BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//RLASKEY//CALENDEROUS//EN
CALSCALE:GREGORIAN
METHOD:PUBLISH
BEGIN:VEVENT
DTSTAMP:20171215T101245Z
LAST-MODIFIED:20170906T184607Z
DTSTART:20170808T190000Z
DTEND:20170808T200000Z
UID:event1780@bu.edu
URL:http://physics.bu.edu/events/show/1780
SUMMARY:Pseudoparticle approach to strongly correlated systems
DESCRIPTION:Featuring Garry Goldstein\, University of Cambridge \nHosted by
: Claudio Rebbi\n\nPart of the Condensed Matter Theory Seminar Series.\n\nI
n the ﬁrst part we present a slave-particle meanﬁeld study of the mixed
boson+fermion quantum dimer model introduced by Punk\, Allais\, and Sachde
v [PNAS 112\, 9552 (2015)] to describe the physics of the pseudogap phase i
n cuprate superconductors. Our analysis naturally leads to four charge e fe
rmion pockets whose total area is equal to the hole doping p\, for a range
of parameters consistent with the t−J model for high temperature supercon
ductivity. Here we ﬁnd that the dimers are unstable to d-wave superconduc
tivity at low temperatures. The region of the phase diagram with d-wave rat
her than s-wave superconductivity matches well with the appearance of the f
our fermion pockets. In the superconducting regime\, the dispersion contain
s eight Dirac cones along the diagonals of the Brillouin zone. \n\nIn the s
econd part we present an approach for studying systems with hard constraint
s such that certain positive semideﬁnite operators must vanish. The diﬃ
culty with mean-ﬁeld treatments of such cases is that imposing that the c
onstraint is zero only in average is problematic for a quantity that is alw
ays non-negative. We reformulate the hard constraints by adding an auxiliar
y system such that some of the states to be projected out from the total sy
stem are at ﬁnite negative energy and the rest at ﬁnite positive energy
\, we dubbed this method the soft constraint. This auxiliary system comes w
ith an extra coupling that we are free to vary and that parametrizes a whol
e family of mean-ﬁeld theories. We argue that this variational-type param
eter for the family of mean-ﬁeld theories should be ﬁxed by matching a
given experimental observation\, with the quality of the resulting mean-ﬁ
eld approximation measured by how it ﬁts other data. We test these ideas
in the well-understood single-impurity Kondo problem\, where we ﬁx the pa
rameter via the TK (Kondo temperature) obtained from the magnetic susceptib
ility value\, and score the quality of the approximation by its predicted W
ilson ratio. We also study a continuum version of the soft constraint metho
d. To show its usefulness we study the Laughlin fractional quantum Hall sta
tes at ﬁlling fraction ν = 1 2m+1. We produce the Landau Ginzburg Cherns
Simons theory for hard core bosons(arising in this formulation of the Laug
hlin functions0 for these ﬁlling fractions. We handle the hard core const
raint explicitly and show that to leading order it is equivalent to a theor
y of soft core bosons with an eﬀective density density interaction betwee
n the particles of the form of a Coulomb interaction coming from the electr
ons in the original theory and an eﬀective interaction coming from the ha
rd core constraint that can be tuned by hand which we denote by λ(q\,ω).
We show that we can use this eﬀective interaction to reproduce the known
spectra of both inter Landau level and intra Landau level neutral excitatio
ns for the Laughlin states as well as the energy gap to charged excitations
and the electromagnetic response tensor.
LOCATION:SCI 328\, 590 Commonwealth Avenue\, 02215
STATUS:CONFIRMED
CLASS:PUBLIC
END:VEVENT
BEGIN:VEVENT
DTSTAMP:20171215T101245Z
LAST-MODIFIED:20170906T184620Z
DTSTART:20170810T150000Z
DTEND:20170810T160000Z
UID:event1781@bu.edu
URL:http://physics.bu.edu/events/show/1781
SUMMARY:The many body localization transition: A dynamical phase transition
triggered by a quantum avalanche
DESCRIPTION:Featuring Marcus Muller\, Paul Scherer Institute\, Villigen Swi
tzerland\nHosted by: Claudio Rebbi\n\nPart of the Condensed Matter Theory S
eminar Series.\n\nI will review recent progress on the understanding of man
y-body localization\, i.e.\, the phenomenon of absence of thermalization in
interacting\, disordered quantum systems. After discussing the restricted
domain of existence of strict many body-localization and the role of rare e
vents\, I will present a real space renormalization group analysis which of
fers a microscopic scenario for the unusual dynamical phase transition whic
h occurs as a localized system is tuned into an ergodic regime. While the t
ransition is associated with a diverging time scale for thermalization\, mo
st spatial correlators remain short range at the critical point. The latter
goes hand in hand with the discontinuous evolution of expectation values o
f local observables. These features distinguish the transition sharply from
standard critical phenomena.
