particular structure of our precursor, which prevents the cleavage of the C-F bond by avoiding interaction with the residual hydrogen originated in the cyclodehydrogenation. This structural protection of the linking bond could be implemented in the synthesis of other sp2-functionalized GNRs.


2019

  • Critical Role of Phenyl Substitution and Catalytic Substrate in the Surface-Assisted Polymerization of Dibromobianthracene Derivatives

    Moreno C., Panighel M., Vilas-Varela M., Sauthier G., Tenorio M., Ceballos G., Peña D., Mugarza A. Chemistry of Materials; 31 (2): 331 - 341. 2019. 10.1021/acs.chemmater.8b03094. IF: 10.159

    Understanding the nature and hierarchy of on-surface reactions is a major challenge for designing coordination and covalent nanostructures by means of multistep synthetic routes. In particular, intermediates and final products are hard to predict since the reaction paths and their activation windows depend on the choice of both the molecular precursor design and the substrate. Here, we report a systematic study of the effect of the catalytic metal surface to reveal how a single precursor can give rise to very distinct polymers that range from coordination and covalent nonplanar polymer chains of distinct chirality to atomically precise graphene nanoribbons and nanoporous graphene. Our precursor consists on adding two phenyl substituents to 10,10′-dibromo-9,9′-bianthracene, a well-studied precursor in the on-surface synthesis of graphene nanoribbons. The critical role of the monomer design in the reaction paths is inferred from the fact that the phenyl substitution leads to very distinct products in each one of the studied metallic substrates. © 2018 American Chemical Society.


  • Nanoarquitecturas de grafeno con precisión atómica

    C. Moreno, A. Garcia-Lekue, A. Mugarza Revista Española de Física; 33 (4) 2019. noDOI-Moreno.

    La síntesis de arquitecturas covalentes en superficie y con precisión atómica se ha erigido como uno de los métodos más prometedores para dotar de nuevas funcionalidades al grafeno, un material ya de por si superlativo en múltiples aspectos. La fortaleza de este método estriba en su flexibilidad para crear una vasta gama de arquitecturas grafénicas. Para llegar a la estructura final buscada se diseñan moléculas precursoras prácticamente a la carta. El diseño de estas moléculas contiene dos aspectos comunes, la parte que dará conformación a la estructura final y la parte que servirá para enlazar dichas moléculas de forma ordenada como si se tratase de piezas de Lego. El resultado final son tiras de grafeno semiconductoras con un alto potencial en aplicaciones en nanoelectrónica o fotónica. También ha sido posible materializar el hito de sintetizar grafeno nanoporoso con precisión atómica. Esto podría suponer un gran avance en el desarrollo de nuevas membranas más ligeras, con velocidades de filtrado mayores, con un menor coste energético de filtrado y con un mayor grado de selectividad.


  • Unraveling the Impact of Halide Mixing on Perovskite Stability

    Hieulle J., Wang X., Stecker C., Son D.-Y., Qiu L., Ohmann R., Ono L.K., Mugarza A., Yan Y., Qi Y. Journal of the American Chemical Society; 141 (8): 3515 - 3523. 2019. 10.1021/jacs.8b11210. IF: 14.695

    Increasing the stability of perovskites is essential for their integration in commercial photovoltaic devices. Halide mixing is suggested as a powerful strategy toward stable perovskite materials. However, the stabilizing effect of the halides critically depends on their distribution in the mixed compound, a topic that is currently under intense debate. Here we successfully determine the exact location of the I and Cl anions in the CH 3 NH 3 PbBr 3-y I y and CH 3 NH 3 PbBr 3-z Cl z mixed halide perovskite lattices and correlate it with the enhanced stability we find for the latter. By combining scanning tunneling microscopy and density functional theory, we predict that, for low ratios, iodine and chlorine incorporation have different effects on the electronic properties and stability of the CH 3 NH 3 PbBr 3 perovskite material. In addition, we determine the optimal Cl incorporation ratio for stability increase without detrimental band gap modification, providing an important direction for the fabrication of stable perovskite devices. The increased material stability induced by chlorine incorporation is verified by performing photoelectron spectroscopy on a half-cell device architecture. Our findings provide an answer to the current debate on halide incorporation and demonstrate their direct influence on device stability. © Copyright 2019 American Chemical Society.


