Staff directory Sergio Osvaldo Valenzuela

Sergio Osvaldo Valenzuela

ICREA Research Professor and Group Leader
sov(ELIMINAR)@icn2.cat
Physics and Engineering of Nanodevices

Publications

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.

    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


  • Impact of the: In situ rise in hydrogen partial pressure on graphene shape evolution during CVD growth of graphene

    Gebeyehu Z.M., Arrighi A., Costache M.V., Sotomayor-Torres C.M., Esplandiu M.J., Valenzuela S.O. RSC Advances; 8 (15): 8234 - 8239. 2018. 10.1039/c7ra13169k.

    Exposing graphene to a hydrogen post-etching process yields dendritic graphene shapes. Here, we demonstrate that similar dendritic structures can be achieved at long growth times without adding hydrogen externally. These shapes are not a result of a surface diffusion controlled growth but of the competing backward reaction (etching), which dominates the growth dynamics at long times due to an in situ rise in the hydrogen partial pressure. We have performed a systematic study on the growth of graphene as a function of time to identify the onset and gradual evolution of graphene shapes caused by etching and then demonstrated that the etching can be stopped by reducing the flow of hydrogen from the feed. In addition, we have found that the etching rate due to the in situ rise in hydrogen is strongly dependent on the confinement (geometrical confinement) of copper foil. Highly etched graphene with dendritic shapes was observed in unconfined copper foil regions while no etching was found in graphene grown in a confined reaction region. This highlights the effect of the dynamic reactant distribution in activating the in situ etching process during growth, which needs to be counteracted or controlled for large scale growth. © The Royal Society of Chemistry 2018.


  • Strongly anisotropic spin relaxation in graphene-transition metal dichalcogenide heterostructures at room temperature

    Benítez L.A., Sierra J.F., Savero Torres W., Arrighi A., Bonell F., Costache M.V., Valenzuela S.O. Nature Physics; 14 (3): 303 - 308. 2018. 10.1038/s41567-017-0019-2.

    A large enhancement in the spin-orbit coupling of graphene has been predicted when interfacing it with semiconducting transition metal dichalcogenides. Signatures of such an enhancement have been reported, but the nature of the spin relaxation in these systems remains unknown. Here, we unambiguously demonstrate anisotropic spin dynamics in bilayer heterostructures comprising graphene and tungsten or molybdenum disulphide (WS2, MoS2). We observe that the spin lifetime varies over one order of magnitude depending on the spin orientation, being largest when the spins point out of the graphene plane. This indicates that the strong spin-valley coupling in the transition metal dichalcogenide is imprinted in the bilayer and felt by the propagating spins. These findings provide a rich platform to explore coupled spin-valley phenomena and offer novel spin manipulation strategies based on spin relaxation anisotropy in two-dimensional materials. © 2017 The Author(s).


  • Thermoelectric spin voltage in graphene

    Sierra J.F., Neumann I., Cuppens J., Raes B., Costache M.V., Valenzuela S.O. Nature Nanotechnology; 13 (2): 107 - 111. 2018. 10.1038/s41565-017-0015-9.

    In recent years, new spin-dependent thermal effects have been discovered in ferromagnets, stimulating a growing interest in spin caloritronics, a field that exploits the interaction between spin and heat currents 1,2 . Amongst the most intriguing phenomena is the spin Seebeck effect 3-5, in which a thermal gradient gives rise to spin currents that are detected through the inverse spin Hall effect 6-8 . Non-magnetic materials such as graphene are also relevant for spin caloritronics, thanks to efficient spin transport 9-11, energy-dependent carrier mobility and unique density of states 12,13 . Here, we propose and demonstrate that a carrier thermal gradient in a graphene lateral spin valve can lead to a large increase of the spin voltage near to the graphene charge neutrality point. Such an increase results from a thermoelectric spin voltage, which is analogous to the voltage in a thermocouple and that can be enhanced by the presence of hot carriers generated by an applied current 14-17 . These results could prove crucial to drive graphene spintronic devices and, in particular, to sustain pure spin signals with thermal gradients and to tune the remote spin accumulation by varying the spin-injection bias. © 2017 The Author(s).