LOCATION:SCI 328\, 590 Commonwealth Avenue\, 02215
STATUS:CONFIRMED
CLASS:PUBLIC
END:VEVENT
BEGIN:VEVENT
DTSTAMP:20171215T101245Z
LAST-MODIFIED:20170906T184708Z
DTSTART:20170811T150000Z
DTEND:20170811T170000Z
UID:event1782@bu.edu
URL:http://physics.bu.edu/events/show/1782
SUMMARY:Out-of-equilibrium states and quantum annealing speed up in non-con
vex optimization and learning problems
DESCRIPTION:Featuring Riccardo Zecchina\n\nPart of the Condensed Matter The
ory Seminar Series.\n\nQuantum annealers aim at solving non-convex optimiza
tion problems by exploiting cooperative tunneling effects to escape local m
inima. A key challenge is to identify classes of non-convex optimization pr
oblems for which this scheme can induce an exponential speed up compared t
o thermal annealing. We show that this happens for a wide class of optimiza
tion problems which are also central to machine learning. Their energy land
scape is exponentially dominated by local minima that trap classical therma
l annealers\, while quantum annealing converges efficiently to rare dense r
egions of optimal solutions. For non convex problems\, these will not in g
eneral correspond to configuration dominating the Gibbs measure.
LOCATION:SCI 328\, 590 Commonwealth Avenue\, 02215
STATUS:CONFIRMED
CLASS:PUBLIC
END:VEVENT
BEGIN:VEVENT
DTSTAMP:20171215T101245Z
LAST-MODIFIED:20170814T133931Z
DTSTART:20170816T150000Z
DTEND:20170816T160000Z
UID:event1783@bu.edu
URL:http://physics.bu.edu/events/show/1783
SUMMARY:Entanglement entropy of finite-temperature pure quantum states and
thermodynamic entropy as a Noether invariant
DESCRIPTION:Featuring Sho Sugiura\, University of Tokyo\nHosted by: Anatoli
Polkovnikov\n\nPart of the Condensed Matter Theory Seminar Series.\n\nEntr
opy is a fundamental concept in statistical mechanics. In this talk\, I inv
estigate two aspects of entropy. First\, we reveal a universal behavior of
entanglement entropy of the pure quantum states which are in thermal equili
brium states. Second\, we characterize thermodynamic entropy as a Noether i
nvariant under a non-uniform time translation. \n\nIn the first part\, we f
ocus on the size dependence of the entanglement entropy of the pure quantum
states which can fully describe thermal equilibrium as long as one focuses
on local observables. The thermodynamic entropy can also be recovered as t
he entanglement entropy of small subsystems. When the size of the subsystem
increases\, however\, quantum correlations break the correspondence and ma
ndate a correction to this simple volume-law. The elucidatione of the size
dependence of the entanglement entropy is thus of essential importance in l
inking quantum physics with thermodynamics\, and in analyzing recent experi
ments on ultra-cold atoms. In this talk\, we derive an analytic formula of
the entanglement entropy for a class of pure states called cTPQ states repr
esenting thermal equilibrium. We find that our formula applies universally
to any sufficiently scrambled pure state representing thermal equilibrium\,
i.e.\, general energy eigenstates of non-integrable models and states afte
r quantum quenches. Our universal formula can be exploited as a diagnostic
tool for chaotic systems; we can distinguish integrable models from chaotic
modelsand detect many-body localization with high accuracy. \n\nIn the sec
ond part\, we investigate a thermally isolated quantum many-body system wit
h an external control represented by a time-dependent parameter. We formula
te a path integral in terms of thermal pure states and derive an effective
action for trajectories in a thermodynamic state space\, where the entropy
appears with its conjugate variable. In particular\, for quasi-static opera
tions\, the symmetry for the uniform translation of the conjugate variable
emerges in the path integral. This leads to the entropy as a Noether invari
ant.
LOCATION:SCI 352\, 590 Commonwealth Avenue\, 02215
STATUS:CONFIRMED
CLASS:PUBLIC
END:VEVENT
BEGIN:VEVENT
DTSTAMP:20171215T101245Z
LAST-MODIFIED:20170906T183707Z
DTSTART:20170913T150000Z
DTEND:20170913T160000Z
UID:event1816@bu.edu
URL:http://physics.bu.edu/events/show/1816
SUMMARY:Correlations in driven-dissipative quantum systems
DESCRIPTION:Featuring Michiel Wouters\, University of Antwerp\nHosted by: A
natoli Polkovnikov\, Dries Sels\n\nPart of the Condensed Matter Theory Semi
nar Series.\n\nRecent advances in experimental platforms for photonic quant
um simulators (microcavity polaritons and superconducting circuits) have sp
urred the interest in the theory of driven-dissipative quantum systems. The
long time steady state of such systems is not given by a Boltzmann-Gibbs d
istribution\, but has to be found by solving a Lindblad master equation. \n
\nIn this talk\, I will discuss two topics: for weakly interacting systems\
, the imprint of Landau-Beliaev scattering on the steady state distribution
of a 1D photonic Hubbard model in the weakly interacting regime and the fa
ilure of the truncated Wigner approximation to capture this feature. For st
rongly interacting systems\, I will present results obtained within the Gut
zwiller approximation to the quantum trajectories of a dissipative XYZ mode
l. We show that a ferromagnetic to paramagnetic phase transition\, that is
missing within a mean field treatment\, is recovered within our approach.