2018

  • Bottom-up synthesis of multifunctional nanoporous graphene

    Moreno C., Vilas-Varela M., Kretz B., Garcia-Lekue A., Costache M.V., Paradinas M., Panighel M., Ceballos G., Valenzuela S.O., Peña D., Mugarza A. Science; 360 (6385): 199 - 203. 2018. 10.1126/science.aar2009. IF: 41.058

    Nanosize pores can turn semimetallic graphene into a semiconductor and, from being impermeable, into the most efficient molecular-sieve membrane. However, scaling the pores down to the nanometer, while fulfilling the tight structural constraints imposed by applications, represents an enormous challenge for present top-down strategies. Here we report a bottom-up method to synthesize nanoporous graphene comprising an ordered array of pores separated by ribbons, which can be tuned down to the 1-nanometer range. The size, density, morphology, and chemical composition of the pores are defined with atomic precision by the design of the molecular precursors. Our electronic characterization further reveals a highly anisotropic electronic structure, where orthogonal one-dimensional electronic bands with an energy gap of ∼1 electron volt coexist with confined pore states, making the nanoporous graphene a highly versatile semiconductor for simultaneous sieving and electrical sensing of molecular species. 2017 © The Authors


  • On-surface synthesis of superlattice arrays of ultra-long graphene nanoribbons

    Moreno C., Paradinas M., Vilas-Varela M., Panighel M., Ceballos G., Peña D., Mugarza A. Chemical Communications; 54 (68): 9402 - 9405. 2018. 10.1039/c8cc04830d. IF: 6.290

    We report the on-surface synthesis of graphene nanoribbon superlattice arrays directed by the herringbone reconstruction of the Au(111) surface. The uniaxial anisotropy of the zigzag pattern of the reconstruction defines a one dimensional grid for directing the Ullmann polymerization and inducing periodic arrays of parallel ultra-long nanoribbons (>100 nm), where the periodicity is varied with coverage at discrete values following a hierarchical templating behavior. © 2018 The Royal Society of Chemistry.


  • Structure and electronic states of vicinal Ag(111) surfaces with densely kinked steps

    Ortega J.E., Vasseur G., Piquero-Zulaica I., Matencio S., Valbuena M.A., Rault J.E., Schiller F., Corso M., Mugarza A., Lobo-Checa J. New Journal of Physics; 20 (7, 073010) 2018. 10.1088/1367-2630/aacbb7. IF: 3.579

    Vicinal surfaces exhibiting arrays of atomic steps are frequently investigated due to their diverse physical-chemical properties and their use as growth templates. However, surfaces featuring steps with a large number of low-coordinated kink-atoms have been widely ignored, despite their higher potential for chemistry and catalysis. Here, the equilibrium structure and the electronic states of vicinal Ag(111) surfaces with densely kinked steps are investigated in a systematic way using a curved crystal. With scanning tunneling microscopy we observe an exceptional structural homogeneity of this class of vicinals, reflected in the smooth probability distribution of terrace sizes at all vicinal angles. This allows us to observe, first, a subtle evolution of the terrace-size distribution as a function of the terrace-width that challenges statistical models of step lattices, and second, lattice fluctuations around resonant modes of surface states. As shown in angle resolved photoemission experiments, surface states undergo stronger scattering by fully-kinked step-edges, which triggers the full depletion of the two-dimensional band at surfaces with relatively small vicinal angles. © 2018 The Author(s). Published by IOP Publishing Ltd on behalf of Deutsche Physikalische Gesellschaft.


  • Towards microscopic control of the magnetic exchange coupling at the surface of a topological insulator

    Philipp Rüßmann, Sanjoy K Mahatha, Paolo Sessi, Miguel A Valbuena, Thomas Bathon, Kai Fauth, Sylvie Godey, Aitor Mugarza, Konstantin A Kokh, Oleg E Tereshchenko, Pierluigi Gargiani, Manuel Valvidares, Erika Jiménez, Nicholas B Brookes, Matthias Bode, Gustav Bihlmayer, Stefan Blügel, Phivos Mavropoulos, Carlo Carbone, Alessandro Barla Journal of Physics: Materials; 1 (1): 15002. 2018. 10.1088/2515-7639/aad02a.

    Magnetically doped topological insulators may produce novel states of electronic matter, where for instance the quantum anomalous Hall effect state can be realized. Pivotal to this goal is a microscopic control over the magnetic state, defined by the local electronic structure of the dopants and their interactions.Wereport on the magnetic coupling amongMnor Co atoms adsorbed on the surface of the topological insulator Bi2Te3. Our findings uncover the mechanisms of the exchange coupling between magnetic atoms coupled to the topological surface state in strong topological insulators. The combination of x-ray magnetic circular dichroism and ab initio calculations reveals that the sign of the magnetic coupling at short adatom–adatom distances is opposite forMnwith respect to Co. For both elements, the magnetic exchange reverses its sign at a critical distance between magnetic adatoms, as a result of the interplay between superexchange, double exchange and Ruderman–Kittel–Kasuya– Yoshida interactions.


