Calendar for Summer 2015

  • May 27
  • (In)stability of anti-de Sitter Spacetime
    Nils Deppe, Cornell Physics
    Show Abstract
    The AdS/CFT correspondence is currently a very exciting topic of research, with Maldacena's original paper now having over 10,000 citations. Maldacena's conjecture states that an n-dimensional CFT is dual to an n+1-dimensional gravity theory. In 2011 Garfinkle and Pando Zayas, and Bizon and Rostworowski performed detailed numerical studies of the (in)stability of AdS spacetime to massless scalar field perturbations, which is related to thermalization in the boundary CFT. Since then further work, both analytical and numerical, has been done to better understand the dynamics. Arguably the most important question is whether or not AdS is stable against the formation of black holes for arbitrarily small perturbations. This addresses the question of whether or not strongly coupled CFTs thermalize. I shall present an overview of what has been accomplished to date, followed by a discussion of ongoing work. This will include results on the behavior of massive scalar fields and critical phenomenon in AdS. Using perturbative and numerical results I will argue that black hole formation is inevitable in AdS for any small perturbation, with the only exception being finely tuned initial data that leads to a stable evolution.

    Download Additional Material
  • June 3
  • Ocean Acoustics and the SOFAR Channel
    Brian Worthmann, Applied Physics / Mechanical Engineering
    Show Abstract
    Everyday, we have experience with acoustics, but the physics that allows that to happen can be applied more broadly, with sometimes surprising results. In this talk, I will give a brief introduction to acoustics and its governing equations, discuss the wide variety of acoustics research going on today, and then discuss the field of ocean acoustics more thoroughly. We'll focus on the deep-ocean SOFAR channel, and we'll find that some very simple assumptions about the ocean environment can lead to very interesting acoustic wave propagation physics, with surprising real-world results and applications.

    Download Slides
  • June 10
  • Spintronic Devices and Spin Physics in Bulk Semiconductors
    Marta Luengo-Kovac, Physics
    Show Abstract
    Moore’s law, the observation that the number of transistors on a chip doubles every year, cannot hold indefinitely. As the information density of traditional computers reaches its limit, and their power consumption increases, researchers are looking for alternatives. While there is much interest in quantum computation, the field of spintronics is also a very promising alternative, with several devices already on the market. In this talk, I will discuss several of these devices and how they compare to their traditional counterparts, as well as my own research on creating and manipulating spin polarizations in bulk semiconductors all-electrically.

    Download Slides
  • June 17
  • Engineering the Properties and Defects of InAsSb by Atomic Surface Structure
    Evan Anderson, Material Science, Advisor: Joanna Millunchick
    Show Abstract
    Due to the recent discovery that InAsSb can be synthesized with a bandgap minimum corresponding to a wavelength of approximately 12 microns, it has become appealing for a variety of long wavelength infrared (LWIR) applications, such as renewable energy (thermophotovoltaics), automotive safety (pedestrian sensors), and the military (night vision). For these applications, it is essential to synthesize InAsSb with minimal defects and understand the parameters needed to avoid incorporating point defects such as contaminants, vacancies, and antisites. Since all processes that occur during the growth of InAsSb occur at the surface of the material, the arrangement of atoms on the surface (surface reconstructions) must be well understood to control the incorporation of point defects. I will discuss the use of a combination of computational and experimental techniques to achieve this goal.
  • June 24
  • Galaxy Evolution in X-Ray Selected Clusters and Groups in Dark Energy Survey Data
    Yuanyuan Zhang, Physics, Advisor: Timothy McKay
    Show Abstract
    The Dark Energy Survey (DES) embarked on a 5 year survey of 5000 deg^2 sky in 2013, providing a vast data set for studying the evolution of galaxy clusters and groups. Galaxy clusters and groups contain enormous amount of baryonic matter and dark matter, leaving prints across the electro-magnetic spectrum. Using optical imaging data from Dark Energy Survey, we have confirmed ~ 200 clusters and groups discovered with XMM-Newton archival data. In turn, this X-ray selected sample provides a well-understood data set for studying their optical content with Dark Energy Survey data. A subsample of the confirmed clusters and groups have been used to study the evolution of central galaxies. The study supports previous speculation that central galaxies grow slower than the prescription from simple semi-analytical modeling. We also show that intra-cluster light may play a greater role than previously assumed.

