Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session J7: Focus Session: Molecules on Surfaces & in Devices |
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Sponsoring Units: GMAG Chair: Herre van der Zant, TU Delft Room: 106 |
Tuesday, March 4, 2014 2:30PM - 2:42PM |
J7.00001: Spin interactions in Graphene-Single Molecule Magnets Hybrids Christian Cervetti, Angelo Rettori, Maria Gloria Pini, Andrea Cornia, A\~na Repoll\'es, Fernando Luis, Stephan Rauschenbach, Martin Dressel, Klaus Kern, Marko Burghard, Lapo Bogani Graphene is a potential component of novel spintronics devices owing to its long spin diffusion length. Besides its use as spin-transport channel, graphene can be employed for the detection and manipulation of molecular spins. This requires an appropriate coupling between the sheets and the single molecular magnets (SMM). Here, we present a comprehensive characterization of graphene-Fe$_{4}$ SMM hybrids. The Fe$_{4}$ clusters are anchored non-covalently to the graphene following a diffusion-limited assembly and can reorganize into random networks when subjected to slightly elevated temperature. Molecules anchored on graphene sheets show unaltered static magnetic properties, whilst the quantum dynamics is profoundly modulated. Interaction with Dirac fermions becomes the dominant spin-relaxation channel, with observable effects produced by graphene phonons and reduced dipolar interactions. Coupling to graphene drives the spins over Villain's threshold, allowing the first observation of strongly-perturbative tunneling processes. Preliminary spin-transport experiments at low-temperature are further presented. [Preview Abstract] |
Tuesday, March 4, 2014 2:42PM - 2:54PM |
J7.00002: Effect of electron-vibron coupling on electron transport via a single-molecule magnet Fe4 Alexander McCaskey, Yoh Yamamoto, Michael Warnock, Xiaoliang Zhong, Kyungwha Park Recently, single-molecule junctions consisting of individual single-molecule magnets (SMMs) bridged between electrodes have been fabricated in three-terminal devices, and magnetic anisotropy of SMMs has been shown to be affected by electron transport through the SMMs. In such junctions, vibrational modes of the SMM can couple to electronic charge and/or spin degrees of freedom, and the coupling influences magnetic and transport properties of the SMM. An effect of the electron-vibron coupling on transport has been extensively studied in small molecules, but not yet for junctions of SMMs. In this talk, we present our calculations of the electron-vibron coupling in a SMM Fe4 based on density-functional theory, and an effect of the coupling on electron transport. In addition, we compare our results with experimental data. [Preview Abstract] |
Tuesday, March 4, 2014 2:54PM - 3:06PM |
J7.00003: First-principles study of a single-molecule magnet $Mn_{12}$ monolayer on the graphene surface. Xiangguo Li, Hai-Ping Cheng Electronic structures of single-molecule magnets $Mn_{12}$ on graphene surfaces are studied using spin-polarized density-functional theory. Charge transfer between molecule and graphene, densities of states, and magnetization are fully analyzed. We also report effects of various ligands and strain. Our results suggest that graphene can be p-doped upon $Mn_{12}$ adsorption, and the doping level is closely related to the choice of ligands in molecule. In addition, we find that the strain in graphene plays an important role in modulating the doping level. [Preview Abstract] |
Tuesday, March 4, 2014 3:06PM - 3:18PM |
J7.00004: Studies of Mn$_{12}$-Ph Single Molecule Magnets by LT-STM and Modeling of Magnetic Stability Under Perturbation K. Reaves, P. Han, K. Iwaya, T. Hitosugi, D. Packwood, H.G. Katzgraber, H. Zhao, K.R. Dunbar, K. Kim, W. Teizer We study Mn$_{12}$O$_{12}$(C$_6$H$_5$COO)$_{16}$(H$_2$O)$_4$ (Mn$_{12}$-Ph) single-molecule magnets on a Cu(111) surface using low temperature scanning tunneling microscopy, LT-STM. We report the observation of Mn$_{12}$-Ph in isolation and in thin films, deposited through vacuum spray deposition onto clean Cu(111). The local tunneling current observed within the molecular structure shows a strong bias voltage dependency, which is distinct from that of the Cu surface. Furthermore, we identify an internal inhomogeneity in the bias behavior within a single molecule. To further understand the stability of the magnetic properties of the molecules while on the surface, we develop a theoretical model to study the stability of the net magnetic moment under deformation of the spin-spin interaction graph. We develop a spin Hamiltonian-type model to predict magnetic moments that are intrinsically robust under random shape deformations to the spin-graph structure. This spin moment is shown to be a weak topological invariant for molecules with sufficiently many spin centers, approximately 20 to 50. [Preview Abstract] |
Tuesday, March 4, 2014 3:18PM - 3:30PM |