2017

  • Electronic Structure of Titanylphthalocyanine Layers on Ag(111)

    Lerch A., Fernandez L., Ilyn M., Gastaldo M., Paradinas M., Valbuena M.A., Mugarza A., Ibrahim A.B.M., Sundermeyer J., Höfer U., Schiller F. Journal of Physical Chemistry C; 121 (45): 25353 - 25363. 2017. 10.1021/acs.jpcc.7b09147. IF: 4.536

    We have investigated the electronic structures of axially oxo functionalized titanylphthalocyanine (TiOPc) on Ag(111) by X-ray and ultraviolet photoelectron spectroscopies, two-photon photoemission, X-ray absorption spectroscopy, and X-ray magnetic circular dichroism. Furthermore, we use complementary data of TiOPc on graphite and planar copper phthalocyanine (CuPc) on Ag(111) for a comparative analysis. Both molecules adsorb on Ag(111) in a parallel orientation to the surface, for TiOPc with an oxygen-up configuration. The interaction of nitrogen and carbon atoms with the substrate is similar for both molecules, while the bonding of the titanium atom to Ag(111) in the monolayer is found to be slightly more pronounced than in the CuPc case. Ultraviolet photoemission spectroscopy reveals an occupation of the lowest unoccupied molecular orbital (LUMO) level in monolayer thick TiOPc on Ag(111) related to the interaction of the molecules and the silver substrate. This molecule-metal interaction also causes an upward shift of the Ag(111) Shockley state that is transformed into an unoccupied interface state with energies of 0.23 and 0.33 eV for the TiOPc monolayer and bilayer, respectively, at the Brillouin zone center. © 2017 American Chemical Society.


  • Growth of Twin-Free and Low-Doped Topological Insulators on BaF2(111)

    Bonell F., Cuxart M.G., Song K., Robles R., Ordejón P., Roche S., Mugarza A., Valenzuela S.O. Crystal Growth and Design; 17 (9): 4655 - 4660. 2017. 10.1021/acs.cgd.7b00525. IF: 4.055

    We demonstrate the growth of twin-free Bi2Te3 and Sb2Te3 topological insulators by molecular beam epitaxy and a sizable reduction of the twin density in Bi2Se3 on lattice-matched BaF2(111) substrates. Using X-ray diffraction, electron diffraction and atomic force microscopy, we systematically investigate the parameters influencing the formation of twin domains and the morphology of the films, and show that Se- and Te-based alloys differ by their growth mechanism. Optimum growth parameters are shown to result in intrinsically low-doped films, as probed by angle-resolved photoelectron spectroscopy. In contrast to previous approaches in which twin-free Bi2Se3 films are achieved by increasing the substrate roughness, the quality of our Bi2Te3 is superior on the flattest BaF2 substrates. This finding indicates that, during nucleation, the films not only interact with the topmost atomic substrate layer but also with buried layers that provide the necessary stacking information to promote a single twin, an observation that is supported by ab initio calculations. © 2017 American Chemical Society.


  • Partial oxidation in a dense phase sub-monolayer of Fe-phthalocyanine on Ag(110)

    Bartolomé E., Bartolomé J., Sedona F., Herrero-Albillos J., Lobo J., Piantek M., García L.M., Panighel M., Mugarza A., Sambi M., Bartolomé F. Solid State Phenomena; 257: 219 - 222. 2017. 10.4028/www.scientific.net/SSP.257.219. IF: 0.493

    In this contribution we report on the structural and magnetic changes along a catalytic cycle of a new dense, “quasi-squared” FePc submonolayer phase (R3) evaporated on Ag(110). X-ray magnetic circular dichroism (XMCD) experiments at the Fe L2,3 edge were performed on four samples: the as-evaporated phase (R3), two differently oxygenated samples (OX1 and OX2) and the annealed phase (ANN). It is concluded that all characterized phases display planar anisotropy, and the values of ms eff/nh are one order of magnitude larger than mL/nh. By oxidation, the isotropic moment increases from 7.2 x 10-2 μB/hole to 1.8 x 10-1 μB/hole, which is about a factor of 2 smaller than the increase achieved for the low-density phase. © 2017 Trans Tech Publications, Switzerland.


