News & Events

Speaker: Rodney S. Ruoff
Affiliation: Ulsan Center for Multidimensional Carbon Materials, Korea
Abstract Details: We have focused recently on preparing large area Cu foils with (111) orientation by converting as- received polycrystalline Cu foils. The large area Cu (111) foils have been used to prepare large area Cu/Ni alloy (111) foils, and also foils involving other elements such that the foil surface lattice has a close lattice match to graphene and to hexagonal boron nitride (h-BN). (i) Our use of these foils to enable the growth of large area single crystal single-layer graphene and bilayer graphene will be presented. (ii) A brief update on work on attempting to make diamane from AB stacked bilayer (and trilayer and multilayer) graphene by either fluorination or hydrogenation will be given. (iii) We have recently been making polymers that can be converted to diamond and/or carbon rich sp3-bonded carbon materials, and this will be briefly presented. This work was supported by IBS-R019-D1.
About the Speaker: Rodney S. Ruoff, Distinguished Professor, UNIST Department of Chemistry and the School of Materials Science and Engineering, is director of the Center for Multidimensional Carbon Materials (CMCM), an IBS Center located at the Ulsan National Institute of Science and Technology (UNIST) campus. Prior to joining UNIST he was the Cockrell Family Regents Endowed Chair Professor at the University of Texas at Austin from September, 2007. He earned his Ph.D. in Chemical Physics from the University of Illinois-Urbana in 1988, and he was a Fulbright Fellow in 1988-89 at the Max Planck Institute für Strömungsforschung in Göttingen, Germany. He was at Northwestern University from January 2000 to August 2007, where he was the John Evans Professor of Nanoengineering and director of NU’s Biologically Inspired Materials Institute. He has co-authored over 430 peer-reviewed publications related to chemistry, physics, materials science, mechanics, and biomedical science, and is a Fellow of the Materials Research Society, the American Physical Society, and the American Association for the Advancement of Science. He was awarded the 2014 MRS Turnbull Award."
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Speaker: Thomas G. Pedersen
Affiliation: Aalborg University, Denmark
Abstract Details: In photodetectors and solar cells, optically generated excitons must be ionized to separate electrons and holes. If the excitons are strongly bound, thermal ionization is inefficient. A strong electric field, however, can greatly enhance the ionization rate. We study this process theoretically for mono- and multilayer transition-metal dichalcogenides within a modified Wannier exciton model. The effects of dimensionality and screening on the exciton binding energy are discussed. In the presence of a strong electric field, the exciton energies become complex resonances. We extract the ionization (tunnelling) rate using two complementary approaches: complex scaling and hypergeometric resummation [1]. By applying these techniques to Mo and W based compounds we compute the field- dependence of the ionization rate for both monolayer and multilayer photodetectors, thereby obtaining a fundamental limit for the photoresponse rate [2]. 1. H. Mera, T.G. Pedersen, and B.K. Nikolic, Nonperturbative quantum physics from low-order perturbation theory, Phys. Rev. Lett. 115, 143001 (2015). 2. T.G. Pedersen et al. Exciton ionization in multilayer transition-metal dichalcogenides, submitted.
