News & Events

Speaker: Andrivo Rusydi
Affiliation: NUS, Department of Physics
Abstract Details: Graphene and two-dimensional systems in general are believed to be a promising candidate for future electronic devices, because of its extremely high electronic mobility compared to semiconductors used in most electronics today. Despite this outstanding characteristics, a pure graphene material is gapless, thus cannot yet be used to function as a transistor or a diode. One way to generate a gap, and possibly add magnetic properties to graphene is through adsorption of certain atoms (adatoms). Along this way, however, one may need to characterize both transport and magnetic properties in separate challenging experiments using different instruments. Here, we present our theoretical study that suggests a way to interpret simultaneously both transport and magnetic properties of graphene with adatoms solely through optical conductivity measurement. The key idea is that there is an intimate connection between the low- and the high-energy behavior of the optical conductivity, from which we can deduce whether the system is gapped or gapless, whether or not the adatoms are magnetic, and what sort of magnetic ordering the adatoms may form.
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Speaker: Steven G. Louie
Affiliation: University of California at Berkeley and Lawrence Berkeley National Laboratory
Abstract Details: Experimental and theoretical studies of atomically thin quasi two-dimensional materials (typically related to some parent van der Waals layered crystals) and their nanostructures have revealed that these systems can exhibit highly unusual behaviors. In this talk, we discuss some theoretical studies of the electronic, transport and optical properties of such systems. We present results on graphene and graphene nanostructures as well as other quasi 2D systems such as monolayer or few-layer transition metal dichalcogenides (e.g., MoS2, MoSe2, WS2, and WSe2). Owing to their reduced dimensionality, these systems present opportunities for unusual manifestation of concepts/phenomena that may not be so prominent or have not been seen in bulk materials. Symmetry and many-body interaction effects often play a critical role in shaping qualitatively and quantitatively their properties. Several phenomena are discussed, exploring their physical origin and comparing theoretical predictions with experimental data.
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Speaker: Yuanbo Zhang
Affiliation: Fudan University
Abstract Details: Two-dimensional crystals have emerged as a new class of materials that may impact future science and technology. Graphene is one of the few examples that show great potential. In this talk, I will first discuss the physics and material aspect of graphene. Drawing from our experiences in graphene study, I will then discuss other 2D materials, including black phosphorus thin film – a new material with excellent semiconductor properties.
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Abstract Details: See abstract details
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Speaker: Francisco (Paco) Guinea (ICMM-CSIC, Spain)
Abstract Details: Graphene is an extremely thin crystalline membrane, which also makes it perhaps the most anharmonic material in nature. Recent experiments and models are reviewed exploring the consequences of this anharmonicity, with emphasis on the dependence of graphene's stiffness on its environment. In addition, the role of in plane strains in determining graphene's electronic mobility will be discussed, studied by combining new experimental results and models for the deformations of graphene in different setups.
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Speaker: Stephen J. Pennycook
Affiliation: University of Tennessee
Abstract Details: n Feynman's famous 1959 lecture "There's Plenty of Room at the Bottom" he challenged us to improve the electron microscope 100 times, so we could just look at the thing. Are we there yet? With the spectacular advances in aberration correction of the last decade, we have improved image resolution to well below 1Å and gained sensitivity to light atoms in both imaging and spectroscopy. But today's microscope is only 20 times better than in Feynman's time – and so we remain far from fulfilling his dream. We cannot just look at point defects and determine their three-dimensional configuration, or their diffusion pathways. Our lateral resolution still far exceeds the fundamental limit set by thermal vibrations, and our depth resolution remains in the nanometer range. In this talk I will show examples where further improvements in resolution would indeed enable us to just look at the thing.
About the Speaker: Stephen J. Pennycook is a Professor in the Dept. of Materials Science and Engineering, University of Tennessee and the Dept. of Materials Science and Engineering, North Carolina State University. Before his retirement from Oak Ridge National Laboratory in December 2013 he was Corporate Fellow in the Materials Science and Technology Division and leader of the Scanning Transmission Electron Microscopy Group. He received his PhD in physics from the Cavendish Laboratory, University of Cambridge in 1978, joining Oak Ridge National Laboratory in 1982. Pennycook is a Fellow of the American Physical Society, the American Association for the Advancement of Science, the Microscopy Society of America, the Institute of Physics and the Materials Research Society, and is recipient of the Microbeam Analysis Society Heinrich Award, the Materials Research Society Medal, the Institute of Physics Thomas J. Young Medal and Award and the Materials Research Society Innovation in Characterization Award. He has 38 books and book chapters, over 400 publications in refereed journals and has given over 200 invited presentations on the development and application of atomic resolution Z-contrast microscopy and electron energy loss spectroscopy.
