Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session G18: Two Dimensional Topological Insulators II: Graphene and Related Materials |
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Sponsoring Units: DCMP Chair: Shaffique Adam, Yale University Room: 320 |
Tuesday, March 19, 2013 11:15AM - 11:27AM |
G18.00001: Is graphene on the edge of being a topological insulator? Jose Gonzalez We show that, at sufficiently large strength of the long-range Coulomb interaction, a mass term breaking parity (so-called Haldane mass) is dynamically generated in the many-body theory of Dirac fermions describing the graphene layer. While the tendency towards a conventional excitonic instability is stronger than for the dynamical breakdown of parity at spatial dimension D greater than 2, we find that the situation is reversed at D = 2. The need to regularize the many-body theory in a gauge-invariant manner (taking the limit D = 2 from below) is what leads to the dominance of the parity-breaking pattern in graphene. We compute the critical coupling for the generation of a parity-breaking mass from the finite radius of convergence of the ladder series supplemented with electron self-energy corrections, finding a value quite close to the effective interaction strength for graphene in vacuum after including Fermi velocity renormalization and static RPA screening of the Coulomb interaction. [Preview Abstract] |
Tuesday, March 19, 2013 11:27AM - 11:39AM |
G18.00002: ABSTRACT WITHDRAWN |
Tuesday, March 19, 2013 11:39AM - 11:51AM |
G18.00003: Excahnge and correlation energy of electrons dressed with circularly-polarized light in graphene and three-dimensional topological insulators Andrii Iurov, Godfrey Gumbs We have formulated a theory for investigating the conditions which are required to achieve entangled states of electrons on graphene and three-dimensional (3D) topological insulators. We consider the quantum entanglement of spins by calculating the exchange energy. A gap is opened up at the Fermi level between the valence and conduction bands at zero doping when graphene as well as 3D topological insulators are irradiated with circularly-polarized light. This energy band gap is dependent on the intensity and frequency of the applied electromagnetic field. The electron-photon coupling also gives rise to a unique energy dispersion of the dressed states which is different from either graphene or the conventional two-dimensional electron gas (2DEG). In our calculations, we obtain the dynamical polarization function for imaginary frequencies. The polarization function is determined by both the energy dispersion and the overlap of pseudo-spin wave functions. The correlation energy is calculated in the random phase approximation (RPA). The application of the derived results to quantum computation will be discussed. [Preview Abstract] |
Tuesday, March 19, 2013 11:51AM - 12:03PM |
G18.00004: Stabilizing topological phases in graphene via random adsorption Jiang Jiang, Zhenhua Qiao, Haiwen Liu, Junren Shi, Qian Niu We study the possibility of realizing topological phases in graphene with randomly distributed adsorbates. When graphene is subjected to periodically distributed adatoms, the enhanced spin-orbit couplings can result in various topological phases. However, at certain adatom coverages, the intervalley scattering renders the system a trivial insulator. Using both finite-size scaling method and transport calculation, we show that when the adatom distribution becomes random, the intervalley scattering is weakened, but other quantities (e.g. spin-orbit couplings, and exchange field) are not affected. This finding points out that the topological states are graphene-favored ground states in the presence of randomly distributed adtoms. \\[4pt] Hua Jiang, Zhenhua Qiao, Haiwen Liu, Junren Shi and Qian Niu, Phys. Rev. Lett. \textbf{109}, 116803 (2012). [Preview Abstract] |
Tuesday, March 19, 2013 12:03PM - 12:15PM |
G18.00005: Bound States of Conical Singularities in Graphene-Based Topological Insulators Andreas Ruegg, Chungwei Lin We investigate the electronic structure induced by wedge-disclinations (conical singularities) in a honeycomb lattice model realizing Chern numbers $\gamma=\pm 1$. We establish a correspondence between the bound state of (i) an isolated $\Phi_0/2$-flux, (ii) an isolated pentagon $(n=1)$ or heptagon $(n=-1)$ defect with an external flux of magnitude $n\gamma \Phi_0/4$ through the center and (iii) an isolated square or octagon defect without external flux, where $\Phi_0=h/e$ is the flux quantum. Due to the above correspondence, the existence of isolated electronic states bound to the disclinations is robust against various perturbations. These results are also generalized to graphene-based time-reversal invariant topological insulators. [Preview Abstract] |
Tuesday, March 19, 2013 12:15PM - 12:27PM |
