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Physics and Astronomy

Mechanical Properties of Suspended Graphene Sheets

Scott Michael Berkley ('09), Ian Frank ('08), David Tanenbaum

We studied the mechanical properties of suspended graphene sheets through measurements of effective spring constant (k). Graphene is a plane of carbon bound in a hexagonal pattern that is a single atom thick. Manual exfoliation is used to suspend stacks of graphene sheets across trenches etched into a silicon dioxide surface. The dimensions of the suspended sheets were measured by an atomic force microscope (AFM). Thickness varied from 2 to 80 nm (about 4 to 400 layers). By pushing the AFM tip of known k onto the graphene and measuring how much the graphene bends relative to the bending of the tip, we are able to infer the k of the graphene. We studied suspended graphene sheets with k’s of .6 to 8 N/m. Measurements were taken across the suspended structures and the k was calculated for each point on rectangular grids. The spatial variation in spring constant fit with qualitative expectations for some of the suspended sheets, but we also observed asymmetries in the spring constant which we attributed to non- uniform tension and surface irregularities. Due to this non-uniform tension, the effective spring constant of the suspended sheets cannot be modeled with simple beam theory.
Funding provided by: NSF Center for Nanoscale Systems (at Cornell University)

CFD Simulation of the Evolution of a High-Viscosity Fluid

Robert Bryant Foresman ('08), Catalin D. Mitescu

Highly viscous fluids exhibit characteristic behavior on an experimental time scale suited to high-speed digital image capture, providing a good method for validating results obtained by computational methods. We have explored the surface of a viscous filament, described as a thin cylinder of fluid evolving under the effects of surface tension and gravity, as well as the Rayleigh-Taylor instability – the characteristic- wavelength surface deformation of a fluid interface in a gravitational field. We utilize the finite-element computational fluid dynamics (CFD) program Fluent™, in which the fluid is meshed into an array of cells that discretize governing continuity equations. The two-phase, time-dependent Volume of Fluid (VOF) model was used to resolve the fluid domain. We observe the Rayleigh-Taylor instability in a fluid with a viscosity of 0.050 Pa s. The viscous filament calculation, in progress, is particularly time-consuming because of requisite mesh fineness and the small time-step size critical to cell residual convergence. Further parallel simulation on multiple processors should yield computational results suitable for comparison with experimental data.
Funding provided by: Rose Hills Foundation

Maskless Nanolithography With an Atomic Force Microscope (AFM)

Ian Ward Frank ('08), Marko Loncar*
*School of Engineering and Applied Sciences, Harvard University, Cambridge, MA

We have demonstrated that an AFM can be used to selectively oxidize substrates that can then be wet etched to form Reactive Ion Etching (RIE) masks. This method of lithography is advantageous due to its speed and low cost in comparison to Electron Beam Lithography and Focused Ion Beam etching. Line widths as low as 70 nm have been regularly obtained and further optimization promises line widths as low as 10 nm. The AFM has also been used to oxidize silver and zinc-oxide nanowires, possibly leading to the creation of gratings that would allow photons to be coupled in and out of the nanowires in their capacity as waveguides.
Funding provided by: NSF (NNIN, Harvard University)

Blazar Observation: Photometry

Daniel Ethan Sedlacek ('09), Caroline Fernandez ('09), Alma Zook

We report on the summer 2007 monitoring of blazars using Pomona College's 1-meter telescope at Table Mountain Observatory. The blazer 3C454 had an important “flaring” event toward the end of the summer. The flaring is believed to be as strong as when the same object flared up in 2005. We will be reporting on the implications and importance of the visible-light data gathered for all objects and especially blazer 3C454. Our work was funded in part by SURP grants from Pomona College.
Funding provided by: SURP

Using Frequency Modulation in an Optical Atom Trap

Eric Stutz ('10), Dwight Whitaker

When certain elements are cooled to within billionths of a degree of absolute zero, the atoms coalesce into a single quantum state, forming a Bose Einstein Condensate (BEC). The Whitaker lab is interested in exploring the phase transition between a normal cloud of cooled atoms and a BEC. In order to investigate this phenomena, we need to be able to accurately measure both the temperature and the density of the  atomcloud. We are able to measure both the temperature and the number of atoms, but the cloud of trapped atoms is too small to observe directly. we have been developing the use of parametric resonance - vibrating the atom trap and looking for resonant frequencies that eject the atoms – to measure the trap volume. Previously we have used amplitude modulation, which undulates the depth of the trap, with mixed results. This summer, however, we were able to successfully use frequency modulation – wiggling the trap from side to side – to test for resonance. We found this method to be more precise, allowing us for the first time to resolve the resonance occurring at one half natural frequency. We are now able to study BEC's much more thoroughly.
Funding provided by: SURP (Seaver)

VRI Blazar Monitoring Using Polarimetry at the Table Mountain Observatory

Caroline Anne Fernandez ('09), Daniel Sedlacek ('09), Alma Zook

We report on continued blazar monitoring at Pomona College’s Table Mountain Observatory during the summer of 2007. Our observations include follow-up work using polarimetry from the previous summer, specifically monitoring object 3C454 towards the end. This object got brighter during this time period, similar to the summer approximately two summers ago. Our work was funded in part by SURP grants from Pomona College.
Funding provided by: SURP (Richter)

Extremely Metal-Poor Damped Lyman-A Systems at High Redshift

Irene Toro Martinez ('09), Dan Beeler ('09), Bryan E. Penprase, Wallace Sargent*, Jason Xavier Prochaska**
*Department of Astronomy, California Institute of Technology, Pasadena, CA **Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA

Damped Lyman-a systems (DLAs), massive clouds of gas that can be observed as absorption in the spectra of background quasars, are used to probe the early universe. Metal-poor DLAs are intriguing because they provide information on the conditions in which the first stars and galaxies formed. I present results from the thorough spectral analysis of several DLAs in the lines of sight of 11 optically bright quasars at redshifts 2 < z < 4 selected from the Sloan Digital Sky Survey for their low metallicity potential. The observations were made with Keck/Echellete Spectrograph and Imager on 16 March 2007, and the data were reduced with the IDL programs cont_fit and ioncheck. The DLA HI column densities range from 20.30 < log(NHI / [c -2]) > 21.25, and very low abundances of CII, OI, SiII, AlII, AlIII, FeII, MgII, MnII, and SI are reported. The DLA in the line of sight towards SDSS0814+5029 is extremely metal-poor, with [CII/H] = -3.45, just 0.0003 of the solar carbon content, and [OI/H] = -3.50, [SiII/H] = -2.95, [AlII/H] = -3.45, and [FeII/H] = -2.95. These abundances, and those in other DLAs in our sample, are the lowest found to date.
Funding provided by: SURP (Richter - ITM)

Research at Pomona