News & Events

Speaker: Prof Qian Niu
Affiliation: University of Texas, Austin/Peking University
Abstract Details: In this talk Prof Niu will discuss how to apply external conditions, such as Rashba and Zeeman coupling as well as interlayer bias, to induce bulk energy gaps at the Dirac points, and show what kind of topological phases can appear in the resulting insulator.
About the Speaker: Qian Niu is a Trull Centennial Professor of Physics at The University of Texas at Austin, from which he is currently on leave to serve as the director of the International Center for Quantum Materials at Peking University. He has worked on the theories on quantum Hall effects, quasicrystals, ultracold atoms, spin transport, and graphene materials, with an emphasis on topological and geometric phase effects in quantum transport. He obtained a B.S. from Peking University and a Ph.D. from the University of Washington at Seattle, and did postdoctoral work at the University of Illinois at Urbana-Champaign and the University of California at Santa Barbara before joining the faculty of UT Austin in 1990.
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Speaker: Irina V. Grigorieva
Affiliation: University of Manchester, UK
Abstract Details: The possibility to induce magnetic response in graphene by introduction of defects has been generating much interest, as this would expand the already impressive list of its special properties and allow novel devices where charge and spin manipulation could be combined. In this talk I will review our recent experiments where we show that point defects in graphene – (i) fluorine adatoms in concentrations x gradually increasing to stoichiometric fluorographene CF_{x-1.0} and (ii) irradiation defects (vacancies) – carry magnetic moments with spin 1/2. Both types of defects lead to notable paramagnetism but no magnetic ordering could be detected down to liquid helium temperatures. The induced paramagnetism dominates graphene’s low-temperature magnetic properties, despite the fact that maximum response we could achieve was limited to one moment per approximately 1000 carbon atoms. This limitation is explained by clustering of adatoms and, for the case of vacancies, by the requirement to keep graphene’s structural stability. Our work clarifies the controversial issue of graphene’s magnetism and sets limits for other graphitic compounds.
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Speaker: Michael Fogler
Affiliation: University of California San Diego
Abstract Details: Graphene is a novel plasmonic medium whose electronic and optical properties can be conveniently controlled by electrostatic gates. Near-field nano-imaging shows that at technologically relevant infrared frequencies common graphene/Si oxide/Si back-gated structures support surface plasmons with wavelength of the order of 200 nm and the propagation length several times this distance. Such plasmons represent concentration of electromagnetic energy on the spatial scale two orders of magnitude smaller than the photon wavelength. Both the amplitude and the wavelength of the plasmons are shown to be tunable by the gate voltage. Plasmon standing waves arise when plasmons launched by a sharp tip of a scanned probe interfere with their reflection off sample edges and inhomogeneities. These interference patterns are shown to depend on the location of the tip and the shape of the sample. Theoretical modeling provides quantitatively accurate description of the plasmonic interference patterns. Plasmonic dispersion and damping, extracted from the spatial decay of the interference fringes sheds light on the exotic electrodynamics of Dirac quasiparticles in graphene. Reference: Z. Fei et al, Nature 487, 82 (2012)
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Speaker: Andre Geim
Affiliation: University of Manchester
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Speaker: Jong-Hyun Ahn
Affiliation: SKKU Advanced Institute of Nanotechnology, Korea
Abstract Details: The outstanding physical and chemical properties of graphene, a single atomic layer of carbon atoms, have attracted significant attention. One of the most important advantages of graphene in the field of electronics is its superb charge carrier mobility. The mobility of ideal exfoliated graphene spans an extraordinarily large range, from 10,000-15,000 cm2/V·s on SiO2 insulating substrates to 200,000 cm2/V·s in suspended structures, suggesting that graphene may potentially outperform established inorganic materials in certain applications, such as high-frequency transistors. Although useful devices have been prepared based on exfoliated graphene, the tiny size of exfoliated graphene particles limits the practical utility of such graphene in electronics applications. Recent studies designed to address this issue have explored the preparation of large-area high-quality graphene via epitaxial growth or chemical vapor deposition (CVD). Many research groups have reported the fabrication of graphene-based transistors via the epitaxial growth of graphene directly on rigid insulating silicon carbide (SiC) wafers. These transistors operate at high frequencies, up to 100 GHz. Other researchers have synthesized graphene on Ni or Cu catalysts using CVD methods and have demonstrated the utility of device integration on a variety of substrates using transfer techniques. The CVD approach is attractive because it permits fabrication over large areas and expands the applicability of graphene to flexible or fully stretchable devices on thin plastic or elastomeric substrates. In this talk, I will discuss recent progress in graphene film preparation and its various applications in the field of electronics, focusing on techniques to integrate them into devices on compliant substrates. Although significant engineering challenges including band gap opening and improved reliability still exist, it could create interesting opportunities for developing future electronic applications because many basic aspects of technically feasible approaches are now emerging.