LOCATION:SCI 328\, 590 Commonwealth Avenue\, 02215
STATUS:CONFIRMED
CLASS:PUBLIC
END:VEVENT
BEGIN:VEVENT
DTSTAMP:20171215T101245Z
LAST-MODIFIED:20170912T161810Z
DTSTART:20170915T160000Z
DTEND:20170915T173000Z
UID:event1833@bu.edu
URL:http://physics.bu.edu/events/show/1833
SUMMARY:"Resonant x-ray spectroscopy of topological semimetals"
DESCRIPTION:Featuring Stefanos Kourtis\, Boston University\, Physics Depart
ment\n\nPart of the Condensed Matter Theory Seminar Series.\n\nAngle-resolv
ed photoemission spectroscopy (ARPES) has so far been the definitive method
for the characterization of materials as topological Weyl or Dirac semimet
als\, via direct visualization of band touchings in the bulk and nontrivial
states at the boundary. However\, the unconventional and potentially usefu
l properties of these materials are a consequence of a singular Berry curva
ture distribution\, which cannot be resolved in ARPES. Furthermore\, contro
l over the number and position of Weyl nodes -- the most important property
from an application standpoint -- can only be achieved with sizable electr
omagnetic fields\, which curtail the resolving power of ARPES\, thereby ren
dering the band structure inaccessible to this technique. We show how reson
ant x-ray scattering (RXS) offers a viable path for (a) deducing the band s
tructure of materials in arbitrarily large magnetic fields and (b) visualiz
ing the Berry curvature distribution in momentum space via polarization con
trol of incoming and outgoing x-ray beams. By low-energy modeling of specif
ic material candidates based on ab initio band structure calculations\, we
derive the corresponding RXS spectra and highlight the salient features ste
mming from topological nontriviality. The proposed measurements are within
the resolving capabilities of current instrumentation.
LOCATION:SCI 352\, 590 Commonwealth Avenue\, 02215
STATUS:CONFIRMED
CLASS:PUBLIC
END:VEVENT
BEGIN:VEVENT
DTSTAMP:20171215T101245Z
LAST-MODIFIED:20170919T145442Z
DTSTART:20170922T160000Z
DTEND:20170922T173000Z
UID:event1836@bu.edu
URL:http://physics.bu.edu/events/show/1836
SUMMARY:"Interfering directed paths & the sign phase transition"
DESCRIPTION:Featuring Christopher Baldwin\, Boston University\, Physics Dep
artment\n\nPart of the Condensed Matter Theory Seminar Series.\n\nWe consid
er the problem of interfering directed paths in disordered media\, and disc
uss the physical contexts in which this problem arises. An important proper
ty of the path sum is the statistics of its sign. The average sign may tend
to zero ("sign-disordered") or remain finite ("sign-ordered") at long dist
ance\, depending on dimensionality and the concentration of negative scatte
ring sites x. We discuss the analogy with a dynamical out-of-equilibrium Is
ing field. We further show that the sign-ordered phase is always unstable f
or any non-zero x in 2D by identifying rare destabilizing events\, and pres
ent numerical evidence that it is stable in 3D. These results have conseque
nces for several different physical systems\, which we conclude by describi
ng.
LOCATION:SCI 352\, 590 Commonwealth Avenue\, 02215
STATUS:CONFIRMED
CLASS:PUBLIC
END:VEVENT
BEGIN:VEVENT
DTSTAMP:20171215T101245Z
LAST-MODIFIED:20170911T151308Z
DTSTART:20170925T150000Z
DTEND:20170925T163000Z
UID:event1829@bu.edu
URL:http://physics.bu.edu/events/show/1829
SUMMARY:"Lieb-Schultz-Mattis Theorem and its generalizations from the Persp
ective of the Symmetry Protected Topological Phase"
DESCRIPTION:Featuring Cenke Xu \, University of California - Santa Barbara\
nHosted by: Anders Sandvik\n\nPart of the Condensed Matter Theory Seminar S
eries.\n\nWe ask whether a local Hamiltonian with a featureless (fully gapp
ed and\nnondegenerate) ground state could exist in certain quantum spin sys
tems. \nWe address this question by mapping the vicinity of certain quantum
critical point (or gapless phase) of the d−dimensional spin system under
study to the boundary of a (d+1)−dimensional bulk state\, and the lattic
e symmetry of the spin system acts as an on-site symmetry in the field theo
ry that describes both the selected critical point of the spin system\, and
the corresponding boundary state of the (d+1)−dimensional bulk. If the c
orresponding bulk state is a trivial state instead of a bosonic symmetry pr
otected topological (SPT) state\, then a featureless ground state of the sp
in system is allowed; if the corresponding bulk state is indeed a nontrivia
l SPT state\, then it likely excludes the existence of a featureless ground
state of the spin system. From this perspective we identify the spin syste
ms with SU(N) and SO(N) symmetries on one\, two and three dimensional latti
ces that permit a featureless ground state. We also verify our conclusions
by other methods\, including an explicit construction of these featureless
spin states.