  • Symmetry forbidden morphologies and domain boundaries in nanoscale graphene islands

    Parreiras S.O., Gastaldo M., Moreno C., Martins M.D., Garcia-Lekue A., Ceballos G., Paniago R., Mugarza A. 2D Materials; 4 (2, 025104) 2017. 10.1088/2053-1583/aa70fa. IF: 6.937

    The synthesis of graphene nanoislands with tailored quantum properties requires an atomic control of the morphology and crystal structure. As one reduces their size down to the nanometer scale, domain boundary and edge energetics, as well as nucleation and growth mechanisms impose different stability and kinetic landscape from that at the microscale. This offers the possibility to synthesize structures that are exclusive to the nanoscale, but also calls for fundamental growth studies in order to control them. By employing high-resolution scanning tunneling microscopy we elucidate the atomic stacking configurations, domain boundaries, and edge structure of graphene nanoislands grown on Ni(1 1 1) by CVD and post-annealed at different temperatures. We find a non-conventional multistep mechanism that separates the thermal regimes for growth, edge reconstruction, and final stacking configuration, leading to nanoisland morphologies that are incompatible with their stacking symmetry. Whole islands shift their stacking configuration during cooling down, and others present continuous transitions at the edges. A statistical analysis of the domain structures obtained at different annealing temperatures reveals how polycrystalline, ill-defined structures heal into shape-selected islands of a single predominant stacking. The high crystallinity and the control on morphology and edge structure makes these graphene nanoislands ideal for their application in optoelectronics and spintronics. © 2017 IOP Publishing Ltd.


2016

  • High Temperature Ferromagnetism in a GdAg2 Monolayer

    Ormaza M., Fernández L., Ilyn M., Magana A., Xu B., Verstraete M.J., Gastaldo M., Valbuena M.A., Gargiani P., Mugarza A., Ayuela A., Vitali L., Blanco-Rey M., Schiller F., Ortega J.E. Nano Letters; 16 (7): 4230 - 4235. 2016. 10.1021/acs.nanolett.6b01197. IF: 13.779

    Materials that exhibit ferromagnetism, interfacial stability, and tunability are highly desired for the realization of emerging magnetoelectronic phenomena in heterostructures. Here we present the GdAg2 monolayer alloy, which possesses all such qualities. By combining X-ray absorption, Kerr effect, and angle-resolved photoemission with ab initio calculations, we have investigated the ferromagnetic nature of this class of Gd-based alloys. The Curie temperature can increase from 19 K in GdAu2 to a remarkably high 85 K in GdAg2. We find that the exchange coupling between Gd atoms is barely affected by their full coordination with noble metal atoms, and instead, magnetic coupling is effectively mediated by noble metal-Gd hybrid s,p-d bands. The direct comparison between isostructural GdAu2 and GdAg2 monolayers explains how the higher degree of surface confinement and electron occupation of such hybrid s,p-d bands promote the high Curie temperature in the latter. Finally, the chemical composition and structural robustness of the GdAg2 alloy has been demonstrated by interfacing them with organic semiconductors or magnetic nanodots. These results encourage systematic investigations of rare-earth/noble metal surface alloys and interfaces, in order to exploit them in magnetoelectronic applications. © 2016 American Chemical Society.


  • Manipulating the Topological Interface by Molecular Adsorbates: Adsorption of Co-Phthalocyanine on Bi2Se3

    Caputo M., Panighel M., Lisi S., Khalil L., Santo G.D., Papalazarou E., Hruban A., Konczykowski M., Krusin-Elbaum L., Aliev Z.S., Babanly M.B., Otrokov M.M., Politano A., Chulkov E.V., Arnau A., Marinova V., Das P.K., Fujii J., Vobornik I., Perfetti L., Mugarza A., Goldoni A., Marsi M. Nano Letters; 16 (6): 3409 - 3414. 2016. 10.1021/acs.nanolett.5b02635. IF: 13.779

    Topological insulators are a promising class of materials for applications in the field of spintronics. New perspectives in this field can arise from interfacing metal-organic molecules with the topological insulator spin-momentum locked surface states, which can be perturbed enhancing or suppressing spintronics-relevant properties such as spin coherence. Here we show results from an angle-resolved photemission spectroscopy (ARPES) and scanning tunnelling microscopy (STM) study of the prototypical cobalt phthalocyanine (CoPc)/Bi2Se3 interface. We demonstrate that that the hybrid interface can act on the topological protection of the surface and bury the Dirac cone below the first quintuple layer. © 2016 American Chemical Society.


