The seminars are regularly held on Wednesday, 11:00-12:00 in S11-02-07 unless otherwise announced.

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Date / Speaker | Title / Abstract |
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03 Dec 2014 Stephan Roche (ICN2 - Catalan Institute of Nanoscience Nanotechnology) |
From material substance to shape to property In this talk, I will discuss charge and spin transport in complex forms of graphene (chemically reduced, polycrystalline graphene, chemically functionalized) of relevance for current and future applications in flexible electronics, energy harvesting and spintronics. The crucial contribution of multiscale simulation will be illustrated, demonstrating an achieved high level of predictive capability for very large system sizes (with up to 1 billion atoms), reaching the experimental and technology scales. One illustration will be the quantitative analysis on the transport properties of structural imperfections produced during the wafer-scale production of graphene through chemical growth (CVD), or the mechanical/chemical exfoliation and chemical transfer to versatile substrates, followed by the device fabrication. Fundamental properties of charge mobilities in polycrystalline graphene, accounting the variability in average grain sizes and chemical reactivity of grain boundaries as observed in real samples grown by CVD will be presented, together with their relevance for device optimization and diversification of applied functionalities such as chemical sensing. In a second part, I will focus on spin transport in graphene functionalized by adatom deposits (gold, thallium). Unique spin dynamics phenomena in graphene, such as the formation of the Quantum Spin Hall state and a crossover to the Spin Hall effect under adatom segregation will be shown, as well as the role of spin-pseudospin entanglement in driving the spin relaxation mechanism in the ultraclean graphene limit. These results could open unprecedented perspectives for achieving proofs of concepts of spin manipulation, contributing to the progress towards non-charge based revolutionary information processing and computing. [1] L. E. F. Foa Torres, S. Roche, and J. C. Charlier, Introduction to Graphene- Based Nanomaterials: From Electronic Structure to Quantum Transport (Cambridge University Press, Cambridge, 2014). |

19 Nov 2014 Boris Yakobson (Rice University, USA) |
From material substance to shape to property Connecting the underlying chemical processes with the growth and emergent form remains an unsurmountable problem in life sciences [0]. In materials research, the current outlook is more optimistic. Establishing such connection, from the basic interatomic forces to growing nanostructure shape and properties becomes a real possibility. We will discuss several important examples of current interest including theory of carbon nanotubes chirality [1], growth and morphology of graphene [2] and other important 2D-materials [3], including the shape of equilibrium or growing islands, polycrystallinty and grain boundaries, and the unexpected functionality they bring about in electronics, magnetism, energy storage, and catalysis. [0] On Growth and Form, by D’Arcy W. Thompson (Cambridge U, 1917). |

29 Oct 2014 Johan Nilsson (University of Gothenburg, Sweden) |
Free fermion description of a paramagnetic Mott insulator Note: Seminar held at the Graphene Theory Common, S16, Level 6 A scheme is presented that enables a description of a paramagnetic Mott insulator in terms of free fermions. The main idea is to view the physical fermions as a part of a multi-band system and to allow for a correlation between the physical fermions and the auxiliary ones. Technically this is implemented through a non-linear canonical transformation, which is conveniently formulated in terms of Majorana fermions. The transformed Hamiltonian is in the next stage approximated with a free fermion theory. The approximation step is variational and provides an upper bound on the ground state energy at zero or the Free energy at finite temperature. In this way we are able to extend the domain of applicability of mean field theory and free fermions. |

28 Oct 2014 Petr Král (University of Illinois at Chicago, USA) |
Nanofluidics in carbonaceous nanostructures: Molecular drag, sensing, filtration, and self-assembly Note: Seminar held at the Graphene Theory Common, S16, Level 6 at 14:00 In this talk, we review many nanofluidic phenomena predicted and observed at the interfaces of carbonaceous nanostructures [1]. First, we discuss material drag effects around nanotubes and graphene induced by electronic currents, coupling of moving molecules, and mechanical vibrations. Then, we describe electronic sensing of a fluid motion around such nanostructures. Ultrasensitive molecular sensing was also reported at graphene grain boundaries. In a similar way, molecules passing through graphene nanopores can be sensed electronically. These nanopores could be used in water purification, molecular filtration, and DNA sequencing. Finally, we describe the self-assembly of carbon nanostructures by water droplets and nanotubes, mention experiments with water confined between graphene layers, and discuss the formation of filled micelles on the surfaces of carbon nanotubes. [1] P. Král and B. Wang, Material Drag Phenomena in Nanotubes, Chem. Rev. 113, 3372 (2013). |