LOCATION:SCI 328\, 590 Commonwealth Avenue\, 02215
STATUS:CONFIRMED
CLASS:PUBLIC
END:VEVENT
BEGIN:VEVENT
DTSTAMP:20171215T101245Z
LAST-MODIFIED:20170908T135842Z
DTSTART:20170927T150000Z
DTEND:20170927T160000Z
UID:event1830@bu.edu
URL:http://physics.bu.edu/events/show/1830
SUMMARY:"Iterative Compression-Decimation Scheme for Tensor Network Optimiz
ation"
DESCRIPTION:Featuring Zhicheng Yang\, Boston University\, Physics Departmen
t\n\nPart of the Condensed Matter Theory Seminar Series.\n\nMotivated by st
atistical physics models connected to computation problems\, we devise a te
nsor network technique that is suited to problems with or without translati
on invariance and with arbitrary boundary conditions. We introduce a compre
ssion-decimation algorithm as an efficient iterative scheme to optimize ten
sor networks that encode generalized vertex models on regular lattices. The
algorithm first propagates local constraints to longer ranges via repeated
contraction-decomposition sweeps over all lattice bonds\, thus achieving c
ompression on a given length scale. It then decimates the lattice via coars
e-graining tensor contractions. Repeated iterations of these two steps allo
w us to gradually collapse the tensor network while keeping the tensor dime
nsions under control\, such that ultimately the full tensor trace can be ta
ken for relatively large systems. As a benchmark\, we demonstrate the effic
iency of the algorithm by computing the ground state entropy density of the
planar ice model and the eight-vertex model. We then apply it to reversibl
e classical computational problems based on a recently proposed vertex mode
l representation of classical computations [Nat. Commun. {\bf8}\, 15303 (20
17)]. Our protocol allows us to obtain the exact number of solutions for co
mputations where a naive enumeration would take astronomically long times\,
suggesting that the algorithm is a promising practical tool for the soluti
on of a plethora of problems in physics and computer science.
LOCATION:SCI 328\, 590 Commonwealth Avenue\, 02215
STATUS:CONFIRMED
CLASS:PUBLIC
END:VEVENT
BEGIN:VEVENT
DTSTAMP:20171215T101245Z
LAST-MODIFIED:20170912T160128Z
DTSTART:20171004T150000Z
DTEND:20171004T160000Z
UID:event1832@bu.edu
URL:http://physics.bu.edu/events/show/1832
SUMMARY:"Stability of quantum statistical ensembles with respect to local m
easurements"
DESCRIPTION:Featuring Walter Hahn\, Skolkovo Institute of Science and Techn
ology\, Moscow\nHosted by: Anatoli Polkovnikov\n\nPart of the Condensed Mat
ter Theory Seminar Series.\n\nWe introduce a stability criterion for quantu
m statistical ensembles describing macroscopic systems. An ensemble is call
ed "stable" when a small number of local measurements cannot significantly
modify the probability distribution of the total energy of the system. We a
pply this criterion to lattices of spins-1/2\, thereby showing that the can
onical ensemble is nearly stable\, whereas statistical ensembles with much
broader energy distributions are not stable. In the context of the foundati
ons of quantum statistical physics\, this result justifies the use of stati
stical ensembles with narrow energy distributions such as canonical or micr
ocanonical ensembles.
LOCATION:SCI 328\, 590 Commonwealth Avenue\, 02215
STATUS:CONFIRMED
CLASS:PUBLIC
END:VEVENT
BEGIN:VEVENT
DTSTAMP:20171215T101245Z
LAST-MODIFIED:20171006T140420Z
DTSTART:20171006T160000Z
DTEND:20171006T173000Z
UID:event1820@bu.edu
URL:http://physics.bu.edu/events/show/1820
SUMMARY:"Diabatic approach to creating ground states of certain critical mo
dels"
DESCRIPTION:Featuring Kartiek Agrawal\, Princeton University\nHosted by: An
atoli Polkovnikov\n\nPart of the Condensed Matter Theory Seminar Series.\n\
nWe propose a spatio-temporal quench protocol that allows for the fast prep
aration of ground states of gapless models with Lorentz invariance. Assumin
g the system initially resides in the ground state of a corresponding massi
ve model\, we show that a supersonically-moving ‘front’ that locally qu
enches the mass\, leaves behind it (in space) a state arbitrarily close to
the ground state of the gapless model. Importantly\, our protocol takes tim
e O (L) to produce the ground state of a system of size ∼ Ld (d spatial d
imensions)\, while a fully adiabatic protocol requires time ∼ O( L2) to p
roduce a state with exponential accuracy in L. We provide proof-of-concept
by solving the proposed quench exactly for a system of free bosons in arbit
rary dimensions\, and for free fermions in d = 1. We also discuss the role
of interactions and UV effects on the free-theory idealization\, before num
erically illustrating the usefulness of the approach using simulations on c
lassical phonons and on the quantum Heisenberg spin-chain with a mass induc
ed by local magnetic fields.
LOCATION:SCI 352\, 590 Commonwealth Avenue\, 02215
STATUS:CONFIRMED
CLASS:PUBLIC
END:VEVENT
BEGIN:VEVENT
DTSTAMP:20171215T101245Z
LAST-MODIFIED:20171002T180812Z
DTSTART:20171013T160000Z
DTEND:20171013T173000Z
UID:event1821@bu.edu
URL:http://physics.bu.edu/events/show/1821
SUMMARY:"Coherent optical control and nonlinear probing of strongly correla
ted materials"
DESCRIPTION:Featuring Roman Mankowsky\, Paul Scherrer Institute \nHosted by
: Wanzheng Hu\n\nPart of the Condensed Matter Theory Seminar Series.\n\nMid
-infrared optical pulses can resonantly drive selected vibrational modes in
solids to deform their crystal structure. In strongly correlated materials
\, this method has been used to melt electronic and magnetic order\, drive
insulator-to-metal transitions\, and transiently induce superconductivity.
