Physics
and Engineering Seminar
Series - Spring 2008
(All
talks are at 1:00 PM in Room S-3-126 unless otherwise noted.)
Wednesday,
May 21st:
Several Approaches to
Understanding Persistent Patterns in Time-Periodic
Advection-Diffusion Problems
Andrew C. Poje
Physics
Department
City University of New York
Abstract: The characterization of the behavior of a passive, diffusing
scalar advected by a prescribed, smooth velocity field is of primary
importance in applications ranging from micro-fluidic mixers to global
climate dynamics. Above and beyond its obvious practical importance,
the 'scalar turbulence' exhibited by solutions of the linear
advection-diffusion equation serves as a simple phenomenological model
for the nonlinear dynamics of the velocity field itself.
Despite the linearity of the governing equation, much remains unknown
about the behavior of solutions to the advection-diffusion equation,
even in the relatively simple case of time-periodic planar flows and
their associated maps. In this talk, we will review some well-known
results and then concentrate on three related approaches to
understanding the formation of persistent patterns, often called strange eigenmodes, of the scalar field. These approaches involve (1) the introduction of a relatively novel measure (χ2)
which allows us to identify the dynamical stages involved in the
emergence of these patterns and the scaling laws governing their
overall decay rates, (2) a formal averaging procedure for integrable
flows which transforms the time dependent problem to a time-independent
system with renormalized velocity and diffusivity and (3) numerical
identification of the Green's Function for the full, time-periodic
advection-diffusion equation.
Wednesday,
May 14th:
Interferometric UV
Lithography:
From Optical Fibers to Biomembranes
Anup Sharma
Physics
Department
Alabama A&M University
Abstract: Interferometric Lithography is used to produce sub-micron
scale periodic structures in glass optical fibers as well as
biomembranes and other substrates of biological interest. Normal as
well as slanted Bragg-gratings fabricated by this technique within the
core of optical fibers are fabricated and used for health
monitoring/distributed sensing in carbon-polymer composite structures.
Interferometric UV lithography is also used to produce microarrays on
phospholipids bilayer membranes as well as other polymer substrates.
Phospholipids are one of the major constituents of biological cell
membranes and lipidic films have found extensive uses as simple models
of cell membranes. Model biomembranes are known to be excellent
platforms for biosensing.
The experiments described here were motivated by the promise of
interferometric techniques to extend membrane photolithography to
produce nanometer scale features. Holographic gratings and microarrays
are recorded in azo-dye (NBD)-labeled phospholipid thin films using 244
nm UV light. Diffraction efficiency of these gratings shows extreme
sensitivity to humidity and can increase reversibly by two orders of
magnitude in air, which is saturated with water vapor. This effect is
related to the unique characteristics of phospholipid molecules to
undergo hydration-dependent structural reorganizations and
self-assembly. Using established techniques, diffraction
characteristics of the membrane-grating can be made sensitive to
molecules recognized by a biological probe immobilized on the bilayer.
Interferometric lithography can give much finer engineering of
biomembranes than can be accomplished with use of masks. This has the
potential for high-density biosensor designs.
Wednesday,
May 7th:
Dynamics of Coupled Cellular
Oscillators
Premananda Indic
University
of Massachusetts Medical School
Worcester, Massachusetts
Abstract: I will discuss how circadian rhythms are generated
in
the mammalian brain, how such rhythms can break down in certain
pathological conditions, and new methods for quantifying and restoring
circadian rhythmicity. I will show that biological tissues composed of
a population of coupled cellular oscillators can exhibit a variety of
complex rhythmic behaviors. Based on theory of coupled oscillators we
derive conditions for collective synchronization and desynchronized
states of a network and apply this theoretical formulation to
understand the dynamics of cellular oscillators in the suprachiasmatic
nucleus – a circadian clock located in the hypothalamus of
the
mammalian brain. Our analysis reveals different salient characteristics
of the biological clock that can help to understand the control of
circadian rhythmicity and its restoration in pathological conditions
such as jet lag and disorders of the sleep/wake cycle.
Wednesday,
April 30th:
Plasmonic Enhancement of
Spontaneous Emission Efficiency
Greg Sun
University
of Massachusetts, Boston
Abstract: A great deal of interest has been directed towards the issue
of using surface plasmons for enhancing the efficiency of spontaneous
emission (fluorescence). With all the results reported, it is not easy
to predict what degree of improvement one can obtain in a given
material system – there are too many parameters influencing
the
overall luminescence efficiency and it is far from clear whether the
schemes involved in experiments are actually optimal for the given
emitters and collection optics. We have developed a comprehensive
theory aimed at answering two simple questions: what is the optimum
configuration of the surface Plasmon structure for an emitter with a
given radiative efficiency in a given light-collection geometry, and
what, if any, is the improvement that such apparatus will offer? In
this talk, I will present results of optimal plasmonic enhancement that
is provided by either a metal sheet or an array of metal nanoparticles
that are deposited on an emitting device.