15 Oct 2014 Kurt Stokbro (Quantum Wise and Copenhagen University) |
QuantumWise and the modelling of atomic-scale transport Note: Seminar held at the Graphene Theory Common, S16, Level 6 As device features near atomic dimensions, simulations of electrical currents need to be based on a quantum-mechanical description rather than a classical one. New phenomena appear which can be exploited for novel device characteristics, but also fundamental challenges arise when the influence of single defects can have devastating effects. The very definition of electrical current should be based on the quantum conductance, but in order to compare measurements and calculations accurately, a realistic atomistic description of the device configuration is required in order to properly describe impurities and defects. Although atomic-scale calculations of ballistic tunneling currents are becoming mainstream over the last decade, many challenges remain. Tight-binding models may work for some systems, but fail to capture the electronic structure of metallic systems, or interfaces combining metals and semiconducting materials, in which cases first principles [1] or semi-empirical [2] approaches becomes necessary. For transistor applications it is necessary to include gates and dielectric screening regions, and in other cases we may need to consider sequential tunneling in the weak coupling limit [3], rather than the coherent tunneling picture. Moreover, all of the above needs to be carried out for large-scale systems that might involve thousands of atoms. We will provide an overview of the state-of-the-art atomic-scale modeling techniques, and show examples of how our software Atomistix ToolKit is used used to study a wide variety of nanoelectronic device structures, such as graphene field-effect transistors, conductance of nanowires, molecular junction diodes, contact resistance of metal-semiconductor interfaces, leakage currents in ultrathin oxide layers, and magnetic tunnel junctions. The latter involves noncollinear calculations with spin-orbit coupling and the calculation of spin-transfer torque. Recent developments on the electron-phonon interaction will also be discussed.The fact that in graphene the density of states vanishes at the Fermi level invalidates the usual arguments for the screening of the nonlocal part of the long-range Coulomb repulsion. Consequently, the latter has to be taken into account for a realistic modeling of correlation effects. Here, we solve the Kane-Mele model with an additional Coulomb repulsion using auxiliary-field quantum Monte Carlo techniques on lattices with up to 18x18 unit cells. The Coulomb repulsion favors short-range sublattice charge fluctuations which compete with the onset of antiferromagnetic order driven by the onsite repulsion. As a result, in the model with onsite and nonlocal repulsion, the critical interaction for the transition to the antiferromagnetic phase is significantly enhanced. However, the overall topology of the phase diagrams remains unchanged upon including a long-ranged Coulomb tail. A systematic finite-size scaling is consistent with the view that, similar to the case of a Hubbard interaction, the transition from the quantum spin Hall phase to the antiferromagnet falls into the 3D XY universality class, and that the transition from the semimetal to the antiferromagnetic insulator is of the Gross-Neveu Heisenberg type. Hence, the long-ranged Coulomb repulsion is (marginally) irrelevant for the considered model. [1] M. Brandbyge, J.L. Mozos, P. Ordejón, J. Taylor, and K. Stokbro, Phys. Rev. B, 65, 165401 (2002) |

01 Oct 2014 F. F. Assaad (Wurzburg University, Germany) |
Phase diagram of the Kane-Mele Coulomb model Note: Seminar held at the Graphene Theory Common, S16, Level 6 The fact that in graphene the density of states vanishes at the Fermi level invalidates the usual arguments for the screening of the nonlocal part of the long-range Coulomb repulsion. Consequently, the latter has to be taken into account for a realistic modeling of correlation effects. Here, we solve the Kane-Mele model with an additional Coulomb repulsion using auxiliary-field quantum Monte Carlo techniques on lattices with up to $18 \times 18$ unit cells. The Coulomb repulsion favors short-range sublattice charge fluctuations which compete with the onset of antiferromagnetic order driven by the onsite repulsion. As a result, in the model with onsite and nonlocal repulsion, the critical interaction for the transition to the antiferromagnetic phase is significantly enhanced. However, the overall topology of the phase diagrams remains unchanged upon including a long-ranged Coulomb tail. A systematic finite-size scaling is consistent with the view that, similar to the case of a Hubbard interaction, the transition from the quantum spin Hall phase to the antiferromagnet falls into the 3D XY universality class, and that the transition from the semimetal to the antiferromagnetic insulator is of the Gross-Neveu Heisenberg type. Hence, the long-ranged Coulomb repulsion is (marginally) irrelevant for the considered model. |

25 Sep 2014 Gabriel Aeppli (ETHZ & EPFL, Switzerland & UC London) |
Support and competition from other ordered states for high temperature superconductivity Note: Seminar held at 11:30 a.m. at the Graphene Theory Common, S16, Level 6 We show X-ray, neutron, NMR and STM results on how mass, charge and spin density wave states coexist with and influence high temperature superconductivity in two layered systems - cuprates and intercalated graphite. |

17 Sep 2014 Oleg V. Yazyev (Federal Institute of Technology Lausanne, Switzerland) |
A theorist’s view of polycrystalline graphene: from atomic structure to electronic transport properties There is growing evidence of the polycrystalline nature of graphene samples at micrometer length scales. Grain boundaries and dislocations, intrinsic topological defects of polycrystalline materials, inevitably affect all kinds of physical properties of graphene [1]. This talk reviews our theoretical efforts directed towards understanding the atomic structure and electronic transport properties of polycrystalline graphene. I will introduce a general approach for constructing dislocations in graphene characterized by arbitrary Burgers vectors and grain boundaries covering the complete range of possible misorientation angles. By means of first-principles calculations we address the thermodynamic properties of grain boundaries revealing energetically favorable large-angle configurations as well as dramatic stabilization of small-angle configurations via the out-of-plane deformation, a remarkable feature of graphene as a two-dimensional material [2]. In the rest of my talk I will focus on the electronic transport properties of polycrystalline graphene. Ballistic charge-carrier transmission across the periodic grain boundaries is governed primarily by momentum conservation. Two distinct transport behaviors are predicted − either perfect reflection or high transparency with respect to low-energy charge carriers depending on the grain boundary periodicity [3]. It is also shown that topologically trivial line defects can be engineered and offer opportunities for generating valley polarized charge carriers [4]. Beyond the momentum conservation picture we find that the transmission of low-energy charge carriers can be dramatically suppressed in the small-angle limit [5]. This counter- intuitive behavior is explained by resonant backscattering involving localized electronic states of topological origin. Finally, the relations between the structure of strongly disordered large-angle grain boundaries and their transport properties are discussed [6]. These results demonstrate that dislocations and grain boundaries are intrinsic topological defects that dramatically affect the transport properties of graphene and can also be used for engineering novel functional devices. [1] Yazyev, O. V. & Chen, Y. P. Polycrystalline graphene and other two-dimensional materials. Nature Nanotechnology, published online (http://dx.doi.org/10.1038/nnano.2014.166) |