  • Superparamagnetism-induced mesoscopic electron focusing in topological insulators

    Sessi P., Rüßmann P., Bathon T., Barla A., Kokh K.A., Tereshchenko O.E., Fauth K., Mahatha S.K., Valbuena M.A., Godey S., Glott F., Mugarza A., Gargiani P., Valvidares M., Long N.H., Carbone C., Mavropoulos P., Blügel S., Bode M. Physical Review B; 94 (7, 075137) 2016. 10.1103/PhysRevB.94.075137. IF: 0.000

    Recently it has been shown that surface magnetic doping of topological insulators induces backscattering of Dirac states which are usually protected by time-reversal symmetry [Sessi, Nat. Commun. 5, 5349 (2014)10.1038/ncomms6349]. Here we report on quasiparticle interference measurements where, by improved Fermi level tuning, strongly focused interference patterns on surface Mn-doped Bi2Te3 could be directly observed by means of scanning tunneling microscopy at 4 K. Ab initio and model calculations reveal that their mesoscopic coherence relies on two prerequisites: (i) a hexagonal Fermi surface with large parallel segments (nesting) and (ii) magnetic dopants which couple to a high-spin state. Indeed, x-ray magnetic circular dichroism shows superparamagnetism even at very dilute Mn concentrations. Our findings provide evidence of strongly anisotropic Dirac-fermion-mediated interactions and demonstrate how spin information can be transmitted over long distances, allowing the design of experiments and devices based on coherent quantum effects in topological insulators. © 2016 American Physical Society.


2015

  • Exchange bias of TbPc2 molecular magnets on antiferromagnetic FeMn and ferromagnetic Fe films

    Nistor C., Krull C., Mugarza A., Stepanow S., Stamm C., Soares M., Klyatskaya S., Ruben M., Gambardella P. Physical Review B - Condensed Matter and Materials Physics; 92 (18, 184402) 2015. 10.1103/PhysRevB.92.184402. IF: 3.736

    Improving the magnetic stability of single-molecule magnets is a key challenge facing molecular spintronics. We use x-ray magnetic circular dichroism to explore the possibility of magnetically stabilizing TbPc2 molecules by attaching them to ultrathin Fe and FeMn films. We show that TbPc2 deposited on antiferromagnetic FeMn films exhibits magnetic hysteresis and exchange bias as a consequence of coupling to the uncompensated interfacial Fe spins. The FeMn-thickness dependence of the coercive field and exchange bias of TbPc2 is similar to that of inorganic ferromagnetic/antiferromagnetic systems. The magnetic remanence is comparable with the fraction of molecules attached to pinned interfacial Fe spins. The Tb magnetic moments are antiferromagnetically coupled to the Fe thin films as well as to the uncompensated Fe spins at the FeMn interface. The sign of the coupling changes from antiferromagnetic to ferromagnetic after doping the interface with electron-donor Li atoms. ©2015 American Physical Society.


  • Spin-flip and element-sensitive electron scattering in the BiAg2 surface alloy

    Schirone S., Krasovskii E.E., Bihlmayer G., Piquerel R., Gambardella P., Mugarza A. Physical Review Letters; 114 (16, 166801) 2015. 10.1103/PhysRevLett.114.166801. IF: 7.512

    Heavy metal surface alloys represent model systems to study the correlation between electron scattering, spin-orbit interaction, and atomic structure. Here, we investigate the electron scattering from the atomic steps of monolayer BiAg2 on Ag(111) using quasiparticle interference measurements and density functional theory. We find that intraband transitions between states of opposite spin projection can occur via a spin-flip backward scattering mechanism driven by the spin-orbit interaction. The spin-flip scattering amplitude depends on the chemical composition of the steps, leading to total confinement for pure Bi step edges, and considerable leakage for mixed Bi-Ag step edges. Additionally, the different localization of the occupied and unoccupied surface bands at Ag and Bi sites leads to a spatial shift of the scattering potential barrier at pure Bi step edges. © 2015 American Physical Society.


  • Substrate-induced stabilization and reconstruction of zigzag edges in graphene nanoislands on Ni(111)

    Garcia-Lekue A., Olle M., Sanchez-Portal D., Palacios J.J., Mugarza A., Ceballos G., Gambardella P. Journal of Physical Chemistry C; 119 (8): 4072 - 4078. 2015. 10.1021/jp511069y. IF: 4.772

    We combine experimental observations by scanning tunneling microscopy (STM) and density functional theory (DFT) to reveal the most stable edge structures of graphene on Ni(111) as well as the role of stacking-driven activation and suppression of edge reconstruction. Depending on the position of the outermost carbon atoms relative to hollow and on-top Ni sites, zigzag edges have very different energies. Triangular graphene nanoislands are exclusively bound by the more stable zigzag hollow edges. In hexagonal nanoislands, which are constrained by geometry to alternate zigzag hollow and zigzag top edges along their perimeter, only the hollow edge is stable, whereas the top edges spontaneously reconstruct into the (57) pentagon-heptagon structure. Atomically resolved STM images are consistent with either top-fcc or top-hcp epitaxial stacking of graphene and Ni sites, with the former being favored by DFT. Finally, we find that there is a one-to-one relationship between the edge type, graphene stacking, and orientation of the graphene islands. © 2015 American Chemical Society.


