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BEGIN:VEVENT
DTSTAMP:20171215T101318Z
LAST-MODIFIED:20170728T165201Z
DTSTART:20170801T193000Z
DTEND:20170801T203000Z
UID:event1775@bu.edu
URL:http://physics.bu.edu/events/show/1775
SUMMARY:Investigating Effects of Morphology and Flagella Dynamics on Swimmi
ng Kinematics of Different Helicobacter Species Using Single Cell Imaging
DESCRIPTION:Featuring Maira Constantino\n\nPart of the PhD Final Oral Exams
.\n\nExamining Committee: Rama Bansil\, Shyam Erramilli\, Kirill Korolev\,
Karl Ludwig\, Kevin Black\n\nAbstract:\n\nThis work explores the effects o
f body shape and configuration of flagella on motility of Helicobacter pylo
ri\, a helical-shaped bacterium that inhabits the viscoelastic gastric muco
sa and causes gastritis\, ulcers and gastric cancer. Although it is well kn
own that different shapes produce different hydrodynamic drag thus altering
the speed and that helical shapes generate additional thrust this has not
been quantitatively established for flagellated bacteria. Using fast time-r
esolution and high-magnification two-dimensional phase-contrast microscopy
to simultaneously image and track individual helical-shaped H. pylori and i
ts rod-shaped isogenic mutant in broth and mucin solutions\, the shape as w
ell as rotational and translational speed can be determined. Henry Fu and M
ehdi Jabbarzadeh used the experimental data to validate the method of regul
arized Stokeslets by directly comparing the observed speeds to numerical ca
lculations. The results show that due to relatively slow body rotation rate
s\, the helical shape makes at most a 15% contribution to speeds. In order
to explore the effects of flagella configuration on motility\, three membe
rs of the Helicobacter species were examined: H. suis (bipolar\, multiple f
lagella)\, H. cetorum (bipolar\, single flagellum) and H. pylori (unipolar\
, multiple flagella). Due to the large number of flagella in each pole\, th
e flagellar bundle of H. suis could be visualized while it swam in broth an
d mucin. Results show that regardless of media\, the flagella bundles can a
ssume one of two configurations interchangeably: extended away from the bod
y or wrapped around it. H. suis predominantly swims with the lagging flagel
la extended behind the body and the leading flagella wrapped around it\, bu
t cases where both bundles are extended or both are wrapped have also been
observed. Using the same experimental technique described earlier\, the res
ults show that bipolar bacteria gain a speed advantage over unipolar bacter
ia. In addition\, the effects of pH on motility of H. suis in broth and muc
in were investigated. Similarly to previous findings on H. pylori\, the res
ults show that H. suis swims faster in mucin than broth at same pH and that
it stops swimming in mucin at pH4 due to gelation.
LOCATION:SCI 352\, 590 Commonwealth Avenue\, 02215
STATUS:CONFIRMED
CLASS:PUBLIC
END:VEVENT
BEGIN:VEVENT
DTSTAMP:20171215T101318Z
LAST-MODIFIED:20170724T132556Z
DTSTART:20170811T161500Z
DTEND:20170811T181500Z
UID:event1779@bu.edu
URL:http://physics.bu.edu/events/show/1779
SUMMARY:Statistical Physics\, Financial and Complex Systems and Networks
DESCRIPTION:Featuring Asher Mullokandov\n\nPart of the PhD Final Oral Exams
.\n\nExamining Committee: H.E. Stanley\, William Skocpol\, Kirill Korolev\,
Irena Vodenska\, Raj Mohanty
LOCATION:SCI 328\, 590 Commonwealth Avenue\, 02215
STATUS:CONFIRMED
CLASS:PUBLIC
END:VEVENT
BEGIN:VEVENT
DTSTAMP:20171215T101318Z
LAST-MODIFIED:20170815T205644Z
DTSTART:20170911T170000Z
DTEND:20170911T180000Z
