Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session Q1: Focus Session: Graphene: Moire, Superlattices, and Twisted Bilayers |
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Sponsoring Units: DMP Chair: Walt de Heer, Georgia Institute of Technology Room: 001A |
Wednesday, March 4, 2015 2:30PM - 2:42PM |
Q1.00001: Bilayer graphene moire pattern amplitude vs. twist angle identified using scanning tunneling microscopy Joshua Thompson, Pijush Ghosh, Paul Thibado, Mehdi Neek-Amal, Francois Peeters, V.D. Wheeler, R.L. Myers-Ward, C.R. Eddy, Jr., D.K. Gaskill Twisted stacked layers of graphene have unique electronic properties. The layers produce a moire pattern with a wavelength determined by the twist angle. In addition, however, the surface of the moire pattern also has an increased corrugation amplitude when compared to untwisted AB-stacked graphene. The deformation strains the top-layer graphene lattice and realizes, in a natural way, triaxial stress creation as proposed by Guinea et al. Consequently, very large pseudo-magnetic fields can be created depending on the amplitude of the corrugations. Until now, no relation has been found between the moire twist angle and its corrugation amplitude. We found, using scanning tunneling microscopy, as the wavelength of the moire pattern increases so does its amplitude. Our experiments are supported by first-principles directed elasticity theory. The membrane corrugation amplitude at arbitrary twist angle is found to be a consequence of a competition between the van der Waals bonding energy and the energy required to bend the graphene. [Preview Abstract] |
Wednesday, March 4, 2015 2:42PM - 2:54PM |
Q1.00002: Electronic properties and van Hove singularities of observed moir{\'e} patterns of dislocated graphene on HOPG Oguz Gulseren, H. Sener Sen, Dilek Yildiz, Oguzhan Gurlu Highly Oriented Pyrolitic Graphite (HOPG) can be described as stacked graphene layers. Due the weak van der Waals interaction between the layers, topmost layer of HOPG can be rotated or shifted by chemical or mechanical means. With rotation of the topmost layer, super periodic structures called as moir{\'e} patterns are formed. In this work, moir{\'e} patterns on HOPG surfaces due to dislocated graphene layers were studied. A simple geometric investigation of the atomic structure of the moir{\'e} patterns revealed that different atomic moir{\'e} periodicities result in similar geometric moir{\'e} periods. Our calculations showed that the band structure of moir{\'e} patterns even though exhibits the fingerprints of those of twisted bilayer graphene system, like the preserved Dirac cone at the K point of moir{\'e} Brillouin zone, it has several new emerging features like van Hove singularities and linear or flat bands depending on the moir{\'e} periodicity. Our results show that most of the moir{\'e} patterns observed on graphene/HOPG system do not have a purely electronic or structural origin, but both. Moreover, our results show that van Hove singularities in these systems with different twist angles have different origins in their respective band structure. [Preview Abstract] |
Wednesday, March 4, 2015 2:54PM - 3:06PM |
Q1.00003: Transport measurements of van Hove singularity in twisted bilayer graphene. Junxi Duan, Jinhai Mao, Yuhang Jiang, Guohong Li, Mona Zebarjadi, Eva Y. Andrei Crossing between the Fermi energy and a van Hove singularity causes electronic instabilities which may result in new correlated phases such as superconductivity, magnetism and charge density waves. These crossings are usually rare and difficult to control as they are determined by the chemical composition and structure of the material. However, as was recently demonstrated in STM measurements, twisted graphene layers display van Hove singularities whose energy can be controlled by tuning the twist angle between them. Combining this flexibility with the ability to tune the Fermi energy by gating, makes it possible to bring these two energies together. But in order to induce an observable electronic instability the crossing energy must be uniform across the sample, which is difficult to achieve due to the potential fluctuation induced by insulating substrates. To investigate the possibility of a gate induced phase transition we fabricated devices with gated and precisely twisted graphene layers on minimally invasive substrates including hBN and suspended samples. We will report our findings on transport and STM measurements that revealed the global and local effects of gate induced band structure reconstruction. [Preview Abstract] |
Wednesday, March 4, 2015 3:06PM - 3:18PM |
Q1.00004: Electronic signatures of stacking domain boundaries in twisted bilayer graphene Fernando Gargiulo, Bastien Grosso, Gabriel Aut\`{e}s, Oleg Yazyev Experiments on bilayer graphene (BLG) show the coexistence of regions of inequivalent stacking order (AB and BA) separated by domain boundaries, topological defects akin to solitons [1]. The Frenkel-Kontorova model predicts that in-plane atomic displacements result in stacking domain boundaries of few nanometers width. We show how a hexagonal network of stacking domain boundaries naturally arises in twisted BLG in the limit of small twist angle (below ca.~1$^{\circ}$). Equilibrium configurations of twisted BLG have been produced by means of classical force-field simulations. Atomic displacements diminish the area of AA stacking regions and extend the AB and BA stacking regions to triangular domains separated by boundaries of 7--8~nm width. Large-scale tight-binding simulations unveil the electronic properties of such twisted BLG models. A charge density depletion is the low-energy signature of stacking domain boundaries with electronic states mostly confined in AB and BA domains. Zero-energy states at the network nodes reach an asymptotic localization for vanishing twist angles. We propose STM experiments for confirming our predictions.\\[4pt] [1] L. Brown et al., Nano Lett. 12, 1609 (2012).\\[0pt] [2] J. S. Alden et al., PNAS 110, 11256(2013).\\[0pt] [3] J. Linang et al., Nano Lett. (2013). [Preview Abstract] |