2017

  • 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.


  • Spin precession in anisotropic media

    Raes B., Cummings A.W., Bonell F., Costache M.V., Sierra J.F., Roche S., Valenzuela S.O. Physical Review B; 95 (8, 085403) 2017. 10.1103/PhysRevB.95.085403. IF: 3.836

    We generalize the diffusive model for spin injection and detection in nonlocal spin structures to account for spin precession under an applied magnetic field in an anisotropic medium, for which the spin lifetime is not unique and depends on the spin orientation. We demonstrate that the spin precession (Hanle) line shape is strongly dependent on the degree of anisotropy and on the orientation of the magnetic field. In particular, we show that the anisotropy of the spin lifetime can be extracted from the measured spin signal, after dephasing in an oblique magnetic field, by using an analytical formula with a single fitting parameter. Alternatively, after identifying the fingerprints associated with the anisotropy, we propose a simple scaling of the Hanle line shapes at specific magnetic field orientations that results in a universal curve only in the isotropic case. The deviation from the universal curve can be used as a complementary means of quantifying the anisotropy by direct comparison with the solution of our generalized model. Finally, we applied our model to graphene devices and find that the spin relaxation for graphene on silicon oxide is isotropic within our experimental resolution. © 2017 American Physical Society.


  • The 2017 Magnetism Roadmap

    Sander D., Valenzuela S.O., Makarov D., Marrows C.H., Fullerton E.E., Fischer P., McCord J., Vavassori P., Mangin S., Pirro P., Hillebrands B., Kent A.D., Jungwirth T., Gutfleisch O., Kim C.G., Berger A. Journal of Physics D: Applied Physics; 50 (36, 363001) 2017. 10.1088/1361-6463/aa81a1. IF: 2.588

    Building upon the success and relevance of the 2014 Magnetism Roadmap, this 2017 Magnetism Roadmap edition follows a similar general layout, even if its focus is naturally shifted, and a different group of experts and, thus, viewpoints are being collected and presented. More importantly, key developments have changed the research landscape in very relevant ways, so that a novel view onto some of the most crucial developments is warranted, and thus, this 2017 Magnetism Roadmap article is a timely endeavour. The change in landscape is hereby not exclusively scientific, but also reflects the magnetism related industrial application portfolio. Specifically, Hard Disk Drive technology, which still dominates digital storage and will continue to do so for many years, if not decades, has now limited its footprint in the scientific and research community, whereas significantly growing interest in magnetism and magnetic materials in relation to energy applications is noticeable, and other technological fields are emerging as well. Also, more and more work is occurring in which complex topologies of magnetically ordered states are being explored, hereby aiming at a technological utilization of the very theoretical concepts that were recognised by the 2016 Nobel Prize in Physics. Given this somewhat shifted scenario, it seemed appropriate to select topics for this Roadmap article that represent the three core pillars of magnetism, namely magnetic materials, magnetic phenomena and associated characterization techniques, as well as applications of magnetism. While many of the contributions in this Roadmap have clearly overlapping relevance in all three fields, their relative focus is mostly associated to one of the three pillars. In this way, the interconnecting roles of having suitable magnetic materials, understanding (and being able to characterize) the underlying physics of their behaviour and utilizing them for applications and devices is well illustrated, thus giving an accurate snapshot of the world of magnetism in 2017. The article consists of 14 sections, each written by an expert in the field and addressing a specific subject on two pages. Evidently, the depth at which each contribution can describe the subject matter is limited and a full review of their statuses, advances, challenges and perspectives cannot be fully accomplished. Also, magnetism, as a vibrant research field, is too diverse, so that a number of areas will not be adequately represented here, leaving space for further Roadmap editions in the future. However, this 2017 Magnetism Roadmap article can provide a frame that will enable the reader to judge where each subject and magnetism research field stands overall today and which directions it might take in the foreseeable future. The first material focused pillar of the 2017 Magnetism Roadmap contains five articles, which address the questions of atomic scale confinement, 2D, curved and topological magnetic materials, as well as materials exhibiting unconventional magnetic phase transitions. The second pillar also has five contributions, which are devoted to advances in magnetic characterization, magneto-optics and magneto-plasmonics, ultrafast magnetization dynamics and magnonic transport. The final and application focused pillar has four contributions, which present non-volatile memory technology, antiferromagnetic spintronics, as well as magnet technology for energy and bio-related applications. As a whole, the 2017 Magnetism Roadmap article, just as with its 2014 predecessor, is intended to act as a reference point and guideline for emerging research directions in modern magnetism. © 2017 IOP Publishing Ltd.