2014

  • Coupling of single, double, and triple-decker metal-phthalocyanine complexes to ferromagnetic and antiferromagnetic substrates

    Lodi Rizzini, A.; Krull, C.; Mugarza, A.; Balashov, T.; Nistor, C.; Piquerel, R.; Klyatskaya, S.; Ruben, M.; Sheverdyaeva, P.M.; Moras, P.; Carbone, C.; Stamm, C.; Miedema, P.S.; Thakur, P.K.; Sessi, V.; Soares, M.; Yakhou-Harris, F.; Cezar, J.C.; Stepanow, S.; Gambardella, P. Surface Science; 630: 361 - 374. 2014. 10.1016/j.susc.2014.07.008. IF: 1.870


  • Fieldlike and antidamping spin-orbit torques in as-grown and annealed Ta/CoFeB/MgO layers

    Avci, C.O.; Garello, K.; Nistor, C.; Godey, S.; Ballesteros, B.; Mugarza, A.; Barla, A.; Valvidares, M.; Pellegrin, E.; Ghosh, A.; Miron, I.M.; Boulle, O.; Auffret, S.; Gaudin, G.; Gambardella, P. Physical Review B - Condensed Matter and Materials Physics; 89: 21 - 214419. 2014. 10.1103/PhysRevB.89.214419. IF: 3.664


  • Metallic thin films on stepped surfaces: lateral scattering of quantum well states

    Schiller, F. ; Abd El-Fattah, Z. M.; Schirone, S. ; Lobo-Checa, J. ; Urdanpilleta, M. ; Ruiz-Osés, M.; Cordón, J.; Corso, M.; Sanchez-Portal, D.; Mugarza, A. ; Ortega, J. E. New Journal of Physics; 16: 123025. 2014. 10.1088/1367-2630/16/12/12302. IF: 3.671


  • Metallic thin films on stepped surfaces: Lateral scattering of quantum well states

    Schiller F., El-Fattah Z.M.A., Schirone S., Lobo-Checa J., Urdanpilleta M., Ruiz-Osés M., Cordón J., Corso M., Sánchez-Portal D., Mugarza A., Ortega J.E. New Journal of Physics; 16 (123025) 2014. 10.1088/1367-2630/16/12/123025.

    Quantum well states of Ag films grown on stepped Au(111) surfaces are shown to undergo lateral scattering, in analogy with surface states of vicinal Ag(111). Applying angle resolved photoemission spectroscopy we observe quantum well bands with zone-folding and gap openings driven by surface/interface step lattice scattering. Experiments performed on a curved Au(111) substrate allow us to determine a subtle terrace-size effect, i.e., a fine step-density-dependent upward shift of quantum well bands. This energy shift is explained as mainly due to the periodically stepped crystal potential offset at the interface side of the film. Finally, the surface state of the stepped Ag film is analyzed with both photoemission and scanning tunneling microscopy. We observe that the stepped film interface also affects the surface state energy, which exhibits a larger terrace-size effect compared to surface states of bulk vicinal Ag(111) crystals. © 2014 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.


  • Spin tuning of electron-doped metal-phthalocyanine layers

    Stepanow, S.; Lodi Rizzini, A.; Krull, C.; Kavich, J.; Cezar, J.C.; Yakhou-Harris, F.; Sheverdyaeva, P.M.; Moras, P.; Carbone, C.; Ceballos, G.; Mugarza, A.; Gambardella, P. Journal of the American Chemical Society; 136 (14): 5451 - 5459. 2014. 10.1021/ja501204q. IF: 11.444


  • Spin-dependent electron scattering at graphene edges on Ni(111)

    Garcia-Lekue, A.; Balashov, T.; Olle, M.; Ceballos, G.; Arnau, A.; Gambardella, P.; Sanchez-Portal, D.; Mugarza, A. Physical Review Letters; 2014. 10.1103/PhysRevLett.112.066802. IF: 7.728


  • Structure and magnetism of Tm atoms and monolayers on W(110)