Wednesday, March 4, 2015 3:18PM - 3:30PM |
Q1.00005: Study of the optical phonons on gated twisted bilayer graphene Ting Fung Chung, Rui He, Tai-lung Wu, Yong P. Chen In twisted bilayer graphene (tBLG), the low-energy van-Hove singularities (vHs) in the density of states (DOS) can be continuously tuned by twisting the two layers, leading to distinct electronic and optical properties compared to Bernal-stacked BLG (AB-BLG). This effect has been explored using resonance Raman scattering, showing enhanced Raman G and ZO' (low frequency, layer breathing vibration) bands when the vHs energy resonates with excitation laser energy. We have studied the influence on vHs and Raman bands in gated tBLG devices (at resonant twist angle $\sim$13$^{\circ}$ under a 532 nm laser light). We observed that the G band splits with increasing doping, attributed to asymmetric doping of charge carriers in the two layers. The strongly quenched G band intensity at high doping level is ascribed to the suppression of resonant interband transitions between the two saddle points (in conduction and valence bands) which are displaced in the momentum space by gate-tuning. We have also measured the doping dependence of ZO' band and R band in tBLG. Our results demonstrate that electric-field can be used to tune the optoelectronic and vibrational properties in tBLG devices. [Preview Abstract] |
Wednesday, March 4, 2015 3:30PM - 3:42PM |
Q1.00006: Electronic Band Structure Modification upon Doping in Twisted Bilayer Graphene Shengqiang Huang, Matthew Yankowitz, Kanokporn Chattrakun, Arvinder Sandhu, Brian LeRoy In twisted bilayer graphene, the electronic band structure and phonon dispersion depend on the rotation angle between the layers. From 9 to 15 degrees, the Raman G peak measured with a 532 nm laser is enhanced due to an increased density of states. The enhancement is a maximum at a critical angle of about 12 degrees where the laser energy matches the energy separation between the van Hove singularities in conduction and valance bands. We conduct a systematic study of the G peak enhancement upon doping up to charge densities of 2*10$^{\mathrm{13}}$ cm$^{\mathrm{-2}}$. The G peak enhancement drops monotonously upon doping for angles smaller than the critical angle. In contrast, for angles larger than the critical angle, the G peak enhancement increases with doping at first before decreasing at higher doping levels. The charge density where the maximum enhancement occurs, scales with the rotation angle above the critical angle. This indicates that the band structure of twisted bilayer graphene is modified and the Fermi velocity is reduced upon doping. [Preview Abstract] |
Wednesday, March 4, 2015 3:42PM - 3:54PM |
Q1.00007: The strong coupling limit of twisted bilayer graphene at large twist angles Hridis Pal, Stephen Spitz, Markus Kindermann Using a recently proposed long-wavelength theory for twisted graphene bilayers near commensuration [1], we show that a strong distortion of energy bands can occur in such systems at large angles of twist. This is in contrast with the previously accepted belief that such non-trivial physics can arise only at extremely small angles of twist. At large angles, sufficiently close to commensuration, the system is driven to a non-perturbative regime (in the effective interlayer coupling), and it becomes locally gapped. We discuss the implications of this for the energy spectrum of the bilayer. [1] Theory of Twisted Bilayer Graphene Near Commensuration, Hridis K. Pal, Steven Carter, and M. Kindermann, arXiv:1409.1971. [Preview Abstract] |
Wednesday, March 4, 2015 3:54PM - 4:06PM |
Q1.00008: Twisted Graphene Nanostructures Satrio Gani, Yudistira Virgus, Enrico Rossi Recent advances in fabrication techniques have made possible the realization of graphene nanostructures with atomic precision. Some of the nanostructures realized are completely novel. We study the electronic properties of such novel graphene nanostructures when deposited on two dimensional crystals. In particular we study the case when the two dimensional crystal is graphene, or bilayer graphene. We obtain results for the nanostructure electronic spectrum and find how the spectrum is affected by the coupling between the nanostructure and the two-dimensional substrate. In particular we study how the ``twist'' angle between the graphene nanostructure and the two-dimensional crystal affects the spectrum of the nanostructure. [Preview Abstract] |
Wednesday, March 4, 2015 4:06PM - 4:18PM |