15 Sep 2014 Massimo Spina (Federal Institute of Technology Lausanne, Switzerland) |
Low-temperature solution-processed nanowires of methyl-ammonium lead iodide (CH3NH3PbI3) for photosensing applications. Note: Seminar takes place at the Graphene Theory Common, S16, Level 6 Methyl-ammonium lead trihalide perovskites (CH3NH3PbX3, X=Br, Cl, I) are solution-processable semiconductors that have recently demonstrated to be excellent materials for optoelectronic applications such as solar cells, lasers and light-emitting diodes. However, despite the intense research to improve the performances of those devices, most of the fundamental questions concerning why these materials are so efficient in generating and transporting photoinduced charges are still unanswered. To investigate the basic physical properties of these materials, besides bulk single crystals, we synthetized nanowires of CH3NH3PbI3. For the first time the photoconductivity of this one- dimensional form of perovskite has been assessed and the influence of parameters such as the number of grain boundaries and the degradation due to the loss of the methyl-ammonium cation have been evaluated. Finally, the interface of this material with graphene has been examined and exploited to improve the sensitivity of photodetectors made with this two-dimensional material. |

10 Sep 2014 Junqiao Wu (University of California, Berkeley) |
Inter-layer coupling and thermal conduction in 2D semiconductors Note: Seminar takes place at the Graphene Theory Common, S16, Level 6 Semiconducting two-dimensional (2D) materials have become a focus of research in recent years. One of the unique properties of these materials is the sensitivity of their electronic structure and vibrational spectrum to inter-layer coupling, despite the weak van der Waals interaction between neighboring layers. In this talk, I will discuss our research in controlling inter-layer coupling in homo- and hetero-structures of 2D semiconductors by physical means. We demonstrated that the inter-layer coupling and the resultant physical properties can be modulated thermally and mechanically, and is sensitive to in-plane crystal structure. We employ a diamond anvil cell to apply high hydrostatic pressures onto 2D structures up to 20 GPa, and probe the resultant optical reflection, absorption and emission and Raman spectrum. In addition, we develop a method to measure thermal conduction along different in-plane directions in 2D materials, from which we probe the anisotropy of lattice thermal conductivity and electrical conductivity. |

09 Sep 2014 Aris Alexandradinata (Princeton University, USA) |
More surprises in Bismuth, and next-generation spinless topological insulators without time-reversal symmetry Note: Seminar starts at 14:00 The 2D topological insulator is distinguished from ordinary insulators by the quantum spin Hall effect, which results in an enhanced magnetic susceptibility. Due to its strong diamagnetism, Bismuth is a promising candidate for such a phase of matter. We report the observation of edge states on Bismuth bilayers, which validate theoretical predictions that 2D Bismuth is indeed a topological insulator. Bismuth thus joins a growing list of experimentally-realized topological insulators, which depend essentially on spin-orbit coupling and/or time-reversal symmetry. To move beyond this paradigm, we theoretically propose the first-known 3D topological insulators without spin-orbit coupling, and with surface modes that are protected only by point groups, i.e., not needing time-reversal symmetry. Our findings greatly expand the range of electronic materials that may host topological phases, and has exciting implications for intrinsically spinless systems such as photonic crystals and ultra-cold atoms. If time permits, I will also introduce topological phases of matter without robust boundary states; they are uniquely distinguished by the crystal-analog of Berry phases. |

29 Aug 2014 T. Vuletić (Zagreb Institute of Physics, Croatia) |
Static conformation and dynamics of polyelectrolytes Biomacromolecules are mostly polyelectrolytes (PE), dissociating into polyions and small counterions. Their long-range electrostatic interaction leads to arrangements different than for neutral polymers and leads both to difficulties in understanding these systems [1] and to distinctive technical applications (gene therapy, gene chips, DNA sequencing) [2]. All studies necessarily reflect effects of both polyions and the ionic cloud, all the while being designed to distinguish between the effects of the two by, e.g., studying the polyion conformation in varying (counter)ion atmospheres, or by studying the changes to the atmosphere that may occur with variation in polyion length, stiffnes or concentration. Two most prominent issues are counterion (atmosphere) condensation and the electrostatic contribution to the polyion persistence length. For the last decade we have adressed these by studying the structure and dynamics of two semirigid (bio)PEs, DNA and HA (hyaluronic acid) [3] and comparing these to results on flexible PE, polystyrene sulfonate (PSS). We employed dielectric/impedance spectroscopy (DS), diffusion measurements by fluorescence correlation spectroscopy (FCS) and structural studies by small- angle X-ray scattering (SAXS). We have demonstrated the complementarity of DS and FCS dynamics study of PE conformations to well-established SAXS structural studies of DNA, HA and PSS samples – obtaining the same parameter, polyion mesh size ξ by both approaches. We have also quantitated and confirmed counterion condensation concepts with DS and FCS of monodisperse dsDNA fragments. Currently, by SAXS and polarizing microscopy we are studying the generality of the equation of state for PEs – whether it should be simply derived from the free, uncondensed counterion concentration. That is, we found that HA generates 4-5 times weaker pressure per free counterion than DNA or PSS. The latter are strong PEs and counterions atmospheres are “rarified” due to the condensation. On the contrary, HA should be a weak polyelectrolyte where no condensation occurs and all the counterions are free to contribute to the osmotic pressure – which appears not to be the case. [1] P.-G. de Gennes et al., J. Phys. (Paris) 37, 1461 (1976); G. S. Manning, Q. Revs. Biophys. 11, 179 (1978); T. Odijk, Macromolecules 12, 804 (1979); A. V. Dobrynin and M. Rubinstein, Prog. Polym. Sci. 30, 1049 (2005); G.C. Wong and L. Pollack, Annu. Rev. Phys. Chem. 61, 171 (2010). A.K. Mazur and M.Maaloum PRL 112, 068104 (2014). |

