Staff directory Roberto Robles Rodríguez

Roberto Robles Rodríguez

SO Senior Postdoctoral Researcher
Severo Ochoa
roberto.robles(ELIMINAR)@icn2.cat
Theory and Simulation

Publications

2018

  • Spin Proximity Effects in Graphene/Topological Insulator Heterostructures

    Song K., Soriano D., Cummings A.W., Robles R., Ordejón P., Roche S. Nano Letters; 18 (3): 2033 - 2039. 2018. 10.1021/acs.nanolett.7b05482.

    Enhancing the spin-orbit interaction in graphene, via proximity effects with topological insulators, could create a novel 2D system that combines nontrivial spin textures with high electron mobility. To engineer practical spintronics applications with such graphene/topological insulator (Gr/TI) heterostructures, an understanding of the hybrid spin-dependent properties is essential. However, to date, despite the large number of experimental studies on Gr/TI heterostructures reporting a great variety of remarkable (spin) transport phenomena, little is known about the true nature of the spin texture of the interface states as well as their role on the measured properties. Here, we use ab initio simulations and tight-binding models to determine the precise spin texture of electronic states in graphene interfaced with a Bi2Se3 topological insulator. Our calculations predict the emergence of a giant spin lifetime anisotropy in the graphene layer, which should be a measurable hallmark of spin transport in Gr/TI heterostructures and suggest novel types of spin devices. © 2018 American Chemical Society.


2017

  • Building Complex Kondo Impurities by Manipulating Entangled Spin Chains

    Choi D.-J., Robles R., Yan S., Burgess J.A.J., Rolf-Pissarczyk S., Gauyacq J.-P., Lorente N., Ternes M., Loth S. Nano Letters; 17 (10): 6203 - 6209. 2017. 10.1021/acs.nanolett.7b02882. IF: 12.712

    The creation of molecule-like structures in which magnetic atoms interact controllably is full of potential for the study of complex or strongly correlated systems. Here, we create spin chains in which a strongly correlated Kondo state emerges from magnetic coupling of transition-metal atoms. We build chains up to ten atoms in length by placing Fe and Mn atoms on a Cu2N surface with a scanning tunneling microscope. The atoms couple antiferromagnetically via superexchange interaction through the nitrogen atom network of the surface. The emergent Kondo resonance is spatially distributed along the chain. Its strength can be controlled by mixing atoms of different transition metal elements and manipulating their spatial distribution. We show that the Kondo screening of the full chain by the electrons of the nonmagnetic substrate depends on the interatomic entanglement of the spins in the chain, demonstrating the prerequisites to build and probe spatially extended strongly correlated nanostructures. © 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.


  • Voltage-Induced Coercivity Reduction in Nanoporous Alloy Films: A Boost toward Energy-Efficient Magnetic Actuation

    Quintana A., Zhang J., Isarain-Chávez E., Menéndez E., Cuadrado R., Robles R., Baró M.D., Guerrero M., Pané S., Nelson B.J., Müller C.M., Ordejón P., Nogués J., Pellicer E., Sort J. Advanced Functional Materials; 27 (32, 1701904) 2017. 10.1002/adfm.201701904. IF: 12.124

    Magnetic data storage and magnetically actuated devices are conventionally controlled by magnetic fields generated using electric currents. This involves significant power dissipation by Joule heating effect. To optimize energy efficiency, manipulation of magnetic information with lower magnetic fields (i.e., lower electric currents) is desirable. This can be accomplished by reducing the coercivity of the actuated material. Here, a drastic reduction of coercivity is observed at room temperature in thick (≈600 nm), nanoporous, electrodeposited Cu–Ni films by simply subjecting them to the action of an electric field. The effect is due to voltage-induced changes in the magnetic anisotropy. The large surface-area-to-volume ratio and the ultranarrow pore walls of the system allow the whole film, and not only the topmost surface, to effectively contribute to the observed magnetoelectric effect. This waives the stringent “ultrathin-film requirement” from previous studies, where small voltage-driven coercivity variations were reported. This observation expands the already wide range of applications of nanoporous materials (hitherto in areas like energy storage or catalysis) and it opens new paradigms in the fields of spintronics, computation, and magnetic actuation in general. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim


2016

  • AFM Imaging of Mercaptobenzoic Acid on Au(110): Submolecular Contrast with Metal Tips