Q1.00009: The Interdependent Atomic and Electronic Structures of Graphene on Hexagonal Boron Nitride Jeil Jung, Ashley DaSilva, Allan MacDonald, Shaffique Adam Recent progress in preparing well controlled 2D van der Waals heterojunctions has opened up a new frontier in materials physics. I will address the intriguing energy gaps that are sometimes observed when a graphene (G) sheet is placed on a hexagonal boron nitride (hBN) substrate, demonstrating that they are produced by an interesting interplay between structural and electronic properties, including electronic many-body exchange interactions. Our theory is able to explain the observed gap behavior by accounting first for the structural relaxation of graphene's carbon atoms when placed on a hBN substrate and then for the influence of the substrate on low-energy $\pi$-electrons located at relaxed carbon atom sites. All three contributions of the moire pattern pseudospin Hamiltonian play a role in defining the features of the moire bands including the degeneracy of the mini-Dirac cones and the particle-hole asymmetry. We find that the effective anisotropic strains arising from virtual hopping are associated with effective magnetic fields on the order of $\sim$10 T and they dominate over the pseudomagnetic vector potentials generated by the moire strains due to partial commensuration. [Preview Abstract] |
Wednesday, March 4, 2015 4:18PM - 4:30PM |
Q1.00010: Transport Properties of Dual-Gated Bilayer Graphene/Hexagonal Boron Nitride Moir\'e Superlattices Bin Cheng, Peng Wang, Cheng Pan, Tengfei Miao, Yong Wu, Chun Ning Lau, Marc Bockrath Moir\'e superlattices of monolayer and bilayer graphene and hexagonal boron nitride (hBN) show Hofstadter's butterfly physics under an applied magnetic field [1-3]. However, such bilayer graphene systems on boron nitride with both back and top gates have not been studied yet, where the properties of the fractal Hofstadter spectrum are potentially tunable by varying the perpendicular electric field applied to the bilayer system. Using layer stacking and edge contacts [4] we fabricate such devices. We will report our latest data from these encapsulated hBN/aligned bilayer-graphene/hBN devices. \\[4pt] [1] P. A. Ponomarenko et al., Nature 497, 594-597 (2013).\\[0pt] [2] C. R. Dean, et al., Nature 497, 598-602(2013).\\[0pt] [3] B. Hunt, et al., Science 340, 1427(2013).\\[0pt] [4] L. Wang. et al., Science 342, 6158(2014) [Preview Abstract] |
Wednesday, March 4, 2015 4:30PM - 4:42PM |
Q1.00011: Electronic Structure of Graphene Moire Superlattice on Hexagonal Boron Nitride Guorui Chen, Mengqiao Sui, Jeil Jung, Yijun Yu, Wei Yang, Kenji Watanabe, Takashi Taniguchi, Guangyu Zhang, Shaffique Adam, Yuanbo Zhang The electronic structure of graphene is strongly modified by the Moire superlattice when placed on a hexagonal boron nitride (hBN) substrate at certain alignment angle. The folding of the original graphene energy bands into the superlattice Brillouin zone generates additional band degeneracy points (referred to as second generation Dirac points) apart from the original Dirac point. Here we show that the band modification by the Moire superlattice has further implications, and a new generation of Dirac points are observed at high doping levels. By systematically probing the new Dirac points at varying alignment angles, we show that the third generation Dirac points are general features of Graphene Moire superlattice on hBN. We discuss the electronic structure of graphene Moire superlattice determined from the observed degeneracy of the three generations of Dirac points. [Preview Abstract] |
Wednesday, March 4, 2015 4:42PM - 4:54PM |
Q1.00012: Valley order and loop currents in graphene on hexagonal boron nitride Bruno Uchoa, Valeri Kotov, Markus Kindermann In this letter, we examine the role of Coulomb interactions in the emergence of macroscopically ordered states in graphene supported on hexagonal boron nitride substrates. Due to incommensura- tion effects with the substrate and interactions, graphene can develop gapped low energy modes that spatially conform into a triangular superlattice of quantum rings. In the presence of these modes, we show that Coulomb interactions lead to spontaneous formation of chiral loop currents in bulk and to macroscopic spin-valley order at zero temperature. We show that this exotic state breaks time reversal symmetry and can be detected with interferometry and polar Kerr measurements. [Preview Abstract] |
Wednesday, March 4, 2015 4:54PM - 5:30PM |
Q1.00013: Cloning of Dirac fermions and hierarchy of Hofstadter states in graphene superlattices Invited Speaker: Leonid Ponomarenko Superlattices have attracted great interest because their use may make it possible to modify the spectra of two-dimensional electron systems and, ultimately, create materials with tailored electronic properties. It has recently been realised that hexagonal boron nitride can be used to creates a smooth periodic potential for Dirac electrons in graphene. This potential arises due to the lattice mismatch between the two materials and misorientation of their crystallographic axis, which result in the formation of a moir\'{e} pattern. When graphene is placed on boron nitride and subjected to a magnetic field, self-similarity appears in the form of numerous replicas of the original Dirac spectrum, and their quantization gives rise to a fractal pattern of Landau levels, referred to as the Hofstadter butterfly. We employ transport measurements and capacitance spectroscopy in magnetic field up to 30 T to probe the density of states and energy gaps in this spectrum. The Hofstadter butterfly shows up as recurring Landau fan diagrams in high fields. Electron-electron interactions lead to suppression of quantum Hall ferromagnetism, a reverse Stoner transition at commensurable fluxes and additional ferromagnetism within replica spectra. [Preview Abstract] |
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