    Download Slides
  • July 1
  • Information Divergence Estimation in Signal Processing and Machine Learning
    Kevin Moon, Electrical Engineering: Systems, Advisor: Alfred Hero
    Show Abstract
    Information divergence is a measure of difference between probability distributions and is important in the fields of machine learning, signal processing, statistics, and information theory. Special cases of divergence include the common measures of information known as entropy and mutual information. This talk presents multiple applications of divergence functional estimation primarily in the context of sunspot image classification, focusing on the problems of dimensionality reduction, extending existing machine learning tasks to probability distributions as features, and estimating the optimal probability of error (the Bayes error) of a classification problem. We then present a simple, computationally tractable non-parametric estimator ofa wide variety of divergence functionals that achieves parametric convergence rates under certain smoothness conditions. This estimator is demonstrated by estimating bounds on the Bayes error for a classical machine learning data set

    Download Slides
  • July 8
  • Partonic Dynamics in High-Energy Proton-Proton Collisions at PHENIX
    Joseph Osbourne, Physics, Advisor: Christine Aidala
    Show Abstract
    In general we think of the\ proton as composed of 3 point like valence quarks, but in reality the structure of the proton is significantly more complicated. Investigating the interactions of quarks and gluons, collectively known as partons, to better understand nucleon structure is one of the primary goals of the PHENIX experiment at the Relativistic Heavy Ion Collider (RHIC). RHIC is the only facility in the world that is capable of colliding transversely or longitudinally polarized proton beams, at center of mass energies of 62-510 GeV. The PHENIX detector is well suited to study partonic interactions due to its electromagnetic calorimetry at midrapidity and forward rapidity. In this talk I will give an introduction to the surprisingly complex structure of the proton and discuss the most recent data set taken at PHENIX in which a new detector was installed to better probe the dynamics of partons within strong-force bound states.