    Hauptmann N., Robles R., Abufager P., Lorente N., Berndt R. Journal of Physical Chemistry Letters; 7 (11): 1984 - 1990. 2016. 10.1021/acs.jpclett.6b00684. IF: 8.539

    A self-assembled monolayer of mercaptobenzoic acid (MBA) on Au(110) is investigated with scanning tunneling and atomic force microscopy (STM and AFM) and density functional calculations. High-resolution AFM images obtained with metallic tips show clear contrasts between oxygen atoms and phenyl moieties. The contrast above the oxygen atoms is due to attractive covalent interactions, which is different than previously reported high-resolution images, where Pauli repulsion dominated the image contrast. We show that the bonding of MBA to the substrate occurs mainly through dispersion interactions, whereas the thiol-Au bond contributes only a quarter of the adsorption energy. No indication of Au adatoms mediating the thiol-Au interaction was found in contrast to other thiol-bonded systems. However, MBA lifts the Au(110)-(2 × 1) reconstruction. © 2016 American Chemical Society.


  • Following the steps of a reaction by direct imaging of many individual molecules

    Van Vörden D., Wortmann B., Schmidt N., Lange M., Robles R., Brendel L., Bobisch C.A., Möller R. Chemical Communications; 52 (49): 7711 - 7714. 2016. 10.1039/c6cc02959k. IF: 6.567

    The dehydrogenation and dechlorination of FeOEP-Cl on Cu(111) has been studied in detail by scanning tunneling microscopy. Although, it is not possible to follow the reaction of an individual molecule, the complete pathway of the reaction with 22 inequivalent intermediate states and the rates of the involved processes are revealed. This is achieved by combining the analysis of a large data set showing thousands of molecules in the different stages of the reaction with numerical simulations. © 2016 The Royal Society of Chemistry.


  • How disorder affects topological surface states in the limit of ultrathin Bi2Se3 films

    Song K., Soriano D., Robles R., Ordejon P., Roche S. 2D Materials; 3 (4, 045007) 2016. 10.1088/2053-1583/3/4/045007. IF: 9.611

    We present a first-principles study of electronic properties of ultrathin films of topological insulators (TIs) and scrutinize the role of disorder on the robustness of topological surface states, which can be analysed through their spin textures. The presence of twin grain boundaries is found to increase the band gap of the film, while preserving the spin texture of states in first conduction and valence bands. Differently, partial hydrogenation of one surface not only results in some self-doping effect, but also provokes some alteration of the spin texture symmetry of the electronic states. The formation of a new Dirac cone at M-point of the Brillouin zone of the hydrogenated surface, together with a modified spin texture characteristics are consistent with a dominant Dresselhaus spin-orbit interaction type, more usually observed in 3D materials. Our findings indicate that defects can either be detrimental or beneficial for exploring spin transport of surface states in the limit of ultrathin films of TIs, which maximizes surface over bulk phenomena. © 2016 IOP Publishing Ltd.


  • Noncontact atomic force microscopy and density functional theory studies of the (2×2) reconstructions of the polar AlN(0001) surface

    Chaumeton F., Robles R., Pruneda M., Lorente N., Eydoux B., Bouju X., Gauthier S., Martrou D. Physical Review B; 94 (16, 165305) 2016. 10.1103/PhysRevB.94.165305.

    Combined experimental and theoretical studies permit us to determine new protocols for growing by molecular beam epitaxy the technologically interesting N-rich aluminum nitride (AlN) surfaces. This is achieved by dosing the precursor gases at unusually low rates. With the help of calculated structures by using density functional theory and Boltzmann distribution of the reconstructed cells, we proposed to assign the measured surface obtained with a growth rate of 10 nm/h to a (2×2) reconstructed surface involving one additional N atom per unit cell. These N-rich AlN surfaces could open new routes to dope AlN layers with important implications in high-power and temperature technological applications. © 2016 American Physical Society.


  • On-Surface Engineering of a Magnetic Organometallic Nanowire

    Ormaza M., Robles R., Bachellier N., Abufager P., Lorente N., Limot L. Nano Letters; 16 (1): 588 - 593. 2016. 10.1021/acs.nanolett.5b04280. IF: 13.779

    The manipulation of the molecular spin state by atom doping is an attractive strategy to confer desirable magnetic properties to molecules. Here, we present the formation of novel magnetic metallocenes by following this approach. In particular, two different on-surface procedures to build isolated and layer-integrated Co-ferrocene (CoFc) molecules on a metallic substrate via atomic manipulation and atom deposition are shown. The structure as well as the electronic properties of the so-formed molecule are investigated combining scanning tunneling microscopy and spectroscopy with density functional theory calculations. It is found that unlike single ferrocene a CoFc molecule possesses a magnetic moment as revealed by the Kondo effect. These results correspond to the first controlled procedure toward the development of tailored metallocene-based nanowires with a desired chemical composition, which are predicted to be promising materials for molecular spintronics. © 2015 American Chemical Society.