Free Electron Lasers have been key in identifying the lattice dynamics unde
rlying these transitions.\n\nIn this talk\, I will first review nonlinear p
hononics and discuss its role in the phenomenon of light-induced supercondu
ctivity. I will then present recent studies\, in which resonant lattice exc
itation was used to control the polarization of ferroelectric materials on
femtosecond timescales. These studies are a first step toward ultrafast rev
ersible switching. In the last part of the lecture\, I will focus on the ex
citation of highly nonlinear lattice dynamics\, a new technique which allow
ed us to reconstruct the interatomic lattice potential of a solid and bench
mark first principle calculations.
LOCATION:SCI 352\, 590 Commonwealth Avenue\, 02215
STATUS:CONFIRMED
CLASS:PUBLIC
END:VEVENT
BEGIN:VEVENT
DTSTAMP:20171215T101245Z
LAST-MODIFIED:20170913T200604Z
DTSTART:20171018T150000Z
DTEND:20171018T160000Z
UID:event1814@bu.edu
URL:http://physics.bu.edu/events/show/1814
SUMMARY:"Cluster Synchronization of Chaotic Systems in Large Networks"
DESCRIPTION:Featuring Louis Pecora\, United States Naval Research Laborator
y\nHosted by: David Campbell\n\nPart of the Condensed Matter Theory Seminar
Series.\n\nSynchronization of two chaotic systems is a well-known phenomen
on\, although it still seems like an oxymoron. But when many oscillators (c
haotic or not) are linked together in a network causing them to interact\,
they can synchronize to other oscillators in the network\, but not necessar
ily all of them. The network can break up into clusters of synchronized sy
stems where the oscillators in the same cluster synchronize\, but not to os
cillators in other clusters even though all clusters are interconnected and
influence each other. I show that such patterns of synchronization can be
related to the symmetries of the network. One can use computational group
theory to find all the symmetries and hence\, clusters. Such symmetries ca
n be huge in number (e.g. 10^66 symmetries in a network of 100 oscillators)
\, but can be found in short time. I'll show how to analyze such systems\,
determine the stability of the cluster patterns\, and find interesting patt
erns of desynchronization bifurcations. I'll show how one can generate such
patterns in an electro-optical system. The methods I present will include
the construction of synchronization patterns that do not arise from symmetr
ies\, but can be built up from symmetry clusters make symmetries the fundam
ental concept in many cluster patterns.
LOCATION:SCI 328\, 590 Commonwealth Avenue\, 02215
STATUS:CONFIRMED
CLASS:PUBLIC
END:VEVENT
BEGIN:VEVENT
DTSTAMP:20171215T101245Z
LAST-MODIFIED:20171017T174315Z
DTSTART:20171020T160000Z
DTEND:20171020T170000Z
UID:event1847@bu.edu
URL:http://physics.bu.edu/events/show/1847
SUMMARY:"Kibble-Zurek Dynamics of a Driven Dissipative System: Universality
Beyond the Markovian Regime"
DESCRIPTION:Featuring Tamiro Villazon\, Boston University\, Physics Departm
ent\n\nPart of the Condensed Matter Theory Seminar Series.\n\nPhase transit
ions in open systems provide a rich avenue for the study of many-body non-e
quilibrium phenomena. These systems exhibit unique properties depending on
the detailed nature of their system-bath interactions\, which may nontrivia
lly alter their critical dynamics\, and which in turn are amenable to study
by slow ramps across the critical point. In this talk\, I will discuss Kib
ble-Zurek dynamics in a paradigmatic two-mode driven-dissipative system of
a pair of non-degenerate parametrically coupled\, optomechanical oscillator
s. I will detail the key theoretical aspects of the model and it's predicti
ons on universality\, which have been confirmed in experiments by our colla
borators in the Vengalattore group at Cornell University.
LOCATION:SCI 352\, 590 Commonwealth Avenue\, 02215
STATUS:CONFIRMED
CLASS:PUBLIC
END:VEVENT
BEGIN:VEVENT
DTSTAMP:20171215T101245Z
LAST-MODIFIED:20171018T193831Z
DTSTART:20171024T153000Z
DTEND:20171024T163000Z
UID:event1848@bu.edu
URL:http://physics.bu.edu/events/show/1848
SUMMARY:"Exciting dynamics in multiple time dimensions"
DESCRIPTION:Featuring Ivar Mar\, Argonne National Lab\n\nPart of the Conden
sed Matter Theory Seminar Series.\n\nExternally driving a dynamical system\
, be it quantum or classical\, effectively increases the number of its time
dimensions. In this talk I will describe how these extra time dimensions
can be harnessed to synthesize topological insulators purely in time domain
\, describe their possible applications for energy conversion and quantum e
ngineering\, and point out connections to localization and chaos.
