News & Events

Speaker: F. F. Assaad
Affiliation: Wurzburg University, Germany
Abstract Details: 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.
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Abstract Details: See Abstract details here
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Speaker: Gabriel Aeppli
Affiliation: ETHZ & EPFL, Switzerland & UC London
Abstract Details:

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.

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Abstract Details:

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Speaker: Oleg V. Yazyev
Affiliation: Federal Institute of Technology Lausanne, Switzerland
Abstract Details:

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Speaker: Massimo Spina
Affiliation: Federal Institute of Technology Lausanne, Switzerland
Abstract Details: 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.
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Speaker: Junqiao Wu
Affiliation: University of California, Berkeley
Abstract Details: 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.
About the Speaker: Professor Junqiao Wu received a B.S. from Fudan University and a M.S. from Peking University, China, both in physics. He obtained a Ph.D. degree in applied physics from the University of California, Berkeley for work on nitride semiconductors and highly mismatched semiconductor alloys. He did postdoctoral research in the Department of Chemistry at Harvard University on phase transitions in transition metal oxide nanomaterials. He began his faculty appointment in the Department of Materials Science and Engineering at the University of California, Berkeley in 2006. His honors include the Berkeley Fellowship, the 29th Ross N. Tucker Memorial Award, the Berkeley Presidential Chair Fellowship, the U.S. NSF Career Award, the U.S. DOE Early Career Award, and the U.S. Presidential Early Career Award for Scientists and Engineers (PECASE). He has published more than 100 widely cited papers. The Wu group explores novel properties and applications of strongly correlated electron materials with reduced dimensions, phase transitions at the nanoscale, and optoelectronic, thermal and thermoelectric properties of semiconductor alloys and interfaces. More information can be found at http://mse.berkeley.edu/~jwu
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Speaker: Aris Alexandradinata
Affiliation: Princeton University, USA
Abstract Details: 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.
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Speaker: T. Vuleti?
Affiliation: Zagreb Institute of Physics, Croatia
Abstract Details: 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 ? 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). [2] D. Branton et al., Nature Biotechnology 26, 1146 (2008); A.Buxboim et al., Nano Lett. 9, 909 (2009); C.R.Safinya et al., Top Curr Chem. 296, 191(2010); C.A.Merchant and M.Drndi? Methods Mol Biol.;870:211-26 (2012). [3] T. Vuleti? et al., Phys. Rev. Lett., 97, 098303(2006); Phys. Rev. E 75, 021905.(2007); Europhys. Lett., 81, 68003 (2008); Phys. Rev. E, 82, 011922 (2010); Phys. Rev. E 83, 041803 (2011); Macromolecules 46, 1107 (2013)
About the Speaker: T. Vuleti? (Ph.D. Uni Zagreb, postdoc Uni Paris XI) is building his research group at the Institute of physics in Zagreb. He is now looking beyond basic studies of polyelectrolytes towards the application of this knowledge in hybrid bio/nano systems, e.g. DNA and graphene in gene chips or nanopore systems. He is also the secretary of Croatian Biophysical Society (http://biofizika.hr) and Chair of International School of Biophysics (http://soft.ifs.hr/school)
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Speaker: Vivek B. Shenoy
Affiliation: University of Pennsylvania, USA
Abstract Details: 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). [2] R. Mukherjee, A. Thomas, D. Datta, E. Singh, J. Li, O. Eksik, V. B. Shenoy, and N. Koratkar Defect Induced Plating of Lithium Metal within Porous Graphene Networks NATURE COMMUNICATIONS 5:3710 (2014). [3] D. Datta, J. Li and V. B. Shenoy Defective graphene as a high-capacity anode material for Na- and Ca-ion batteries ACS NANO, 6(3): 1788-1795 (2014).
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