Forum Schedule Spring 2017

Fridays 3:45pm - 4:45pm BPB-217

Date Speaker Topic (click down-arrow to see abstract)
Jan 20 David Pappas
host: Eunja Kim
Electric-field Noise from Ion-Trap Electrode Surfaces

Electric-field noise from the surfaces of ion-trap electrodes couples to the ion's charge causing heating of the ion's motional modes. This heating limits the fidelity of quantum gates implemented in quantum information processing experiments. The exact mechanism that gives rise to electric-field noise from surfaces is not well-understood and remains an active area of research. In this talk, I will detail experiments intended to measure ion motional heating rates with exchangeable surfaces positioned in close proximity to the ion, as a sensor to electric-field noise. We have prepared samples with various surface conditions, characterized in situ with scanned probe microscopy and electron spectroscopy, ranging in degrees of cleanliness and structural order. The heating-rate data, however, show no significant differences between the disparate surfaces that were probed. These results suggest that the driving mechanism for electric-field noise from surfaces is more than that due to thermal excitations alone.

David Pappas a Group Leader at the NIST in the Quantum Electrical Metrology Division in Boulder, CO. He is an APS Fellow, a Senior Member of the IEEE, Silver Medal awardee at NIST, and was awarded an NSF Young Investigator award. He received a B.A. in Physics from the University of Colorado in Boulder and Ph.D. at the University of California, Irvine working in the area of magnetism and quantum effects at crystalline surfaces. At UCI David worked as an IBM Graduate Research Fellowship and went on to do post-doctoral work first at IBM and then at the Naval Research Laboratory as an Office of Naval Technology Research Fellow. David was an Assistant Professor at Virginia Commonwealth University and then returned to Boulder to work at NIST in the magnetic data storage project. He continued basic research into the physics of magnetic phase transitions at surfaces and also developed real-time imaging of magnetic tapes for forensic analysis in collaboration with the FBI. He is currently working in the area of reducing loss and improving coherence of macroscopic quantum systems using superconducting materials as well as reducing anomalous heating of ions in ion traps.

Jan 27 David Johnston
host: Jason Steffen
Deep Learning, Artificial Intelligence and the Internet of Optimization

Recent advances in the fields of optimization and machine learning are ushering in an age where algorithms, connected in global networks are involved in sophisticated decision making and are taking over a larger part of the human economy. Deep learning, a renaissance in the field of neural networks, is now fully competitive with humans on many tasks in voice and image recognition, natural language processing and translation. Reinforcement learning systems, aided in part by deep learning, are now capable of piloting cars and aircraft, orchestrating robotic motion, out-competing humans at sophisticated strategy games such as Poker and Go and can learn to do this largely without human interaction and training. While the excellent performance of such systems are demonstrable, the reasons behind the effectiveness of deep learning is still being worked out. The field of mathematical optimization is experiencing a boom of it's own. Optimization, the workhorse behind much of machine learning, is just as in important in it's own right. New algorithms such as the Alternate Direction Method of Multipliers enable the concept of an Internet of Optimization; a network where parties not only exchange information but seek out and discover economic trade-offs in order to arrive at an optimal solution toward a shared goal. Applications include the running of the smart grid, distributed routing of vehicles, operations of new markets and the automated running of supply chains. In this talk, we'll discuss these advances, how they are being applied and some of the open questions remaining. Besides applications in industry, we'll emphasize how scientists can benefit by mastering this new set of tools to increase their own productivity.

Feb 3 Jack Lissauer
NASA Ames Research Center
host: Jason Steffen
Kepler's Multiple Planet Systems

More than one-third of the 4700 planet candidates found by NASA’s Kepler spacecraft are associated with target stars that have more than one planet candidate, and such “multis” account for the vast majority of candidates that have been verified as true planets.The large number of multis tells us that flat multiplanet systems like our Solar System are common. Virtually all of the candidate planetary systems are stable, as tested by numerical integrations that assume a physically motivated mass-radius relationship. Statistical studies performed on these candidate systems reveal a great deal about the architecture of planetary systems, including the typical spacing of orbits and flatness. The characteristics of several of the most interesting confirmed Kepler multi-planet systems will also be discussed.