20 Aug 2014 Vivek B. Shenoy (University of Pennsylvania, USA) |
Multiscale simulations of CVD growth of 2D Materials Note: Seminar held at S16-06 (Graphene Theory Common) Crystalline 2D materials such as graphene, boron nitride, transition metal dichalcogenides and composites of these materials have received attention for their potential applications in logic, energy storage and optoelectronics. Chemical Vapor deposition has become a common method for large-scale synthesis of these materials. This growth process is influenced by thermodynamic, kinetic, and material parameters, often leading to diverse island shapes including dendrites, squares, stars, hexagons, butterflies, and lobes. Here, we introduce a phase-field model that provides a unified description of these diverse growth morphologies of graphene and compare the model results with new experiments[1]. Our model explicitly accounts for the anisotropies in the energies of growing graphene edges, kinetics of attachment of carbon at the edges, and the crystallinity of the underlying copper substrate (through anisotropy in surface diffusion). We show that anisotropic diffusion has a very important, counterintuitive role in the determination of the shape of islands, and we present a “phase diagram” of growth shapes as a function of growth rate for different copper facets. Our results are shown to be in excellent agreement with growth shapes observed for high symmetry facets such as (111) and (001) as well as for high-index surfaces such as (221) and (310). I will also talk about our work on using defects in graphene and other 2D materails to enhance energy storage capacity [2]. [1] E. Meca, J. Lowengrub, H. K. Kim, C. Mattevi, and V. B. Shenoy Epitaxial Graphene Growth and Shape Dynamics on Copper: Phase-Field Modeling and Experiments NANO LETTERS 13(11) 5692-5697 (2013). |

Date / Speaker | Title / Abstract |
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07 Aug 2014 Rupert Oulton (Imperial College London) |
Laser Science in a Nanoscopic Gap Semiconductor diode lasers have overcome numerous technological limitations in the 50 years since their first demonstration to become faster, brighter and smaller; however, scaling their size beyond the diffraction limit of light is a more recent achievement, with still uncertain consequences. A number of demonstrations from around the world, now show the capability of metal-based lasers to create and sustain coherent light well below the diffraction limit, by generating and amplifying surface waves on metal surfaces, known as Surface Plasmon Polaritons. This seminar introduces the motivations and construction of "plasmonic" lasers and discusses their limitations and potential applications. In particular, such laser devices could be the most efficient and compact method of delivering optical energy to the nanoscale. There are two benefits: firstly, the efficiently generated (focused) coherent laser field can be extremely intense; and secondly, vacuum fluctuations within the laser cavity are considerably stronger than in free space. Consequently, plasmonic lasers have the unique ability to drastically enhance both coherent and incoherent light-matter interactions, bringing fundamentally new capabilities to photonic technologies. While there is a great deal of research ahead for plasmonic lasers systems, this work highlights the feasibility of nano-scale light sources and the potential to do laser science at the nanoscale. |

06 Aug 2014 Alessandra Lanzara (UC Berkeley and Berkeley Lab) |
Ultrafast manipulation of electronic structure in 2D materials Recent developments of table top laser sources have changed the way we study and control material properties, previously achieved through chemical substitutions or with static perturbations such as pressure, electric and magnetic fields. In this talk I will present few examples of how the emerging tool of time and angle resolved photoemission spectroscopy is changing the way we study the properties of a variety of 2D materials and how this tool will allow measurements of devices under working conditions; access to hidden states of matter, as well as design of new photo functional materials. The case of unconventional superconductors and graphene will be discussed in details illustrating the power of this unique time-resolved spectroscopy. |

05 Aug 2014 Gregory R Stewart (University of Florida) |
Specific Heat of Iron Based Superconductors at Tc - A Global Correlation Note: Seminar held at S14-06-20 In the study of the new iron based superconductors, there are not many global correlations. For example, the early statement that Tc is a maximum (in the 1111 materials) when the As-Fe-As bond angle is the 'regular' tetrahedron value of 109.47 degrees has not been born out in, e. g., the 111 LiFeAs. However, one correlation pointed out by Bud'ko, Ni and Canfield (Phys. Rev. B 79, 220516 (2009)) based on samples of the 122 iron based superconductors has since (Kim et al., J. Phys.: Conds. Mat. 23, 222201 (2011)) been confirmed in both the 111 and the 11 iron based superconductors. This correlation concerns the discontinuity, DC, at the superconducting transition temperature and its dependence on Tc. More recent data, and a comparison of this iron based superconductor behavior with BCS superconductors, will be given. This correlation keeps on being borne out and serves as a.) a metric for sample quality; b.) a test whether a newly discovered iron-containing superconductor is of the same class of superconductor as the other known iron based superconductors and c.) a challenge to theorists to explain this behavior. |