UID:event1798@bu.edu
URL:http://physics.bu.edu/events/show/1798
SUMMARY:COUPLED WIRE CONSTRUCTION OF NON-ABELIAN CHIRAL SPIN LIQUIDS
DESCRIPTION:Featuring Po-Hao Huang\n\nPart of the PhD Final Oral Exams.\n\n
Dissertation Committee: Claudio Chamon\, Andrei Ruckenstein\, Anatoli Polk
ovnikov\, Emanuel Katz\, Michael El-Batanouny\n\nAbstract:\n\nTopologically
ordered states of matter have attracted much interest in the past decade.\
nThe search for emergent point-like non-Abelian anyons electronic systems i
s one of the\nmost exciting problems in condensed matter physics\, since it
opens many possibilities for\ntopological quantum computing. In this talk\
, I will construct two-dimensional non-Abelian\ntopologically ordered state
s by strongly coupling arrays of one-dimensional quantum wires\nvia interac
tions. In our scheme\, all charge degrees of freedom are gapped\, so the co
n-\nstruction can use either quantum wires or quantum spin chains as buildi
ng blocks\, with\nthe same end result. Hence\, the resulting non-Abelian sp
in liquid states have vanishing\nquantum Hall conductivities\, but nonzero
thermal ones. The construction gaps the degrees\nof freedom in the bulk\, w
hile leaving decoupled states at the edges that are described by\nconformal
eld theories (CFT) in (1 + 1)-dimensional space and time. We prescribe\, i
n\nparticular\, how to arrive at all the edge states described by the unita
ry CFT minimal\nmodels with central charges c < 1.
LOCATION:SCI TBA\, 590 Commonwealth Avenue\, 02215
STATUS:CONFIRMED
CLASS:PUBLIC
END:VEVENT
BEGIN:VEVENT
DTSTAMP:20171215T101318Z
LAST-MODIFIED:20170925T130542Z
DTSTART:20170928T193000Z
DTEND:20170928T203000Z
UID:event1837@bu.edu
URL:http://physics.bu.edu/events/show/1837
SUMMARY:SOLUTIONS TO LINEAR PROBLEMS IN ABERRATED OPTICAL SYSTEMS
DESCRIPTION:Featuring William Shain\n\nPart of the PhD Final Oral Exams.\n\
nExamining Committee: Bennett Goldberg\, Jerome Mertz\, David Campbell\, St
eve Ahlen\, Thomas Bifano\n\nAbstract:\n\nOne of the greatest challenges in
understanding any physical system is extracting as much information as pos
sible. In optics\, the laws that govern the interaction of a system with li
ght are frequently linear: Maxwell’s equations lead to linear wave equati
ons\, and in general one can write the evolution of the quantities of inter
est as a linear propagation in space and time. This linearity imposes a con
straint on how one recovers information about an object of interest; for ex
ample\, one can expand the field-of-view by taking multiple images (reducin
g the total framerate) or by lowering the numerical aperture (reducing the
resolution). In each case\, one aspect of information is sacrificed for ano
ther. Therefore\, one can efficiently probe a system through optical means
by carefully choosing what aspects of the light are measured\, depending on
the problem of interest. This can be attained through direct manipulation
of the optical propagator\, by using a Spatial Light Modulator to perform p
hysical linear transformations on the light field. In this thesis\, I demon
strate the usefulness of a MEMS Deformable Mirror (DM) in implementing the
necessary linear transformation\, and apply the methodology to three system
s of interest: recovering imaging capabilities in porous rock\, expanded im
aging of neuronal activity in brain tissue\, and high-throughput particle i
dentification in label-free flow cytometry. In each case\, I show how caref
ul control of the light travelling through the microscope allows for optimi
zed recovery of the desired information.
