Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session J2: Invited Session: Topological States and Plasmonics in Graphene |
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Sponsoring Units: DCMP Chair: Qian Niu, University of Texas at Austin Room: Ballroom II |
Tuesday, March 19, 2013 2:30PM - 3:06PM |
J2.00001: Topological insulator gap in graphene with heavy adatoms Invited Speaker: Ruqian Wu It is important to search an effective approach to expand the spin-orbit coupling gap of graphene for the realization of the two-dimensional topological insulator (TI) state. We found that heavy In or Tl adatoms may dramatically enhance the gap to detectable values of order 7 or 20 meV, large enough for the realization of quantum spin Hall effect in experimental conditions. However, In and Tl atoms may easily coalesce on graphene due to their weak binding energies and shallow segregation barriers. We proposed a new way to produce a two-dimensional spin-orbit coupling gap using the impurity bands that are mediated through graphene. First principles calculations predict that the gaps generated by osmium and iridium exceed 200 meV over a broad range of adatom coverage The position of the Fermi level can be manipulated by using external electric field and co-adsorbates. The mechanism at work is expected to be rather general and may open the door to designing new TI phases in many materials. [Preview Abstract] |
Tuesday, March 19, 2013 3:06PM - 3:42PM |
J2.00002: Engineering topological states in graphene systems Invited Speaker: Zhenhua Qiao In this talk, I will introduce our recent progress on engineering various topological states in graphene systems. The presentation includes two parts: (i) We show that in monolayer graphene, Rashba spin-orbit coupling (SOC) together with Zeeman field can open a nontrivial bulk gap to host the quantum anomalous-Hall effect [1]. We further show that this can be realized via doping magnetic metal atoms on graphene [2,3]. In Bernal stacking bilayer graphene, an interlayer potential difference breaks the inversion symmetry and opens a bulk gap to support the quantum valley-Hall effect. We find that Rashba SOC can induce a topological phase transition from the quantum valley-Hall effect to a Z2 topological insulator [4]. When the Zeeman field is further considered, a rich variety of topological phases emerge. (ii) When the mass term (e.g., sublattice potential in monolayer graphene, or interlayer potential difference in bilayer graphene) varies spatially, topologically protected 1D kink states arise along zero lines. We demonstrate that such 1D kink state exhibits zero bend resistance for arbitrary turns in its propagating path [5]. We further point out that similar kink states can be tailored in graphene nanoroads in boron nitride sheets [6]. When the kink current experiences a crossing junction composed of four zero lines, we find the splitting of the 1D kink state at the bifurcation point obeys an explicit law of current partition [7].\\[4pt] References:\\[0pt] [1] Z.H. Qiao \textit{et al.}, Phys. Rev. B 82, 161414(R) (2010)\\[0pt] [2] J. Ding \textit{et al.}, Phys. Rev. B 84, 195444 (2011)\\[0pt] [3] H. Jiang \textit{et al.}, Phys. Rev. Lett. 109, 116803 (2012)\\[0pt] [4] Z.H. Qiao \textit{et al.}, Phys. Rev. Lett. 107, 256801 (2011)\\[0pt] [5] Z.H. Qiao \textit{et al.}, Nano Letters 11, 3453 (2011)\\[0pt] [6] J. Jung \textit{et al.}, Nano Letters 12, 2936 (2012)\\[0pt] [7] Z.H. Qiao \textit{et al.}, to be submitted. [Preview Abstract] |
Tuesday, March 19, 2013 3:42PM - 4:18PM |
J2.00003: Giant Rashba effect and spin polarization of Dirac fermions in graphene Invited Speaker: Oliver Rader Graphene in spintronics has so far meant a material with low spin-orbit coupling which could be used as high-performance spin current leads. If the spin-orbit interaction could be enhanced by an external effect, the material could serve also as an active element in a spintronics device such as the Das-Datta spin field effect transistors. We show that by intercalation of Au under graphene grown on Ni(111), a Rashba-type spin-orbit splitting of $\sim$ 100 meV can be created in a wide energy range while the Dirac cone is preserved and becomes slightly p-doped. We discuss different superstructures of Au under the graphene which are observed in the experiment. Ab initio calculations indicate that a sharp