  • Reversible 2D Phase Transition Driven by an Electric Field: Visualization and Control on the Atomic Scale

    Wortmann B., Vörden D.V., Graf P., Robles R., Abufager P., Lorente N., Bobisch C.A., Möller R. Nano Letters; 16 (1): 528 - 533. 2016. 10.1021/acs.nanolett.5b04174. IF: 13.779

    We report on a reversible structural phase transition of a two-dimensional system that can be locally induced by an external electric field. Two different structural configurations may coexist within a CO monolayer on Cu(111). The balance between the two phases can be shifted by an external electric field, causing the domain boundaries to move, increasing the area of the favored phase controllable both in location and size. If the field is further enhanced new domains nucleate. The arrangement of the CO molecules on the Cu surface is observed in real time and real space with atomic resolution while the electric field driving the phase transition is easily varied over a broad range. Together with the well-known molecular manipulation of CO adlayers, our findings open exciting prospects for combining spontaneous long-range order with man-made CO structures such as "molecule cascades" or "molecular graphene". Our new manipulation mode permits us to bridge the gap between fundamental concepts and the fabrication of arbitrary atomic patterns in large scale, by providing unprecedented insight into the physics of structural phase transitions on the atomic scale. © 2015 American Chemical Society.


  • Spin-polarised edge states in atomic Mn chains supported on Cu2N/Cu (100)

    Choi D.-J., Robles R., Gauyacq J.-P., Rubio-Verdú C., Lorente N., Ignacio Pascual J. Journal of Physics Condensed Matter; 28 (23, 23LT01) 2016. 10.1088/0953-8984/28/23/23LT01. IF: 2.209

    Scanning tunnelling microscopy and density functional theory studies of manganese chains adsorbed on Cu2N/Cu (100) reveal an unsuspected electronic edge state at ∼ 1 eV above the Fermi energy. This Tamm-like state is strongly localised to the terminal Mn atoms of the chain and fully spin polarised. However, no equivalence is found for occupied states, and the electronic structure at ∼ -1 eV is mainly spin unpolarised due to the extended p-states of the N atoms that mediate the coupling between the Mn atoms in the chain. The spin polarisation of the edge state is affected by the antiferromagnetic ordering of the chains leading to non-trivial consequences. © 2016 IOP Publishing Ltd.


  • Structural and magnetic properties of FeMnx chains (x=1-6) supported on Cu2 N/Cu (100)

    Choi D.-J., Robles R., Gauyacq J.-P., Ternes M., Loth S., Lorente N. Physical Review B; 94 (8, 085406) 2016. 10.1103/PhysRevB.94.085406.

    Heterogeneous atomic magnetic chains are built by atom manipulation on a Cu2N/Cu (100) substrate. Their magnetic properties are studied and rationalized by a combined scanning tunneling microscopy (STM) and density functional theory (DFT) work completed by model Hamiltonian studies. The chains are built using Fe and Mn atoms ontop of the Cu atoms along the N rows of the Cu2N surface. Here, we present results for FeMnx chains (x=1-6) emphasizing the evolution of the geometrical, electronic, and magnetic properties with chain size. By fitting our results to a Heisenberg Hamiltonian we have studied the exchange-coupling matrix elements J for different chains. For the shorter chains, x≤2, we have included spin-orbit effects in the DFT calculations, extracting the magnetic anisotropy energy. Our results are also fitted to a simple anisotropic spin Hamiltonian and we have extracted values for the longitudinal-anisotropy D and transversal-anisotropy E constants. These parameters together with the values for J allow us to compute the magnetic excitation energies of the system and to compare them with the experimental data. © 2016 American Physical Society.


