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DTSTAMP:20170823T061013Z
LAST-MODIFIED:20170801T174843Z
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\n\nPart of the Condensed Matter Theor
y Seminar Series.\n\nIn the ﬁrst part we present a slave-particle meanﬁ
eld study of the mixed boson+fermion quantum dimer model introduced by Punk
\, Allais\, and Sachdev [PNAS 112\, 9552 (2015)] to describe the physics of
the pseudogap phase in cuprate superconductors. Our analysis naturally lea
ds to four charge e fermion pockets whose total area is equal to the hole d
oping p\, for a range of parameters consistent with the t−J model for hig
h temperature superconductivity. Here we ﬁnd that the dimers are unstable
to d-wave superconductivity at low temperatures. The region of the phase d
iagram with d-wave rather than s-wave superconductivity matches well with t
he appearance of the four fermion pockets. In the superconducting regime\,
the dispersion contains eight Dirac cones along the diagonals of the Brillo
uin zone. \n\nIn the second part we present an approach for studying system
s with hard constraints such that certain positive semideﬁnite operators
must vanish. The diﬃculty with mean-ﬁeld treatments of such cases is th
at imposing that the constraint is zero only in average is problematic for
a quantity that is always non-negative. We reformulate the hard constraints
by adding an auxiliary system such that some of the states to be projected
out from the total system are at ﬁnite negative energy and the rest at
nite positive energy\, we dubbed this method the soft constraint. This au
xiliary system comes with an extra coupling that we are free to vary and th
at parametrizes a whole family of mean-ﬁeld theories. We argue that this
variational-type parameter for the family of mean-ﬁeld theories should be
ﬁxed by matching a given experimental observation\, with the quality of
the resulting mean-ﬁeld approximation measured by how it ﬁts other data
. We test these ideas in the well-understood single-impurity Kondo problem\
, where we ﬁx the parameter via the TK (Kondo temperature) obtained from
the magnetic susceptibility value\, and score the quality of the approximat
ion by its predicted Wilson ratio. We also study a continuum version of the
soft constraint method. To show its usefulness we study the Laughlin fract
ional quantum Hall states at ﬁlling fraction ν = 1 2m+1. We produce the
Landau Ginzburg Cherns Simons theory for hard core bosons(arising in this f
ormulation of the Laughlin functions0 for these ﬁlling fractions. We hand
le the hard core constraint explicitly and show that to leading order it is
equivalent to a theory of soft core bosons with an eﬀective density dens
ity interaction between the particles of the form of a Coulomb interaction
coming from the electrons in the original theory and an eﬀective interact
ion coming from the hard core constraint that can be tuned by hand which we
denote by λ(q\,ω). We show that we can use this eﬀective interaction t
o reproduce the known spectra of both inter Landau level and intra Landau l
evel neutral excitations for the Laughlin states as well as the energy gap
to charged excitations and the electromagnetic response tensor.
LOCATION:SCI SCI 328\, 590 Commonwealth Avenue\, 02215
STATUS:CONFIRMED
CLASS:PUBLIC
END:VEVENT
BEGIN:VEVENT
DTSTAMP:20170823T061013Z
LAST-MODIFIED:20170801T175116Z
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\n\nPart of the Condensed Matter Theory Seminar Series.\n\nI will r
eview recent progress on the understanding of many-body localization\, i.e.
\, the phenomenon of absence of thermalization in interacting\, disordered
quantum systems. After discussing the restricted domain of existence of str
ict many body-localization and the role of rare events\, I will present a r
eal space renormalization group analysis which offers a microscopic scenari
o for the unusual dynamical phase transition which occurs as a localized sy
stem is tuned into an ergodic regime. While the transition is associated wi
th a diverging time scale for thermalization\, most spatial correlators rem
ain short range at the critical point. The latter goes hand in hand with th
e discontinuous evolution of expectation values of local observables. These
features distinguish the transition sharply from standard critical phenome
na.
LOCATION:SCI SCI 328\, 590 Commonwealth Avenue\, 02215
STATUS:CONFIRMED
CLASS:PUBLIC
END:VEVENT
BEGIN:VEVENT
DTSTAMP:20170823T061013Z
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\n\nPart of the Cond
ensed Matter Theory Seminar Series.\n\nEntropy is a fundamental concept in
statistical mechanics. In this talk\, I investigate two aspects of entropy.
First\, we reveal a universal behavior of entanglement entropy of the pure
quantum states which are in thermal equilibrium states. Second\, we charac
terize thermodynamic entropy as a Noether invariant under a non-uniform tim
e translation. \n\nIn the first part\, we focus on the size dependence of t
he entanglement entropy of the pure quantum states which can fully describe
thermal equilibrium as long as one focuses on local observables. The therm
odynamic entropy can also be recovered as the entanglement entropy of small
subsystems. When the size of the subsystem increases\, however\, quantum c
orrelations break the correspondence and mandate a correction to this simpl
e volume-law. The elucidatione of the size dependence of the entanglement e
ntropy is thus of essential importance in linking quantum physics with ther
modynamics\, and in analyzing recent experiments on ultra-cold atoms. In th
is talk\, we derive an analytic formula of the entanglement entropy for a c
lass of pure states called cTPQ states representing thermal equilibrium. We
find that our formula applies universally to any sufficiently scrambled pu
re state representing thermal equilibrium\, i.e.\, general energy eigenstat
es of non-integrable models and states after quantum quenches. Our universa
l formula can be exploited as a diagnostic tool for chaotic systems; we can
distinguish integrable models from chaotic modelsand detect many-body loca
lization with high accuracy. \n\nIn the second part\, we investigate a ther
mally isolated quantum many-body system with an external control represente
d by a time-dependent parameter. We formulate a path integral in terms of t
hermal pure states and derive an effective action for trajectories in a the
rmodynamic state space\, where the entropy appears with its conjugate varia
ble. In particular\, for quasi-static operations\, the symmetry for the uni
form translation of the conjugate variable emerges in the path integral. Th
is leads to the entropy as a Noether invariant.
LOCATION:SCI 352\, 590 Commonwealth Avenue\, 02215
STATUS:CONFIRMED
CLASS:PUBLIC
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