UC Davis
Physics REU Program, Summer 2006
(Students' names link to their final papers. There are some
formatting
problems,
particularly with figures, but you can still get a lot more detail
about the projects
from them.)
Condensed Matter Experiment
Melina Blees (Carleton College;
advisor Kai Liu)
made cobalt wires, 200 nanometers in diameter, by pulsed
electrodeposition. To test the samples, she measured resistance as a
function of applied magnetic field, finding a sharp difference when
the field is perpendicular or parallel to the wires. This observed
magnetoresistance confirmed that the fabrication process was successful.
The next step in the work will be to make wires with layers of copper
and cobalt interspersed. The magnetic moment of a thin cobalt layer may
be used as a memory bit. Magnetic scattering depending on the cobalt's
moment would affect the wire's resistance. The key advantage of the
technique is that an applied current might be used both for reading the
magnetization and for switching it, with ultimate applications in magnetic
recording technology.
Anna Smith (Seattle
University; advisor Rena
Zieve) worked on a variety of tasks related to studies of
vortex motion in superfluid helium. She began by fabricating experimental
cells: preparing fine wires for the measurements, stretching each along
the axis of a long cylinder, gluing it in place, and checking for leaks.
Later she wound a spare superconducting magnet for the experiments.
It proved necessary before the end of the summer, and Anna installed it
on the refrigerator. Anna also worked on computer models of a moving
vortex, both running lengthy simulations and writing a companion program
to make the main simulation code easier to debug.
Bryce Kalmbach (University of Washington;
advisor Rena
Zieve)
measured the stability angle of a binary granular mixture in two
dimensions. The two grain shapes were single spheres, and hexagons
formed by welding seven of the spheres together. A homogeneous pile of
hexagons reaches a much larger angle before avalanching than does a pile
of spheres. Mixing a small fraction of spheres in with the hexagons
sharply reduces the stability angle, while mixing a small fraction
of hexagons in with the spheres has little effect. Bryce took careful
measurements, investigating and eliminating various sources of noise.
By using spheres and hexagons of different colors, he related the behavior
of the stability angle to the tendency of the spheres to cluster near
the center of the container, slightly below the surface of the pile.
The strong effect of a few spheres added to a pile of hexagons suggests
that this central region is key to triggering avalanches.
Condensed Matter Theory
Ben Heldt (Santa Clara University; advisor
Sergey Savrasov
programmed a Pade approximation technique to approximate functions.
If a function is known on the imaginary axis, the technique finds
the analytic continuation to the real axis, where the values may have
a physical interpretation. The approximation depends on a weighted
sum over the roots of a derived polynomial. As the number of known
values of the function increases, so does the order of the polynomial.
Ben found that the algorithm, while mostly effective, becomes unstable
for very high order polynomials. A possible fix is to change the basis
used to describe polynomials.
The Hubbard model describes electrons which are located at discrete,
fixed sites and interact in several ways: through their spins, Coulomb
repulsion, and the possibility of "hopping" from one site to another.
Changing the various interaction strengths and occasionally adding additional
interaction terms allow the model to help
investigate different types of magnetism and even superconductivity.
Helen Craig (American River College; advisors
Warren Pickett and Richard
Scalettar) wrote and ran code to calculate ground states
for two variants of the Hubbard model. The goal was to construct a phase
diagram by determining the ground state for different values of the
interaction strengths. Helen found both charge and spin density wave phases
in different regions.
Biological Physics
Scott
Clark (Oregon State University; advisors Daniel Cox and Rajiv
Singh)
did a statistical analysis of the stability of different protein
configurations. Since proteins are normally surrounded by water,
the calculation assumes that some of the amino acids that compose the
protein are hydrophobic and tend to point towards the inside of the protein,
while others are hydrophilic and tend to point outwards into the water.
These considerations determine how exactly the protein folds into a
betahelix. Scott's calculation estimated favorable positions for each
amino acid, and approximated the volume of the resulting configuration.
A standard technique for tracking molecules in biological systems is
attaching a fluorescent group, or fluorophore. Ideally the fluorophore
has little effect on the original molecule's structure and function.
Unfortunately, the label does sometimes disrupt protein function,
and even measuring the degree of disruption is difficult. Andrea Herrick (Ohio Northern University;
advisor Xiangdong
Zhu) worked on an entirely noninvasive method for
tracing reactions, by measuring slight changes in a solution's optical
reflectivity. Andrea worked on setting up a new microscope, particularly
writing software to control the microscope as it scans a microarray.
Each microarray contains thousands of distinct compounds in isolated
positions, and make possible an economy of scale when testing large
numbers of possible reactions.
High Energy Experiment
Alessandra Borgia (SUNY Binghamton;
advisor Mani
Tripathi)
) worked on C.A.C.T.U.S., a groundbased gamma ray
telescope using a former solar plant in the southern California desert.
Alessandra worked on Monte Carlo simulations modeling the background noise
from the night sky. Her Monte Carlo technique generated photon showers
that would impinge on the detector. Next she used ray tracing to
determine where each photon was detected, and compiled histograms of
the photon distribution for different showers. The work confirmed the
importance of cleaning the mirrors and highlighted the problem of diffuse
light from the nearby town, military base, and airport.
Nuclear Physics
In an encounter between two relativistic heavy ions, many pairs of
nucleons collide. Analysis of the result depends heavily on statistical
calculations.
Suzanne Pittman (San Joaquin Delta
Community College;
advisors
Manuel Calderon de la Barca Sanchez and Daniel Cebra
)
used the Glauber Model to estimate the number of expected
collisions as a function of the nearest approach distance of the ions'
centers. To relate the calculations to experimental results from
Brookhaven's Relativistic Heavy Ion Collider, Suzanne then worked out
the implications of her calculation for how many charged particles a
collision would likely generate. She compared data from three experimental
runs, one with known trigger inefficiency, and found that trigger
efficiency has less effect on charged particle creation than expected.
Cosmology
Josh Lande (Marlboro College; advisor Tony Tyson
) wrote a computer model of the Deep Lens Survey.
The survey attempts to locate dark matter in the nearby universe by noting
its bending of light. The bending is observed as gravitational lensing,
or distortions in the images that appear when the light reaches us.
Josh generated artificial images of pieces of the sky, including the
effects of gravitational lensing, the atmosphere, and noise. A next step
is to compare the simulation and actual data, a way to uncover possible
systematic errors in the real data.