Wednesday,
April 23rd:
Magnetic and Magnetoelectric
Properties
of Jahn-Teller Crystals
Michael D. Kaplan
Simmons
College, Boston
Abstract: I will start with a brief introduction to the use
of spin
chains for quantum communication. Following this, I will describe
some methods to perfect this communication using certain reasonbable
strategies when one uses chains in their ferromagnetic ground state. I
will then briefly point out the use of antiferromagnetic spin chains
for the same purpose. Next, I will discuss the use of bosons
hopping
freely in a one dimensional lattice to generate entanglement between
the ends of the lattice. I will also describe the generation of
entanglement from the
ground state of a chain of qudits coupled by purely exchange
interactions.
Wednesday,
April 2nd:
Quantum Communication and
Entanglement Distribution Through
Spin Chains and Allied Systems
Sougato Bose
University
College, London
Abstract: I will start with a brief introduction to the use
of spin
chains for quantum communication. Following this, I will describe
some methods to perfect this communication using certain reasonbable
strategies when one uses chains in their ferromagnetic ground state. I
will then briefly point out the use of antiferromagnetic spin chains
for the same purpose. Next, I will discuss the use of bosons
hopping
freely in a one dimensional lattice to generate entanglement between
the ends of the lattice. I will also describe the generation of
entanglement from the
ground state of a chain of qudits coupled by purely exchange
interactions.
Wednesday,
March 12th:
What's
New in Quantum Nano-Electro-Mechanical Systems
Kurt Jacobs
University
of Massachusetts, Boston
Abstract: I'll present highlights from the recent Gordon conference on
Quantum NEMS.
Wednesday,
March 5th:
Plasmonic-Electronic
Transduction
Robert Peale
University
of Central Florida
Abstract: Nanophotonics is an active research area in which bound and
tightly-confined electromagnetic waves on metallic surfaces, so called
surface plasmons, play a major role. Plasmonic integrated circuits have
been suggested, but with plasmons excited and detected using free-space
radiation that requires bulky external optics. Direct electronic
generation and detection of plasmons on surfaces and in two dimensional
conductors would eliminate the need for external optics. Interaction
between plasmons and electron transport is known at long-wave IR (LWIR)
and THz frequencies, but not in the optical range where most
nanophotonics research is concentrated. Hence, a new set of materials
and devices must be developed for efficient generation, confinement,
propagation, and detection of 2D low-frequency plasmons. Our
investigations on the direct interactions between 2D plasmons and
electron transport, relevant to future plasmonic transceivers and
networks, will be presented.
Wednesday,
February 13th:
Thermalisation
and its Mechanism in Closed Quantum Systems
Maxim Olchanii
University
of Massachusetts, Boston
Abstract: Time dynamics of isolated many-body quantum systems has long
been an elusive subject, perhaps most
urgently needed in the foundations of quantum statistical mechanics.
Only very recently experimentalists have
begun performing detailed studies of this matter. In generic systems,
one expects the nonequilibrium
dynamics to lead to thermalization: a relaxation to states where the
values of macroscopic quantities are
stationary, universal with respect to widely differing initial
conditions, and predictable through the time-tested
recipe of statistical mechanics. The relaxation mechanism is not
obvious, however; for example, dynamical
chaos cannot play the key role as it does in classical systems since
quantum evolution is linear. That new rules
could apply to isolated quantum systems was underscored by recent
studies suggesting that statistical mechanics
may give wrong predictions for their relaxation. Here we demonstrate
that a generic quantum many-body
system does relax to a state well-described by standard statistical
mechanical prescription. Moreover, we show
that time evolution itself plays a merely auxiliary role and that
thermalization happens instead at the level of
individual eigenstates, as first proposed by M. Srednicki. Due to this
eigenstate thermalization scenario,
thermalization joins the list of processes that, like scattering, seem
intrinsically temporal, and yet in quantum
mechanics effectively become time-independent problems.
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Physics
and Engineering Seminar
Series - Fall 2007
(All
talks are at 1:00 PM in Room S-3-126 unless otherwise noted.)