31 Jul 2014 Marcin Mucha Kruczynski (Bath University) |
Modifying electronic structure of graphene with external perturbations Because of its atomic thickness and elastic properties, electronic structure of graphene can be significantly modified with external perturbations. In this talk, I will discuss some theoretical models constructed to capture changes in the band and Landau level structure in (mono- and bilayer) graphene due to strain, external electric field and presence of a substrate. In particular, I will talk about strain-induced changes in the topology of the low-energy band structure of bilayer graphene and signatures of pseudo-magnetic field in electronic transport through strained graphene ribbons. I will also discuss formation of minibands in graphene on incommensurate hexagonal substrates (e.g. h-BN) and some peculiarities of the band structure of bilayer graphene in strong perpendicular electric fields. |

23 Jul 2014 Feng-Chuan Chuang (National Sun Yat-Sen University, Taiwan) |
Prediction of Large-Gap Two-Dimensional Topological Insulators Consisting of Bilayers of Group III Elements with Bi We use first-principles electronic structure calculations to predict a new class of two-dimensional (2D) topological insulators (TIs) in binary compositions of group III elements (B, Al, Ga, In, and Tl) and bismuth (Bi) in a buckled honeycomb structure. We identify band inversions in pristine GaBi, InBi, and TlBi bilayers, with gaps as large as 560 meV, making these materials suitable for room-temperature applications. Furthermore, we demonstrate the possibility of strain engineering in that the topological phase transition in BBi and AlBi could be driven at ~6.6% strain. The buckled structure allows the formation of two different topological edge states in the zigzag and armchair edges. More importantly, isolated Dirac-one edge states are predicted for armchair edges with the Dirac point lying in the middle of the 2D bulk gap. A room-temperature bulk band gap and an isolated Dirac cone allow these states to reach the long-sought topological spin-transport regime. Our findings suggest that the buckled honeycomb structure is a versatile platform for hosting nontrivial topological states and spin-polarized Dirac fermions with the flexibility of chemical and mechanical tunability. [1] Feng-Chuan Chuang, Liang-Zi Yao, Zhi-Quan Huang, Yu-Tzu Liu, Chia-Hsiu Hsu, Tanmoy Das, Hsin Lin, and Arun Bansil, Nano Lett. 14, 2505 (2014). |

09 Jul 2014 Giovanni Vignale (University of Missouri) |
Many-body effects in doped graphene layers The peculiar band structure of graphene, coupled with electron-electron interactions, is responsible for the breakdown of the Fermi liquid concept in the undoped material. Interesting many-body effects are also predicted to occur in doped graphene layers, where the Fermi liquid picture still applies with an enhanced Fermi velocity. In this talk I review some of these effects, which should be observable in optical and infrared spectroscopies, magnetic susceptibility measurement, and thermal transport measurements. Due to the lack of Galilean invariance, both the plasmon frequency and the Drude weight in the optical conductivity are significantly enhanced relative to the conventional RPA values. The orbital magnetic susceptibility, which vanishes in the free-electron approximation, is found to be positive, i.e. paramagnetic, with a value that is completely controlled by the electron-electron interaction. The quasiparticle lifetime is long and leads to a large electronic component of the thermal conductivity, which strongly violates the Wiedemann-Franz law. I review these theoretical predictions vis-a-vis the current state of the experiment. |

02 Jul 2014 Alexandra Carvalho (NUS, GRC) |
Phosphorene Black phosphorus has recently been brought into the limelight following the unveiling of the curious properties of its monolayer form, phosphorene. While most known 2D materials have hexagonal structures, resembling graphene, phosphorene has a curious waved-like structure that gives its properties an anisotropic "twist". In less than one year, the studies of black phosphorus and phosphorene have been multiplying, with exciting results. It has been shown to be a direct-gap or nearly-direct gap semiconductor both on monolayer and multi-layer form that can become an indirect-gap semiconductor, a semimetal or a metal application of uniaxial stress. Experiments in multi-layer material yield carrier mobilities of about 1000 cm2/(Vs). And recently, the isolation of the monolayer form has been announced. The list continues to grow as theoretical studies add high optical absorption, superconductivity, thermoelectricity... In this talk, I will consider what makes phosphorene so special. As the recent developments are reviewed, we will turn to density functional theory models for further insight into the electronic and optical properties of this material. |

26 Jun 2014 Mei-Yin Chou (Georgia Tech, USA & Academia Sinica, TW) |
Physics of Few-Layer Graphene: From Neutrino-like Oscillations to Hofstadter Butterflies Note: Seminar starts at 15:00 at the Graphene Theory Common (S16, L6). The Dirac-Weyl Hamiltonian for massless fermions describes the low-energy quasiparticles in graphene. Going beyond the monolayer, intriguing physics has been found in few-layer graphene systems. In this talk, I will focus on our recent theoretical and computational studies of a few representative systems. In particular, the quasiparticle states in rotated bilayer graphene systems act as massless fermions with two “flavors”, and interlayer coupling induces neutrino-like oscillations and anisotropic transport. In addition, a rare fractal-like “butterfly” energy spectrum arises under an external magnetic field. These two- dimensional atomic layer systems provide a unique platform to probe the rich physics involving multiple interacting massless fermions. |

25 Jun 2014 Andrivo Rusydi (NUS, Department of Physics) |
Probing transport and magnetic properties of graphene with adatoms through optical conductivity 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. |