2016

  • Determination of the spin-lifetime anisotropy in graphene using oblique spin precession

    Raes B., Scheerder J.E., Costache M.V., Bonell F., Sierra J.F., Cuppens J., Van De Vondel J., Valenzuela S.O. Nature Communications; 7 ( 11444) 2016. 10.1038/ncomms11444. IF: 11.329

    We determine the spin-lifetime anisotropy of spin-polarized carriers in graphene. In contrast to prior approaches, our method does not require large out-of-plane magnetic fields and thus it is reliable for both low-and high-carrier densities. We first determine the in-plane spin lifetime by conventional spin precession measurements with magnetic fields perpendicular to the graphene plane. Then, to evaluate the out-of-plane spin lifetime, we implement spin precession measurements under oblique magnetic fields that generate an out-of-plane spin population. We find that the spin-lifetime anisotropy of graphene on silicon oxide is independent of carrier density and temperature down to 150 K, and much weaker than previously reported. Indeed, within the experimental uncertainty, the spin relaxation is isotropic. Altogether with the gate dependence of the spin lifetime, this indicates that the spin relaxation is driven by magnetic impurities or random spin-orbit or gauge fields.


  • Spin Hall Effect and Origins of Nonlocal Resistance in Adatom-Decorated Graphene

    Van Tuan D., Marmolejo-Tejada J.M., Waintal X., Nikolić B.K., Valenzuela S.O., Roche S. Physical Review Letters; 117 (17, 176602) 2016. 10.1103/PhysRevLett.117.176602. IF: 7.645

    Recent experiments reporting an unexpectedly large spin Hall effect (SHE) in graphene decorated with adatoms have raised a fierce controversy. We apply numerically exact Kubo and Landauer-Büttiker formulas to realistic models of gold-decorated disordered graphene (including adatom clustering) to obtain the spin Hall conductivity and spin Hall angle, as well as the nonlocal resistance as a quantity accessible to experiments. Large spin Hall angles of ∼0.1 are obtained at zero temperature, but their dependence on adatom clustering differs from the predictions of semiclassical transport theories. Furthermore, we find multiple background contributions to the nonlocal resistance, some of which are unrelated to the SHE or any other spin-dependent origin, as well as a strong suppression of the SHE at room temperature. This motivates us to design a multiterminal graphene geometry which suppresses these background contributions and could, therefore, quantify the upper limit for spin-current generation in two-dimensional materials. © 2016 American Physical Society.


2015

  • Graphene spintronics: The European Flagship perspective

    Roche S., Åkerman J., Beschoten B., Charlier J.-C., Chshiev M., Dash S.P., Dlubak B., Fabian J., Fert A., Guimarães M., Guinea F., Grigorieva I., Schönenberger C., Seneor P., Stampfer C., Valenzuela S.O., Waintal X., Van Wees B. 2D Materials; 2 (3, 030202) 2015. 10.1088/2053-1583/2/3/030202. IF: 0.000

    Wereview current challenges and perspectives in graphene spintronics, which is one of themost promising directions of innovation, given its room-temperature long-spin lifetimes and the ability of graphene to be easily interfaced with other classes ofmaterials (ferromagnets, magnetic insulators, semiconductors, oxides, etc), allowing proximity effects to be harvested. The general context of spintronics is first discussed togetherwith open issues and recent advances achieved by theGraphene SpintronicsWork Package consortiumwithin theGraphene Flagship project. Based on such progress, which establishes the state of the art, several novel opportunities for spinmanipulation such as the generation of pure spin current (through spinHall effect) and the control of magnetization through the spin torque phenomena appear on the horizon. Practical applications arewithin reach, but will require the demonstration of wafer-scale graphene device integration, and the realization of functional prototypes employed for determined applications such as magnetic sensors or nano-oscillators. This is a specially commissioned editorial from the Graphene Flagship Work Package on Spintronics. This editorial is part of the 2DMaterials focus collection on 'Progress on the science and applications of twodimensionalmaterials,' published in association with theGraphene Flagship. It provides an overviewof key recent advances of the spintronicswork package aswell as the mid-term objectives of the consortium. © 2015 IOP Publishing Ltd.