Wednesday,
November 28th:
Random
Waves and Quantum
Statistical Mechanics
Eric J. Heller
Harvard
University
Abstract: The Berry hypothesis. that
eigenstates of classically ergodic systems are locally given
by
random superpositions of plane waves of fixed kinetic energy may be an
alternative formulation of quantum statistical mechanics. By
systematically incorporating boundary conditions as constraints we move
closer to the question of statistical mechanics, which must deal with
constraints. We derive the quantum version of the equivalence
of
ensembles in terms of unfamiliar asymptotic limits of Bessel functions.
In the end, the ambiguity of whether the Berry hypothesis is a
prescription for quantum statistical mechanics comes down to the
question of degrees of freedom which are frozen out by quantization and
finite temperature. However quantum statistical mechanics has no
statistical answer for
this either - one must solve the Schroedinger equation.
Wednesday,
November 21st:
Cold Atoms and Out of
Equilibrium
Quantum Dynamics
Anatoli Polkonikov
Boston
University
Abstract: Recent experimental progress in cold atom physics allows us
to get nearly isolated many-particle systems with tunable interactions.
This opens new possibilities to experimentally study quantum dynamics
far from equilibrium. In particular, one can now address such
fundamental problems of condensed matter physics as thermalization,
role of integrability, decoherence and quantum measurements in
many-body interacting systems, etc. Such experiments also motivate a
lot of theoretical attention to these problems.
In this talk I will first review some recent experiments in quantum
dynamics of cold atoms. Then I will concentrate on a particular example
of slow (adiabatic) dynamics and will discuss how quantum adiabatic
theorem is related to the thermodynamic adiabatic theorem of
statistical physics. I will argue that in low-dimensional gapless
systems the thermodynamic adiabatic theorem can fail in two different
ways. I will illustrate general statements with specific examples of
tuning a system across a quantum phase point and slow dynamics in
weakly interacting superfluids. If time permits, I will also describe
another example of slow dynamics in moving superfluids. I will present
a non-equilibrium phase diagram and show that it qualitatively depends
on the dimensionality of the system. The predictions of this work are
in excellent agreement with recent experiments.
Wednesday,
November 14th:
Matter-waves Coherence Loss
and
Revival Induced by an Atom-optics Kicked Rotor
Alexya Tonyushkin
Harvard
University
Abstract: We implemented an atomic kicked rotor in a de Broglie wave
interferometer. The dynamics of the matter-wave's coherence is
investigated by looking at the echo signal of the interferometer. The
overlap of a reference interferometer signal and its perturbed version
is analogous to a fidelity amplitude measurement of our interferometer.
We directly observed that at quantum resonances the fidelity decay
saturates at a finite value after just a small number of kicks. We
discuss the applications of the scheme to high precision measurements
and quantum simulations.
Wednesday,
November 7th:
Supersolid
State of Matter
Boris Svistunov
University
of Massachusetts – Amherst
Abstract: The discovery of the phenomenon of supersolidity in helium, a
non-dissipative transport of He-4 atoms through He-4 crystals, is an
amazing recent achievement in the low-temperature physics.
While some of the mechanisms of supersolidity in He-4, such as
superfluid grain boundaries and superfluid screw dislocations, are now
established at the level of ab initio simulations, a microscopic
interpretation of the phenomenon of non-classical rotational inertia
originally observed by Kim and Chan is still lacking, and actually gets
even more controversial in light of recent experiments by other groups.
Wednesday,
October 31st: (4 p.m. seminar)
Left
handed light
Srinivas
Sridhar
Northeastern
University
Abstract: We discuss negative refraction at microwave and optical
frequencies in 1D and 2D metallic and dielectric photonic crystal
media. Here negative refraction is due to the anomalous dispersion
characteristics of the medium. In negative index media, the fields and
the wavevector obey a left-handed relationship. The experiments show
that materials with tailor-made negative or positive refractive indices
over broad spectral ranges can be designed and fabricated. We have also
demonstrated that negative refraction leads to some novel optical
elements for imaging, such as flat lenses and focusing by plano concave
lenses. A general theory of imaging by a flat lens without optical axis
has been developed, leading to specification of the characteristics
required of the flat lens metamaterial. Features of images formed using
negatively refracting optical elements, including sub-wavelength
resolution, are discussed. Potential applications for imaging and
communications at microwave and optical frequencies are discussed.
Work supported by the National Science Foundation and the Air Force
Research Laboratories, Hanscom.
Wednesday,
October 24th:
New
Frontiers in Trapping of
Ultra-Cold Atoms and Molecules
Mark G. Raizen
University
of Texas at Austin
Abstract: The method of laser cooling has opened the door to low
temperature physics of dilute gases. Despite the great
success of
this method, it has been limited to a small set of atoms in the
periodic table. I will describe in this talk new approaches to trapping
of atoms and molecules that do not require lasers and are hence
applicable to most atoms. I will then describe how the
trapped
atoms can be further cooled in an experimental realization of Maxwell's
demon. Applications of these methods to atom optics and to
fundamental tests will be described.