    Download Slides
  • July 15
  • Measuring the Stark Shift in the Two-Photon Cross Section of Fluorescent Proteins
    Elisabeth Maret, Applied Physics, Advisor: Jennifer Ogilvie
    Show Abstract
    Two-photon fluorescence imaging has vast improvements over its one-photon counter part for imaging biological systems, including reduced contributions due to scattering, z-axis dependent focusing, ease of detection geometry and reduced out of focus photobleaching. Central to the development and versatility of fluorescence imaging is green fluorescent protein (GFP) and its ability to be expressed in various mammalian systems without the need for species-specific cofactors. Manipulation of GFP’s chromophore amino acid structure has also enabled the creation of many GFP mutants with a range of emissions wavelengths spanning from the blue to the red. Stark spectroscopy has revealed that GFP mutant emission wavelengths are dependent on the local electric field produced by the chromophore’s amino acid sequence, suggesting a dependence on the chromophore’s difference dipole moment. Coincidentally, two-photon absorption is also dependent on the difference dipole moment, suggesting that changing the electric field environment of the protein can shift the emissions spectrum of the protein’s two-photon response. If it is possible to characterize and monitor the shift in two-photon emissions from an applied electric field, this effect offers many potential improvements to two-photon fluorescence imaging of biological processes, like neuron action potential events, which involve the movement of ions across the neuronal membrane.
  • July 22
  • Hyperbolic Metamaterials Based on Graphene-Dielectric Multilayers
    You-Chia Chang, Applied Physics, Advisor: Ted Norris
    Show Abstract
    Hyperbolic metamaterials (HMMs) are artificially-structured materials engineered to create hyperbolic dispersion for optical waves. They have attracted attention because of the supported propagating high-k modes and the enhanced photonic density of states. While metal is the most common conducting constituent element in HMMs, graphene provides another useful building block, i.e., a truly two-dimensional (2D) conducting sheet whose conductivity can be controlled by doping. In this presentation, I will first introduce an ellipsometry technique we developed specifically for characterizing optical properties of 2D materials. Following that, I will present our recent experimental realization of a multilayer structure of alternating graphene and aluminum oxide layers, a structure similar to the metal-dielectric multilayers commonly used in creating visible-wavelength HMMs. Our characterization with an infrared ellipsometer demonstrates that the metamaterial experiences a optical topological transition from elliptic to hyperbolic dispersion at mid-infrared frequencies.
  • July 29
  • Ultra-thin, Smooth and Low-loss Al-doped Ag Film and its Application in Optoelectronic Devices
    Cheng Zhang, EECS, Advisor: L. Jay Guo
    Show Abstract
    Thin Ag films are widely used \ in various devices such as solar cells, plasmonic devices and meta-materials due to its good conductivity and low loss in the visible and NIR regions. However, it is very difficult to obtain a very thin and smooth Ag film due to its tendency to form 3D islands during the film growth. We have developed an effective approach to achieve ultra-thin and smooth Ag films by co-depositing a small amount of Al during the film deposition. The film can be as thin as 6 nm on various substrates without any wetting layer, and with sub-nm roughness. It has significantly improved thermal and long-term stability compared with pure Ag. Al-doped Ag has been employed in applications such as semi-transparent conductor for organic solar cells, long range surface plasmonic waveguide, and hyperbolic metamaterials.
  • Aug 5
  • NOTE: This week has NO talk.
  • Aug 12
  • Now That We Found the Higgs Boson, What Next?
    Zhengkang (Kevin) Zhang, MCTP / Physics, Advisor: James Wells
    Show Abstract
    The discovery of the Higgs boson in 2012 completed the verification of the Standard Model of particle physics, but there are good reasons to believe there is new physics and to search for it. Effective field theories (EFTs), based on the idea of renormalization group, provide a model-independent framework to study the effects of new particles if they are too heavy to be directly observed at colliders. Though the idea is old and well-known in many areas of physics, subtleties arise when it is applied to particle physics which have not been fully appreciated by the community. I will give a pedagogical overview of particle physics in the post-Higgs-discovery era, highlighting recent efforts toward consistent use of EFTs when comparing theory calculations with data.
  • Aug 19
  • Radiations, Photons and Interference
    Uttam Paudel, Physics, Advisor: Duncan Steel
    Show Abstract
    Willis Lamb once said that a license should be required for the use of the word “photon”, and he would not hand out very many written licenses. So far Quantum Electrodynamics (QED) has been a complete theory of electromagnetic (EM) fields that in essence has explained every aspect of light-matter interactions. Yet there are numerous rich phenomena that arise in light-matter interactions at low energy scales that were previous unknown and are actively being studied by the quantum optics community. In addition, quantum optics has been playing a leading role in the testing of the axioms of quantum mechanics, while also exploring ways to build new technologies exploiting quantum phenomena. In this talk, I will give a historical review of the development of the idea of photons and elucidate why there was some resistance in the quantum optics community to the notion of a quantized EM field, some even claiming that QED was unnecessary to explain the low energy EM field. I will also discuss photon statistics and photon interference, drawing examples from some historical experiments that played a pivotal role in clearing some of the doubts that people had about the nature of the EM field. If time permits, I will give a brief introduction to my work in a related field.
  • Aug 26
  • Glaciers, Ice Sheets and Sea Level Rise
    Yue Ma
    Show Abstract
    Ice sheets are an important component of our climate system because they not only cover a large proportion of the earth's surface, but also are an integral part of the global climate system. As the biggest source of the world's freshwater, the annual discharge from the ice sheets into the oceans is comparable with the annual discharge of the Amazon River. Radically altering the flux of cold freshmelt water from ice sheets has the potential to alter the thermohaline ocean circulation. Moreover, if the ice sheets were to completely melt they contain enough water to raise sea level by more than 70m. Hence, even small changes in ice sheet volume can have devastating effects on coastal communities. Mass is lost from ice sheets through surface and basal melting and iceberg calving. Currently, iceberg calving accounts for nearly 50% of the mass lost from both the Antarctic and Greenland ice sheets. However, the complexity, heterogeneity and diversity of processes involved in understanding calving have hindered attempts to develop parameterizations of calving that can be implemented in numerical ice-sheet-climate models to predict ice sheet retreat, severely limiting ice sheet model performance and introducing large uncertainties into sea level rise projections corresponding to past, present and future climate changes. Here we examine the calving law using a 2D full Stokes finite element model along the center flow of an idealized tidewater glacier in order to answer the question: why are glaciers the way they are?

Maintained by Christopher Barnes (barnchri[at]umich[dot]edu)
PGSS at the University of Michigan, Summer, 2016