24 Jun 2014 Steven G. Louie (University of California at Berkeley and Lawrence Berkeley National Laboratory) |
Novel Electronic and Optical Phenomena in van der Waals Layers: Graphene and Beyond Graphene Note: Seminar starts at 15:00. 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. |

24 Jun 2014 Yuanbo Zhang (Fudan University) |
Two-dimensional Materials Beyond Graphene 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. |

18 Jun 2014 Francisco (Paco) Guinea (ICMM-CSIC, Spain) |
Strains: structural and electronic properties of graphene 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. |

11 Jun 2014 Stephen J. Pennycook (University of Tennessee) |
Fulfilling Feynman’s dream: “Make the electron microscope 100 times better” – Are we there yet? In 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.” |

02 Jun 2014 Marcin Mucha-Kruczynski (Bath University, UK) |
Electronic structure of graphene on hBN Note: Seminar starts at 15:20 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. |

02 Jun 2014 Simon Bending (Bath University, UK) |
Superconductivity in Few Molecular Layer NbSe2 Note: Seminar starts at 14:40 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. |

02 June 2014 Enrico Da Como (Bath University, UK) |
Ultrafast optical spectroscopy of few-layer graphene Note: Seminar starts at 14:00. 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. |

21 May 2014 Howard Lee (Caltech, USA) |
Gate-Tunable Oxide Plasmonic Nanocircuits and Plasmonic Photonic Crystal Fibers Note: Seminar starts at 12:00 p.m. 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. |

14 May 2014 Jeil Jung (GRC, NUS) |
Surface interactions and crystal incommensurability in 2D materials: the case of graphene on hexagonal boron nitride Note: Seminar starts at 14:00 When atomically thin two-dimensional materials are layered they often form incommensurate non-crystalline structures manifested in moiré patterns found when examined by scanning probes. In this talk I will address theoretically on a methodology based on first principles calculations to build models that incorporate surface interactions between layered materials for general lattice mismatch or angle misalignment. When applied in a heterojunction consisting of graphene and hexagonal boron nitride, we can derive an effective moire pseudospin superlattice Hamiltonian that provides an intuitive platform to understand the role of the substrate on the electronic properties of graphene. I will use this model to obtain information of experimental interest such as the band structure and (local) density of states. |

Date / Speaker | Title / Abstract |
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30 Apr 2014 Mirco Milletari (Graphene Research Center, NUS) |
Electronic Correlations in Graphene: a bosonization study We reconsider the problem of electron correlations in the low energy theory of Graphene. We derive the low energy interacting theory directly from the lattice model and show that, unlike in previous analysis, off diagonal density-density interaction terms are also present. Using functional bosonization techniques, we show that the model with a long range Coulomb interaction and zero chemical potential can be solved in an essentially exact way. We derive the interacting Green's function, the anomalous dimension and the Fermi velocity renormalization. While some of these results have already been derived by various authors within a renormalization group approach, in our scheme the renormalized Fermi velocity turns out to be finite and independent from the high energy cut-off. Other observables like the momentum distribution function and the interacting density of states are also obtained and show typical Luttinger liquid behaviour. |

23 Apr 2014 Vincent Sacksteder (Sch. of Phys. & Math. Sci., NTU) |
Spin diffusion equations for hole-doped quantum wells and graphene Note: There will be two talks in this seminar by the same speaker. We obtain the spin-orbit interaction and spin-charge coupled transport equations of a two-dimensional heavy hole gas under the influence of strain and anisotropy. We predict an enhanced spin lifetime associated with a spin helix standing wave similar to the Persistent Spin Helix which exists in the two-dimensional electron gas with equal Rashba and Dresselhaus spin-orbit interactions. We also briefly discuss spin diffusion in graphene with adsorbed impurities. Collaborators: Andrei Bernevig, Quansheng Wu, Aires Ferreira, Ivan Shelykh |

23 Apr 2014 Vincent Sacksteder (Sch. of Phys. & Math. Sci., NTU) |
Bulk effects on surface conduction in 3-D topological insulators Note: There will be two talks in this seminar by the same speaker. We report on intensive numerical studies of topological conduction on the surface of 3-D TIs, including bulk effects. We highlight a bulk effect which provides a second mechanism of protection against disorder, and also bulk-induced deviations from diffusive scattering. Collaborators: Andrei Bernevig, Quansheng Wu, Aires Ferreira, Ivan Shelykh |

26 Mar 2014 Philip Phillips (UIUC, USA) |
Gravity and Strange Metals Note: Seminar held at S14-06-20. Since its discovery in 1986, high temperature superconductivity has frustrated experimentalists and theorists alike. The frustration comes from the fact that since the interactions are strong, none of the simple cartoon models based on the standard theory of metals apply. Consequently, there is no clear way of interpreting experiments. Further, as there is no agreed-upon way of solving problems in which the interactions are strong, theorists continue to make progress largely based on the strength of their personalities, that is, proof by intimidation. I will describe a new approach to the strongly correlated problem based on a mapping to a gravitational dual. I will emphasize the rudiments of this mapping focusing on the origins of this new approach and give examples of how it has been applied to the classic Mott problem and superconductivity. |

25 Mar 2014 Philip Phillips (UIUC, USA) |
Unparticles in strongly correlated electron matter Note: Seminar starts at 10:30 a.m. and held at the Graphene Theory Common (S16, Level 6) One of the open problems in strong correlation physics is whether or not Luttinger's theorem works for doped Mott insulators. I will begin this talk by using this theorem to count particles and show that it fails in general for the Mott state. The failure stems from the divergent self energy that underlies Mottness. When such a divergence is present, charged degrees of freedom are present that have no particle interpretation. Such degrees of freedom are well described by the unparticle construction of Howard Georgi's. I will show how a gravity dual can be used to determine the scaling dimension of the unparticle propagator. I will close by elucidating a possible superconducting instability of unparticles and demonstrate that unparticle stuff is likely to display fractional statistics in the dimensionalities of interest for strongly correlated electron matter. |

