Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session C15: Graphene: Adatoms, Doping, and Magnetism |
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Sponsoring Units: DCMP DMP Chair: Olaf van 't Erve, Naval Research Laboratory Room: 314 |
Monday, March 14, 2016 2:30PM - 2:42PM |
C15.00001: Proximity induced exchange interaction in graphene-YIG devices Johannes Christian Leutenantsmeyer, Alexey Kaverzin, Magdalena Wojtaszek, Bart J. van Wees The proximity of two materials with radically different properties can give rise to a new physical phenomenon present only in the direct vicinity to the interface. Graphene is a perfect candidate for observing proximity effects as being ultimately thin and therefore ultimately sensitive for such interactions. Ferromagnetism is one of the desired properties for spintronics applications of graphene. It is absent in the pristine state, however, one can artificially induce magnetic ordering by bringing graphene in the proximity of ferrimagnetic insulating material, such as yttrium iron garnet (YIG). In this work we show that a monolayer of graphene placed on top of YIG adopts the exchange interaction induced by YIG and thus becomes ferromagnetic even at room temperatures. The proximity induced exchange interaction results in an effective magnetic field that influences directly the spin transport in graphene seen in a spin precession measurements. We are able to fit the measured Hanle dependences with extended solutions of Bloch diffusion equations and extract the value of the effective exchange field that is around 200 mT. Our findings open up a new route for creating novel all graphene in plane spin valve devices for spintronics applications. [Preview Abstract] |
Monday, March 14, 2016 2:42PM - 2:54PM |
C15.00002: Magneto-plasmons in graphene in a periodically modulated magnetic field Yuxuan Jiang, Owen Vail, Jeremy Yang, Jamey Gigliotti, Claire Berger, Walter de Heer, Dmitry Smirnov, Zhigang Jiang We present infrared magneto-spectroscopy study of graphene in a periodically modulated magnetic field. Corrugated trenches are first patterned on SiC substrate using e-beam lithography and plasma etching. Multilayer epitaxial graphene is then grown over the trenches, forming a periodic structure. In the presence of an external magnetic field, the perpendicular component of the field is spatially modulated, and the modulation strength can be tuned by varying the trench packing density or the titling angle of the field with respect to the substrate. Experimentally, enhanced absorption and salient deviations from single Lorentzian lineshape are observed, resulting from the periodic modulation. The deviated spectra can be well fitted by two Lorentzians, suggestive of the presence of two active modes. In addition, as the packing density of the trenches increases, the coupling between the corrugated graphene structures becomes stronger, resulting in a broadened spectrum. [Preview Abstract] |
Monday, March 14, 2016 2:54PM - 3:06PM |
C15.00003: Theory for disorder-induced magnetodrag in one-channel model for graphene Navneeth Ramakrishnan, Derek Ho, Shaffique Adam Recent work has shown the presence of disorder induced magnetoresistance that persists far away from charge neutrality, even in effective one-band systems [1]. This effect manifests itself in the magnetic field dependence of Coulomb drag, an effect that existing theoretical treatments have not considered [2]. In the presence of disorder, we calculate the magnetodrag as a function of the parameters of the disorder distribution and compare our results with the available experimental data in graphene and 2D electron gases. We comment on the relevance of these results in explaining the large magnetodrag at charge neutrality in graphene. \begin{thebibliography}{99} \bibitem{ping}J. Ping, I. Yudhistira, N. Ramakrishnan, S. Cho, S. Adam, and M. S. Fuhrer, Phys. Rev. Lett. 113, 047206 (2014). \bibitem{narozhny} B. N. Narozhny, A. Levchenko, arXiv:1505.07468 (2015). \end{thebibliography} [Preview Abstract] |
Monday, March 14, 2016 3:06PM - 3:18PM |