  • Hot-Carrier Seebeck Effect: Diffusion and Remote Detection of Hot Carriers in Graphene

    Sierra J.F., Neumann I., Costache M.V., Valenzuela S.O. Nano Letters; 15 (6): 4000 - 4005. 2015. 10.1021/acs.nanolett.5b00922. IF: 13.592

    We investigate hot carrier propagation across graphene using an electrical nonlocal injection/detection method. The device consists of a monolayer graphene flake contacted by multiple metal leads. Using two remote leads for electrical heating, we generate a carrier temperature gradient that results in a measurable thermoelectric voltage VNL across the remaining (detector) leads. Due to the nonlocal character of the measurement, VNL is exclusively due to the Seebeck effect. Remarkably, a departure from the ordinary relationship between Joule power P and VNL, VNL ∼ P, becomes readily apparent at low temperatures, representing a fingerprint of hot-carrier dominated thermoelectricity. By studying VNL as a function of bias, we directly determine the carrier temperature and the characteristic cooling length for hot-carrier propagation, which are key parameters for a variety of new applications that rely on hot-carrier transport. (Figure Presented). © 2015 American Chemical Society.


  • Spin Hall effects

    Sinova J., Valenzuela S.O., Wunderlich J., Back C.H., Jungwirth T. Reviews of Modern Physics; 87 (4): 1213 - 1260. 2015. 10.1103/RevModPhys.87.1213. IF: 29.604

    Spin Hall effects are a collection of relativistic spin-orbit coupling phenomena in which electrical currents can generate transverse spin currents and vice versa. Despite being observed only a decade ago, these effects are already ubiquitous within spintronics, as standard spin-current generators and detectors. Here the theoretical and experimental results that have established this subfield of spintronics are reviewed. The focus is on the results that have converged to give us the current understanding of the phenomena, which has evolved from a qualitative to a more quantitative measurement of spin currents and their associated spin accumulation. Within the experimental framework, optical-, transport-, and magnetization-dynamics-based measurements are reviewed and linked to both phenomenological and microscopic theories of the effect. Within the theoretical framework, the basic mechanisms in both the extrinsic and intrinsic regimes are reviewed, which are linked to the mechanisms present in their closely related phenomenon in ferromagnets, the anomalous Hall effect. Also reviewed is the connection to the phenomenological treatment based on spin-diffusion equations applicable to certain regimes, as well as the spin-pumping theory of spin generation used in many measurements of the spin Hall angle. A further connection to the spin-current-generating spin Hall effect to the inverse spin galvanic effect is given, in which an electrical current induces a nonequilibrium spin polarization. This effect often accompanies the spin Hall effect since they share common microscopic origins. Both can exhibit the same symmetries when present in structures comprising ferromagnetic and nonmagnetic layers through their induced current-driven spin torques or induced voltages. Although a short chronological overview of the evolution of the spin Hall effect field and the resolution of some early controversies is given, the main body of this review is structured from a pedagogical point of view, focusing on well-established and accepted physics. In such a young field, there remains much to be understood and explored, hence some of the future challenges and opportunities of this rapidly evolving area of spintronics are outlined. © 2015 American Physical Society.


