|
Graduate Student Seminar Series,
2006-2007 |
The talk provides an
introduction to variational perturbation theory (VPT), which converts divergent
weak-coupling series into exponentially fast convergent strong-coupling
expansions. In order to perform the resummation, one introduces artificial
variational parameters whose influence is then optimized according to the
principle of minimal sensitivity. As an introductory example, the ground-state
energy of the quantum-mechanical anharmonic oscillator is considered.
Subsequently, a two-neuron system is examined. The system can display periodic
activity in form of a limit cycle when the temporal delays in the system exceed
a critical value. First, the limit cycle and its frequency are calculated by
means of perturbation expansions and it is shown that they are divergent
series. Two methods, Shohat resummation and VPT, are then used to resum the
series, and the accuracy of the nonperturbative results is evaluated.
The causality-induced
relationship between the frequency dependence of attenuation and phase velocity
represented by the Kramers-Kronig relations have been investigated in the
context of biomedical ultrasound for a number of years. Our purpose is to
examine the validity of the Kramers-Kronig relations under conditions in which
phase cancellation at a phase sensitive aperture induces an apparent
attenuation. In the context of medical imaging and tissue characterization,
aberrations arising from tissue inhomogeneities that represent local variations
in phase velocity are often the source of such artifacts. As distinct from
interference effects in the ultrasonic field, which represent lossless
redistributions of energy, signal loss at a phase sensitive receiving aperture
is an irreversible loss of information. We explore the question of whether the
causality-induced link between attenuation and dispersion remains valid when
the signal loss arises as a consequence of inadequacies in the process of
receiving the distorted ultrasonic field rather than from intrinsic losses
within the medium under study. In order to explore this question, we measure
the frequency-dependent apparent attenuation and phase velocity in media that
exhibit the approximately linear with frequency attenuation coefficient and
logarithmic with frequency phase velocity. Flat and parallel plates of Lucite
(polymethyl methacrylate) and Lexan (polycarbonate resin thermoplastic) were
machined with various step sizes on one of their flat sides. The thicker,
thinner, and stepped regions of these plates were insonified by a matched pair
of single element planar transducers. The resulting apparent attenuation
coefficient and phase velocity values are compared with the corresponding
predictions from the Kramers-Kronig relations.
Torsion balances are
novel instruments which have a long history of measuring delicate forces.
Through clever design physicists are able to do remarkable things with these
seemingly simple instruments. Due to this fact many scientists around the world
employ a version of these instruments in order to make feeble measurements.
This talk is focused on the motivations behind the construction of 2 torsion
balances in our physics department. These instruments will be used to measure a
variety of natural phenomena ranging from geophysics to particle physics. I
will discuss physical phenomena to be measured and the design of the balance
which will be used to make the measurements.
Measurements from many laboratories indicate that on average the phase velocity of ultrasonic waves propagating in bone decreases with increasing frequency. This negative dispersion in bone is inconsistent with the nearly local approximation to the Kramers-Kronig relations that relates attenuation to dispersion. We hypothesized that observed negative dispersion in bone might result from the interference between the fast and the slow compressional waves that are supported in bone. We carried out simulations in which two modes each of which characterized with positive dispersion were generated and propagated. Consequent phase spectroscopy analysis of the simulated data yielded the result that we have anticipated. For cases in which the fast and slow waves were separated in time, the positive dispersion was recovered for both modes. However, when the fast and slow waves overlapped and the resultant mixed mode was analyzed as a single wave, the phase spectroscopy algorithm yielded negative dispersion. We then applied Bayesian probability theory to solve the inverse problem: extracting the underlying properties of bone. Simulated mixed mode signals were analyzed using Bayesian probability. The calculations were implemented using Markov chain Monte Carlo with simulated annealing to draw samples from the marginal posterior probability for each parameter.
Binary systems are
believed to be the most promising sources for both the ground and space based
laser interferometer gravitational wave detectors. Because of the spin-orbit
and spin-spin coupling, the orbital motion and gravitational wave form for
spinning binaries are much more complicated than the non-spinning case. During
this talk, I'll first briefly review the history of the gravitational wave
detection and the equation of motion for binary systems, then I'll talk about
our recent result on computing the 3.5 post-Newtonian order spin-orbit and
spin-spin radiation reaction.
"Relaxation
studies of the intermetallic ZrNiHx were performed using hydrogen NMR in the
beta (x=0.85) and gamma phases (x=2.6 and 3.0). Correlation times for atomic
diffusion in the hydride were determined based on the temperature dependence of
spin-lattice and spin-spin relaxation times. The hydrogen motion is shown to be
thermally activated over the temperature range 300-550K, and the activation
energies for diffusion are determined." These types of measurements and
analysis can be used on metal hydrides in general to advance our knowledge of
hydride systems and aid research in hydrogen based alternative fuels.