G18.00006: Topological kink states at the tilt boundary in gated multi-layer graphene Eun-Ah Kim, Abolhassan Vaezi, Yufeng Liang, Darryl Ngai, Li Yang Search for new realization of symmetry protected topological states with protected edge states is an active area of research. We show that a tilt boundary in gapped multi-layer graphene supports topologically protected gapless kink states. We investigate such kink states from two perspectives: the microscopic perspective of tight-binding model and an ab-initio calculation on bilayer, and the perspective of symmetry protected topological (SPT) states for general multi-layer. We show that the bilayer tilt boundary supports gapless kink states that are undeterred by strain concentrated at the boundary. Further we establish the kink states as concrete examples of edge states of {\it time-reversal symmetric} $Z$-type SPT, protected by $T$ and two $U(1)$ symmetries in the absence of inter-valley mixing. Recent observations of such boundaries in multi-layer samples suggest that transport through these topological kink states might explain the long standing puzzle of sub-gap conductance. We discuss possible topological phase transitions upon breaking subset of symmetries from SPT perspective. [Preview Abstract] |
Tuesday, March 19, 2013 12:27PM - 12:39PM |
G18.00007: Topological Proximity Effects in Graphene Nanoribbon Heterostructures Gufeng Zhang, Xiaoguang Li, Guangfen Wu, Jie Wang, Dimitrie Culcer, Efthimios Kaxiras, Zhenyu Zhang Topological insulators (TI) are bulk insulators that possess robust chiral conducting states along their interfaces with normal insulators. A tremendous research effort has recently been devoted to TI-based heterostructures, in which conventional proximity effects give rise to many exotic physical phenomena. Here we establish the potential existence of ``topological proximity effect'' at the interface of a topological graphene nanoribbon (GNR) and a normal GNR. Specifically, we show that the location of the topological edge states exhibits versatile tunability as a function of the interface orientation, as well as the strengths of the interface coupling and spin-orbit coupling in the normal GNR. For zigzag and bearded GNRs, the topological edge state can be tuned to be either at the interface or outer edge of the normal ribbon. For armchair GNR, the potential location of the topological edge state can be further enriched to be at the edge of or within the normal ribbon, at the interface, or diving into the topological GNR. We also discuss potential experimental realization of the predicted topological proximity effects, which may pave the way for integrating the salient functionality of TI and graphene in future device applications. [Preview Abstract] |
Tuesday, March 19, 2013 12:39PM - 12:51PM |
G18.00008: Designer quantum spin Hall phase transition in molecular graphene Pouyan Ghaemi, Sarang Gopalakrishnan, Taylor Hughes Graphene was the first material predicted to be a time-reversal-invariant topological insulator; however, the insulating gap is immeasurably small owing to the weakness of spin-orbit interactions in graphene. A recent experiment demonstrated that designer honeycomb lattices with graphene-like ``Dirac'' band structures can be engineered by depositing a regular array of carbon monoxide atoms on a metallic substrate. Here, we argue that by growing such designer lattices on metals or semiconductors with strong spin-orbit interactions, one can realize an analog of graphene with strong intrinsic spin-orbit coupling, and hence a highly controllable two-dimensional topological insulator. We estimate the range of substrate parameters for which the topological phase is achievable, and consider the experimental feasibility of some candidate substrates. [Preview Abstract] |
Tuesday, March 19, 2013 12:51PM - 1:03PM |
G18.00009: Topological Classification of Crystalline Insulators with Point Group Symmetry Di Xiao, Priyamvada Jadaun, Qian Niu, Sanjay Banerjee We show that in crystalline insulators point group symmetry alone gives rise to a topological classification based on the quantization of electric polarization. Using $C_3$ rotational symmetry as an example, we first prove that the polarization is quantized and can only take three inequivalent values. Therefore, a $Z_3$ topological classification exists. A concrete tight-binding model is derived to demonstrate the $Z_3$ topological phase transition. Using first-principles calculations, we identify graphene on BN substrate as a possible candidate to realize the $Z_3$ topological states. To complete our analysis we extend the classification of band structures to all 17 two-dimensional space groups. This work will contribute to a complete theory of symmetry conserved topological phases and also elucidate topological properties of graphene like systems. [Preview Abstract] |
Tuesday, March 19, 2013 1:03PM - 1:15PM |