C15.00004: Induced magnetism in exfoliated graphene via proximity effect with yttrium iron garnet thin films Mario Amado, Yang Li, Angelo Di Bernardo, Antonio Lombardo, Andrea C. Ferrari, Jason Robinson The recent discovery of the quantum anomalous Hall effect (QAHE) in magnetically doped topological insulators cooled below in the milikelvin regime represents breakthrough in the field of spintronics. Theoretically, the QAHE should occur in graphene proximity coupled to a ferromagnetic insulato but with the promise of much higher operating temperatures for practical applications. Hints of proximity-induced magnetism in graphene coupled to yttrium iron garnet (YIG) films have been reported although the QAHE remains unobserved; the lack of a fully developed plateau in graphene/YIG devices can be attributed to poor interfacial coupling and therefore a dramatically reduced magnetic proximity effect. Here we report the deposition and characterisation of epitaxial thin-films of YIG on lattice-matched gadolinium gallium garnet substrates by pulsed laser deposition. Pristine exfoliated graphene flakes transferred mechanically onto the YIG are reported alongside results that correlate the effects of YIG morphology on the electronic and crystal properties of graphene by electrical (low temperature magnetoresistance measurements in Hall-bar-like configuration) and optical (Raman) means. [Preview Abstract] |
Monday, March 14, 2016 3:18PM - 3:30PM |
C15.00005: The magnetic ratchet effect in bilayer graphene Edward McCann, Narjes Kheirabadi, Vladimir Fal'ko Experiments [1] have measured a magnetic ratchet effect for electrons in hydrogenated monolayer graphene, an effect in which a d.c. electric current is generated from an a.c. electric field in the presence of an in-plane magnetic field and spatial asymmetry. Here, we describe the theory of the magnetic ratchet effect in bilayer graphene. The Boltzmann kinetic equation [2,3] is used to relate the d.c. current to the scattering probability of electrons in bilayer graphene. Taking into account details of the low-energy band structure of bilayer graphene, including interlayer hopping parameters, we compare contributions arising from gate- and disorder-induced spatial asymmetry, illustrating that bilayer and multilayer graphenes are natural candidates for the study of non-linear transport effects. [1] C. Drexler et al, Nature Nanotechnology 8, 104 (2013). [2] V. I. Fal'ko, Sov. Phys. Solid State 31, 561 (1989). [3] S. A. Tarasenko, Phys. Rev. B 83, 035313 (2011). [Preview Abstract] |
Monday, March 14, 2016 3:30PM - 3:42PM |
C15.00006: Magnetic nanostructures on graphene Xiaojie Liu, Cai-Zhuang Wang, Hai-Qing Lin, Myron Hupalo, Patricia A. Thiel, Kai-Ming Ho, Michael C. Tringides The calculations also show that Fe clusters on graphene exhibit ferromagnetic order but have smaller magnetic moments compared to the corresponding free-standing clusters. By contrast, Mn clusters on graphene exhibit ferrimagnetic coupling and enhanced magnetic moment compared to their free-standing clusters. Adsorption of Fe and Mn nanostructures also induces magnetic moments in graphene, and the induced magnetic moment on each carbon atom in graphene is correlated with the distortion of the graphene lattice. The origin of the magnetic moment changes in the clusters upon adsorption can be attributed to the electron redistribution due to the interaction with graphene. [Preview Abstract] |
Monday, March 14, 2016 3:42PM - 3:54PM |
C15.00007: Understanding Magnetic Trimer Interactions in (Cr,Mn)-Substituted Graphene Jason T. Haraldsen, Charles B. Crook, Gregory Houchins, Jian-Xin Zhu, Costel Constantin, Alexander V. Balatsky We investigate the magnetic interactions within a graphene superlattice produced by three directly substituted transition-metal atoms (specifically chromium and manganese). Using a first principles approach, we calculate the electronic and magnetic properties for this system assuming an equilateral trimer configuration with varying atomic separations. Through an examination of the electronic band structure, density of states, and Millikan populations (magnetic moment) for each atom, we find that the presence of magnetic impurities establishes a distinct magnetic moment in the graphene lattice, where the interactions are dependent on the spatial and magnetic characteristic between the magnetic atoms and the carbon atoms, which leads to either ferromagnetic or antiferromagnetic behavior. Furthermore, we use magnetization mapping to show that the substituted atoms induce an overall magnetic moment in the graphene lattice, which may help guide the discussion on spintronic graphene. [Preview Abstract] |
Monday, March 14, 2016 3:54PM - 4:06PM |