19 March 2014 Kazu Suenaga (Nanotube Research Center, AIST, Japan) |
Atomic scale imaging and spectroscopy of low-dimensional materials with interrupted periodicities In the Nanotube Research Center at AIST, we have been developing the top-level facilities of electron microscopy which enables the atomic resolution analysis of low-dimensional materials. Point defects and edge structures of graphene have been intensively studied with atomic precision in the last decade [1-4]. Because the studies of atomic defects and boundaries are of general interest in the fundamental researches and becoming more and more crucial for technological applications of any nanoscale materials, the atomic scale studies can be also expanded to the other low-dimensional materials. Here I demonstrate some examples for atomic-scale imaging and spectroscopy of various low-dimensional materials with interrupted periodicities. Active 4|8 defects are most recently found to be responsible for plastic deformation of hexagonal boron-nitride (h-BN) layers [5]. Vacancies and edges with radical bonds are also successfully assigned in h-BN [6, 7]. Doping and boundary behaviors of single-layered dichalcogenides (MX2) are intensively studied because they indeed govern the phase transition behaviors between 2H and 1T phases [8, 9]. Possible nano-device assembly made of metallic and semiconducting MoS2 single layers will be also proposed. [1] A. Hashimoto et al., Nature, 430 (2004) pp.870-873 |

27 Feb 2014 Amir O. Caldeira (Universidade Estadual de Campinas - Instituto de Física "Gleb Wataghin") |
Cats, Decoherence and Quantum Measurement Note: Seminar starts at 16:00 and held at the CQT Seminar Room, S15-03-15. In this talk it is our intention to review the basic ideas of how entanglement relates to the so-called Schrödinger cat state and present a paradigmatic situation where states very similar to that one can be created. The example we have chosen is the SQUID ring which depending on the external bias allows us to implement a wealth of interesting physical situations to be treated. We shall argue that in these situations the question of dissipation is really relevant and the concept of decoherence naturally arises. Once we have accomplished that we discuss some possible implications of decoherence to the quantum theory of measurement . As a matter of fact, we shall employ an alternative measure of quantum correlation which goes beyond entanglement – the quantum discord – with the same purpose. We finally present recent experimental results performed with twin photons which corroborate our predictions. This Colloquium is jointly organised with the Center For Quantum Technologies, NUS. |

26 Feb 2014 Matthias Droth (University of Konstanz, Germany) |
Electron Spin Relaxation in Graphene Nanoribbon Quantum Dots Note: Seminar held at Graphene Theory Common (S16, Level 6). Armchair graphene nanoribbons (aGNR) are promising as a host material for electron spin qubits because of their potential for scalability and long coherence times [1]. The spin lifetime T1 is limited by spin relaxation, where the Zeeman energy is absorbed by lattice vibrations [2], mediated by spin-orbit and electron-phonon coupling. We have calculated T1 by treating all couplings analytically and find that T1 can be in the range of seconds for several reasons: (i) Van Vleck cancellation; (ii) weak spin-orbit coupling; (iii) low phonon density; (iv) vanishing coupling to out-of-plane modes due to the electronic structure of the aGNR. Owing to the vanishing nuclear spin of 12C, T1 is a good measure for overall coherence. These results and recent advances in the controlled production of graphene nanoribbons [3] make this system interesting for classical and quantum spin- tronics applications. [1] B. Trauzettel, D. V. Bulaev, D. Loss, and G. Burkard, Nature Phys. 3, 192-196 (2007). |

20 Feb 2014 Paolo E. Trevisanutto (Graphene Centre & SSLS) |
Ab initio approaches to spectroscopic properties and quantum transport Note: Seminar starts at 10:00 a.m. and held at the Graphene Theory Common (S16, Level 6). Abstract unavailable. |

12 Feb 2014 Ma Ping Nang (CQT, Singapore) |
Quantum Monte Carlo: Directed Worm Algorithm Note: Seminar starts at 14:00. Quantum Monte Carlo (QMC) algorithms based on a world-line representation are among the most powerful numerical-exact techniques for the simulation of non-frustrated spin models and of bosonic models at finite temperature. The directed worm algorithm (DWA) is one of these continuous-time numerical-exact QMC methods that proves to be very efficient. In this talk, I shall focus on the DWA simulation of boson Hubbard model, directly applicable to realistic bosonic optical lattices of actual sizes (~200^3). Its capability to directly compare with the experiments proves DWA to be in-challengable by any other numerical method. Last but not least, I shall discuss about the ALPS C++/Python implementation of DWA. |

