News & Events

Speaker: Shivaji Sondhi
Affiliation: Shivaji Sondhi
Abstract Details: This blackboard talk is aimed at theoretical condensed matter physicists.  We study three-dimensional Dirac fermions with weak finite-range scalar potential disorder. We show that even though disorder is perturbatively irrelevant at 3D Dirac points, nonperturbative effects from rare regions give rise to a nonzero density of states and a finite mean free path, with the transport at the Dirac point being dominated by hopping between rare regions. As one moves in chemical potential away from the Dirac point, there are interesting intermediate-energy regimes where the rare regions produce scattering resonances that determine the DC conductivity. We also discuss the interplay of disorder with interactions at the Dirac point. Attractive interactions drive a transition into a granular superconductor, with a critical temperature that depends strongly on the disorder distribution. In the presence of Coulomb repulsion and weak retarded attraction, the system can be a Bose glass. Our results apply to all 3D systems with Dirac points, including Weyl semimetals.
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Speaker: Nuno M. R. Peres
Affiliation: University of Minho, Portugal
Abstract Details: We discuss how to induce surface plasmon-polaritons in graphene aimed at increasing the radiation absorption of the material.
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About the Speaker:

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Speaker: Yshai Avishai
Affiliation: Ben-Gurion University, Israel
Abstract Details: It was shown by Dirac about 80 years ago that if there is a magnetic charge g leading to a central magnetic field B = g r/r^3, then g must be quantized as 2eg = n c h/(2pi) (n = 0, 1, 2 . . . is the monopole number). The corresponding ”hydrogen atom problem” (a spinless electron in the field of a magnetic charge) was solved by Igor Tamm just a few months after Dirac’s paper. Here I approach this problem from a ”condensed matter point of view” using a tight binding model. The motivation is threefold: First, the physics is rather beautiful and involves interesting relations with spherical geometry and the theory of graphs. Second, the notion of magnetic monopole is quite relevant in condensed matter physics. Among others, it serves as a useful tool for constructing translation invariant many electron wave functions in the FQHE (such as Laughlin’s and Moore Read’s N electron wave functions). Third, I will show that under some conditions, this seemingly inaccessible system can be mapped on a realistic physical system. When the sites upon which the electron resides and hops form a highly symmetric object, the energy spectrum is calculated analytically as function of n and displays a beautiful pattern, which is entirely distinct from that of the Hofstadter butterfly. The systematics of level degeneracy is unusual and poses some challenges to the theory of point symmetry groups. The spectrum of an electron hopping on the sites of a Fullerene reveals a set of magic (monopole) numbers ni . Within the same tight-binding geometry, this problem is now confronted with that of a spin-full electron subject to an electric field of a point charge E = q r/r^3 which is responsible for Rashba type spin-orbit interaction. The spectrum is calculated analytically as function of the (dimensionless) spin-orbit strength and displays rich and beautiful pattern with some unexpected symmetries in which physics and geometry interlace. I then expose a remarkable relation between the two distinct physical problems: The energy spectrum in the second system at a certain symmetry point is identical with the energy spectrum in the first system at monopole number n = 1. Thus, it is principally possible to test the physics of an experimentally inaccessible system (electron in the field of magnetic monopole) in terms of an experimentally accessible one (an electron subject to spin-orbit force induced by central electric field).
