Gotham-Metro Condensed Matter Meeting

Agenda

Abstracts

Science Alliance Workshop:  Introduction to Networking

Monica Kerr, PhD, Director of Science Alliance at the New York Academy of Sciences

In preparation for this meeting, you have likely read the latest scientific articles in the field and scoped out the most interesting talks and posters. Perhaps you have also thought about pursuing a collaboration with another lab, getting your hands on an important tool or reagent, or even finding that all-important postdoc position! But if the thought of networking makes you want to hide behind the posters or cheese dip, join us over breakfast for a fun and interactive session that will show you that building your professional network does not have to be intimidating, all you need is a little advance preparation. You will learn the skill of developing an ‘elevator speech’ and will have the opportunity to practice your own before embarking on the meeting. In advance of the session, you may want to think about someone you would like to network with in order to make the exercises that you will be doing more concrete.

Quantum Imaging of Topologically Ordered Matter

Hari Manoharan, PhD, Stanford University

Deforming a material and restoring it precisely back to its starting point intuitively implies that the material before and afterwards is identical. This is true classically, and was believed to be true in general until recently in the history of quantum mechanics. Even if all the atoms, electrons, and other ingredients are returned exactly to where they started, we now know that the restored material can differ from the undeformed material by nontrivial quantum mechanical phase factors. These geometric or Berry phases have garnered increasing appreciation in recent years, and in condensed matter they arise from the topology of electronic states and embedded degeneracies. Such considerations have helped to identify new ground states consisting of topologically ordered matter, which on a practical level can be exploited in quantum devices and quantum computing strategies. This talk will overview new experiments from our lab, employing scanning tunneling microscopy and atomic manipulation, that directly visualize and control topological order in several materials and nanostructures.

Mircofluidics for Making and Studying New Soft Materials

David Weitz, PhD, Harvard University

Microfluidic devices offer a new way of very precisely controlling the flow and mixing of fluids. These can be used to create new and interesting structured fluids. They can also be used for novel biotechnological applications. These will be described.

Fireside Chat

Prominent physicist, R. Shankar, PhD of Yale University, will lead this interactive discussion with the audience. The topic of discussion may range from the speaker's views on the future of physics in the US to stories depicting adventures in research.

Short Talks

Single-Frame Holographic Particle Image Velocimetry

Lisa Dixon, Fook Chiong Cheong and David G. Grier, New York University

We present a novel method for single frame particle image velocimetry of micron scale spheres based on holographic video microscopy. Our approach takes advantage of the blurring that recorded holograms suffer when a sphere moves during the exposure period of the camera. By measuring the angular variance in intensity of the blurred hologram, we extract a model-independent metric for the particle velocity. We find this to be accurate for speeds that permit characterization of other properties of the sphere, such as radius and refractive index through Lorenz-Mie mocroscopy. Single-frame holographic velocimetry yields information on the dynamics of a particle, without sacrificing any other measurements.

Carbon Nanofilms on Insulating Substrates

S.V.Samsonau and A.M. Zaitsev, The College of Staten Island/The City University of New York

Methods of growth of uniform highly conductive carbon nanofilms on large area insulating substrates remain the major challenge of anticipated carbon-based nanoelectronics. Development of such a method is the aim of the presented research. Carbon nanofilms were grown by the catalyst-assisted deposition from methane on diamond, sapphire and quartz substrates at temperatures in the range from 650 to 900ºC. A specially designed all-graphite furnace was use for experiments. Kinetic of the growth was monitored by in-situ measurements of conductance of the growing films. In order to assess the type of conductance of the nanofilms, the temperature dependence of their conductance was measured from room down to liquid nitrogen temperature. The temperature dependence of conductance indicates that the carbon nanofilms consist of amorphous (semiconductor-like) and metallic carbon. The developed method of growth of carbon nanofilms promises to be inexpensive and compatible with existing Si-wafer based electronic technology.

Transport Experiments with Dirac Electrons

J. Checkelsky, L. Li, N.P. Ong, Y.S. Hor and R.J. Cava, Princeton University

Dirac electrons, charged spin ½ particles which are characterized by a linear energy-momentum dispersion relation, have been recently shown to exist in solid state systems. The surface of the 3D topological insulators and the 2D system graphene are two such systems. The ubiquity of quantum spin Hall physics and protected, current carrying states in low-dimensional Dirac systems has driven interest in observing transport currents in these materials. Transport experiments are presented here characterizing both systems to high magnetic fields. First, we report an insulating state induced in graphene at the Dirac point in magnetic field indicating that the protected states can be destroyed. Second, in the topological insulator Bi2Se3 we report conducting, metallic in-gap states. This demonstrates the existence of robust current carrying modes originating from the surface band structure.

