Description
Poster presenters:
Mr. ALBA, vincenzo:
Entanglement entropy of two disjoint blocks in critical Ising
models
I will present results about the scaling of the Renyi and
entanglement entropy of two disjoint blocks of critical Ising
models, as function of their sizes and separations. I will
present analytic results based on conformal field theory that
are quantitatively checked in numerical simulations of both
the quantum spin chain and the classical two dimensional
Ising model. Theoretical results match the ones obtained
from numerical simulations only after taking properly into
account the corrections induced by the finite length of the
blocks.
Mr. HAMZELOUI, Saeed:
Dr. HOU, Chang-Yu:
KAILASVUORI, Janik:
Finite Conductivity Minimum in Bilayer Graphene without
Charge Inhomogeneities
Boltzmann transport theory fails near the linear band-
crossing of single-layer graphene and near the quadratic
band-crossing of bilayer
graphene. We report on a numerical study which assesses
the role of inter-band coherence in transport when the
Fermi level lies near the
band-crossing energy of bilayer graphene. We find that
interband coherence enhances conduction, and that it plays
an essential role in
graphene's minimum conductivity phenomena. This
behavior is qualitatively captured by an approximate theory
which treats inter-band
coherence in a relaxation-time approximation. On the basis
of this short-range-disorder model study, we conclude that
electron-hole puddle
formation is not a necessary condition for finite conductivity
in graphene at zero average carrier density.
Mr. LAHTINEN, Ville:
Interacting anyons and gauge field driven topological phase
transitions
The exact solvability of the honeycomb lattice model offers
an ideal
platform to study phase transitions between different
topological
phases. Here we show the existence of a previously
undiscovered
topological phase that supports chiral Abelian anyons. It
appears due
to a presence of a vortex lattice that we can relate to a
staggering
of the model's couplings. By considering the effect of
coupling
distortions on the low-energy theory of Dirac fermions, we
show that
different phase transition driving perturbations translate to
different chiral gauge fields. These lead to distinct Fermi
surface
evolutions that characterize the phase transitions. Finally,
we
demonstrate how the transition can also be understood as
arising due
to interactions between the anyonic vortices.
Mr. LEVKIVSKYI, Ivan:
Noise-Induced Phase Transition in the Electronic Mach-
Zehnder
Interferometer
Recently, Aharonov-Bohm (AB) effect in electronic
Mach-Zehnder (MZ) interferometers has attracted much
attention among
experimental and theoretical physicists. These
interferometers, for
the first time experimentally realized in the group of
Heiblum [1],
utilize quantum Hall edge states in place of optical beams,
and
quantum point contacts (QPC) as beam splitters, to partition
edge
channels. Theoretical attempts to explain experimentally
observed
puzzling lobe-type behavior of the visibility of AB oscillations
as a
function of voltage bias [2-5], have led to a number of
publications
[6-9]. They have focused on the filling factor ν=1 state, and
suggested different mechanisms of dephasing, including the
resonant
interaction with a counter-propagating edge state [6], the
dispersion
of the Coulomb interaction potential [7], and non-Gaussian
noise
effects [8,9]. To date, however, all the experiments,
reporting
multiple side lobes in the visibility function of voltage bias,
have
been done at filling factor ν=2. We will argue that, in fact,
there
are two main mechanisms of dephasing in MZ
interferometers. One
mechanism [10], due to spontaneous emission of edge
magneto-plasmons,
leads to a size effect, which explains the lobes and many
other
details of experiments [2-5]. According to the second
mechanism [11],
dephasing in electronic MZ interferometers is due to an
external
non-equilibrium noise source. Experimentally [2], such a
noise is
created with the help of an additional QPC with the
transparency T
that partitions incident edge channels. We predict that a
phase
transition occurs at T=1/2, where the visibility function of
voltage
bias sharply changes its behavior. An important role in this
phenomenon is played by a non-Gaussianity of noise, which
is typically
negligible because of a weak coupling. It turns out that MZ
interferometers are strongly coupled to noise. They,
therefore, can be
considered efficient detectors of full counting statistics [12].
References:
[1] Y. Ji et al., Nature (London) 422, 415 (2003).
[2] I. Neder et al., Phys. Rev. Lett. 96, 016804
(2006).
[3] E. Bieri et al., Phys. Rev. B 79, 245324 (2009).
[4] P. Roulleau et al., Phys. Rev. B 76, 161309(R)
(2007).
[5] L.V. Litvin et al., Phys. Rev. B 75, 033315 (2007).
[6] E.V. Sukhorukov, and V.V. Cheianov, Phys. Rev. Lett.
99, 156801 (2007).
[7] J.T. Chalker, Y. Gefen, and M.Y. Veillette, Phys. Rev. B
76, 085320 (2007).
