The Third Gotham-Metro Condensed Matter Meeting

Overview

The biannual Gotham-Metro Condensed Matter Meeting was launched in April 2009 to provide a forum for researchers studying condensed matter physics at institutions across the greater New York City metropolitan area. The conference is organized by graduate students and highlights their work through presentations and poster sessions, and features keynote presentations in both hard and soft condensed matter.

The third event was held on April 9, 2010, and featured keynote presenters Hari Manoharan of Stanford University and David Weitz of Harvard University. Additionally, six students provided a sampling of the research underway at New York area institutions. The day ended in an interactive discussion with the audience on the realities of being a physicist, led by R. Shankar of Yale University.

Hard matter sessions

In "Quantum Imaging of Topologically Ordered Matter," Hari Manoharan explained how his lab uses scanning tunneling microscopy to measure Berry phases in nanostructures. When a material undergoes a topological deformation and is then restored to the original state, the final state is classically understood to be identical to the original. However, if the deformation traces a closed loop in phase space encircling a singularity in the potential energy surface, the system may acquire a non-trivial quantum mechanical phase factor. The observability of this geometric (or Berry) phase was long debated, but is now widely accepted. To observe the Berry phase, an experiment must measure the quantum mechanical wavefunction itself, including the phase information that is lost in the directly observable probability distribution function. The Manoharan lab has devised several experiments to discern the phase of the wavefunction (Moon, Mattos, et al. 2008; Moon, Lutz, et al. 2008; Moon et al. 2009), and has now applied these methods to measure the Berry phase in several materials and nanostructures, including a square quantum corral and graphene.

Since its first experimental realization in 2004 (Novoselov et. al. 2004), research on graphene has grown exponentially to make it one of the most popular topics in physics. Graphene, a single atomic layer of carbon atoms bonded in a hexagonal lattice, has several unusual and unique properties, including zero effective mass electrons, high electron mobility, and large mechanical strength. Due to the favorable electronic properties, graphene is widely under investigation as a semi-conductor to replace silicon in electronics applications. Several of the student presentations at the Gotham-Metro Condensed Matter Meeting focused on further elucidating the properties of graphene, and one focused on developing a scalable method of production.

Milan Begliarbekov presented the results from extensive optical and electrical characterizations of graphene samples in "Characterization of Graphitic Thin Films and Top-Gated Klein Transistors." The overall goal of Begliarbekov's work is to study coherent electrical transport in graphene systems to assess their applicability for electronic applications. The results presented include an analysis of Raman spectroscopy to identify the number of graphene layers in a sample, and a novel method to determine the edge-state purity. Using these results, Begliarbekov and his colleagues built a nanoscopic field-effect transistor in which they observe evidence of Klein tunneling. This work was performed at Stevens Institute of Technology as a collaboration of the NanoPhotonics Laboratory, directed by Stefan Strauf, and the NanoElectronics Laboratory, led by Eui-Hyeok Yang.

Adina Luican, a student with Eva Andrei in the Rutgers University Department of Physics & Astronomy, looks beyond single layer graphene to study how the angle between layers affects the electronic properties of stacked graphene layers. The presentation "Landau Level Quantization and Slow-Down of Electrons in Graphene Layers" begins with a visualization of the striking Moiré patterns created by stacking hexagonal lattices. The superperiod of these structures decrease with increasing angle of rotation between the two layers. The Moiré patterns of stacked graphene are visible in a topograph obtained by scanning tunneling microscopy; the twist angle between the layers is identified from the size of the superperiod. With this information, the electronic properties of stacked graphene layers as a function of rotation angle can be studied. The energy separation between the van Hove singularities on the two layers is dependent on the degree of rotation angle. Luican's experimental data matches the theory, and demonstrates that the distance between van Hove singularities increases with twist angle. In the presence of a magnetic field, the density of states of the stacked graphene layers develops quantized Landau levels. Utilizing the Landau level sequence, the Fermi velocity renormalization is shown to be dependent on the rotation angle between layers.

Joseph Checkelsky, a student in the laboratory of N.P. Ong at Princeton University, presented his work on the transport properties in both graphene and topological insulators in "Transport Experiments with Dirac Electrons." Unlike "normal" materials, where charge is carried by massive particles (e.g., electrons), in Dirac materials the charge carriers are massless, leading to a photon-like dispersion relation. This dispersion leads to interesting electronic properties. The quantum spin Hall effect (QSHE) was predicted to exist in graphene by Kane and Mele (Kane, 2005), but experiments found the spin-orbital coupling too small for the state to be observed. QSHE was first observed in a layer of HgTe sandwiched between CdTe layers (König, 2007), and later in a 3D topological insulator Bi₂Se₃.Ca_x (Hsieh, 2009). Checkelsky's approach is to chemically dope the Bi₂Se₃ with calcium to move the chemical potential down from the conductance band into the energy gap. Once the desired property is engineered, thin crystals of the material are transferred to a dielectric substrate. They then use an electric field to sweep chemical potential through the gap and find the in-band states to be metallic. Measurements on graphene reported show that the conductance is suppressed at high magnetic field precluding the observation of QSHE in graphene.