Feb 10 Amanda Truitt
Arizona State University
host: Jason Steffen
The Diversity of Chemical Composition in Stars and Implications for Planetary Habitability

I have investigated how stars of different mass and composition evolve and how the stellar evolution impacts the location of the habitable zone (HZ) around a star. Current research into the habitability of exoplanets focuses mostly on the concept of the "classical HZ" – the range of distances from a star over which liquid water could exist on a planet's surface. This location is determined by the host star's luminosity and spectral characteristics, which are derived from the stellar composition. Though stellar evolution is not the only consideration to understand planetary habitability, it is nonetheless one of the most important and physically well-understood factors. I have created over 900 stellar evolution models to represent a range of spectral types with variations in the specific oxygen, carbon, magnesium, and neon abundances. My catalog is designed for use by the astrobiology and exoplanet communities to characterize the evolution of host stars and their surrounding HZs for real planetary candidates of interest. I will discuss how stellar evolution provides insight into how we define the HZ, how these regions co-evolve with their parent star over time, and how this will apply to the continued search for Earth-like planets in the future.

Feb 17 Nick Higginbottom
University of Southampton, UK
host: Daniel Proga
Radiation Hydrodynamic simulations of Coronae and Disk winds in X-ray Binaries

A number of X-ray binaries exhibit clear evidence for the presence of disk winds in the high/soft state. A promising driving mechanism for these outflows is mass loss driven by the thermal expansion of X-ray heated material in the outer disk atmosphere. Here we present the results and future goals of an ongoing project to model these winds using a radiation hydrodynamic approach. We show that the interplay between heating and cooling rates cause significant changes to the nature of a thermally driven wind, and reasonable values of these rates can produce a wind of sufficient density to potentially destabilise the inner disk and provide a mechanism for state change.

Feb 24

Mar 3 Mark Tuckerman
New York University
host: Qiang Zhu
Exploration and learning of free energy landscapes of molecular crystals and oligopeptides

Theory, computation, and high-performance computers are playing an increasingly important role in helping us understand, design, and characterize a wide range of functional materials, chemical processes, and biomolecular/biomimetic structures. The synergy of computation and experiment is fueling a powerful approach to address some of the most challenging scientific problems. In this talk, I will describe the efforts we are making in my group to develop new computational methodologies that address specific challenges in free energy exploration and generation. In particular, I will describe our recent development of enhanced free energy based methodologies for predicting structure, polymorphism, and defects in atomic and molecular crystals, for exploring first-order phase transitions, and for determining conformational equilibria of oligopeptides. The strategies we are pursuing include heterogeneous multiscale modeling and learning techniques, which allow “landmark” locations (minima and saddles) on a high-dimensional free energy surface to be mapped out, and temperature-accelerated methods, which allow relative free energies of the landmarks to be generated efficiently and reliably. I will then discuss new schemes for using machine learning techniques to represent and perform computations using multidimensional free energy surfaces.

Mar 10

Mar 17 XiaoCheng Zeng
University of Nebraska, Lincoln
host: Qiang Zhu
Computer-Aided Nanomaterial and Nanostructure Design: Nanoice, Gold-clusters, Superhydrophobicity

In this talk I shall report several research findings from my group over the past few years, including: (1) New phases of low-dimensional nano-ice and ice clathrate; (2) superhydrophobic phenomena at the nanoscale, e.g. Lotus effect at nanoscale; and (3) growth pattern of small-sized gold clusters and nanogold catalysis.

Understanding physical and chemical properties of confined water at nanoscale has implications for diverse phenomena at the intersection between chemistry, biological sciences, and physics, such as boundary lubrication in nanofluidic devices and synthesis of antifreeze proteins for ice-growth inhibition. Gas hydrates are solid-state materials typically consisting of a host ice frame with nanoscale cages that serve to trap small guest molecules such as methane. Methane hydrate is one of the most significant energy sources; and gas hydrate formation is also a critical issue in deep-water gas/oil pipelines. Molecular simulations of low-dimensional gas hydrate formation may bring new insights into bulk gas hydrate formation. Bulk gold is known to be catalytically inert whereas gold nanoparticles can exhibit exceptional catalytic properties towards CO oxidation. We have performed a systematic study of catalytic activities of sub-nanometer gold clusters, hollow-cage clusters in particular, with or without metal-oxide supports, using density functional theory calculations.