J7.00005: Magnetic Properties of Single Cobalt Atoms on Thin MgO Films Ileana Rau, Susanne Baumann, Chris Lutz, Andreas Heinrich Studies of individual magnetic atoms on surfaces have shown magnetic anisotropies that inhibit thermal transitions over the barrier at cryogenic temperatures. To observe stable magnetic moments in single atoms at room temperatures, as would be desirable for technological applications, it is necessary to further increase the magnetic anisotropy barrier. The size of this barrier is usually limited by the quenching of the orbital moment that occurs because of the binding of the atom to its ligands. We use inelastic electron tunneling spectroscopy in a low temperature scanning tunneling microscope to measure the energy splitting between the atomic spin states of single Cobalt atoms deposited on 1 monolayer MgO on top of a metal substrate. We show that in this crystal environment the orbital moment of the magnetic atom is not quenched. This leads to the maximal spin-orbit induced separation between the spin ground and excited state and a lower bound on the magnetic anisotropy energy barrier for Cobalt of 58meV. [Preview Abstract] |
Tuesday, March 4, 2014 3:30PM - 3:42PM |
J7.00006: Metallocene Molecular Clusters Studied with Scanning Tunneling Microscopy and Spectroscopy Jeonghoon Kwon, Ungdon Ham, Minjun Lee, Seong Joon Lim, Young Kuk Atomic spins and molecular magnets have been actively reported using Scanning Tunneling Microscope(STM) in recent studies. One can even assemble an artificial magnet by STM manipulation. Manganocene((C$_{\mathrm{5}}$H$_{\mathrm{5}})_{\mathrm{2}}$Mn), a sandwich complex of metallocene, is composed of one manganese atom and two cyclopentadianyl ligands. This molecule is known to reveal not only high spin number S $=$ 5/2 at room temperature but also two structural states: monomer and molecular chain. In this presentation, we report STM images and spectroscopic results of these monomers and dimers. We try to map the molecular electronic state and the spin texture. The molecule is adsorbed on an insulating layer to decouple the spin state from the metallic substrate. We will present that manganocene can become a basic element of a spin chain. [Preview Abstract] |
Tuesday, March 4, 2014 3:42PM - 4:18PM |
J7.00007: Scanning SQUID microscopy with single electron spin sensitivity Invited Speaker: Denis Vasyukov Superconducting interference devices (SQUIDs) have been traditionally used for studying fundamental properties of magnetic materials and superconductors. Although widely used in scanning magnetic microscopy, their progress towards detection of small magnetic moments was stagnating of late due to limitations imposed by conventional designs of planar SQUIDs and contemporary lithography techniques, restricting sample-to-sensor distance smaller than $\sim$ 0.5 micron and SQUIDs diameters smaller than $\sim$ 200 nm. These limitations were overcome by the invention of a SQUID-on-tip device [1], subsequent realization of a SQUID-on-tip microscope [2], and by creation of an ultra-small sensor with spatial resolution of 20 nm and sensitivity to a single electron spin per 1 Hz bandwidth [3]. In this talk I will describe the principles of scanning SQUID magnetometry, its applications to study superconductors and its potential for magnetic nano-scale imaging of novel materials.\\[4pt] [1] Self-aligned nanoSQUID on a tip. A. Finkler, Y. Segev, Y. Myasoedov, M. L. Rappaport, L. Ne'eman, D. Vasyukov, E. Zeldov, M. E. Huber, J. Martin and A. Yacoby, Nano Letters 10 (2010) 1046.\\[0pt] [2] Scanning superconducting quantum interference device on a tip for magnetic imaging of nanoscale phenomena. A. Finkler, D. Vasyukov, Y. Segev, L. Ne'eman, E. O. Lachman, M. L. Rappaport, Y. Myasoedov, E. Zeldov and M. E. Huber, Rev. Sci. Instrum. 83 (2012) 073702.\\[0pt] [3] A scanning superconducting quantum interference device with single electron spin sensitivity. D. Vasyukov, Y. Anahory, L. Embon, D. Halbertal, J. Cuppens, L. Ne'eman, A. Finkler, Y. Segev, Y. Myasoedov, M. L. Rappaport, M. E. Huber and E. Zeldov, Nature Nanotechnology 8 (2013) 639; Magnetic sensors: A tip for better sensing. D. Koelle, Nature Nanotechnology 8 (2013) 617. [Preview Abstract] |
Tuesday, March 4, 2014 4:18PM - 4:30PM |
J7.00008: Tailoring the Kondo Effect in Composite Magnetic Systems Deung-Jang Choi, Shichao Yan, Jacob Burgess, Steffen Rolf-Pissarczyk, Sebastian Loth We manipulate different transition metal atoms and build chains using a sub-Kelvin scanning tunneling microscope (STM). The chains composed of transition metal atoms form a highly correlated spin singlet ground state exhibiting a Kondo resonance. By studying temperature and magnetic field dependence, we confirm the Kondo effect in this composite system. We find that the occurrence of the Kondo resonance sensitively depends on the length of the atomic chain and the spin anisotropy energy of each atom. We construct chains with different elemental composition and obtain various spin ground states. In this way, we can tailor the singlet ground state on and off and also modify its strength. The addition of vector magnetic fields to the atomically assembled nanostructures provides another parameter that can continuously tune the behavior of atomic spins. This opens up a fruitful experimental model spin system to be conquered and makes it possible to engineer many-body effects in prototypical spin structures at atomic dimensions. [Preview Abstract] |
