We have made a new type of X-ray Diffractometer that uses CCD technology developed for X-ray astrophysics. The instrument provides X-ray diffraction and fluorescence information from unprepared samples. This can be used to identify minerals and provide surface texture information. The instrument is designed to minimize mass, power, and risk for planetary exploration.

Location: Physics Bldg., Room 401
Coffee: 3:15 p.m.

In this talk I show results of cosmological simulations suggesting a possible identification of at least some dwarf spheroidal galaxies in the Local Group as the fossils of the first galaxies (``pre-reionization fossils''). I also revisit the problem of gas accretion onto minihalos after reionization. I show that primordial minihalos with v_{cir}<20 km/s stop accreting gas after reionization, as it is usually assumed, but in virtue of their increasing concentration and the decreasing temperature of the intergalactic medium as redshift decreases, they may have a late phase (at redshift z<2) of gas accretion and possibly star formation. As a result we expect that pre-reionization fossils have a more complex star formation history than previously envisioned. The dwarf spheroidal galaxy Leo~T fits with this scenario. Another prediction of the model is the existence of a population of gas rich minihalos that never formed stars. A subset of compact high-velocity clouds may be identified as such objects but the bulk of them may still be undiscovered.

Location: Physics Bldg., Room 401

Hao successfully defended his thesis proposal on November 14, 2008.

TITLE:
Theoretical Comparison of Optical Approaches to Quantum Logic Gates

ABSTRACT:
Quantum computing has been of intense interest over the last 10 years because of its promising ability to do high-speed factoring and its potential for the efficient simulation of quantum dynamics. It could be implemented in many different ways using optical techniques. A better understanding of the advantages and disadvantages of these approaches would allow the experimental groups working in this area to optimize their choice of experiment and to concentrate on the approaches that are most likely to succeed. We propose to systematically analyze a number of promising optical implementations for quantum logic gates using standard density matrix theory. Basic issues of theoretical comparison of optical approaches to quantum logic gate will be presented here.

Andrew successfully defended his dissertation on November 18, 2008.

TITLE:
Step Dynamics and the Morphological Evolution of Nanostructures

ABSTRACT:
Surface diffusion of atoms plays a significant role in the evolution of the shape of a material in the nanoscale regime because it becomes the dominant mechanism for mass transport as the surface area to volume ratio increases. In this dissertation, I present a theoretical study of the shape evolution of a particular nanostructure: the nanowire. At non-zero temperatures, atomic steps are always present on the surface of a nanostructure and their growth and motion causes a change of shape and can cause the wire to break.

I have studied the morphological evolution of nanowires before and after a break develops, the resulting segments of a broken nanowire whose tips are nanoneedles. My approach is to develop and compare a step motion model and atomistic simulations and test them against available experimental data and classical continuum theory. My approach also allowed me to derive the scaling relations and exponents describing the morphological evolution of nanowires and study the microscopic nature of the instability that causes them to break.

Jeff successfully defended his dissertation on November 19, 2008.

TITLE:
Displacement Damage-Induced Electrical and Structural Effects in Gallium Arsenide Solar Cells Following Ion Irradiation

ABSTRACT:
For nearly two decades, deviations between experimental data and the nonionizing energy loss (NIEL) have been observed for GaAs devices. In particular, previous data has suggested that electrical parameters associated with GaAs solar cells can follow different energy dependences with NIEL but only at the higher proton energies. In this paper, displacement damage-induced electrical and structural effects in GaAs solar cells were monitored before and after irradiation with various ions. The radiation-induced defects responsible for causing electrical changes were characterized using illuminated current-voltage, deep level transient spectroscopy (DLTS), and electron beam induced current (EBIC) while the structural changes were monitored using transmission electron microscopy (TEM). The EBIC images showed the existence of radiation-induced active recombination volumes or defect clusters after irradiation with high energy protons (E ≥ 10 MeV) and 22 MeV silicon ions, which were not produced by lower energy protons. The TEM images revealed strain related defects that correspond to the same irradiation conditions for which the defect clusters were observed, and therefore, the defects in the TEM images are associated with those observed in the EBIC images. These defects were not observed prior to irradiation so the lattice strain in the material is definitely associated with irradiation-induced lattice defects. HRTEM imaging has shown that the disordered regions are not amorphous but probably most likely a cluster of vacancies and a surrounding region rich in interstitials, which is produced when a large number of neighboring atoms are displaced in collision cascades known as the displacement spike. The formation of the U-band defect as determined by DLTS seems to evolve under the same irradiation conditions as the defects in the images. This very broad U-band peak is consistent with what would be expected from defect clusters. From analyses of the recoil spectra, high energy recoils appear to be responsible for the formation of these disordered regions and these regions are independent of the total displacement damage energy deposited. This study has shown that NIEL scaling is only violated for incident ion energies when the defect clusters are observed.

What is a physicist? A case is made for defining a physicist as anyone with a bachelor's degree (or higher) in physics. Under this definition, a large fraction of physicists are hidden, that is, they have left, or never belonged to, the traditional lot of Ph.D. academicians. Data from the Statistical Research Center at the American Institute of Physics and from a survey of members of the national physics honor society, Sigma Pi Sigma, show the vast array of actual career paths taken by physicists. From spandex to blackberries to bioinformatics to flight control to wind energy to spintronics, physicists can be found in nearly every job sector with some of the coolest careers around.

Location: Physics Bldg., Room 401

Antonia successfully defended her dissertation on November 20, 2008.

TITLE:
Temperature Change And Water Vapor Feedback In The Atmosphere. A Comprehensive Assessment Using The Atmospheric Infrared Sounder Instrument On NASA Aqua Satellite

ABSTRACT:
Global surface temperature has increased ~0.2 degree Celsius per decade in the past 30 years. Observational data recorded from 1850 to 2007 indicate that the warmest 11 years have occurred between 1995 and 2006. The 2007 report of the Intergovernmental Panel on Climate Change concluded that there is a "very high confidence - 95% confidence - that the global average net effect of human activities since 1750 has been one of warming" and that "most of the observed increase in global average temperatures since the mid-20th century is very likely - 90% confidence - due to the observed increase in anthropogenic greenhouse gas concentrations".

Processes in the climate system that can either amplify or dampen the climate response to an external forcing such as an increase in anthropogenic greenhouse gas concentrations, and directly or indirectly affect the Earth's radiation budget at the top of the atmosphere are normally referred to as "climate feedbacks". Among all trace gases, water vapor is the most sensitive to temperature variations. In fact, water vapor absorbs strongly in the vibration-rotation and pure rotation bands at wavelengths in which a large portion of infrared emission occurs at temperature characteristic of the Earth's surface and atmosphere.

In the present study, we exploit the uniform spatial coverage and high vertical resolution of the Atmospheric InfraRed Sounder database of temperature and water vapor profiles to perform a detailed investigation of the covariance between temperature and water vapor. Differently from the previous studies, who only analyzed the overall tropically averaged water vapor and temperature relationship, we make a more comprehensive analysis by investigating this relationship on a local basis. By doing so, we explore the horizontal gradient of this relationship in the tropics, in order to better confine its range of variability and the interplay of the physical processes underneath it. An overall conclusion on the sign and magnitude of the tropical sensitivity to surface temperature variations by mean of water vapor feedback will be assessed.

Due to the Thanksgiving Holiday, there will be no seminar on Wednesday, November 26, 2008.