  • Surface cis Effect: Influence of an Axial Ligand on Molecular Self-Assembly

    Knaak T., Gopakumar T.G., Schwager B., Tuczek F., Robles R., Lorente N., Berndt R. Journal of the American Chemical Society; 138 (24): 7544 - 7550. 2016. 10.1021/jacs.6b03710. IF: 13.038

    Adding ligands to molecules can have drastic and unforeseen consequences in the final products of a reaction. Recently a surface trans effect due to the weakening of a molecule-surface bond was reported. Here, we show a surface cis effect where an axial ligand at adsorbed transition-metal complexes enables lateral bonding among the molecules. In the absence of this ligand, the intermolecular interaction is repulsive and supramolecular patterns are not observed. Fe-tetramethyl-tetraazaannulene on Au(111) was investigated using low-temperature scanning tunneling microscopy and spectroscopy along with density functional theory calculations. At low coverages, the molecules remain isolated. Exposure to CO leads to axial CO bonding and induces reordering into extended clusters of chiral molecular trimers. The changed self-assembly pattern is due to a CO-induced modification of the molecular structure and the corresponding charge transfer between the molecule and the substrate, which in turn changes the lateral intermolecular forces. © 2016 American Chemical Society.


  • Theoretical studies of Rashba and Dresselhaus effects in hybrid organic-inorganic perovskites for optoelectronic applications

    Pedesseau L., Kepenekian M., Robles R., Sapori D., Katan C., Even J. Proceedings of SPIE - The International Society for Optical Engineering; 9742 ( 97421B) 2016. 10.1117/12.2213618. IF: 0.000

    In this paper, we propose a description of the Rashba-Dresselhaus effect in Hybrid Organic Perovskite (HOP). We show how the loss of the inversion symmetry leads to the loss of the spin degeneracy. An example of structure where both Rashba and Dresselhaus operate is illustrated with the formamidinium tin iodide CH(NH2)2SnI3. The control of this effect is as well addressed by two examples. A first example concerns the control with the temperature and is demonstrated for the 2D HOP Bz2PbCl4 (Bz = benzylammonium). Then the control with an external field is established for the 3D HOP CH3NH3PbBr3. © COPYRIGHT SPIE. Downloading of the abstract is permitted for personal use only.


2015

  • Assembly of ferrocene molecules on metal surfaces revisited

    Ormaza M., Abufager P., Bachellier N., Robles R., Verot M., Le Bahers T., Bocquet M.-L., Lorente N., Limot L. Journal of Physical Chemistry Letters; 6 (3): 395 - 400. 2015. 10.1021/jz5026118. IF: 7.458

    Metallocene (MCp2) wires have recently attracted considerable interest in relation to molecular spintronics due to predictions concerning their half-metallic nature. This exciting prospect is however hampered by the little and often-contradictory knowledge we have concerning the metallocene self-assembly and interaction with a metal. Here, we elucidate these aspects by focusing on the adsorption of ferrocene on Cu(111) and Cu(100). Combining low-temperature scanning tunneling microscopy and density functional theory calculations, we demonstrate that the two-dimensional molecular arrangement consists of vertical- and horizontal-lying molecules. The noncovalent T-shaped interactions between Cp rings of vertical and horizontal molecules are essential for the stability of the physisorbed molecular layer. These results provide a fresh insight into ferrocene adsorption on surfaces and may serve as an archetypal reference for future work with this important variety of organometallic molecules. (Figure Presented). © 2015 American Chemical Society.


  • FeCoCp3 Molecular Magnets as Spin Filters

    Abufager P.N., Robles R., Lorente N. Journal of Physical Chemistry C; 119 (22): 12119 - 12129. 2015. 10.1021/acs.jpcc.5b01839. IF: 4.772

    Metallorganic molecules have been proposed as excellent spin filters in molecular spintronics because of the large spin polarization of their electronic structure. However, most of the studies involving spin transport have disregarded fundamental aspects such as the magnetic anisotropy of the molecule and the excitation of spin-flip processes during electron transport. Here, we study a molecule containing a Co and an Fe atom stacked between three cyclopentadienyl rings, which presents a large magnetic anisotropy and a S = 1. These figures are superior to other molecules with the same transition metal and improves the spin-filtering capacities of the molecule. Nonequilibrium Green's functions calculations based on density functional theory predict excellent spin-filtering properties both in tunnel and contact transport regimes. However, exciting the first magnetic state drastically reduces the current's spin polarization. Furthermore, a difference of temperature between electrodes leads to strong thermoelectric effects that also suppress spin polarization. Our study shows that in principle good molecular candidates for spintronics need to be confronted with inelastic and thermoelectric effects. (Graph Presented). © 2015 American Chemical Society.