G18.00010: Surface band topology of Ge on Ag(111) Athanasios Dimoulas, Evangelos Golias, Evangelia Xenogiannopoulou, Dimitra Tsoutsou, Polixronis Tsipas, Sigiava Giamini While compelling evidence for silicene on Ag (111) has been recently published [1], the existence of germanene remains elusive. We have performed MBE growth of (sub) monolayer Ge on single crystal Ag (111) substrates, supported by DFT calculations, with the aim to obtain germanene. RHEED data indicate a $\left( {\sqrt 3 \times \sqrt 3 } \right)R30^{0}$ superstructure, while \textit{in-situ} ARPES reveals a rich surface band structure consisting of linearly, highly dispersive cone-like features with hexagonal and snow-flake warping clearly imaged in the constant energy contour plots k$_{\mathrm{x}}$-k$_{\mathrm{y}}$. Unlike the case of graphene-like 2D crystals where Dirac cones are expected at the K-points, here the cone-like features appear at the center ($\Gamma$ points) of the surface Brillouin zone similar to what is observed in topological insulators. This suggests the possibility to witness a non-trivial surface band topology triggered by intrinsic spin-orbit coupling as predicted [2] for 2D honeycomb Ge lattices or by strong Ge and Ag p orbital hybridization in an ordered surface alloy Ag$_{2}$Ge.\\[4pt] [1] P. Vogt et al., PRL 108, 155501 (2012);\\[0pt] [2] C.C-Liu et al., PRL 107, 076802 (2011) [Preview Abstract] |
Tuesday, March 19, 2013 1:15PM - 1:27PM |
G18.00011: Polarization-driven topological insulator transition in a GaN/InN/GaN quantum well M.S. Miao, Q. Yan, C.G. Van de Walle, W.K. Lou, L.L. Li, K. Chang Topological insulators (TIs), a new state of quantum matter, have recently attracted significant attention, both for their fundamental research interest and for their potential device applications. Although many families of TI materials have been found, the realization of TI in conventional semiconductors remains elusive, mainly due to their sizable gaps and small spin-orbit interactions (SOI). Based on advanced first-principles calculations combined with an effective low-energy k$\cdot$p Hamiltonian, we show that the intrinsic polarization of materials can be utilized to simultaneously reduce the energy gap and enhance the SOI, driving the system to a TI state. The proposed system consists of ultrathin InN layers embedded into GaN, a layer structure that is experimentally achievable. We found that the TI transition happens at GaN/InN/GaN quantum well with 3 to 4 InN atomic layers. Since polarization fields occur in many materials, a similar mechanism may apply to other systems as well. Our approach may pave the way toward integrating controllable TIs with conventional semiconductor devices. [Preview Abstract] |
Tuesday, March 19, 2013 1:27PM - 1:39PM |
G18.00012: Electronic Structure calculations in a 2D SixGe1-x alloy under an applied electric field Jos\'e Eduardo Padilha, Renato B. Pontes, Leandro Seixas, Ant\^onio J.R. da Silva, Adalberto Fazzio The recent advances and promises in nanoscience and nanotechnology have been focused on hexagonal materials, mainly on carbon-based nanostructures. Recently, new candidates have been raised, where the greatest efforts are devoted to a new hexagonal and buckled material made of silicon, named Silicene. This new material presents an energy gap due to spin-orbit interaction of approximately 1.5 meV, where the measurement of quantum spin Hall effect(QSHE) can be made experimentally. Some investigations also show that the QSHE in 2D low-buckled hexagonal structures of germanium is present. Since the similarities, and at the same time the differences, between Si and Ge, over the years, have motivated a lot of investigations in these materials. In this work we performed systematic investigations on the electronic structure and band topology in both ordered and disordered SixGe1-x alloys monolayer with 2D honeycomb geometry by first-principles calculations. We show that an applied electric field can tune the gap size for both alloys. However, as a function of electric field, the disordered alloy presents a W-shaped behavior, similarly to the pure Si or Ge, whereas for the ordered alloy a V-shaped behavior is observed. [Preview Abstract] |
Tuesday, March 19, 2013 1:39PM - 1:51PM |
G18.00013: Reflection from surface step defect in topological insulator nanofilm Thakshila M. Herath, Prabath Hewageegana, Vadim M. Apalkov Ultrathin topological insulator nanofilm with a step-like defect, which divides two regions of nanofilm with different thicknesses, is considered. Electron, propagating along the nanofilm surface, is reflected from the step. We calculate the reflectance of such electron for different parameters of the nanofilm and different parameters of the defect. We demonstrate that such system has an interesting property. Namely, the incident electron wave not only produces the reflected and transmitted waves, but also generates the mode, localized at the step-like defect. Such mode results in an enhancement of the electron density at the defect by $\sim$20\%. The strength of such enhancement depends on the parameters of the nanofilm and the height of the step. [Preview Abstract] |
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