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Speaker: Warren E. Picket
Affiliation: UCDavis, USA
Abstract Details: The subjects of graphene science, oxide nanostructures, and the topological nature of materials comprise three of the most active topics in materials physics at this time. After working on oxide polar interfaces (LAO/STO) for some time, we asked whether non-polar interfaces between oxides held the possibility for surprises. Studying VO2/TiO2 multilayers, we discovered computationally a new type of 2D point 'Fermi surface' system: linear (Weyl) dispersion in one direction, quadratic (Dirac) dispersion in the perpendicular direction. Such a system, which we dubbed semi-Dirac for simplicity, lies midway between graphene and conventional zero-gap semiconductors in many respects. Unlike both, however, it displays extreme anisotropy in its electronic structure and and low-energy propeties. Alas, unlike the poit of essence in graphene, this point is non-topological. Our studies will be reviewed in this talk.
About the Speaker: Warren Pickett obtained his B.S. (Physics, Math) and M.S. (Physics) degrees at Wichita State University before going to SUNY-Stony Brook, New York to complete his PhD in theoretical condensed matter physics in 1975. He had postdoctoral research appointments at the University of Bristol, England, at UC Berkeley, and at Northwestern University, where his research centered on the electronic structure of crystalline metals and semiconductors, on semiconductor interfaces, and on superconductivity. In 1979 he accepted a research scientist position at the Naval Research Laboratory in Washington DC. Early in his years at NRL, Warren increased his activity in large scale calculations of the properties of condensed phases, using the NRL ASC supercomputer and early Cray machines (Boeing) to carry out a study of, for example, the heavy fermion superconductor UBe13, which has 28 atoms in the unit cell. He was elected Fellow of the American Physical Society in 1989, and shared 2nd Prize in the IBM Supercomputing Competition in 1990. At NRL Pickett was awarded the E. O. Hulburt Award in 1990 and the Sigma Xi Technical Achievement Award in Pure Science in 1993. He was appointed Senior Scientist at NRL in 1992. He received Alan Berman Research Publication Awards at NRL in 1983, 1998,1989, and 1992. In 1997 Warren took a professorship in physics at UC Davis, where he is a participant in an interdisciplinary program in nanophases as well as working in condensed matter physics research programs. Warren Pickett has served on the editorial board of the Journal of Superconductivity since its foundation in 1989, and on the editorial board of Chemical Design Automation News from 1993-1998. He was a member of the Council of the American Physical Society from 1996-1999. In 2001 he became Editor of the Journal of the Physics and Chemistry of Solids, published by Elsevier Press. His research focuses on moving toward a first principles description of the properties of complex crystalline materials, and he has begun collaborations with solid state chemists and chemical engineers as well as solid state theorists and experimentalists. Recently his interest in materials has expanded to include magnetic materials that become superconducting, ferroelectric insulators that become ferromagnetic, and more generally novel magnetic and superconducting compounds that are of interest to both condensed matter physicists, solid state chemists, and materials scientists. He has over 275 publications in the primary scientific literature, and has active collaborations in Europe, Asia, South America, and across the USA.