2014

  • Fingerprints of inelastic transport at the surface of the topological insulator Bi 2 Se 3: Role of electron-phonon coupling

    Costache, M.V.; Neumann, I.; Sierra, J.F.; Marinova, V.; Gospodinov, M.M.; Roche, S.; Valenzuela, S.O. Physical Review Letters; 2014. 10.1103/PhysRevLett.112.086601. IF: 7.728


  • Graphene spintronics: Puzzling controversies and challenges for spin manipulation

    Roche, S.; Valenzuela, S.O. Journal of Physics D - Applied Physics; 2014. 10.1088/0022-3727/47/9/094011. IF: 2.521


  • Pseudospin-driven spin relaxation mechanism in graphene

    Tuan, D.V.; Ortmann, F.; Soriano, D.; Valenzuela, S.O.; Roche, S. Nature Physics; 10 (11): 857 - 863. 2014. 10.1038/nphys3083. IF: 20.603


2013

  • Electrical detection of spin precession in freely suspended graphene spin valves on cross-linked poly(methyl methacrylate)

    Neumann, I.; Van De Vondel, J.; Bridoux, G.; Costache, M.V.; Alzina, F.; Torres, C.M.S.; Valenzuela, S.O. Small; 9 (1): 156 - 160. 2013. 10.1002/smll.201201194. IF: 7.823


  • Enhanced spin accumulation at room temperature in graphene spin valves with amorphous carbon interfacial layers

    Neumann, I.; Costache, M.V.; Bridoux, G.; Sierra, J.F.; Valenzuela, S.O. Applied Physics Letters; 2013. 10.1063/1.4820586. IF: 3.794


2012

  • Lateral metallic devices made by a multiangle shadow evaporation technique

    Costache, M.V.; Bridoux, G.; Neumann, I.; Valenzuela, S.O. Journal of Vacuum Science and Technology B: Microelectronics and Nanometer Structures; 30: 4. 2012. .


2011

  • Enhanced spin signal in nonlocal devices based on a ferromagnetic CoFeAl alloy

    Bridoux, G.; Costache, M. V.; Van de Vondel, J. ; Neumann, I.; Valenzuela, S. O. Applied Physics Letters; 2011. .


  • Generation of pure spin currents in a single electron transistor with a superconducting island

    Costache, M.V.; Valenzuela, S.O. Proceedings of SPIE - The International Society for Optical Engineering; 2011. 10.1117/12.890231 .


  • Magnon-drag thermopile

    Costache, M.V.; Bridoux, G.; Neumann, I.; Valenzuela, S.O. Nature Materials; 2011. .


2010

  • Experimental spin ratchet

    Costache, M.V.; Valenzuela, S.O. SCIENCE; 330: 1645 - 1648. 2010. 10.1126/science.1196228.


2009

  • Large-amplitude driving of a superconducting artificial atom : IIInterferometry, cooling, and amplitude spectroscopy

    Oliver, W.D.; Valenzuela, S.O. Quantum Information Processing; 8: 261 - 281. 2009. 10.1007/s11128-009-0108-y.


  • Nonlocal electronic spin detection, spin accumulation and the spin hall effect

    Valenzuela, S.O. International Journal of Modern Physics B; 23: 2413 - 2438. 2009. 10.1142/S021797920905290X.


  • Pulse imaging and nonadiabatic control of solid-state artificial atoms

    Bylander, J.; Rudner, M.S.; Shytov, A.V.; Valenzuela, S.O.; Berns, D.M.; Berggren, K.K.; Levitov, L.S.; Oliver, W.D. Physical Review B - Condensed Matter and Materials Physics; 80 2009. 10.1103/PhysRevB.80.220506.


2008

  • Amplitude spectroscopy of a solid-state artificial atom

    D.M. Berns; M.S. Rudner; S.O. Valenzuela; K.K. Berggren; W.D. Oliver; L.S. Levitov; T.P. Orlando Nature; 455: 51 - 57. 2008. 10.1038/nature07262.


  • Microwave-induced cooling of a superconducting qubit

    S.O. Valenzuela; W.D. Oliver; D.M. Berns; K.K. Berggren; L.S. Levitov; T.P. Orlando Science; 314 (5805): 1589 - 1592. 2008. 10.1126/science.1134008 .


  • Quantum Phase Tomography of a Strongly Driven Qubit

    M.S. Rudner; A.V. Shytov; L.S. Levitov; D. M. Berns; W.D. Oliver; S.O. Valenzuela; T.P. Orlando Physical Review Letters; 101: 190502. 2008. 10.1103/PhysRevLett.101.190502.