Staff directory
Aitor Mugarza Ezpeleta
ICREA Research Professor and Group Leader
aitor.mugarza(ELIMINAR)@icn2.cat
Atomic Manipulation and Spectroscopy
- ORCID: 0000-0002-2698-885X
Publications
2017
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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.
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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.
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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.
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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
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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.
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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.
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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
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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.
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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.
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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
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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.
2010
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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.