Tuesday, March 4, 2014 4:30PM - 4:42PM |
J7.00009: Kondo effect signatures in the electronic transport through a single-ion magnet under an applied transverse magnetic field Javier Romero, Edson Vernek, George Martins, Eduardo Mucciolo We study the low-temperature electronic transport properties of a single magnetic ion molecule (SMIM) in the presence of a rhombic ligand field using the numerical renormalization group method. The rhombic ligand environment induces uniaxial and transverse zero-field spin anisotropies in the ion. We find signatures of a Kondo effect caused by the presence of a transverse (zero-field) anisotropy in the molecule. Upon applying a transverse magnetic field to the SIMM, we observe oscillations of the Kondo effect near the diabolical (degeneracy) points of the energy spectrum of the molecule. The field-induced lifting of the ground state degeneracy competes with the interference modulation, resulting in some situations in a suppression of the Kondo peak. [Preview Abstract] |
Tuesday, March 4, 2014 4:42PM - 4:54PM |
J7.00010: ABSTRACT WITHDRAWN |
Tuesday, March 4, 2014 4:54PM - 5:06PM |
J7.00011: A molecular approach to the Kondo problem in Carbon based systems Maria Soriano, David Jacob, Juan Jose Palacios There has been a great effort in recent years to understand the emerging Kondo-like resonances in different magnetic molecules such as MnPc. Theoretical approaches based on atomic models have proven to be very useful for the study of this phenomenon when the magnetic moment is essentially localized on a magnetic atom [1,2]. Nevertheless the Kondo effect can arise in pure carbon-based systems as has been demonstrated experimentally in fullerenes and carbon nanotubes [3,4]. In this communication we present a multiorbital Anderson model where the orbitals are not atomic but molecular orbitals. This model is fully obtained from Density Functional Theory calculation in combination with Green's functions methodologies [5,6]. Standard impurity solver techniques are used to solve the model which is applied to fullerenes and other nanographene structures [7]. \\[4pt] [1] A. Str\'{o}zecka et. al. Phys. Rev. Lett. 109, 147202 (2012);\\[0pt] [2] D. Jacob et. al. Phys. Rev. B 88, 134417 (2013);\\[0pt] [3] J. J. Parks et. al. Phys. Rev. Lett. 99, 026601 (2007);\\[0pt] [4] P. Jarillo-Herrero et. al. Nature 434, 484. (2005);\\[0pt] [5] ANT.G03. www.alacant.dfa.ua.es;\\[0pt] [6] D. Jacob et. al. Phys. Rev. B. 82, 195115 (2010);\\[0pt] [7] J Fernandez-Rossier et. al. Phys. Rev. Lett. 99, 177204 (2007). [Preview Abstract] |
Tuesday, March 4, 2014 5:06PM - 5:18PM |
J7.00012: Spectral evolution of the SU(4) Kondo effect from the single impurity to the two-dimensional lattice Alejandro Lobos, Marcelo Romero, Armando Aligia We describe the evolution of the SU(4) Kondo effect as the dimensionality of the system is gradually increased from the zero-dimensional limit (i.e., impurity) to the two-dimensional (2D) lattice. We derive a Hubbard-Anderson model describing a 2D array of atoms or molecules with two-fold orbital degeneracy, acting as magnetic impurities and interacting with a metallic host. We calculate the differential conductance, observed typically in experiments of scanning tunneling spectroscopy, for different arrangements of impurities on a metallic surface: a single impurity, a periodic square lattice, and several sites of a rectangular cluster. Our results point towards the crucial importance of the orbital degeneracy and agree well with recent experiments in different systems of of iron(II) phtalocyanine molecules deposited on top of Au(111) [N. Tsukahara \textit{et al.}, Phys. Rev. Lett. \textbf{106}, 187201 (2011)]. Our results indicate that this would be the first experimental realization of a 2D SU(4) Kondo-lattice system. [Preview Abstract] |
Tuesday, March 4, 2014 5:18PM - 5:30PM |
J7.00013: Incorporating isolated molybdenum (Mo) atoms into Bilayer Epitaxial Graphene on 4H-SiC(0001) Han Huang, Wen Wan, Hui Li, Swee Liang Wong, Lu Lv, Yongli Gao, Andrew T.S. Wee The atomic structures and electronic properties of isolated Mo atoms in bilayer epitaxial graphene (BLEG) on 4H-SiC(0001) are investigated by low temperature scanning tunneling microscopy (LT-STM). LT-STM results reveal that isolated Mo dopants prefer to substitute C atoms at $\alpha $-sites, and preferentially locate between the graphene bilayers. First-principles calculations confirm that the embedding of single Mo dopants within BLEG is energetically favorable as compared to monolayer graphene. The calculated bandstructures show that Mo-doped BLEG is n-doped, and each Mo atom introduces a local magnetic moment of 1.81 $\mu_{\mathrm{B}}$. Our findings demonstrate a simple and stable method to incorporate single transition metal dopants into the graphene lattice to tune its electronic and magnetic properties for possible use in graphene spin devices. [Preview Abstract] |
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