    Nistor, C.; Mugarza, A.; Stepanow, S.; Gambardella, P.; Kummer, K.; Diez-Ferrer, J.L.; Coffey, D.; de la Fuente, C.; Ciria, M.; Arnaudas, J.I. Physical Review B - Condensed Matter and Materials Physics; 2014. 10.1103/PhysRevB.90.064423. IF: 3.664


2013

  • Scattering of surface electrons by isolated steps versus periodic step arrays

    Ortega, J.E.; Lobo-Checa, J.; Peschel, G.; Schirone, S.; Abd El-Fattah, Z.M.; Matena, M.; Schiller, F.; Borghetti, P.; Gambardella, P.; Mugarza, A. Physical Review B - Condensed Matter and Materials Physics; 87 2013. 10.1103/PhysRevB.87.115425. IF: 3.767


  • Site- and orbital-dependent charge donation and spin manipulation in electron-doped metal phthalocyanines

    Krull, C.; Robles, R.; Mugarza, A.; Gambardella, P. Nature Materials; 12: 337 - 343. 2013. 10.1038/nmat3547. IF: 35.749


2012

  • Effect of surface reconstruction on the photoemission cross-section of the Au(111) surface state

    Borghetti, P.; Lobo-Checa, J.; Goiri, E.; Mugarza, A.; Schiller, F.; Enrique Ortega, J.; Krasovskii, E.E. Journal of Physics Condensed Matter; 24 2012. 10.1088/0953-8984/24/39/395006.


  • Electronic and magnetic properties of molecule-metal interfaces: Transition-metal phthalocyanines adsorbed on Ag(100)

    Mugarza, A.; Robles, R.; Krull, C.; Korytár, R.; Lorente, N.; Gambardella, P. Physical Review B - Condensed Matter and Materials Physics; 85 2012. 10.1103/PhysRevB.85.155437.


  • Exchange biasing single molecule magnets: coupling of TbPc2 to antiferromagnetic layers

    A. Lodi Rizzini; C. Krull; T. Balashov; A. Mugarza; C. Nistor; F. Yakhou; V. Sessi; S. Klyatskaya; M. Ruben; S. Stepanow; P. Gambardella Nano Letters; 12: 5703. 2012. doi.org/10.1021/nl302918d.


2011

  • Coupling Single Molecule Magnets to Ferromagnetic Substrates

    Lodi Rizzini, A. ; Krull, C. ; Balashov, T.; Kavich, J. J.; Mugarza, A.; Miedema, P. S.; Thakur, P. K.; Sessi, V.; Klyatskaya, S.; Ruben, M. ; Stepanow, S. ; Gambardella, P. Physical Review Letters; 2011. .


  • Determination of the photoelectron reference plane in nanostructured surfaces

    Lobo-Checa, J.; Mugarza, A.; Ortega, J.E.; Michel, E.G. New Journal of Physics; 13 2011. 10.1088/1367-2630/13/10/103013.


  • Localization, splitting, and mixing of field emission resonances induced by alkali metal clusters on Cu(100)

    Stepanow, S.; Mugarza, A.; Ceballos, G.; Gambardella, P.; Aldazabal, I.; Borisov, A.G.; Arnau, A. Physical Review B - Condensed Matter and Materials Physics; 83 2011. 10.1103/PhysRevB.83.115101.


  • Mixed-valence behavior and strong correlation effects of metal phthalocyanines adsorbed on metals

    Stepanow, S.; Miedema, P. S.; Mugarza, A.; Ceballos, G. ; Moras, P. ; Cezar, J. C.; Carbone, C. ; de Groot, F. M. F.; Gambardella, P. Physical Review B; 2011. .


  • Self-Assembled Nanometer-Scale Magnetic Networks on Surfaces: Fundamental Interactions and Functional Properties

    Carbone, C.; Gardonio, S.; Moras, P.; Lounis, S.; Heide, M.; Bihlmayer, G.; Atodiresei, N.; Heinz Dederichs, P.; Blügel, S.; Vlaic, S.; Lehnert, A.; Ouazi, S.; Rusponi, S.; Brune, H.; Honolka, J.; Enders, A.; Kern, K.; Stepanow, S.; Krull, C.; Balashov, T.; Mugarza, A.; Gambardella, P. Advanced Functional Materials; 2011. .


  • Spin coupling and relaxation inside molecule-metal contacts.

    Mugarza, A.; Krull, C.; Robles, R.; Stepanow, S.; Ceballos, G.; Gambardella, P. Nature Communications; 2 2011. 10.1038/ncomms1497.