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Speaker: Lifa Zhang
Affiliation: Nanjing Normal University, China
Abstract Details: Recently, a remarkable phenomenon of the phonon Hall effect was observed in a paramagnetic insulator, which is indeed a surprise since phonons as neutral quasiparticles cannot directly couple to magnetic field via Lorentz force. The following theoretical studies showed that through Raman spin-phonon interaction the magnetic field can have an effective force to distort phonon transport, and thus drive a circulating heat. Inspired by the phonon Hall effect, very recently we found chiral phonons in systems that break time reversal or spatial inversion symmetries. In magnetic systems, where time reversal symmetry is broken, phonons generally carry a nonzero angular momentum . At zero temperature, a phonon has a zero-point angular momentum in addition to a zero-point energy. With increasing temperature, the total phonon angular momentum diminishes and approaches zero in the classical limit. The nonzero phonon angular momentum can have a significant impact on the Einstein–de Haas effect. In non-magnetic crystals with inversion symmetry breaking, we find chiral phonons with valley contrasting circular polarization. At valley centers, there is a three-fold rotational symmetry endowing phonons with a quantized pseudo angular momentum, which includes spin and orbital parts. The chiral valley phonons are verified and the selection rules are predicted in monolayer Molybdenum disulfide. Due to valley contrasting phonon Berry curvature, a valley phonon Hall effect can also be observed. [1] L. Zhang, J. Ren, J.-S. Wang, and B. Li, Phys. Rev. Lett. 105, 225901 (2010). [2] L. Zhang and Q. Niu, Phys. Rev. Lett. 112, 085503 (2014). [3] L. Zhang and Q. Niu, Phys. Rev. Lett. 115, 115502 (2015)"
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Speaker: Rajasekhar Balasubramanian
Affiliation: Dept. Civil and Environmental Engineering, NUS
Abstract Details: In recent years, graphene-based materials are gaining considerable attention as novel materials for environmental applications because of their distinct advantages over other carbon materials. Three-dimensional (3D) nanoporous graphene materials have particularly become one of the most active research fields in the last three years. Some of the key attributes of these materials are their well-defined porous structure (high surface area and high pore volume), hydrophobicity and ease of regeneration for their repeated use. These characteristics together with the unique physicochemical properties of two-dimensional (2D) graphene, notably its high electronic and thermal conductivity, and great mechanical strength, can lead to effective technologies to address the pressing global environmental challenges such as “carbon capture and utilization” and “water purification”. This presentation will discuss the major outcomes of our research efforts on the synthesis, characterization and applications of 3D graphene- based materials in the environmental field and also point out new research directions for further development of these materials to promote sustainability.
About the Speaker: Rajasekhar Bala has been a faculty member at NUS since 1996. Prior to joining NUS, he worked at the New York State Department of Health and at Brookhaven National Research Laboratory, USA as a scientist. Prof Bala has broad research interests ranging from urban air quality to climate change mitigation. He and his research group have published 200 research articles in international journals. He has led several multi-national, multi-disciplinary research projects successfully over the years. He is the Editor of the International Journal of Aerosol and Air Quality Research and currently serves as the guest editor of the special issue with the theme “PM2.5 in Asia”. In recognition of his excellence in research and teaching, Prof Bala has received a number of research and educator awards including the Alan Berman Research Publication Award from the US Department of the Navy (2014) and two research awards from the Institution of Chemical Engineers (2015).
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Speaker: Li Linjun
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Speaker: Thomas Schmidt
Affiliation: University of Luxembourg
Abstract Details: It has been shown recently that the interplay of spin-orbit coupling, magnetic fields, and the superconducting proximity effect can lead to the emergence of zero-energy Majorana bound states (MBS) at the ends of a nanowire. These states are interesting because of their non-Abelian exchange statistics and their potential usefulness for quantum computation applications. However, detecting and manipulating them remains a challenge. In the first part of the talk, I will present multi-terminal networks hosting MBS which could be useful for their identification. In particular, I will discuss T-shaped junctions of two Majorana nanowires. When the wires are in the topologically nontrivial regime, three MBS are localized near the outer ends of the wires, while one MBS is localized near the crossing point, and when the lengths of the wires are finite adjacent MBS can overlap. A combination of current and cross-correlation measurements can then be used to reveal the predicted coupling of four MBS in a topological T junction. Interestingly, the elementary transport processes at the central lead are different compared to the outer leads, giving rise to characteristic nonlocal signatures in electronic transport [1]. MBS have also been proposed as building blocks for qubits on which certain operations can be performed in a topologically protected way using braiding. However, the set of these protected operations is not sufficient to realize universal quantum computing. In the second part of the talk, I will show that the electric field in a microwave cavity can induce Rabi oscillations between adjacent MBS. These oscillations can be used to implement an additional single-qubit gate. Supplemented with one braiding operation, this gate allows one to perform arbitrary single-qubit operations [2,3]. [1] Luzie Weithofer, Patrik Recher, and Thomas L. Schmidt, Phys. Rev. B 90, 205416 (2014) [2] Thomas L. Schmidt, Andreas Nunnenkamp, and Christoph Bruder, Phys. Rev. Lett. 110, 107006 (2013) [3] Thomas L. Schmidt, Andreas Nunnenkamp, and Christoph Bruder, New J. Phys. 15, 025043 (2013)
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Abstract Details: Do join us for the New Year Party 2016!