Landau Level Quantization and Slow-Down of Electrons in Twisted Graphene Layers

Adina Luican¹, Guohong Li¹, Alfonso Reina², Jing Kong², Rahul R. Nair³, Kostya S. Novoselov³, Andre. K. Geim³, and Eva.Y. Andrei¹, ¹Rutgers University, ²MIT, ³University of Manchester, UK

A twist between stacked graphene layers produces a super-lattice which for certain rotation angles gives rise to Moiré patterns. These patterns are often seen in STM images, but their effect on the electronic properties is not fully understood. Using scanning tunneling microscopy and spectroscopy, we obtain direct evidence for the electronic structure of twisted graphene layers. The samples were suspended membranes of CVD grown graphene which contain areas with various rotation angles. We find that the density of states on twisted layers develops two Van Hove singularities that flank the Dirac point at an energy that is proportional to the twist angle [1]. In the presence of a magnetic field the density of states develops quantized Landau levels (LL) characteristic of massless Dirac fermions. From the energy and field dependence of the LL sequence we obtain the Fermi velocity and find that it is renormalized by an amount that depends on the angle of rotation. These results are compared with theoretical predictions.

[1] Li, G.; Luican, A.; Lopes dos Santos, J.M. B.; Castro Neto, A. H.; Reina, A.; Kong, J.; Andrei, E.Y. Observation of Van Hove singularities in twisted graphene layers. Nature Physics 2010, 6, 109-113.

Proposed Physical Mechanism of Chromosome Segregation in Caulobacter Crescentus

Edward J. Banigan, University of Pennsylvania, Michael Gelbart, Princeton University, Zemer Gitai, Princeton University, Andrea J. Liu. University of Pennsylvania and Ned S. Wingreen, Princeton University

Chromosome segregation is a fundamental process for all cells, but the force-generating mechanisms that drive chromosome movements in bacteria are especially unclear. In Caulobacter crescentus, recent work has demonstrated that a structure made up of the ParA protein elongates from one cell pole and interacts with ParB, a protein binding to the chromosome near the origin of replication (ori). ParB disassembles ParA, causing ParA to pull ParB, and thus, the ori to the opposite end of the cell. We performed Brownian dynamics simulations of this system in order to uncover the physical mechanism of this motion. We find that motion of the ori is robust to several variations of the model as long as a steady-state concentration gradient of ParA is established in the moving frame of the ParB-decorated chromosome. We suggest that the mechanism is "self-diffusiophoretic'': by disassembling ParA, ParB creates a concentration gradient of ParA so that the ParA concentration is higher in front of the chromosome than behind it. Since the chromosome is attracted to ParA via ParB, it moves up the gradient in the desired direction.

Characterization of Graphitic Thin Films and Top-Gated Klein Transistors

Milan Begliarbekov, Onejae Sul, Nan Ai, Chen-En Tsai, Johanna Heureaux, Eui-Hyeok Yang, and Stefan Strauf, Stevens Institute of Technology

Single and few layer graphitic thin films are very promising for future applications in nanoelectronics and provide an ideal material for the study of quantum transport phenomena. We employed Raman spectroscopy in order to perform layer metrology and identify the number of constituent graphene layers which have been micromechanically exfoliated from natural graphite. Furthermore, we have directly identified zigzag and armchair edges by their characteristic Raman signatures caused by the Kohn anomalies in the graphene spectrum. Building on these results we have fabricated top-gated graphene field-effect transistors by electron beam lithography, which consist of lateral source and drain contacts and an electrically insulated top gate of 50 nm width fabricated on top of a 10 nm aluminum-oxide dielectric layer. Two terminal measurements were then carried out to determine the location of the Dirac point. A room temperature mobility of 3700 cm2V-1s-1 was determined from the transconductance measurements. An n-p-n transistor geometry was created by electrostatic biasing of the top gate with respect to the substrate, which acts as a global back-gate. In this configuration we observed the characteristic signature of chiral Klein tunneling at 4K. The magnitude of Klein-tunneling decreases with increasing temperature until it ceases at around 66 K in our device. The observation of Klein tunneling in these structures is useful in ascertaining the ballistic mean free path for future studies of coherent transport phenomena.

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