2010

  • Correlated electrons step by step: Itinerant-to-localized transition of fe impurities in free-electron metal hosts

    Carbone, C.; Veronese, M.; Moras, P.; Gardonio, S.; Grazioli, C.; Zhou, P.H.; Rader, O.; Varykhalov, A.; Krull, C.; Balashov, T.; Mugarza, A.; Gambardella, P.; Lebèque, S.; Eriksson, O.; Katsnelson, M.I.; Lichtenstein, A.I. Physical Review Letters; 104 2010. 10.1103/PhysRevLett.104.117601.


  • Giant spin and orbital moment anisotropies of a Cu-phthalocyanine monolayer

    Stepanow, S.; Mugarza, A.; Ceballos, G.; Moras, P.; Cezar, J.C.; Carbone, C.; Gambardella, P. Physical Review B - Condensed Matter and Materials Physics; 82 2010. 10.1103/PhysRevB.82.014405.


  • Orbital specific chirality and homochiral self-assembly of achiral molecules induced by charge transfer and spontaneous symmetry breaking

    Mugarza A., Lorente N., Ordejón P., Krull C., Stepanow S., Bocquet M.-L., Fraxedas J., Ceballos G., Gambardella P. Physical Review Letters; 105 (11, 115702) 2010. 10.1103/PhysRevLett.105.115702.

    We study the electronic mechanisms underlying the induction and propagation of chirality in achiral molecules deposited on surfaces. Combined scanning tunneling microscopy and ab initio electronic structure calculations of Cu-phthalocyanines adsorbed on Ag(100) reveal the formation of chiral molecular orbitals in structurally undistorted molecules. This effect shows that chirality can be manifest exclusively at the electronic level due to asymmetric charge transfer between molecules and substrate. Single molecule chirality correlates with attractive van der Waals interactions, leading to the propagation of chirality at the supramolecular level. Ostwald ripening provides an efficient pathway for complete symmetry breaking and self-assembly of homochiral supramolecular layers. © 2010 The American Physical Society.


2009

  • Direct evidence of imprinted vortex states in the antiferromagnet of exchange biased microdisks

    Salazar-Alvarez, G.; Kavich, J.J.; Sort, J.; Mugarza, A.; Stepanow, S.; Potenza, A.; Marchetto, H.; Dhesi, S.S.; Baltz, V.; Dieny, B.; Weber, A.; Heyderman, L.J.; Nogués, J.; Gambardella, P. Applied Physics Letters; 95 2009. 10.1063/1.3168515.


  • La ciencia de superficies: aplicación al estudio de la adsorción del agua

    Mugarza, A.; Herranz, T.; Salmeron, M. Revista Española de Física; 23: 17. 2009. .


  • Water growth on metal and oxids: binding, dissociation and the role of hydroxyl groups

    Salmeron, M.; Bluhm, H.; Tatarkhanov, M.; Kettler, G.; Shimizu, T.K.; Mugarza, A.; Deng, X.; Herranz, T. Faraday Discussions; 141: 221 - 229. 2009. 10.1039/B806516K.


2008

  • Adsorption of water on O(2×2)/Ru(0001): Thermal stability and inhibition of dissociation

    Mugarza, A.; Shimizu, T.K.; Cabrera-Sanfelix, P.; Sánchez-Portal, D.; Arnau, A.; Salmeron, M. Journal of Physical Chemistry C; 112: 14052 - 14057. 2008. 10.1021/jp8026622.


  • Structure and reactions of carbon and hydrogen on Ru(0001): A scanning tunneling microscopy study

    Shimizu, T.K.; Mugarza, A.; Cerdá, J.I.; Salmeron, M. Journal of Chemical Physics; 129 2008. 10.1063/1.2991434.


  • Surface species formed by the adsorption and dissociation of water molecules on a Ru(0001) surface containing a small coverage of carbon atoms studied by scanning tunneling microscopy

    Shimizu, T.K.; Mugarza, A.; Cerdá, J.I.; Heyde, M.; Qi, Y.; Schwarz, U.D.; Ogletree, D.F.; Salmeron, M. Journal of Physical Chemistry C; 112: 7445 - 7454. 2008. 10.1021/jp711097j.


  • Water growth on metals and oxides: Binding, dissociation and role of hydroxyl groups

    Salmeron, M.; Bluhm, H.; Tatarkhanov, M.; Ketteler, G.; Shimizu, T.K.; Mugarza, A.; Deng, X.; Herranz, T.; Yamamoto, S.; Nilsson, A. Faraday Discussions; 141: 221 - 229. 2008. 10.1039/b806516k.


  • Water on surfaces studied by Scanning Probe Microscopies

    A.Verdaguer; A. Mugarza; J. Fraxedas Contributions to Science; 4: 141 - 155. 2008. .