C15.00008: Controlling Adatom Magnetism on Bilayer Graphene by External Field Mukul Kabir, Dhani Nafday, Tanusri Saha-Dasgupta We study the effect of external electric field on the magnetic properties of single Fe adatom and Fe dimer hosted on a bilayer graphene surface grown on a SiO$_2$ substrate within first-principles calculations. We find that electric field perpendicular to the bilayer graphene modulates the charge and spin-state of the single Fe adatom over a wide range. States ranging from 3$d^{6}$, S=2 to 3$d^{10}$, S=0 have been observed for Fe adatom, which may be inaccessible under normal condition. This would be of interest in the context of orbitally controlled Kondo effect. Further, for Fe-dimer, we find that a small electric field is able to tune the magnetic exchange coupling. Interestingly, we also observe an unusual magnetostructural coupling for Fe-dimer, which stabilizes a ferrimagnetic state over a fully compensated antiferromagnetic spin configuration. [Preview Abstract] |
Monday, March 14, 2016 4:06PM - 4:18PM |
C15.00009: Novel electronic properties of hydrogenated graphene: A first-principles calculation Hong-Yan Lu, Rui Wang, Shih-Yang Lin, C. S. Ting We studied the electronic properties of some new kinds of hydrogenated graphenes by first-principles calculations. The designed systems, depending on the position and concentration of the hydrogen atoms, may show interesting band structures that are different from that of the pure graphene. For example, we can obtain semiconductor of a gap about 3eV with flat valance and conduction bands, or semimetal with anisotropic Dirac cones in which the position of Dirac points are shifted from K points, or semimetal with flat band crossing the Dirac cone at the Dirac point. The consequences of these features will be presented. We are able to get good metal with considerable density of states at Fermi level. The phonon dispersions and spectra as well as the electron-phonon couplings of such metallic systems are currently being investigated by the first-principles calculation, their superconductivity transition temperatures Tc should thus be predictable by assuming the electron-phonon coupling as the pairing interaction. We expect that the phonon frequencies are quite large, and the Tc could be high for some of the metallic systems. [Preview Abstract] |
Monday, March 14, 2016 4:18PM - 4:30PM |
C15.00010: Peculiarity of Thiophene/Graphene interface for organic electronic applications Souraya Goumri-Said Interfacial study between thiophene molecule and graphene surface is investigated on the basis of density functional theory. The reported HOMO-LUMO energy gaps, adsorption energy as well as binding energy are showing the existence of intermolecular forces accumulated from the attractive van der Waals forces and Pauli repulsion forces. The interface's separation distance is varied from 1.00{\AA} to 2.50{\AA}. It is noted that, subsequently growing intermolecular forces are very sensitive even to a relatively small change in the interface's separation distance between the molecule and the surface. The electronic density of states, dense electrons population of the thiophene/graphene system is found to be at energy Fermi level with appearance of spin-polarization. A slight magnetic behaviour on thiophene molecule, accompanied by a decrease in the magnetization of graphene surface was observed in the presence of the molecule near to the surface. [Preview Abstract] |
Monday, March 14, 2016 4:30PM - 4:42PM |
C15.00011: Adsorption of Nitric Oxide on Carbon Vacancies in Graphene and its Impact on the Conductivity Jorge Sofo, Sangzi Liang, Gugang Chen, Avetik Harutyunyan The conductance of graphene in FET devices increases when exposed to NO with detection limits down to the part-per-quadrillion level. We explore a possible explanation for this phenomena assuming that NO chemisorbs to vacancies and eventually dissociates. We found that adsorption of NO in graphene vacancies is favorable by 5.3 eV. In order to evaluate the conductivity due to these impurities, we obtain a minimum tight binding model with a Wannier transformation of the Kohn-Sham orbitals obtained by DFT. We evaluate the conductivity using the Kubo-Greenwood formula and the kernel polynomial method. We consider vacancies, NO and N chemisorbed in the vacancies, and O as an adatom on graphene. We found that the conductivity stays the same when NO adsorbs into a vacancy, but it increases when the oxygen atom moves away from the nitrogen atom, either leaving or moving to other parts of the surface, with the former giving a larger increase in conductivity. [Preview Abstract] |
Monday, March 14, 2016 4:42PM - 4:54PM |