Pentacene (Pn) is one
of the most promising molecular materials for organic thin film transistors
(TFTs) due to its strong tendency to form crystalline packing. To successfully
replace Si as active layer in the conventional TFTs, it is very important to
fabricate highly ordered Pn thin films with large crystal domains and low
defect densities. During this talk, I will briefly discuss the polymorphs of Pn
and the transfer printing fabrication method. Then I will talk about our recent
investigations on the effect of transfer printing conditions on the polymorph,
the preferred orientation,the crystalline size and structure perfection of Pn
thin films. Our results show that the optimized crystalline size can be
directly correlated with the improvement of the carrier mobility of TFTs.
Gravitational radiation
is a problem that dates back to the beginning of general relativity. It has
received renewed interest with the proposal and setting up of several
interferometer gravitational wave detectors. The most promising sources for
these detectors are inspiraling compact binary systems. In order to calculate
highly accurate theoretical templates for data analysis of these detectors, we
are using post-Newtonian method to derive the equations of motion for binary
systems. We have been working on the 3.5PN order spin-orbit radiation reaction
contributions and derived the general structure of it. We also show that there
are 12 degrees of freedom which corresponding precisely to the gauge freedom.
Dr. Clark informed us
about the accomplishments and plans of the department.
The motivation of our
project is to supply a cheap, multifunctional, disposable type of imaging
sensor device. There will be 4 types of sensors, each will be capable of
measuring a certain property, i.e. magnetic field intensity, electric field
intensity, opto property and elastic property/acoustic property (of cell). EMR
has been established in 2001 by Dr. Solin, which is an excellent candidate for
the magnetic field sensor. Down the same line, EPC (extraordinary
piezoconductance) and EOC (extraordinary optoconductance, K.A.Wieland is the
key person in this) are established recently. I will briefly cover the
background of these sensors. My focus, however, is on the EEC (extraordinary
electroconductance), which will be responsible for the surface charge density
imaging. Working principles, current results and future directions will be
discussed.
Abstract misplaced
because of the gross incompetence of the grad seminar staff.
How does a brain work?
How do we think? How am I typing this and how are you reading it? These are
poorly formed questions, but still fair ones. Part of the answer lies in
feedforward processing within the brain (sensory--"processing
areas"--motor). However, with ~100 billions neurons in the human brain
there is an enormous amount of feedback in the connections of our brains and
within the brains of our cousins, all animals. My talk will discuss feedback
and demonstrate how we study this phenomenon experimentally in the accessible
systems of the chicken and frog. There will be pretty pictures, and educational
video, and maybe some math and physics.
Observations of
Ultra-Heavy (30 < Z < 40) galactic cosmic rays (GCR) help to distinguish
the possible origins of GCRs. The Trans-Iron Galactic Element Recorder (TIGER)
is designed to measure the charge (Z) and energy of GCRs using a combination of
scintillation counters, Cherenkov counters, and a scintillating fiber
hodoscope. TIGER has accumulated data on two successful flights from McMurdo,
Antarctica: the first launched in December of 2001 with a total flight duration
of 31.8 days and the second in December of 2003 with a total flight duration of
18 days. The combined dataset of the two flights has the statistics and charge
resolution to resolve ~140 particles with Z>30. I will present a preliminary
analysis of the combined data from both flights for Ultra-Heavy GCRs and
discuss the results in the context of different GCR source models.
This past fall I
traveled to Oak Ridge National Lab (ORNL) to use a 3-D atom probe (3DAP) of the
LEAP (Local Electrode Atom Probe) variety. This talk will discuss the technique
as well as the ups and downs that i have had while trying to make sense out of
our results. There will probably be very little math and alot of basic physics
that we all hopefully still understand. This technique shows that you can still
do new experiements although they are based upon 117/118 level physcis
concepts.
We know that physics
helps us understand stuff, but are physics concepts actually useful for
research? Do the lessons learned in stat mech, or E&M have any relevance
for coming up with new ideas for a field where most investigators imagine
mathematical modeling to be akin to hiring models to show off TI-89 calculators
at trade shows. For me, the answer has been an enthusiastic yes. The purpose of
my talk is to convince you that a physics background puts you in a perfect
position to come up with new ideas, especially in fields where most people
don't think about physics. Come to my talk and see the ideas you could, and
would have had, had you joined our lab.