LOCATION:SCI 328\, 590 Commonwealth Avenue\, 02215
STATUS:CONFIRMED
CLASS:PUBLIC
END:VEVENT
BEGIN:VEVENT
DTSTAMP:20171215T101245Z
LAST-MODIFIED:20171011T140140Z
DTSTART:20171025T150000Z
DTEND:20171025T163000Z
UID:event1823@bu.edu
URL:http://physics.bu.edu/events/show/1823
SUMMARY:"Nearly deconfined spinon excitations in the square-lattice spin-1/
2 Heisenberg antiferromagnet"
DESCRIPTION:Featuring Hui Shao\, Beijing Computational Science Research Cen
ter\nHosted by: Anders Sandvik\n\nPart of the Condensed Matter Theory Semin
ar Series.\n\nWe study the spin excitation spectrum of the spin-1/2 square-
lattice\nHeisenberg antiferromagnet with QMC and an improved stochastic\na
nalytic continuation method. The spectra are in excellent agreement\nwith r
ecent neutron scattering experiments on Cu(DCOO)2·4D2O\, where a\nbroad sp
ectral-weight continuum at wavevector q = (\pi\,0) was\ninterpreted as deco
nfined spinons. Our results show a similar\nreduction of the magnon weight
and a large continuum. Upon turning on\nthe Q interaction in the J-Q model\
, we observe a rapid reduction of\nthe magnon weight to zero\, well before
the system undergoes a\ndeconfined quantum phase transition into a non-magn
etic spontaneously\ndimerized state. Based on these results\, we re-interpr
et the picture\nof deconfined spinons at (\pi\,0) in the experiments as nea
rly\ndeconfined spinons—a precursor to deconfined quantum criticality. We
\nalso introduce an effective model of the excitations (one magnon or\ntwo
spinons) which can reproduce the reduction of magnon weight and\nlowered ex
citation energy at (\pi\,0) in the Heisenberg model\, as well\nas the rapid
loss of the (\pi\,0) magnon with increasing Q in the J-Q\nmodel.
LOCATION:SCI 328\, 590 Commonwealth Avenue\, 02215
STATUS:CONFIRMED
CLASS:PUBLIC
END:VEVENT
BEGIN:VEVENT
DTSTAMP:20171215T101245Z
LAST-MODIFIED:20171023T193234Z
DTSTART:20171027T160000Z
DTEND:20171027T173000Z
UID:event1835@bu.edu
URL:http://physics.bu.edu/events/show/1835
SUMMARY:"Putting hydrodynamic into solid state"
DESCRIPTION:Featuring Kin Chung Fong\, Raytheon BBN Technologies and Harvar
d University\nHosted by: Scott Bunch\n\nPart of the Condensed Matter Theory
Seminar Series.\n\nDespite of the strong Coulomb interaction\, electrons i
n simple metal behave as a non-interacting Fermi gas with long-lived quasi-
particle excitation. However\, weak screening near the charge neutrality po
int of the massless Dirac fermions in graphene can lead to a new collective
behavior described by hydrodynamics. By listening to the Johnson noise of
the electrons\, we are able to probe simultaneously the thermal and electri
cal transport of the Dirac fluid and observe how it departs from Fermi liqu
id physics. At high temperature near the neutrality point\, we find a stron
g enhancement of the thermal conductivity and breakdown of Wiedemann-Franz
law in graphene. This is attributed to the non-degenerate electrons and hol
es forming a strongly coupled Dirac fluid.
LOCATION:SCI 352\, 590 Commonwealth Avenue\, 02215
STATUS:TENTATIVE
CLASS:PUBLIC
END:VEVENT
BEGIN:VEVENT
DTSTAMP:20171215T101245Z
LAST-MODIFIED:20171031T145208Z
DTSTART:20171106T160000Z
DTEND:20171106T170000Z
UID:event1853@bu.edu
URL:http://physics.bu.edu/events/show/1853
SUMMARY:"Polarization\, Large Gauge Invariance\, and Quantum Hall Effect on
Lattice"
DESCRIPTION:Featuring Masaki Oshikawa\, University of Tokyo\nHosted by: And
ers Sandvik\n\nPart of the Condensed Matter Theory Seminar Series.\n\nQuant
um systems on a non-simply connected space possess a\n"large" gauge invaria
nce. Laughlin utilized this to explain quantum\nHall effect. Later\, it was
applied to elucidate a universal relation\nbetween filling factor and ener
gy spectrum in quantum many-body\nsystems on periodic lattices (Lieb-Schult
z-Mattis-M.O.-Hastings).\nSomewhat surprisingly\, the large gauge invarianc
e is also deeply\nrelated to modern theory of electric polarization develop
ed by\nResta et al. Combining these ideas together\, we can derive a constr
aint\non the Hall conductivity of a many-particle system on a periodic\nlat
tice\, with a given magnetic flux per plaquette and particle density.