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Speaker: Young-Woo Son
Affiliation: KIAS, Seoul, Korea
Abstract Details: Interactions between graphene systems and applied forces or inserted molecules often result in very interesting consequences that are sometimes useful as well. In this talk, I will present a couple of theoretical works exploring those aspects in various graphene systems. First, I will discuss an ideal strength of graphene as function of doping [1]. There have been several works on variations of electronic structures in graphene systems with external forces. However, the effect of variation of electronic degree of freedom on their mechanical properties has not been explored well. It is shown that the ideal mechanical properties of graphene are strong functions of doping due to distinct electron-phonon interactions in graphene systems. Second, I will also discuss another aspect of realization of interesting electron-phonon interactions in bilayer graphene in spectroscopic measurements [2]. When two layers in bilayer graphene slide each other, interlayer electronic interactions couple to intralayer phonon degrees of freedom that changes Raman and IR spectroscopic signals. Third, if time allowed, I will present a possible explanation on a recent astonishing experiment showing the unimpeded water permeation through graphene oxide membrane [3]. It is shown that the interlayer distance and ice formation are important factors for perfect water penetrations and for complete blocking for other gases and liquids. [1] S. Woo and Y.-W. Son, Phys. Rev. B 87, 075419 (2013). [2] S.-M. Choi, S.-H. Jhi and Y.-W. Son, ACS Nano, in press (2013) [3] D. W. Boukhvalov, M. I. Katsnelson and Y.-W. Son, Nano Lett. in press (2013)
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Speaker: J. Scott Bunch
Affiliation: Boston University
Abstract Details: Graphene, a single layer of graphite, represents the first truly two dimensional atomic crystal. It consists solely of carbon atoms covalently bonded in a hexagonal chicken wire lattice. This unique atomic structure gives it remarkable electrical, mechanical, and thermal properties. It has the highest electrical and thermal conductivity among all materials known. However, it is the mechanical properties of this wonder material that fascinate our group the most. It is the thinnest, stiffest, and strongest material in the world as well as being impermeable to all standard gases. In this talk, I will review some of our recent experimental results on graphene adhesion, atomically thin semipermeable membranes, and the mechanical properties of a new class of ultrathin (~1 nm) oxide membranes. Speaker's Bio: Scott Bunch is currently an Assistant Professor of Mechanical Engineering and a faculty member in the Materials Science and Engineering Program at Boston University and Assistant Professor of Mechanical Engineering and a Fellow of the Materials Science and Engineering Program at University of Colorado at Boulder. He is primarily interested in the mechanical properties of atomically thin materials such as graphene. He received his B.S. degree in Physics from Florida International University (2000) and a Ph.D. in Physics (2008) from Cornell University where he studied the electrical and mechanical properties of graphene. After finishing his Ph.D., he spent 3 months as a postdoctoral researcher in the Laboratory of Atomic and Solid State Physics at Cornell University studying nanoelectromechanical systems. His awards include a Ph.D. fellowship from Lucent Technologies, Bell Laboratories (2000-2004), the DARPA MTO Young Faculty Award (2008), and the NSF CAREER Award (2011).
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Speaker: J. N. B. Rodrigues
Abstract Details: Recently, Tsen et al. [1] demonstrated how one can probe the electronic transport properties of a single grain boundary in graphene. Following this remarkable possibility, in this seminar we will investigate the electronic transport across periodic defect lines. In the continuum low-energy limit, such defects act as infinitesimally thin stripes separating two regions where the Dirac Hamiltonian governs the low-energy phenomena. The behavior of these systems is controlled by the boundary condition imposed by the defect on the massless Dirac fermions. We will demonstrate how this low-energy boundary condition can be computed from the tight-binding model of the defect line. We will illustrate this procedure by considering a simple zigzag oriented defect line solely composed by pentagonal carbon rings: the pentagon-only defect line. The recently observed zz(558) defect line [2], as well as the zz(5757) defect line will also be considered [3,4]. [1] A. W. Tsen, L. Brown, M. P. Levendorf, F. Ghahari, P. Y. Huang, R. W. Havener, C. S. Ruiz-Vargas, D. A. Muller, P. Kim and J. Park, Science 336, 1143 (2012). [2] J. Lahiri, Y. Lin, P. Bozkurt, I. I. Oleynik and M. Batzill, Nature Nanotechnology 5, 326 (2010). [3] J. N. B. Rodrigues, N. M. R. Peres and J. M. B. Lopes dos Santos, PRB 86, 214206 (2012). [4] J. N. B. Rodrigues, N. M. R. Peres and J. M. B. Lopes dos Santos, JPCM 25, 075303 (2013).
