Condensed Matter Theory
cyuLOCK@uci.edu (remove "LOCK")
Professor Yu earned her A. B. (1979) and Ph.D. (1984) from Princeton
University. She did postdoctoral research at the University of Illinois
at Urbana-Champaign and Los Alamos National Laboratory before joining
the faculty at UCI in 1989. She is the recipient of a Sloan Research
Fellowship and a Fellow of the American Physical Society. She has a
broad range of research interests which include
disordered systems, biophysics, noise, and quantum computing.
BIOPHYSICS: INTRACELLULAR TRANSPORT
A cell is like a city. It has all the basic infrastructure that a city
has. For example, it has power plants (mitochondria), workers (proteins),
a library (genome), recycling centers (lysosomes), etc. A cell also
has a transportation system that works like container shipping.
There are interstate highways (microtubules) and local streets
(actin filaments) as well as trucks or motors (kinesin, dynein, and myosin)
that pull large cargo vesicles along the roads. We are working with
Steven Gross' group (UCI Dept. of Cell and Developmental Biology) to
understand how the motors and roads conspire to get cargo vesicles where
they need to go. The motion of the vesicles does not proceed smoothly
in one direction. Rather it can frequently reverse direction along
one road or switch roads as it diffuses through the cell.
In addition, a cargo can be carried by more than one mtor protein.
So how does the cargo get to where it needs to go? To answer this
question, we are using computer simulations. For more information
"Description of transportion inside a living cell"
BIOPHYSICS: DEVELOPMENTAL BIOLOGY
When an embryo develops, chemical signals are used to dictate the
development of the body plan, e.g., where legs, arms, wings and organs
will be. There are spatial and temporal fluctuations in these chemical
signals, yet organisms are able to develop in very symmetrical ways.
For example your eyes are the same size and symmetrically placed on your face.
How is the system able to preserve such symmetry in the presence of
such noise? How does an organ know when to stop growing? How does a
wound know when healing has been completed? We are studying questions
such as these using Monte Carlo simulations of the development
of the wing disc in Drosphila (fruit flies). We have been focussing
on the Fat signalling pathway and whether there is mechanical signalling
between cells via protocadherins which are transmembrane proteins that protrude
from a cell and bind to a protocadherin of a neighboring cell.
Why does a tumor reside where it does within an organ? Location is
traditionally viewed as a random event, yet the statistics of the location
of tumors argues against this being a random occurrence. For example,
about 60% of breast cancer tumors start in the upper outer quadrant
of the breast near the armpit, even though it is estimated that
only 35 to 40% of the breast tissue is in this quadrant. Lung cancer
occurs about 2.5 times more often in the upper than the lower lobes,
even though the upper and lower lobes have roughly the same volume.
The reasons for these significant enhancements imply that there is an
unknown factor that significantly increases the risk of cancer that is
not solely related to genes and toxins.
One of the most exciting developments in cancer treatment is immunotherapy
which tries to get the immune system to attack cancer cells. Immunotherapy
only works if immune cells infiltrate the tumor and recognize the cancer cells.
Tumors need a supporting environment known as the tumor microenvironment in
order to survive and thrive. The tumor microenvironment consists of blood
vessels, immune cells, fibroblasts, extracellular matrix, etc.
(Fibroblasts are cells that synthesize extracellular matrix and fibers
such as collagen; they are the most common cells in connective tissue.)
Using statistical techniques such as maximum entropy,
We are studying the tumor microenvironment to try to understand under what
conditions immune cells infiltrate the tumor.
Disordered systems such as glasses and spin
glasses. The physics of glassy systems is one of the most interesting
and least understood problems in condensed matter physics.
In the field of disordered systems, the Yu group has investigated several
topics including the glass transition, Coulomb glasses, dipolar glasses,
spin glasses, and the low temperature properties of glasses.
THE LOW TEMPERATURE PROPERTIES OF GLASSES
Glasses at low temperatures also present a challenge.
A bunch of molecules in a disordered jumble behaves very
differently from an ordered crystalline array of
those same molecules. This can be seen in
low temperature thermodynamic properties that
tend to be universal, independent
of the particular material and its chemistry. What is it
about the nature of disorder that gives rise to such universal
properties? This is the basic problem of disordered systems.
Professor Yu has studied this question with models in
which defects or tunneling centers interact with one another.
More recently her group investigated the influence of
these two level systems on qubits.
QUANTUM COMPUTING QUBITS
The basic unit of information for any computer is the bit. For a quantum
computer the quantum bit, or "qubit", is a wavefunction
describing a coherent superposition of the |0> and |1> state. Decoherence
of the wavefunction is one of the great obstacles facing the realization
of quantum computers. Josephson junction (JJ) qubits are a leading
candidate for making a quantum computer. A major obstacle to the
realization of quantum computers with Josephson junction qubits is
noise and decoherence. The goal of our research is to elucidate the microscopic
sources of this decoherence and to suggest ways to eliminate or reduce
these culprits. We are working closely with experimentalists.
A Josephson junction consists of 2 superconducting electrodes separated
by a tunnel barrier that is often an insulator. The current passing
through a Josephson junction is superconducting. We are investigating
how fluctuating electronic spins on the surface
can produce magnetic noise and can lead to decoherence of the qubit.
We are also exploring how interactions between the spins can affect
Professor Yu has taught graduate courses in condensed
matter physics, many body theory, and
computational physics as well as undergraduate courses on
electricity and magnetism.