Wednesday,
October 17th:
Quantum
Simulation
Markus Greiner
Harvard
University
Abstract: Recent advances in ultracold atom research
have opened a new research field in which fundamental questions of
modern condensed matter physics can be addressed with ultracold quantum
gases. I will discuss such quantum simulations of condensed matter
systems, present our concept of a quantum gas microscope and report
about first results with our new apparatus.
Thursday,
October 11th:
Monte
Carlo Simulations of Transport Properties in Nanostructures
Rob Kelsall
The
University of Leeds
Abstract: Stochastic (Monte Carlo) methods are well suited to
the
simulation of particle trajectories in a diverse range of
media.
This approach offers numerous advantages over continuum
models: a
high degree of microscopic physical detail can be obtained;
non-equilibrium transport phenomena can be treated without
approximation, and transient responses can be obtained without
significantly extra computational effort. In this
presentation,
the application of Monte Carlo methods to a range of nanostructured
systems will be described, including simulations of electronic
transport in multilayer semiconductor devices, optoelectronic devices,
and conducting organic layers. Results from multiphysics
algorithms, in which Monte Carlo trajectory simulations are
self-consistently coupled with electrostatic (Poisson) and thermal
(heat diffusion) solutions, will be presented.
Application
of Monte Carlo methods to phonon transport simulations will also be
discussed.
Wednesday,
October 10th:
How
do we know what they know?
Arthur Eisenkraft
COSMIC,
University of Massachusetts, Boston
Abstract: Assessment is a crucial component of good
instruction.
Advances in the cognitive and measurement sciences allow us to take a
fresh look at the way in which we assess student
understanding.
Theoretical approaches to assessment can inform practical applications
in the classroom to enhance student performance and success in
courses. A review of recent literature and some concrete
recommendations for course assessments will be discussed.
Wednesday,
October 3rd:
Dimensionality
and dynamics in the behavior of C. elegans
Greg Stephens
Princeton
University
Abstract: A major challenge in analyzing animal behavior is to discover
some underlying simplicity in complex motor actions. Here we
show
that the space of shapes adopted by the nematode C. elegans is
surprisingly low dimensional, with just four dimensions accounting for
95% of the shape variance, and we partially reconstruct `equations of
motion’ for the dynamics in this space. These dynamics have
multiple attractors, and we find that the worm visits these in a rapid
and almost completely deterministic response to weak thermal
stimuli. Stimulus-dependent correlations among the different
modes suggest that one can generate more reliable behaviors by
synchronizing stimuli to the state of the worm in shape space. We
confirm this prediction, effectively ``steering’’
the worm
in real time.
Wednesday,
September 26th:
WKB
Propagation of Gaussian
Wavepackets
Raul Vallejos
Centro
Brasileiro de Pesquisas Fisicas, Rio de Janeiro, Brasil
Abstract: We will analyze the semiclassical evolution of a
Gaussian wavepacket in a chaotic closed system using standard
time-dependent WKB theory (no complex trajectories). We will show that
the Wigner function develops the structure of a classical filament plus
quantum oscillations, with phase and amplitude being determined by
geometric properties of an evolving classical manifold. We also will
discuss the extension of the theory to open systems in the Lindbladian
context.
Wednesday,
September 19th:
Thermalization
in closed quantum systems: open problems and suggested solutions
Maxim Olchanii
University
of Massachusetts, Boston
Abstract: I will begin with a list
of open questions on the foundations
of statistical mechanics of closed quantum systerms. Among
them are: (1) how can linear quantum dynamics provide chaos necessary
for
thermalization, and (2) which constraints should be used in building an
appropriate thermodynamical ensemble, given that in a quantum system
with nondegenerate spectra all integrals of motion are functionally
dependent on the Hamiltonian?
To answer these questions, we perform an ab initio numerical analysis
of a system of hard-core bosons on a lattice and show that these
controversies can be resolved via the Eigenstate Thermalization
Hypothesis [Srednicki, 1994] and its further generalizations for
integrable systems. According to this hypothesis, in quantum
systems thermalization happens at the level of individual eigenstates,
but hidden initially by coherences between them.
In course of time evolution the thermal properties become revealed
through (linear) decoherence. This answeres the question (1).
Our numerical experiemnts with integrable lattices partially resolve
the
controversy (2), indicating that dunctional independence between the
integrals of motion plays no role in quantum thermodynamics and should
probably be superseded by functional independence between their quantum
expectation values.
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