graphene-Au interface at the equilibrium distance accounts for only $\sim$ 10meV spin-orbit splitting and enhancement can occur due to Au atoms in the hollow position that get closer to graphene while preserving the sublattice symmetry. For the system graphene/Ir(111) we observe a large splitting of the Dirac cone as well. The large lattice mismatch of this system allows us to investigate properties of the pseudospin that are related to the structure of minigaps that occur at the zone boundary of the superstructure. We also report on the giant Rashba splitting of an Ir(111) surface state which persists underneath the graphene. Finally, we re-investigate with p(1 $\times$ 1) graphene/Ni(111) and Co(0001) typical examples where the sublattice symmetry breaking by the substrate is believed to lead to a large band gap at the Dirac point. We show that this is not the case and the Dirac point of graphene stays instead intact, and we discuss implications of this finding. [Preview Abstract] |
Tuesday, March 19, 2013 4:18PM - 4:54PM |
J2.00004: Infrared nano-imaging and nano-spectroscopy of graphene plasmons Invited Speaker: Zhe Fei Graphene plasmons, which are collective oscillations of Dirac fermions in graphene, are of broad interests in both fundamental research and technological applications. In this talk, we present first nano-imaging and nano-spectroscopy studies of graphene plasmons using scattering-type scanning near-field optical microscope --a unique technique allowing efficient excitation and high-resolution imaging of graphene plasmons. With this technique, we were able to show that common graphene/SiO2/Si back-gated structure support propagating surface plasmons in the infrared frequencies. The observed plasmons are highly confined surface modes with a wavelength around 200nm that are conveniently tunable by the back gate voltages [Nature 487, 82--85 (2012)]. In addition, we performed nano-spectroscopy of graphene over a broad range of mid-infrared frequencies. Our spectroscopy results provide evidence of strong coupling between graphene plasmons and SiO2 optical phonons [Nano Lett. 11(11), 4701-4705 (2011)]. Finally, we were able to map and characterize various types of line defects inside CVD graphene film by exploring real space patterns of propagating surface plasmons. These line defects, including cracks, wrinkles, and even grain boundaries, trigger distinct plasmonic features due to plasmon interference. Further modeling and analysis unveiled unique electronic properties associated with these line defects. [Preview Abstract] |
Tuesday, March 19, 2013 4:54PM - 5:30PM |
J2.00005: Quantum Anomalous Hall Effects and Topological Phase Transitions in Silicene Invited Speaker: Motohiko Ezawa Silicene is a monolayer of silicon atoms forming a two-dimensional honeycomb lattice, which is experimentally manufactured this year. The low energy theory is described by Dirac electrons, but they are massive due to a relatively large spin-orbit interaction. I will explain the following properties of silicene: 1) The band structure is controllable by applying an electric field [1]. Silicene undergoes a phase transition from a topological insulator to a band insulator by applying external electric field [1]. 2) The topological phase transition can be detected experimentally by way of diamagnetism [7]. 3) There is a novel circular dichroism and spinvalley selection rules by way of photon absorption [6]. 4) Silicene shows a quantum anomalous Hall effects when ferromagnet is attached onto silicone [3]. 5) Silicene shows a photo-induced quantum Hall effects when we apply strong laser onto silicene [8]. 6) Single Dirac cone state emerges when we apply photo-irradiation and electric field, where the gap is open at the K point and closed at the K' point [8].\\[4pt] [1] M. Ezawa, New J. Phys. 14, 033003 (2012).\\[0pt] [2] M. Ezawa, J. Phys. Jpn. 81, 064705 (2012). \\[0pt] [3] M. Ezawa, Phys. Rev. Lett. 109, 055502 (2012)\\[0pt] [4] M. Ezawa, Europhysics Letters 98, 67001 (2012).\\[0pt] [5] M. Ezawa, J. Phys. Soc. Jpn. 81, 104713 (2012).\\[0pt] [6] M. Ezawa, Phys. Rev. B 86, 161407(R) (2012).\\[0pt] [7] M. Ezawa, cond-mat/arXiv:1205.6541 (to be published in EPJB).\\[0pt] [8] M. Ezawa, cond-mat/arXiv:1207.6694.\\[0pt] [9] M. Ezawa, cond-mat/arXiv: 1209.2580. [Preview Abstract] |
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