LOCATION:SCI 352\, 590 Commonwealth Avenue\, 02215
STATUS:CONFIRMED
CLASS:PUBLIC
END:VEVENT
BEGIN:VEVENT
DTSTAMP:20171215T101318Z
LAST-MODIFIED:20171013T200332Z
DTSTART:20171113T170000Z
DTEND:20171113T180000Z
UID:event1846@bu.edu
URL:http://physics.bu.edu/events/show/1846
SUMMARY:Magnetic Field Effects in Low-Dimensional Quantum Magnets
DESCRIPTION:Featuring Adam Iaizzi\n\nPart of the PhD Final Oral Exams.\n\nE
xamining Committee: Anders Sandvik\, David Campbell\, Robert Carey\, Shyam
Erramilli\, Claudio Chamon\n\nAbstract:\n\nWe present a comprehensive study
of a low-dimensional spin-half quantum antiferromagnet\, the J-Q model\, i
n the presence of an external (Zeeman) magnetic field using numerical \nm
ethods\, chiefly stochastic series expansion quantum Monte Carlo with dire
cted loop updates and \nquantum replica exchange. The J-Q model is a many
-body Hamiltonian acting on a lattice of \nlocalized spin-half degrees of f
reedom; it augments the Heisenberg exchange with a \nfour-spin interaction
of strength Q. This model has been extensively studied at zero field\, whe
re \nthe Q term drives a quantum phase transition from a Neel-like state
to a valence-bond solid (a nonmagnetic state consisting of a long-range-
ordered arrangement of local singlet bonds \nbetween sites). This transiti
on is believed to be an example of deconfined quantum criticality\, \nwhere
the excitations are spinons—exotic spin-half bosons. We study the J-Q m
odel in the presence \nof a magnetic field in both one and two dimensions.
In one dimension\, there is metamagnetism above \na critical coupling rati
o (Q/J )min. Metamagnetism is a first-order quantum phase transition \ncha
racterized by discontinuities in the magnetization as a function of field (
magnetization jumps). \n We derive an exact expression for (Q/J)min = 2/9\
, and show that the metamagnetism is caused by \nthe onset of attractive in
teractions between magnons (flipped spins on a polarized\nbackground). We
predict that the same mechanisms will produce metamagnetism in the unfrus
trated \nantiferromagnetic J1-J2 model with anisotropy. Below (Q/J)min\,\n
the saturation transition is continuous and we show that it is governed by
the expected zero-scale-factor universality. In two dimensions\, we also f
ind metamagnetism above a \ncritical coupling ratio (Q/J)min=0.417\, caused
by the same mechanism as in the one-dimensional \ncase. In two dimension
s we also show evidence of an anomalous temperature dependence of specific
\nheat arising from field-induced Bose-Einstein condensation of spinons at
the deconfined quantum \ncritical point.
LOCATION:SCI 328\, 590 Commonwealth Avenue\, 02215
STATUS:CONFIRMED
CLASS:PUBLIC
END:VEVENT
BEGIN:VEVENT
DTSTAMP:20171215T101318Z
LAST-MODIFIED:20171204T215759Z
DTSTART:20171206T133000Z
DTEND:20171206T140000Z
UID:event1860@bu.edu
URL:http://physics.bu.edu/events/show/1860
SUMMARY:RIVETING TWO-DIMENSIONAL MATERIALS: EXPLORING STRAIN PHYSICS IN ATO
MICALLY THIN CRYSTALS WITH MICROELECTROMECHANICAL SYSTEMS
DESCRIPTION:Featuring Jason Christopher\, Boston University\, Department of
Physics\n\nPart of the PhD Final Oral Exams.\n\nABSTRACT\nTwo dimensional
(2D) materials can withstand an order of magnitude more strain than their b
ulk counterparts\, which results in dramatic changes to electrical\, therma
l and optical properties. These changes can be harnessed for technological
applications such as tunable light emitting diodes or field effect transis
tors\, or utilized to explore novel physics like exciton confinement\, pseu
do-magnetic fields (PMFs)\, and even quantum gravity. However\, current te
chniques for straining atomically thin materials offer limited control over
the strain field\, and require bulky pressure chambers or large beam bendi
ng equipment. This dissertation describes the development of micro-electrom
echanical systems (MEMS) as a platform for precisely controlling the magnit
ude and orientation of the strain field in 2D materials. MEMS are a versat
ile platform for studying strain physics. Mechanical\, electrical\, thermal
and optical probes can all be easily incorporated into their design. Furt
her\, because of their small size and compatibility with electronics manufa
cturing methods\, there is an achievable pathway from the laboratory bench
to real-world application. Nevertheless\, the incorporation of atomically
thin crystals with MEMS has been hampered by fragile\, non-planer structure
s and low friction interfaces. We have innovated two techniques to overcom
e these critical obstacles: micro-structure assisted transfer to place the
2D materials on the MEMS gently and precisely\, and micro-riveting to creat
e a slip-free interface between the 2D materials and MEMS. With these advan
cements\, we were able to strain monolayer molybdenum disulfide (MoS2) to g
reater than 1% strain with a MEMS for the first time. The dissertation dev
elops the theoretical underpinnings of this result including original work
on the theory of operation of MEMS chevron actuators\, and strain generated
PMFs in transition metal dichalcogenides\, a large class of 2D materials.