C15.00012: Using Single Adatoms to Sense Screening by Graphene Charge Carriers Jonathan Wyrick, Fabian Natterer, Yue Zhao, Kenji Watanabe, Takashi Taniguchi, Nikolai Zhitenev, Joseph Stroscio As electronic devices approach the nanometer scale it becomes increasingly necessary to understand how small numbers of defects interact with one another and ultimately determine the behavior of a device. In the case of devices with graphene exposed at the surface, defects can be modelled and varied by depositing adatoms. We investigate how Co atoms adsorbed on graphene create and modify the electric potential experienced by graphene carriers using scanning tunneling microscopy and spectroscopy at low temperature. When an STM tip is brought into proximity to a biased graphene sample the electric field between tip and sample is screened by graphene's 2D electron gas, resulting in a local top-gating potential under the tip. This potential can be manipulated by varying a backgate voltage and the sample bias, and can be spatially characterized by imaging charging rings that form around defects when the potential changes their charge state. We find that defect charging rings lying near or even crossing adsorbed Co atoms become distorted and can form secondary charging rings around those atoms. To explain these effects we employ a charging model that incorporates a defect resonance, the screened tip potential, and modified screening in regions localized around Co atoms. [Preview Abstract] |
Monday, March 14, 2016 4:54PM - 5:06PM |
C15.00013: Transport measurement of Li doped monolayer graphene Ali Khademi, Ebrahim Sajadi, Pinder Dosanjh, Joshua Folk, Alexander Stöhr, Stiven Forti, Ulrich Starke Lithium adatoms on monolayer graphene have been predicted to induce superconductivity with a critical temperature near 8 K [1], and recent experimental evidence by ARPES indicates a critical temperature nearly that high [2]. Encouraged by these results, we investigated the effects of lithium deposited at cryogenic temperatures on the electronic transport properties of epitaxial and CVD monolayer graphene down to 3 K. The change of charge carrier density due to Li deposition was monitored both by the gate voltage shift of the Dirac point and by Hall measurements, in low and high doping regimes. In the high doping regime, a saturation density of 2\texttimes 10$^{\mathrm{13}}$ cm$^{\mathrm{-2}}$ was observed independent of sample type, initial carrier density and deposition conditions. No signatures of superconductivity were observed down to 3 K. [1] G. Profeta, et al., Nat Phys 8, 131 (2012). [2] B. M. Ludbrook, et al., PNAS 112 (38), 11795--11799 (2015). [Preview Abstract] |
Monday, March 14, 2016 5:06PM - 5:18PM |
C15.00014: Phase transitions of monolayers on graphene Joshua Kahn, Boris Dzyubenko, Oscar Vilches, David Cobden We have studied physisorbed layers of monatomic and diatomic gases on graphene. We used devices in which few-layer graphene, ranging from monolayer to trilayer, is suspended across a trench between two platinum contacts and are cleaned by thermal and current annealing. We found that the density of adsorbates is revealed by the conductance, similar to the case with nanotubes. The conductance change for a monolayer can be large. On trilayer graphene the adsorbed gases can be seen to exhibit transitions between two-dimensional phases identical to those on bulk graphite, including incommensurate and commensurate solid, fluid and vapor and multiple layers. New features appear in the conductance at the boundaries of the commensurate phase of Kr. We are able to measure single-particle binding energies very accurately and see how it depends on thickness; investigate the effects of changing disorder by gradually current annealing; and search for new phases in the case of monolayer graphene where atoms adsorbed on both sides can interact. We can map out the 2d phase diagrams very quickly by ohmic heating, which gives nearly instantaneous control of the temperature. [Preview Abstract] |
Monday, March 14, 2016 5:18PM - 5:30PM |
C15.00015: Electronic transport experiments on osmium-adatom-decorated graphene Jamie Elias, Erik Henriksen Monolayer graphene is theoretically predicted to inherit a spin-orbit coupling from a dilute coating of certain transition metal adatoms. To explore these predictions we have constructed a cryogenic probe capable of \textit{in situ} thermal annealing of graphene followed immediately by electronic transport measurements and controlled deposition of sub-monolayer coatings of most any metal. Previously a light coating of indium on graphene was investigated, and found to transfer electrons to graphene and reduce the mobility although no evidence of an induced spin-orbit coupling was seen. We are now depositing osmium and tungsten on graphene devices. Our initial results show an unexpected hole-doping and a sizable increase in resistance of the sample. We will report our progress on characterizing these samples by electronic transport measurements. [Preview Abstract] |
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