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Speaker: Marcin Mucha-Kruczynski
Affiliation: Bath University, UK
Abstract Details: The heterostructures of graphene with other hexagonal layered crystals or crystals with hexagonal symmetry facets feature moiré patterns which are the result of incommensurability of the periods of the two two-dimensional lattices, or their misalignment. Using the already realised example of highly aligned graphene/hexagonal boron nitride heterostructures, I will investigate the influence of moiré potential on graphene electrons and formation of the electronic minibands. I will also discuss how competition between the quantising effects of the superlattice potential and magnetic field influences electron states resulting in a complex fractal spectrum.
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Speaker: Simon Bending
Affiliation: Bath University, UK
Abstract Details: The isolation of graphene in 2004 has led to a dramatic renewal of interest in van der Waals bonded transition metal dichalcogenides. These exhibit a wide range of electronic ground states, e.g., superconducting, metallic, charge density wave and semiconducting, which can be tuned by varying the number of molecular layers and electrostatic doping. We describe systematic investigations of superconductivity in few molecular layer NbSe2 field effect transistors. All superconducting devices show multiple resistive transitions which we attribute to disorder in the layer stacking. The conductivity in the normal state and Tc both decrease as the electron concentration is increased with a back gate, consistent with a reduction in the density of states that is predicted theoretically. The magnetic field dependence of different resistive transitions allows values of the zero temperature upper critical field, Hc2(0), and coherence length, xi(0), to be independently estimated and compared. Results are interpreted in terms of available theoretical models.
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Speaker: Enrico Da Como
Affiliation: Bath University, UK
Abstract Details: In this communication, I will talk about how we determine the intrinsic carrier recombination time in exfoliated few-layer samples using femtosecond near-infrared (0.8-0.35 eV photon-energy) pump-probe spectroscopy. The spectra of the different single-flakes (1-, 2-, 3- and 4- layers) show an evolving structure of photoinduced absorption bands superimposed on the bleaching caused by Pauli blocking of the interband optically coupled states. Supported by tight-binding model calculations of the electronic structure, we assign the photoinduced absorption features to inter-subband transitions as the number of layers is increased. Interestingly, the inter-subband photoinduced resonances show a longer dynamics than the interband bleaching, because of their independence from the absolute energy of the carriers with respect to the Dirac point. The dynamic of these inter-subband transitions depends only on the intrinsic carrier lifetime and provides an elegant method to access it in this important class of carbon nanostructures. We report lifetimes from 7 to 5 ps almost independently from the layer number up 4-layer graphene. Above this number of layers the carrier recombination time sets below 4 ps and is identical to what we observe in graphite."
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Speaker: Howard Lee
Affiliation: Caltech, USA
Abstract Details: Down-scaling optics to sub-wavelength dimension is one of the key challenges for developing optical nanocircuits and nanodevices. Plasmonics, the sub-wavelength surface electromagnetic waves that are guided on a metal-dielectric interface, enable a promising approach for achieving such down-scaling due to its extreme light confinement. However, current plasmonic devices encounter significant limitations due to high optical losses and the lack of efficient tunability and functionality. In this talk, I will present two examples of plasmonic structures which can overcome these limits: (1) Gate tunable chip-based active plasmonic nanocircuits, and (2) Photonic crystal fiber-based hybrid plasmon waveguides. I will first present the use of gate-tunable low-loss active materials, transparent conducting oxides, to demonstrate an efficient on-chip nano-scale plasmonic modulator operating via field-effect dynamics. In addition, I will present a plasmonic coherent resonant system used to engineer optical dispersion and to serve as an ultra-compact resonator, color router, and logical device. I will then discuss the integration of plasmonics and “holey” fiber optics for the development of a new class of hybrid plasmonic/photonic waveguides. Such hybrid fibers provide a promising novel platform with controllable optical dispersion and long interaction length for the investigation of plasmonic optical properties and the realization of novel in-fiber devices. These studies open up new directions for enhancing nano-scale light-matter interactions and implementing future nanophotonic communication chips, controllable metamaterials, and hybrid optical fiber systems.
About the Speaker: Dr. Howard Lee is a Postdoctoral Fellow at the California Institute of Technology in Prof. Harry Atwater's group in the Applied Physics Department. He received his PhD in Physics from the Max Planck Institute for the Science of Light in Germany in 2012 and BSc in Applied Physics from the City University of Hong Kong in 2007. His research focuses on developing new techniques in nanophotonics and plasmonics, including developing novel materials and nanostructuring to control the interaction between light and matter at the nanometer length-scale, and to study emergent optical phenomena of novel nanophotonic devices. Dr. Lee is the recipient of the Croucher Postdoctoral Fellowship and his articles have been published in various journals, including Optics Letters, Optics Express, Applied Physics Letters, Advanced Materials, Nano Letters and Science.
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