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Speaker: David Tomanek
Affiliation: Michigan State University, USA
Abstract Details: Significant advances in Materials Science have been achieved by harnessing specific functionalities of nanostructures, such as improved mechanical, electrical and thermal properties, for particular applications. Predictive ab initio calculations suggest that designer nanostructures, such as schwarzites[1] and related foam structures[2] of carbon, may combine low gravimetric density with high stiffness and favorable electrical as well as thermal conductivity. Twisted carbon nanotube ropes [3] may store reversibly much more energy than other energy storage devices [4]. Unusual charge and thermal transport properties can be expected in peapods consisting of doped fullerenes or diamondoids enclosed in a carbon nanotube [5]. Unusual nanostructures such as Se double helices [6] or metallic sulfur chains [7] form inside the void of carbon nanotubes. Successful synthesis of such nanostructures precludes detailed understanding of their microscopic formation mechanism. Since direct observation of such atomic-scale processes is very hard by experimental means, computer simulations are a welcome alternative to gain microscopic insight into the underlying processes. [1] S. Park, K. Kittimanapun, J.-S. Ahn, Y.-K. Kwon and D. Tománek, J. Phys.: Condens. Matter 22, 334220 (2010). [2] Z. Zhu and D. Tománek, Phys. Rev. Lett. 109, 135501 (2012). [3] David Teich, Gotthard Seifert, Sumio Iijima, and David Tománek, Phys. Rev. Lett. 108, 235501 (2012). [4] David Teich, Zacharias G. Fthenakis, Gotthard Seifert, and David Tománek, Phys. Rev. Lett. 109, 255501 (2012). [5] Jinying Zhang, Zhen Zhu, Yanquan Feng, Hitoshi Ishiwata, Yasumitsu Miyata, Ryo Kitaura, Jeremy E. P. Dahl, Robert M. K. Carlson, Natalie A. Fokina, Peter R. Schreiner, David Tománek, and Hisanori Shinohara, Angew. Chem. Int. Ed. 52, 3717 (2013). [6] Toshihiko Fujimori, Renato Batista dos Santos, Takuya Hayashi, Morinobu Endo, Katsumi Kaneko, and David Tománek, ACS Nano 7, 5607 (2013). [7] Toshihiko Fujimori, Aarón Morelos-Gómez, Zhen Zhu, Hiroyuki Muramatsu, Ryusuke Futamura, Koki Urita, Mauricio Terrones, Takuya Hayashi, Morinobu Endo, Sang Young Hong, Young Chul Choi, David Tománek and Katsumi Kaneko, Nature Commun. (2013).
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Speaker: Fengnian Xia
Affiliation: IBM T. J. Watson Research Center, USA
Abstract Details: Graphene has been intensively explored by physicists and engineers due to its unique electronic and photonic properties. In this talk, I will first address the physics of light-graphene interaction within the single-electron framework, followed by a discussion of light excitation of collective oscillations of the carriers, i.e., plasmons in graphene. I will then cover a variety of photonic devices based on these two mechanisms of light-graphene interaction, such as high-bandwidth photodetectors, optical modulators, electro-magnetic wave shielding, optical filters, and linear polarizers. Finally I will talk about the carrier transport at metal-graphene junction, bandgap engineering in graphene, and their possible impacts on electronics based on other twodimensional materials such molybdenum disulfide.
About the Speaker: Fengnian Xia received the B.Eng. degree with the highest honor in electronics engineering from Tsinghua University, Beijing, China, in 1998 and M.A. and Ph.D. degrees in electrical engineering from Princeton University, Princeton, NJ, USA in 2001 and 2005, respectively. He joined IBM Thomas J. Watson research center in Yorktown Heights, NY, USA as a postdoc in March 2005, and currently is a Research Staff Member. His current research focuses on nanophotonics and nanoelectronics using both conventional and emerging materials such as graphene, carbon nanotubes, silicon, germanium, III-Vs, and various combinations of them. He is also actively involved in carrier transport research in low-dimensional systems. Dr. Xia received an IBM corporate award, three IBM research division level awards, and many IBM invention achievement awards. In 2011, he was selected by MIT technology review magazine as a top young innovator under the age of 35.
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About the Speaker: A group of delegates attending ICMAT 2013 will be visiting GRC facilities on Wednesday, 3 July. We warmly invite you to attend the lunch hosted for these delegates. This will be a good time for you to interact and mingle with them for more discussions.
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