Mar 23 12:30PM Eric Coughlin
University of California, Berkeley
host: Rebecca Martin
Stars destroyed by binary supermassive black holes: a numerical and statistical study

When a star gets too close to a supermassive black hole -- a gravitational behemoth that resides at the center of nearly every galaxy -- the tidal force of the black hole overwhelms the self-gravity of the star. The star is subsequently tidally shredded and stretched into a stream of debris that, at later times, returns to the black hole, forms an accretion disk, and temporarily lights up the galactic center. These ``tidal disruption events'' are unique probes of the centers of galaxies, giving us the opportunity to discern properties of quiescent black holes that are otherwise difficult to constrain observationally. I will describe recent work that investigates how a binary companion to the disrupting black hole alters both the tidal disruption process and the subsequent evolution of the tidally-disrupted debris. In particular, I will show that a supermassive black hole binary generates chaotic, three-body interactions that affect the pre-disruption orbit of the star, thereby providing a small enhancement to the tidal disruption rate. The post-disruption hydrodynamics of the disrupted debris are also modified considerably by the presence of the binary companion, leading to complex accretion morphologies and rates that vary over a range of spatial and temporal scales. These results have implications for current and upcoming wide-field surveys, which are predicted to discover hundreds to thousands of tidal disruption events over the next decade.

Mar 24

Mar 31 Mario Flock
NASA Jet Propulsion Laboratory
host: Zhaohuan Zhu
The Inner Rim in Protoplanetary Disks

Many planets orbit within an AU of their stars, raising questions about their origins. Particularly puzzling are the planets found near the silicate sublimation front. We investigate conditions near the front in the protostellar disk around a young intermediate-mass star, using the first global 3-D radiation non-ideal MHD simulations in this context.

The results show magnetorotational turbulence around the sublimation front at 0.5 AU. Beyond 0.8 AU is the dead zone, cooler than 1000 K and with turbulence orders of magnitude weaker. A local pressure maximum just inside the dead zone concentrates solid particles, allowing for efficient growth. Over many orbits, a vortex develops at the dead zone's inner edge, increasing the disk's thickness locally by around 10%.

We synthetically observe the results using Monte Carlo transfer calculations, finding the sublimation front is bright in the near-infrared. The models with vertical magnetic flux develop extended, magnetically-supported atmospheres that reprocess extra starlight, raising the near-infrared flux 20%. The vortex throws a non-axisymmetric shadow on the outer disk.

Radiation-MHD models of the kind we demonstrate open a new window for investigating protoplanetary disks' central regions. They are ideally suited for exploring young planets' formation environment, interactions with the disk, and orbital migration, in order to understand the origins of the close-in exoplanets.

Apr 7 Stephen Lubow
Space Telescope Science Institute
host: Rebecca Martin
Circumbinary Disks

Circumbinary disks are disks that orbit around binary systems. Their properties have been explored over the past few decades both observationally and theoretically. These disks play a vital role in accreting planetary systems, binary star systems, and supermassive black hole binaries. I will describe some dynamical processes involving these disks, including some recent developments.

Apr 14

Apr 21 Andrew Howard
host: Jason Steffen
Earth-size Exoplanets

Earth-size exoplanets and their slightly larger ‘super-Earth’ cousins are the most abundant planets orbiting close to Sun-like stars. These planets have diverse physical compositions, unusual atmospheres, and poorly understood origins. My talk will trace the discovery and early characterization of these small worlds through Doppler and transit surveys, and look forward to future discoveries with instruments such as the Keck Planet Finder.

Apr 28 James Owen
Institute for Advanced Study
host: Zhaohuan Zhu
The Kepler planets: a tale of evaporation

The Kepler mission has transformed our knowledge of the properties of extra-solar planets. It has told us a dominant - perhaps the dominant - population of exoplanets are those close to their parent stars with masses in the range 1-20 Mearth. Unlike the low-mass, close-in planets in our solar system these exoplanets are thought to contain voluminous H/He envelopes. I will discuss the evolution of this population of exoplanets, particularly with regard to mass-loss. It appears that as soon as these planets finish forming, and their parent disc disperses they begin a lifelong period of mass-loss and evaporation which imprints itself on the population we see today.

May 5 Brad Hansen
host: Jason Steffen
Secular effects of distant giant planets on compact planetary systems

Planet searches in recent years have unearthed planets with a wide range of properties, both in terms of mass and separation from their host stars. In particular, radial velocity searches have started to quantify the population of giant planets on scales of 1 Au and larger, while transit searches have unearthed a substantial number of sub-jovian planets on smaller scales. In this talk I will discuss the gravitational interactions between these two populations of planets and what effects this could have on the observed properties of systems which contain both classes around the same star.

May 12

Past forums: Fall 2016  Spring 2016  Fall 2015   Spring 2015   Fall '14   Spring '14   Fall '13   Spring '13   Fall '12 Spring '12   Fall '11   Spring '11   Fall '10   Spring '10   Fall '09   Spring '09   Fall '08