In order for graphene to be successful in electronic applications, a scalable method of production needs to be developed. The goal of the research presented by S.V. Samsonau in "Carbon Nanofilms on Insulating Substrates" is to grow the carbon film directly on a dielectric substrate. This method is technologically simple and inexpensive. Three dielectric materials were tested as substrates—diamond, sapphire, and quartz. In-situ measurements of conductance provide evidence of the growing carbon nanofilm and subsequent measurements of conductance as a function of temperature determines whether the film is metallic or a semi-conductor. Of the three substrates, quartz proved to be the most promising, and provided a uniform carbon film with good conductance. Further research on the properties of the film is necessary to determine if this method produces viable graphene. This work is performed with Alexander Zaitsev in the Nanofabrication Laboratory in the College of Staten Island.

Soft matter sessions

David Weitz's laboratory designs devices to engineer and measure new materials on the micrometer scale, as he explained in "Microfluidics for Making and Studying New Soft Materials." The microfluidic devices impart complete control over the fluid at the microscopic level, making it possible to formulate novel, interesting materials through the formation of droplets. Weitz explained how his group is able to form monodisperse emulsions, and extend the method to form multiple emulsions—droplets within droplets—in any combination of number and size. This method can be used to make vesicles to encapsulate a liquid for controlled release, without waste of the encapsulated liquid, as well as torroidal particles, liquid crystal shells, microgels, magnetic micro-particles, microscale chemistry reaction vessels, and more.

One major limitation of materials created with microfluidics is scale—even at thousands of drops per second, the output of a standard microfluidic device is not sufficient to create large enough quantities of the new material for applications. The Weitz group has solved this problem of scaling up by developing a method of creating chemically robust microfluidic devices with soft lithography, coating the PDMS device with glass. This allows parallelization of the devices in two- and even three-dimensions. The easy-to-replicate, parallel devices quickly scale-up production of the new materials. Soon you may even eat synthetic "caviar"! Weitz has co-founded the company Capsum to use this microfluidic technology to design new materials for food and cosmetics.

Research in David Grier's laboratory at New York University focuses on the interactions, dynamics, and collective properties of colloidal suspensions of micrometer-scale spheres. In particular, the group uses holographic optical trapping and microscopy techniques to analyze the colloidal suspensions. In holographic microscopy, the light scattered from the particles form interference patterns that can be analyzed using Lorenz-Mie scattering theory to determine characteristics of the particles, such as radius and position. In the presentation "Single-Frame Holographic Particle Image Velocimetry," Lisa Dixon explained that in a dynamic system, the interference fringes captured by a camera are blurred in the direction of the motion due to the finite shutter speed, but are present in the perpendicular direction. This pattern allows for determination of the in-plane velocity of the spheres from a single frame. Ms. Dixon showed results from both simulation and an experiment on a dilute colloidal suspension flowing through a thin channel.

In the presentation "Proposed Physical Mechanisms of Chromosome Segregation in Caulobacter crescentus," Edward Banigan described the Brownian dynamics simulations he has developed in the group of Andrea Liu at the University of Pennsylvania. The goal of this work is to elucidate the physical mechanism of chromosome segregation in the bacteria Caulobacter crescentus. During cell replication in this bacterium, one copy of the origin of replication must transverse the cell before division, but the mechanism of this motion is not well understood. Previous work (Fogel, Waldor, 2006) shows evidence that the origin is pulled across the cell by a structure of the protein ParA interacting with the origin-binding protein ParB. During the cell division, a structure of the protein ParA nucleates at the far end of the cell and polymerizes in a band toward the origin. The ParB bound to the origin reacts with the ParA polymer and pulls the origin across the cell as the ParA depolymerizes. The simulations presented by Banigan show that the rate of motion of the origin is dependent only on the rate of disassembly of the ParA structure, and is robust to several changes in the model. This evidence suggests that the motion is driven by self-diffusiophoresis—the ParB is attracted to regions of high concentration of ParA, and the concentration profile of ParA translates along with the ParB, thus the ParB and the origin are pulled across the cell.

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Presentations are available from:

David Weitz (Harvard University)
Edward J. Banigan (University of Pennsylvania)
Milan Begliarbekov (Stevens Institute of Technology)
Joseph Checkelsky (Princeton University)
Lisa Dixon (New York University)
Adina Luican (Rutgers University)
S.V. Samsonau (The College of Staten Island, CUNY)

Sponsors

Silver

Academy Friends

• New York University Materials Research Science & Engineering Center
• Brookhaven National Laboratory
• CUNY Office of the Vice Chancellor of Research