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Speaker: Tomás Palacios
Affiliation: MIT, USA
Abstract Details: Electrical engineering is at a crossroads. For the last fifty years, semiconductors have been driving the development of information technology, which has completely transformed our society. Conventional electronics, however, is reaching scaling and performance limits which jeopardizes future developments. New materials with unique properties are necessary and graphene, a one atom thick layer of sp2 bonded carbon, is at the top of potential candidates. Graphene not only has outstanding transport properties, but it also shows many unique properties not found in any other high performance electronic material. It is flexible, transparent, ultimately scalable, easily transferable to any surface, and its ambipolar conduction offers new possibilities for advanced electronics. In this talk, we describe how the use of these properties allows the development of new devices, which can overcome some of the main limitations of traditional electronics in terms of sensitivity, maximum frequency, and linearity. Several novel devices will be discussed for RF communications and remote sensing, including graphene frequency multipliers, graphene RF mixers and graphene chemical sensors.
About the Speaker: Prof. Palacios studied Telecommunication Engineering at the Universidad Politécnica de Madrid and received his PhD from the University of California – Santa Barbara in 2006. Since that year, he leads the Advanced Semiconductor Materials and Devices Group at the Massachusetts Institute of Technology. His research focuses on the development of new electronic devices based on the combination of new materials (GaN and graphene) and nanotechnologies. Prof. Palacios’ work has been recognized with several awards including the 2011 Presidential Early Career Award for Scientists and Engineers (PECASE), the 2010 Young Scientist Award of the International Symposium on Compound Semiconductors (ISCS), the 2009 ONR Young Investigator Award, the 2009 NSF CAREER Award, the 2008 DARPA Young Faculty Award, as well as multiple best papers awards. He has authored more than 200 contributions on advanced semiconductor devices in international journals and conferences, 40 of them invited, and several book chapters and patents. Since 2006, Prof. Palacios has been leading the development of new applications for graphene-based materials. In 2010, he founded the MIT Center for Graphene Devices and Systems, with the goal to coordinate the graphene work at MIT. He is also the co-founder of the IEEE MTT Nanotechnology Committee and the MTT Distinguished Microwave Lecturer on graphene nanotechnology.
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Speaker: Ricardo W. Nunes
Affiliation: UFMG, Brazil
Abstract Details: Recent theoretical studies (D. Malko et al., PRL 108, 086804, 2012) have revealed the existence of Dirac-cone electronic spectra in a family of carbon materials known as graphynes. Some of these graphynes share with graphene the underlying hexagonal Bravais lattice but not the honeycomb structure. In our study, we concentrate on the so-called alpha-graphynes, which have a 2D crystal structure where a pair of threefold-coordinated carbon atoms, placed on a honeycomb geometry, are connected through chains of twofold coordinated carbons. We show that such geometries can be used to create hydrocarbons with a Dirac-cone electronic structure near the Fermi level. Furthermore, we propose the existence of low-energy stoichiometric antiphase boundaries (APB - an extended 1D defect) in boron nitride. The periodic unit for this 1D defect consists of a tetragon-octagon pair (a 4-8 unit), which is the armchair version of a zigzag-oriented 1D extended defect, with a periodic unit consisting of two pentagons and an octagon (a 5-5-8 unit), that has been recently observed to form in graphene grown on Ni substrates (J. Lahiri et al., Nature Nanotech. 5, 326, 2010). In our study, we show that in BN the 4-8 unit APB has lower formation than the 5-5-8 defect in most growth conditions, and shows the presence of shallow aceptor and donor defect states.