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Speaker: Chu Leiqiang
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Speaker: Sankalpa Ghosh
Affiliation: Indian Institute of Technology, Delhi
Abstract Details: Two aspects of dirac fermions: Electron optics and unconventional band structure due to bound states. Abstract: The talk has two parts. In the first part we shall describe our last few years' work on optical analogy of ballistic electron transport in monolayer and bilayer Graphene. We shall particularly emphasize the electron transport through magnetic barriers, their super lattice under various circumstances and how optical analogy helps us to model such transport. In the second part of the talk we shall describe our recent work on the unconventional band structure of dirac fermions on the surface of three dimensional Topological Insulators due to formation of 'bound states in a potential barrier'. We particularly point out how it can lead to interesting application in fields like Spintronics. Main references: 1. Neetu Agrawal (Garg), Sankalpa Ghosh, and Manish Sharma, Int. J. Mod. Phys. B 27, 1341003 (2013). 2. Manish Sharma and Sankalpa Ghosh, J. Phys.: Condens. Matter 23 055501 (2011). 3. Sankalpa Ghosh and Manish Sharma, J. Phys.: Condensed Matter 21, 292204 (2009). 4. Neetu Agrawal (Garg) et al, J. Phys.: Condens. Matter 24, 175003 (2012). 5. Neetu Agrawal Garg, Sankalpa Ghosh and Manish Sharma, EPJB 86, 317 (2013). 6. Puja Mondal and Sankalpa Ghosh, J. Phys.: Condensed Matter (in press) (arxiv: 1411.2091)"
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Speaker: Aires Ferreira
Affiliation: University of York, UK
Abstract Details: Graphene subjected to chiral disorder is believed to host zero energy modes resilient to localization, as dictated by the renormalization group analysis of the underlying field theory [1]. For disorder in the BDI chiral orthogonal class – such as vacancies and bond disorder – a line of fixed points with conductivity ~e^2/h is predicted. Such an unconventional quantum transport regime is found at variance with recent numerical works, however, which report the localization of all states, including the zero energy modes [2]. In this talk, I introduce an exact polynomial expansion of quantum-mechanical lattice response functions, whose implementation in large-memory machines allows tackling disordered systems with multi-billion (>10^9) atoms and fine meV resolutions. Its application to the honeycomb lattice with random vacancy defects reveals an unprecedentedly robust metallic state in two dimensions. The Kubo conductivity of zero energy modes is found to match graphene’s universal ballistic conductivity - 4e^2/(pi h) - within 1% accuracy, over a wide range of energy level broadenings and vacancy concentrations [3]. These results testify to the power of the novel polynomial expansion, and shed new light on the nature of electronic transport at the Dirac point of graphene.  [1] P.M. Ostrovsky, I.V. Gornyi & A.D. Mirlin, PRB 74, 235443 (2006). P.M. Ostrovsky, et al., PRL 105, 266803 (2010).  [2] G.T. de Laissardiere & D. Mayou, PRL 111, 146601 (2013). A. Cresti, F. Ortmann, T. Louvet, D.V. Tuan & S. Roche, PRL 110, 196601 (2013). Z. Fan, A. Uppstu & A. Harju, PRB 89, 245422 (2014). [3] A. Ferreira & E. Mucciolo, PRL 115, 106601 (2015). 
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