Light transport in two-layer tissues

Speaker: 

Prof. Arnold Kim

Institution: 

UC Merced

Time: 

Monday, October 18, 2004 - 4:00pm

Location: 

MSTB 254

Light propagation in tissues is governed by the theory of radiative transport. The radiative transport equation takes into account absorption and scattering due to inhomogeneities. A two-layer medium is a useful model for tissues because it accounts for the differences in optical properties between the superficial and deep regions of tissues. We are interested in probing only the superficial layer because most pre-cancerous tissues develop there. To do this we introduce an alternate boundary condition that allows for the removal of the bottom layer from the problem. For the case when the top layer is thin, we compute an asymptotic solution. We validate our results by comparing them with numerical solutions.

Simulating Semiconductor Charge Transport

Speaker: 

Prof. Shaoqiang Tang

Institution: 

Peking University

Time: 

Monday, October 25, 2004 - 4:00pm

Location: 

MSTB 254

Computer simulations of charge transport in semiconductor devices (like diodes and micro-chips) are used by the semiconductor industry as a tool for reducing the cost of developing new devices and new process technologies. At the scale of micron or sub-micron, the semiconductor Boltzmann equation is the most exact model. In order to alleviate computing load, macroscopic models have been derived, assuming that the state of the electron gas is described by certain averaged quantities. These models take similar forms as those in fluid mechanics, and we may apply CFD techniques to probe this promising field of academic importance and commercial value.

We shall present some of our recent results. First, by simulating a hydrodynamic model, we demonstrate the (direct) applicability of CFD techniques. Secondly, the continuing trend of scaling-down and speed-up makes the modeling and computing of quantum effect and transient behavior among the top issues in semiconductor research. Careful numerical tests helped identifying well-posedness problem in a quantum hydrodynamic (QHD) model. Viscous QHD model derived from a Wigner Fokker-Planck equation yields more reliable numerical results, and demonstrate interesting nonlinear phenomena, such as negative differential resistance and hysteresis.

Operator spaces with prescribed sets of completely bounded maps

Speaker: 

Prof. Timur Oikhberg

Institution: 

UCI

Time: 

Tuesday, November 2, 2004 - 3:00pm

Location: 

MSTB 254

For a Banach algebras $A$ satisfying certain properties,
we construct an operator space $X$ such that the space of
completely bounded maps $CB(X)$ consists of elements of
$A$ (or, at least, $\pi(A)$, where $\pi$ is a faithful
representation), and their "small" perturbations.

As properties of an operator space are reflected in its
space of completely bounded maps, we construct spaces
with various "pathological" properties. The prime example
here is the space $X$, isometric to a separable Hilbert
space, such that any c.b. maps on any subspace of $X$ is a
sum of a scalar and a Hilbert-Schmidt operator. Other
"strange" spaces include $Y$, completely isomorphic to
$Y \oplus Y$, and such that $CB(Y)$ admits a non-trivial
trace.

Part of this work was done jointly with Eric Ricard.

Ferromagnetic Ordering of Energy Levels and Applications

Speaker: 

Bruno Nachtergaele

Institution: 

UC Davis

Time: 

Thursday, January 13, 2005 - 11:00am

Location: 

MSTB 254

NOTE TIME CHANGE FOT JANUARY 13 ONLY: SEMINAR AT 11 AM

The ferromagnetic Heisenberg model is conjectured to possess the
property of Ferromagnetic Ordering of Energy Levels (FOEL): the smallest
eigenvalues in the invariant subspaces of fixed total spin, S, are
monotonically decreasing in S. I will present a proof of this conjecture
for the one-dimensional case and discuss generalizations to other models
and several applications.

Equality of the edge and bulk Hall conductances in 2D

Speaker: 

Alexander Elgart

Institution: 

Stanford

Time: 

Thursday, November 4, 2004 - 2:00pm

Location: 

MSTB 254

The integral QHE can be explained either as resulting from bulk or
edge currents (or, in reality, as a combination of both). The equality
of the two conductances at zero temperature was recently established
for the case that the Fermi energy falls in the spectral gap of the bulk
system. We define the edge conductance via a suitable time averaging
procedure in the more general case of a bulk system which exhibits
dynamical localization in the vicinity of the Fermi energy, and show
that the two conductances are equal.
This is a joint work with G.-M. Graf and J. Schenker.

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