LOCATION:SCI 328\, 590 Commonwealth Avenue\, 02215
STATUS:CONFIRMED
CLASS:PUBLIC
END:VEVENT
BEGIN:VEVENT
DTSTAMP:20171215T101245Z
LAST-MODIFIED:20171023T160747Z
DTSTART:20171108T160000Z
DTEND:20171108T170000Z
UID:event1850@bu.edu
URL:http://physics.bu.edu/events/show/1850
SUMMARY:"Instabilities of the Sachdev-Ye-Kitaev Model"
DESCRIPTION:Featuring Zhen Bi\, Pappalardo Fellow\, Massachusetts Institute
of Technology\nHosted by: Zhicheng Yang\n\nPart of the Condensed Matter Th
eory Seminar Series.\n\nThe Sachdev-Ye-Kitaev (SYK) model is an intriguing
0+1d strongly interacting disordered model of non-Fermi liquid states with
exact solution and possible holographic duality. In this talk\, I will cons
ider a series of perturbations on the SYK model. We show that the maximal c
haotic non-Fermi-liquid phase described by the ordinary SYK model has margi
nally relevant or irrelevant (depending on the sign of the coupling constan
ts) four-fermion perturbations allowed by symmetry. Changing the sign of on
e of these four-fermion perturbations leads to a continuous chaotic-nonchao
tic quantum phase transition of the system accompanied by a spontaneous tim
e-reversal symmetry breaking. We also studied a generalized 0+1d interactin
g disordered fermion model which has a series of new fixed points with cont
inuously varying exponents. If time permits\, I will talk about a 1+1d mode
l built out of SYK clusters\, which exhibits a continuous transition betwee
n thermal metal and insulator phase at low temperature.
LOCATION:SCI 328\, 590 Commonwealth Avenue\, 02215
STATUS:CONFIRMED
CLASS:PUBLIC
END:VEVENT
BEGIN:VEVENT
DTSTAMP:20171215T101245Z
LAST-MODIFIED:20171114T161116Z
DTSTART:20171117T170000Z
DTEND:20171117T180000Z
UID:event1859@bu.edu
URL:http://physics.bu.edu/events/show/1859
SUMMARY:"A Bosonization Puzzle in Non-linear Transport"
DESCRIPTION:Featuring Carlos Bolech\, University of Cincinnati\n\nPart of t
he Condensed Matter Theory Seminar Series.\n\nIn this talk\, we will critic
ally reexamine the bosonization-debosonization procedure for systems includ
ing localized features or boundaries. First\, by focusing on the case of a
tunneling junction out of equilibrium\, we will show that the conventional
bosonization approach can give results that are not consistent with a direc
t solution of the problem even at the qualitative level when higher-order p
rocesses are considered. Thus highlighting possible inconsistencies that ca
n adversely affect the results of all types of calculations. We will subseq
uently report on a "consistent bosonization-debosonization" procedure that
we have developed to resolve this non-linear transport puzzle and argue tha
t it should be widely applicable [1]. This new framework takes the form of
additional considerations\, related to the regularization of fermionic bili
nears\, that supplement the conventional bosonization procedure and it can
be carried over to the study of other systems. We will then discuss its app
lication to the two-lead Kondo junction [2]\, that besides being a key theo
retical prototype of a non-equilibrium strongly correlated system is also o
f continuing experimental relevance. The discussion will be focused on the
Toulouse limit of that problem. \n\nReferences:\n[1] Nayana Shah and C. J.
Bolech\, Phys. Rev. B 93\, 085440 (2016).\n[2] C. J. Bolech and Nayana Sha
h\, Phys. Rev. B 93\, 085441 (2016).
LOCATION:SCI 352\, 590 Commonwealth Avenue\, 02215
STATUS:CONFIRMED
CLASS:PUBLIC
END:VEVENT
BEGIN:VEVENT
DTSTAMP:20171215T101245Z
LAST-MODIFIED:20171129T180150Z
DTSTART:20171201T170000Z
DTEND:20171201T180000Z
UID:event1863@bu.edu
URL:http://physics.bu.edu/events/show/1863
SUMMARY:"Learning and optimizing quantum control landscapes"
DESCRIPTION:Featuring Alexandre Day\, Boston University\, Department of Phy
sics\n\nPart of the Condensed Matter Theory Seminar Series.\n\nThe ability
to prepare quantum states rapidly and with high fidelity is crucial for qua
ntum technologies. In this talk I will discuss the control problem of a cha
in of coupled qubits. In particular we will see that there exist phase tran
sitions in the quantum state preparation problem and a possible mapping to
a spin-glass model. We will also touch on how various optimization methods\
, including reinforcement learning\, can help discover novel preparation pr
otocols and in particular we will consider their robustness and their resou
rce requirements.
LOCATION:SCI 352\, 590 Commonwealth Avenue\, 02215
STATUS:CONFIRMED
CLASS:PUBLIC
END:VEVENT
BEGIN:VEVENT
DTSTAMP:20171215T101245Z
LAST-MODIFIED:20171201T164740Z
DTSTART:20171204T153000Z
DTEND:20171204T163000Z
UID:event1864@bu.edu
URL:http://physics.bu.edu/events/show/1864
SUMMARY:"Quantum Hall Ferromagnetism in $\nu=0$ Bilayer Graphene"
DESCRIPTION:Featuring Ganpathy Murthy\, University of Kentucky\nHosted by:
Anushya Chandran\n\nPart of the Condensed Matter Theory Seminar Series.\n\n
In the presence of a quantizing magnetic field\, the lowest energy manifold
of bilayer graphene (BLG) has spin\, valley\, and a two-fold orbital degen
eracy. Experimentally\, it can be tuned by applying a parallel magnetic fie
ld and by applying a perpendicular electric field\, which tends to polarize
the layers. At charge neutrality\, which corresponds to $\nu=0$\, the syst
em can choose among a rich set of possible ground states based on the effe
ctive interactions and external tuning parameters. We will set up a simple
model Hamiltonian containing symmetry-allowed terms and analyze it in the H
artree-Fock approximation to obtain several potential ground states\, some
of which have been obtained before (such as the canted antiferromagnet and
the fully layer polarized phase) and one which has not. This new ground sta
te breaks two different U(1) symmetries spontaneously\, and could appear as
an intermediate phase between the canted antiferromagnet and the fully lay
er polarized phases.