05 Feb 2014 Tanmoy Das (LANL, USA & Graphene Centre) |
Engineering topological insulators, Weyl semimetals, superconductors, and spin-orbit order via layer by layer approaches The realization of most of the material properties such as the newly discovered ‘topological insulator’ and spin-orbit locked electronic states is limited to single compound synthesis with appropriate symmetries. Here we propose ways of artificially engineering such three dimensional (3D) bulk properties in layer by layer approaches. In the first example, we show that 3D `topological insulators’ (which act as insulator in the bulk while metallic on the surface) can be designed by growing bilayer of Rashba-type spin-orbit coupled 2D electronic gas on adjacent planes of bilayers.[1] Secondly, we propose two complementary design principles for engineering 3D Weyl semimetals and superconductors (which host relativistic electronic states dispersing in all three spatial directions with very high mobility). We show that such states can be engineered artificially in a layer-by-layer setup which includes even and odd parity orbitals in alternating layers.[2] Finally, we show how electronic interaction can be introduced and tuned in these highly functional 2D layered superlattices which renders a new form of phase, dubbed ‘spin-orbit density wave’.[3] Possible realizations and/or experimental evidences of these proposals, and their fundamental implications will also be discussed. [1] Tanmoy Das, A. V. Balatsky, “Engineering three-dimensional topological insulators in Rashba-type spin-orbit coupled heterostructures”, Nat. Commun. 4, 1972 (2013). |

29 Jan 2014 Joel K.W. Yang (IMRE and SUTD) |
Light, metal, and few-nanometer-precision fabrication Note: Seminar starts at 4:00 p.m. at the Graphene Seminar Room, S16, Level 6. The field of Nanoplasmonics focuses on the phenomenon of light interaction with metal structures that support resonances at optical frequencies. The collective oscillations of charges in these structures give rise to resonances and enhanced fields that depend strongly on the precise geometry, metal gap sizes, and surrounding dielectric media. Due to the significant dimensional dependence of nanoplasmonic structures down to the few-nanometer scale, highly precise nanofabrication capabilities are imperative. In this talk we will discuss some advances in nanofabrication processes involving electron-beam lithography [1], self-assembly [2], and chemical synthesis [3] to control features in these nanostructures that affect their interaction with light. Examples include the control of nanostructure sizes to produce color prints [4], gap sizes for strong localization of light [5], atomic-scale grain boundaries, and nanoscale contacts between metal nanostructures and a dielectric substrates as sources of damping. [1] H. Duan, H. Hu, H.K. Hui, Z. Shen, J.K.W. Yang, “Free-standing sub-10 nm nanostencils for the definition of gaps in plasmonic antennas”, Nanotechnology 2013, 24, 185301 |

22 Jan 2014 Anthony Leggett (UIUC and NUS) |
Prospects for topological quantum computing I give an introduction to the general idea of topologically protected quantum computing. I then review some of the systems, both naturally occurring and purpose-engineered, which have been proposed for its implementation, and try to assess the likelihood of success in each case. |

15 Jan 2014 Ashley da Silva (University of Texas, Austin) |
Enhancing the Purcell factor with graphene multilayers Graphene's response to electromagnetic excitation shows promise for a variety of applications including near perfect light absorption and radiative energy transfer control. At the heart of this response are collective oscillations of the carriers of graphene, which are known as plasmons. I will discuss the plasmon modes of weakly coupled graphene multilayers. These plasmon modes are associated with peaks in the transmission coefficient of electromagnetic waves. A radiative molecule placed near to the surface of such a graphene multilayer will radiate into these modes, thus decaying more rapidly, provided the energy of the radiated modes match the energy range of the plasmon modes. This ability to control radiative energy transfer is quantified by the Purcell factor, which is greatly enhanced in graphene multilayer systems in the THz to IR regimes of the electromagnetic spectrum. I will compare this behavior to that of metallic superlattices, which show enhanced Purcell factor in the optical part of the electromagnetic spectrum. Tuning the graphene Fermi level provides a knob with which to control the plasmon energies. This tunabilitiy as well as the novel energy regime (THz to IR) makes graphene multilayers an exciting system in which to study optical properties. |

13 Jan 2014 G. Vignale (U. Missouri-Columbia, USA) |
Spin-charge conversion in interfacial electron liquids Semiconductor quantum wells, inter-metallic interfaces, layered oxides, and monolayer materials are all promising platforms for the observation of spin-charge conversion due to strong spin-orbit interaction in the quasi two dimensional electron liquid they host. In this talk I focus on two closely related effects that can occur in these materials, namely the conversion of charge current to spin current (spin Hall effect) and the generation of spin polarization from an electric current (Edelstein effect). Together with their inverses (in the sense of Onsager reciprocity relations), these effects constitute a useful set of tools for spintronic applications. The theoretical challenge is to provide a unified treatment of the different mechanisms at work, including spin precession, spin relaxation, electron-impurity scattering and electron-electron scattering. In this talk I show how the SU(2) drift-diffusion theory allows such a unified treatment, and I describe its application to the interpretation of recent experiments on spin-charge conversion at the interface between two non-magnetic metals (Ag/Bi)†. I conclude with a brief review of our recent results on the generation of ordered spin structures from density-modulated structures and on the impact of spatially-dependent spin-orbit couplings on the process of spin-charge conversion. † J. C. Rojas-Sanchez et al., Nature Commun. 4, 2944 (2013). |

08 Jan 2014 Bo Yang (Princeton University, USA) |
Holomorphic Wavefunctions, Jack Polynomials and the Energy Spectrum of the Fractional Quantum Hall States We show that the holomorphic wavefunction descriptions of the fractional quantum hall states, first pioneered by Laughlin, Moore-Read and others, can be more naturally described by the Jack polynomial formalism. Within this formalism not only the ground states of the various fractional quantum hall fluids can be constructed, the techniques can also be applied to zero-mode edge states and both charged and neutral bulk excitations. As an explicit example, we construct model wavefunctions for a family of single-quasielectron states supported by the nu = 1/3 fractional quantum Hall (FQH) fluid. |