  • Rashba and Dresselhaus Effects in Hybrid Organic-Inorganic Perovskites: From Basics to Devices

    Kepenekian M., Robles R., Katan C., Sapori D., Pedesseau L., Even J. ACS Nano; 9 (12): 11557 - 11567. 2015. 10.1021/acsnano.5b04409. IF: 12.881

    We use symmetry analysis, density functional theory calculations, and k·p modeling to scrutinize Rashba and Dresselhaus effects in hybrid organic-inorganic halide perovskites. These perovskites are at the center of a recent revolution in the field of photovoltaics but have also demonstrated potential for optoelectronic applications such as transistors and light emitters. Due to a large spin-orbit coupling of the most frequently used metals, they are also predicted to offer a promising avenue for spin-based applications. With an in-depth inspection of the electronic structures and bulk lattice symmetries of a variety of systems, we analyze the origin of the spin splitting in two- and three-dimensional hybrid perovskites. It is shown that low-dimensional nanostructures made of CH3NH3PbX3 (X = I, Br) lead to spin splittings that can be controlled by an applied electric field. These findings further open the door for a perovskite-based spintronics. © 2015 American Chemical Society.


2014

  • Orbital redistribution in molecular nanostructures mediated by metal-organic bonds

    Yang, Z.; Corso, M.; Robles, R.; Lotze, C.; Fitzner, R.; Mena-Osteritz, E.; Bäuerle, P.; Franke, K.J.; Pascual, J.I. ACS Nano; 8 (10): 10715 - 10722. 2014. 10.1021/nn504431e. IF: 12.033


  • Spin transport in dangling-bond wires on doped H-passivated Si(100)

    Kepenekian, M.; Robles, R.; Rurali, R.; Lorente, N. Nanotechnology; 2014. 10.1088/0957-4484/25/46/465703. IF: 3.672


2013

  • Electron transport through dangling-bond silicon wires on H-passivated Si(100)

    Kepenekian, M.; Novaes, F.D.; Robles, R.; Monturet, S.; Kawai, H.; Joachim, C.; Lorente, N. Journal of Physics Condensed Matter; 25 2013. 10.1088/0953-8984/25/2/025503. IF: 2.355


  • Leakage current in atomic-size surface interconnects

    Kepenekian, M.; Robles, R.; Joachim, C.; Lorente, N. Applied Physics Letters; 103 2013. 10.1063/1.4825375. IF: 3.794


  • 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


  • Surface-induced dechlorination of FeOEP-Cl** on Cu(111)

    Van Vörden, D.; Lange, M.; Schaffert, J.; Cottin, M.C.; Schmuck, M.; Robles, R.; Wende, H.; Bobisch, C.A.; Möller, R. Chemphyschem : a European journal of chemical physics and physical chemistry; 14: 3472 - 3475. 2013. 10.1002/cphc.201300497. IF: 3.349


  • Surface-state engineering for interconnects on H-passivated Si(100)

    Kepenekian, M.; Robles, R.; Joachim, C.; Lorente, N. Nano Letters; 13: 1192 - 1195. 2013. 10.1021/nl304611m. IF: 13.025


  • Surface-supported supramolecular pentamers

    Karan, S.; Wang, Y.; Robles, R.; Lorente, N.; Berndt, R. Journal of the American Chemical Society; 135: 14004 - 14007. 2013. 10.1021/ja405456k. IF: 10.677


  • Vibrational transition rule during a through-bond electron transfer process

    Monturet, S.; Kepenekian, M.; Robles, R.; Lorente, N.; Joachim, C. Chemical Physics Letters; 567: 1 - 5. 2013. 10.1016/j.cplett.2013.02.057. IF: 2.145


2012

  • 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.


  • Energetics and stability of dangling-bond silicon wires on H passivated Si(100)

    Robles, R.; Kepenekian, M.; Monturet, S.; Joachim, C.; Lorente, N. Journal of Physics Condensed Matter; 24 2012. 10.1088/0953-8984/24/44/445004.


  • Spin doping of individual molecules by using single-atom manipulation

    Robles, R.; Lorente, N.; Isshiki, H.; Liu, J.; Katoh, K.; Breedlove, B.K.; Yamashita, M.; Komeda, T. Nano Letters; 12: 3609 - 3612. 2012. 10.1021/nl301301e.


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.