LOCATION:SCI 352\, 590 Commonwealth Avenue\, 02215
STATUS:CONFIRMED
CLASS:PUBLIC
END:VEVENT
BEGIN:VEVENT
DTSTAMP:20171215T101245Z
LAST-MODIFIED:20171207T160336Z
DTSTART:20171208T170000Z
DTEND:20171208T180000Z
UID:event1872@bu.edu
URL:http://physics.bu.edu/events/show/1872
SUMMARY:"The Cluster Truncated Wigner Approximation in Strongly Interacting
Quantum Systems"
DESCRIPTION:Featuring Jonathan Wurtz\, Boston University\, Department of Ph
ysics\n\nPart of the Condensed Matter Theory Seminar Series.\n\nQuantum man
y-body systems exhibit a rich set of interesting behavior\, from many body
localization to thermalization. However\, existing methods are lacking a w
ay to describe generic out-of-equilibrium dynamics of such systems\, mainly
due to the exponential complexity of large systems. In this seminar\, I de
scribe work done on describing non-equilibrium dynamics of strongly interac
ting quantum systems from the perspective of phase space. Within this persp
ective\, linear quantum dynamics with exponential DoF is replaced with appr
oximate nonlinear classical dynamics with polynomial DoF. I will also descr
ibe some results and behavior of the method\, including entanglement growth
and spreading\, and the change between quantum and classical behavior.
LOCATION:SCI 352\, 590 Commonwealth Avenue\, 02215
STATUS:CONFIRMED
CLASS:PUBLIC
END:VEVENT
BEGIN:VEVENT
DTSTAMP:20171215T101245Z
LAST-MODIFIED:20171127T161247Z
DTSTART:20171215T160000Z
DTEND:20171215T170000Z
UID:event1822@bu.edu
URL:http://physics.bu.edu/events/show/1822
SUMMARY:"Properties of Topological semimetals: Chiral magnetic effect and i
nstability under magnetic field"
DESCRIPTION:Featuring Xi Dai\, Hong Kong University of Science and Technolo
gy\nHosted by: Wanzheng Hu\n\nPart of the Condensed Matter Theory Seminar S
eries.\n\nIn the present talk\, I will introduce two important physical pro
perties of topological semimetals. The first one is the chiral magnetic eff
ect\, which is the direct consequence of the presence of Weyl points in the
band structures. I will introduce that the chiral magnetic effect can be d
etected by its coupling to certain phonon modes\, which behave like pseudo
scalars under point group transformations. Such coupling can generate reson
ance between the intrinsic plasmon oscillation and the corresponding phonon
modes\, leading to dramatic modification of the optical response by the ex
ternal magnetic field. The second one is the instability of topological sem
imetal phase under strong magnetic field. In Dirac semi-metal\, the quantum
limit can be easily reached under the magnetic field leading to two types
of phase instability caused by the high degeneracy of the topological zerot
h Landau bands (LBs)\, namely the charge density wave (CDW) phase and polar
ized nematic phase formed by exciton condensation. We also propose that bo
th the DC transport measurement and Raman scattering can be used to disting
uish the above two phases.
LOCATION:SCI 352\, 590 Commonwealth Avenue\, 02215
STATUS:CONFIRMED
CLASS:PUBLIC
END:VEVENT
BEGIN:VEVENT
DTSTAMP:20171215T101245Z
LAST-MODIFIED:20171211T142327Z
DTSTART:20171220T160000Z
DTEND:20171220T170000Z
UID:event1873@bu.edu
URL:http://physics.bu.edu/events/show/1873
SUMMARY:"Propagation of an impurity through a quantum medium"
DESCRIPTION:Featuring Mikhail Zvonarev\, LPTMS\, CNRS\, Orsay\, France\n\nP
art of the Condensed Matter Theory Seminar Series.\n\nI will report on rece
nt theoretical and experimental progress in understanding the dynamics of a
n impurity particle injected into a one-dimensional quantum liquid. I will
show that the momentum distribution of the impurity subject to a constant e
xternal force exhibits characteristic Bragg reflections at the edge of an e
mergent Brillouin zone. As a consequence\, the impurity exhibits periodic d
ynamics that is interpreted as Bloch oscillations\, which arise even though
the quantum liquid is translationally invariant. I will also discuss a qua
ntum flutter phenomenon\, whose essence is that the impurity injected into
a liquid with some initial momentum sheds only a part of it to the backgrou
nd gas\, and forms a correlated state that no longer decays in time; furthe
rmore\, if the initial momentum is large enough\, the impurity undergoes lo
ng-lived oscillations. The value of the impurity's velocity at infinite tim
e lies between zero and the speed of sound in the gas\, and is determined b
y the injection protocol. This way\, the impurity's frictionless motion is
a dynamically emergent phenomenon whose description goes beyond accounting
for the kinematic constraints of Landau's approach to superfluidity.
LOCATION:SCI 328\, 590 Commonwealth Avenue\, 02215
STATUS:CONFIRMED
CLASS:PUBLIC
END:VEVENT
END:VCALENDAR