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Speaker: Hsin Lin
Affiliation: Department of Physics, Northeastern University, USA
Abstract Details: Recent development of topological band theory combined with the predictive power of first-principles calculations has led to discovery of new states of quantum matter as well as their material realization. These topological materials are distinguished by unique physical properties. Due to the time-reversal symmetry, the topological insulators host back-scattering-free spin-polarized surface states that enables viable applications in spintronics. Topological superconductors offer the opportunity to observe Majorana fermions which is a first step towards fault-tolerant quantum computation. Topological materials are interesting from both a fundamental physics and a practical applications point of view. While several families of topological insulators have already been found, the intense world-wide search for new classes of topological materials continues unabated. This interest is driven by the need for materials with greater structural flexibility and tunability to enable viable applications in spintronics and quantum computing. We have used first-principles band theory computations in combination with angle-resolved photoemission experiments to successfully predict many new classes of topologically interesting materials, including Bi2Se3 series, the ternary half-Heusler compounds, thallium-based chalcogenides, Li2AgSb-class, GenBi2mTe3m+n families, quaternary chalcogenides and famatinites as well as the first material realization of topological crystalline insulator in (Pb,Sn)Te class.[1-3] The topological phase transition of silicene and TlBi(Se,S)2 and the properties of the surface states will be demonstrated. [4,5] [1] Y. Xia, D. Qian, D. Hsieh, L.Wray, A. Pal, H. Lin, A. Bansil, D. Grauer, Y. S. Hor, R. J. Cava, and M. Z. Hasan, Nature Physics 5, 398 (2009). [2] H. Lin, L. A. Wray, Y. Xia. S. Y. Xu, S. Jia, R. J. Cava, A. Bansil, and M. Z. Hasan, Nature Materials 9, 546 (2010). [3] H. Lin, R. S. Markiewicz, L. A. Wray, L. Fu, M. Z. Hasan, Physical Review Letters 105, 036404 (2010). [4] S. Y. Xu, Y. Xia, L. A. Wray, S. Jia, F. Meier, J. H. Dil, J. Osterwalder, B. Slomski, A. Bansil, H. Lin, R. J. Cava, and M. Z. Hasan, Science 332, 560 (2011). [5] S. Y. Xu, M. Neupane, Chang Liu, L. A. Wray, Duming M. Zhang, A. Richardella, Nasser Alidoust, M. Leandersson, T. Balasubramanian, J. Sanchez-Barriga, O. Rader, G. Landolt, Bartosz Slomski, J. Dil, Tay-Rong Chang, Horng-Tay Jeng, J. Osterwalder , H. Lin, A. Bansil, N. Samarth, M. Z. Hasan, Nature Physics, in press (2012).
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Speaker: Oki Gunawan
Affiliation: IBM T. J. Watson Research Center, USA
Abstract Details: Photovoltaics (PV) technology is one of the leading candidates for large scale and renewable energy source in the near future. At IBM, we recently developed kesterite Cu2ZnSn(Se,S)4 (CZTSSe)-based solar cell, an emerging thin film PV technology unrestrained by material availability or toxicity issues suffered by other leading technologies such as CuInGaSe (CIGS) and CdTe. We recently demonstrated a CZTSSe cell with world record efficiency of 10.1% and performed various electrical characterization techniques to identify the key performance bottlenecks. I will draw various comparisons between CZTSSe and the well established CIGS technology to gain insights to the current performance limitations in CZTSSe. These findings help to identify key areas of improvements to realize a high performance, tera (1012) watt scale CZTSSe PV technology in the near future.
About the Speaker: Oki Gunawan, received PhD from Dept. of Electrical Engineering, Princeton University and M. Eng and B. Eng from Nanyang Technological University also in Electrical Engineering. He is currently a Research Staff Member in Photovoltaics Science and Technology, at IBM Thomas J Watson Research Center, Yorktown Heights NY. His research activities encompass several areas like optoelectronics, valley-based electronics or “valleytronics”, nanowire CMOS and solar cell, solar cell characterization and currently CZTSSe thin film solar cell. At IBM, he led efforts to develop IBM-PVX system, a new software system for advanced solar cell characterization and he is in charge of electrical device characterization and diagnostics activities. His works have been published, among others in, Nature Physics, Physical Review Letters, Physical Review B, Nano Letters, Applied Physics Letter and Progress in Photovoltaics.
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