Einstein-Maxwell theory
is ordinary general relativity combined with electromagnetism, and this theory
will be discussed briefly. Then an alternative theory will be presented called
the Lambda-renormalized Einstein-Schrodinger theory. This theory is essentially
the original Einstein-Schrodinger theory but with a modification to account for
a quantization effect, and it closely approximates Einstein-Maxwell theory. In
particular, the field equations match the ordinary Einstein and Maxwell
equations except for additional terms which are <10^-16 of the usual terms for worst-case fields accessible to measurement. An exact charged solution has a fractional difference of ><10^-64 compared to the Reissner-Nordstrom solution of Einstein-Maxwell theory. An exact electromagnetic plane-wave solution is identical to its counterpart in Einstein-Maxwell theory. The theory predicts the exact Lorentz-force equation and avoids ghosts. Predictions of periastron advance, deflection of light, and time delay of light show fractional differences of ><10^-56 compared to Einstein-Maxwell theory. Additional fields can be added to the Lagrangian, and these fields may couple to the symmetric metric and electromagnetic vector potential, just as in Einstein-Maxwell theory. When spin-1/2 fields are added, the theory predicts fractional differences in Hydrogen atom energy levels of ><10^-50 compared to Einstein-Maxwell-Dirac theory. Finally, the theory becomes exactly Einstein-Maxwell theory in the limit as the cosmological constant from zero-point fluctuations goes to infinity. >
Most particle theorists
study the properties of elementary particles via the standard model theory of
the strong interaction known as Quantum Chromodynamics (QCD). One of the most interesting
features of QCD is a property called (quark) confinement. Quarks are the
particles that make up protons and neutrons, as well as other particles,
collectively called hadrons. Years of arduous searching on the part of particle
experimentalists has provided not one person a glimpse of quarks running around
free, and all evidence suggests that they ALWAYS congregate in either pairs or
trios, thus confined. Why does this happen? Also, theorists conjecture that
ridiculously high temperatures and densities, corresponding to, say, the
conditions present at the time of the Big Bang, can bust the quarks out of
their hadron jails setting them completely free. This deconfined state of free
quarks running a muck (as well as strong force-mediating gluons) is the
infamous quark-gluon plasma that only those successfully converted to the dark
side research. In our study of the mechanism of confinement/deconfinement, Mike
and I stumbled upon something unexpected (as well as unwelcome), ANOTHER
PHASE??... What does it mean? Is it physical? or is it just some mathematical
abstraction / computational pathology that means absolutely nothing to reality
as we know it?
The goal of my talk is
to provide a broad overview of particle physics. I will start by giving a
description of the most successful theory in the history of science: The
Standard Model. No experiment has ever disagreed with the predictions of the
Standard Model, however we have some strong reasons to believe it is
incomplete. From there, I will discuss the next big experiment: The Large
Hadron Collider (LHC). I will tell you what it is looking for and how it might
shed light on the exoticness of nature through extra dimensions, supersymmetry
or the mysterious Higgs. I hope everyone who comes will have a baseline
understanding of particle physics, and if not, at least can make it past the
first 3 slides of the next relevant seminar.
Deep in a dark (except
for the annoying fluorescent lights) windowless office in a campus far, far
away, research is being pursed to better understand why and how your heart
fills, pumps, and does everything in between to keep you alive. "But
Charles," you interject. "I am not a doctor, so I dont care!"
Well, you should. Not only because you want to know how the universe (and
therefore yourself) works, but because it is Physics that will give us the answers!
In my talk, I'll describe some energetics of pumping and relaxing hearts and
how this can be characterized using methods bastardized from nonlinear
dynamics. I may even describe mechanical correlates and biological
support/consequences for the physical findings. So come and learn some physical
constraints on your heart's lub-dub-ing.
We derive the equations
of motion for binary systems with finite-sized, non-spinning but arbitrarily
shaped bodies. In particular we study the contributions of the internal
structure of the bodies (such as self-gravity) that would diverge if the size
of the bodies were to shrink to zero. Using a set of virial relations accurate
to the first post-Newtonian order that reflect the stationarity of each body,
and redefining the masses to include 1PN and 2PN self-gravity terms, we show
that a class of potentially divergent terms cancel, leaving 2PN equations of
motion that depend only on the masses (modulo tidal effects). This is further
evidence of the Strong Equivalence Principle, and supports the use of
post-Newtonian approximations to derive equations of motion for strong-field
bodies such as neutron stars and black holes.
Extraordinary
Optoconductance (EOC) was realized by our group in 2004. The EOC structure is a
metal-semiconductor hybrid (MSH) device. With optical exposure, the voltage
generated by a laser is enhanced by the geometry of the device compared to a
homogeneous device. Following a brief review of the physics of previously
observed 'EXX' effects, the discussion will focus on a proof of principal
demonstration of EOC with a gain as high as 500% relative to the bare
semiconductor. This is the first example of an EXX phenomenon that is driven by
a bulk rather than an interfacial effect. The following discussion will focus
on the physics of recent advances both theoretically and experimentally in EOC
and extraordinary electroconductance (EEC) and the progress being made towards
nanoscale devices. Time permitting, a nanoarray composed of these sensors for real-time
ultra-high spatial resolution imaging of the properties of cancer cells will be
addressed.
It has been reported
recently that hydrogen forms clatrate hydrate, which is a class of inclusion
compounds with cage like lattice structure in which hydrogen can be stored.,
This system has been viewed as a novel way of storing hydrogen. There has been
much effort to improve storage capacity and thermodynamic stability to use as
an energy storage. However, dynamics of H2 molecule in the cages have have not
been properly understood. We have been trying to explore dynamic properties
(basic physics), such as diffusion, rotation, dipole-dipole interactions etc,
of the trapped H2 molecules inside the cages of clathrate hydrate using NMR. In
this presentation, I'm going to discuss my recent results and give some
background knowledge of clathrates, hydrogen molecule and NMR (as time
permits).
Adam did not actually
give a talk this week.
Last Updated : 6/12/2007 Ben Johnson