We conclude the dissertation with a roadmap to guide and inspire future ph
ysicists and engineers exploring strain in 2D systems and their application
s. The roadmap contains ideas for next-generation fabrication techniques t
o improve yield\, sample quality\, and add capabilities. We have also incl
uded in the roadmap proposals for experiments such as a speculative techniq
ue for realizing topological quantum field theories that mimics recent theo
retical wire construction methods.\n\n![Jason](/resources/event-image/1860/
c555177_small)
LOCATION:SCI 352\, 590 Commonwealth Avenue\, 02215
STATUS:CONFIRMED
CLASS:PUBLIC
END:VEVENT
BEGIN:VEVENT
DTSTAMP:20171215T101318Z
LAST-MODIFIED:20171128T165801Z
DTSTART:20171211T170000Z
DTEND:20171211T180000Z
UID:event1865@bu.edu
URL:http://physics.bu.edu/events/show/1865
SUMMARY:DYNAMICAL PROPERTIES OF CLASSICAL AND QUANTUM SPIN SYSTEMS
DESCRIPTION:Featuring Na Xu\n\nPart of the PhD Final Oral Exams.\n\nExamini
ng Committee: Anders Sandvik\, Anatoli Polkovnikov\, Claudio Chamon\, Alex
Sushkov\, Robert Carey\n\nAbstract:\n\nThe Kibble-Zurek mechanism (KZM) wa
s originally proposed to describe the evolution\nand "freezing" of defects
in the early universe\, but later it was generalized to study other\nquantu
m and classical systems driven by a varying parameter. The basic idea behin
d the\nKZM is that\, as long as the changing rate (velocity) of the paramet
er is below a certain\ncritical velocity\, the system will remain adiabatic
(for quantum systems) or quasi-static\n(for classical systems). The non-eq
uilibrium finite-size scaling (FSS) method based on KZM\nhas been exploited
systematically. Through applying the scaling hypothesis\, we can extract\n
the critical exponents and study the dynamic properties of the system.\nIn
the rest few chapters of this dissertation\, we discuss the applications of
KZM in several\nclassical systems: first\, we study the dynamics of 2D and
3D Ising model under a varying\ntemperature as well as a varying magnetic
eld. Secondly\, we examine the classical Z2 gauge\nmodel\, in which we show
that KZM also works for topological phase transitions. Moreover\,\nwe also
investigate the dynamics of other models with topological ordering only at
T = 0\,\nwhere KZM cannot be applied. Lastly\, we explore the 2D Ising spi
n glass with bimodal\nand gaussian couplings. With bimodal couplings\, we f
ind dual time scales associated with\nthe order parameter and the energy co
rrespondingly\, while in the gaussian case one unique\ntime scale is involv
ed.\nThe systems mentioned above are all classical and the dynamics are app
roached through\nsimulated annealing (SA)\, in which thermal uctuations dri
ves systems to explore the energy\nlandscape in finding the ground state. I
n the last chapter\, we explore the efficiency of Quantum Annealing (QA) \n
on a fully-connected spin glass (or Sherington-Kirkpatrick model)\nwith a t
ransverse field. QA is the counterpart of SA\, where quantum fluctuations d
rive the\nsystem toward the ground state when the quantum terms are reduced
. QA is currently\nwidely explored as a paradigm for quantum computing to s
olve optimization problems.\nHere we compare the scaling of the dynamics (w
ith system size) of the fully-connected spin\nglass through QA versus SA.
LOCATION:SCI 328\, 590 Commonwealth Avenue\, 02215
STATUS:CONFIRMED
CLASS:PUBLIC
END:VEVENT
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