[8] S.-C. Youn, H.-W. Lee, and H.-S. Sim, Phys. Rev. Lett.
100, 196807 (2008).
[9] I. Neder and E. Ginossar, Phys. Rev. Lett. 100, 196806
(2008).
[10] I.P. Levkivskyi, and E.V. Sukhorukov, Phys. Rev. B 78,
045322 (2008).
[11] I.P. Levkivskyi, E.V. Sukhorukov, Phys. Rev. Lett. 103,
036801 (2009).
[12] L.S. Levitov, H. Lee, and G.B. Lesovik, J. Math. Phys.
37, 4845 (1996).
Mr. MACRI, Tommaso:
Dynamics of Cold-Atom Fermi Mixtures
Mrs. MIILBAEVA, Guljamal:
MOZGUNOV, E:
Excitation spectrum of a 2D long-range Bose liquid with a
supersymmetry
Specic model of a 2D Bose liquid with non-relativistic
supersymmetry [1, 2] is studied numerically
by means of a mapping to a classical Langevin dynamics [3,
4]. The model contains dimensionless
coupling constant . At small 1 this model is very similar to
the 2D Bose-lqiuid with pair-wise
logarithmic interaction and thus exibit superuid ground
state. At very large 35 the ground
state nearly breaks translational symmetry: equal-time
density correlations in the emergent ground
state are equivalent to those of the classical 2D crystal at
nonzero temperature. We have studied the
excitation spectrum of this model in the whole range of by
means of the analysis of the dynamic
structure factor S(k, t) computed for the equivalent classical
model, like it was done in Ref. [5] for the
model of quantum dimers at the Rokshar-Kivelson point [6].
The spectrum !(q) we found contains
a plasmon gap !0 at q = 0 and a well-dened roton minimum
at q = q0 = 2
p
n with minimal
excitation energy . The ratio /!0 decreases sharply with in
the whole range of the strongly
coupled Bose liquid 1 < < 35, down to very small values
10−2. However, we could not detect,
with our numerical accurace, a vanishing of the roton gap
before 2D crystallization transition takes
place at = c 37.We thus conclude that the ground-state is
of superuid nature (at T = 0) in the
whole range of < c (however, the critical temperature Tc of
superuid transition drops sharply
with ). In the crystalline state > c no well-dened low-
energy excitations corresponding to
shear modes was found, in agreement with theoretically
expected spectrum !(k) / k2 that suggests
strongly decaying nature of the corresponding
quasiparticles.
[1] M. V. Feigel'man and M. A. Skvortsov. Nucl. Phys. B 506
[FS], 665 (1997), arXiv:cond-mat/9703215v1
[2] C. Kane, S. Kivelson, Lee and Zhang Phys. Rev. b
(1991)
[3] M. V. Feigel'man and A. M. Tsvelik, Sov.Phys. JETP
(1982)
[4] C.L. Henley, J. Phys.: Condens. Matter 16, S891
(2004).
[5] A. M. Lauchli, S. Capponi and F. F. Assaad, J. Stat. Mech.
(2008) P01010
[6] D. Rokhsar and S. Kivelson, Phys. Rev. Lett. 61, 2376
(1988).
Dr. OHANYAN, Vadim:
Spontaneous translational symmetry breaking associate magnetization
plateaus in one-dimensional spin systems with Ising and Heisenberg
bonds
There are well known examples of integrable many-particle systems solvable
within the Bethe ansatz technique,
like 1d Heisenberg chain, Hubbard model, e.t.c. Though, Bethe ansatz, being a
paradigm in modern theoretical and
mathematical physics, is very important by itself, its application in condensed
matter physic is still very restricted.
Particularly, in the physics of magnetism one usually needs to describe
thermodynamic properties of certain quantum
spin-lattice models at ¯nite temperature. At the moment exact treatment of this
problem is feasible only for very
limited number of model within rather complicated and sophisticated techniques
like quantum transfer matrix
and non-liner integral equation. However, majority of real magnetic materials
(even one-dimensional) has lattice
structure which corresponds to no known integrable models. Thus, numerical
calculations are almost the only way
to shed a light into their thermodynamic properties. However, one can change a
little underlying spin system to get
the model, which can be easily solved exactly within classical transfer-matrix
technique. Namely, if one change some
interaction bonds with Ising ones in such a way that Hamiltonian of the
emergent system be a sum of commuting
operator, then one can expand the exponential in the partition function and get
the formal structure which is
suitable to applying the classical transfer matrix method. Thus, one will obtain a
one-dimensional spin system with
clusters of quantum spins and intermediate "classical" spins between them. The
problem of calculation the partition
function of the system yield the problem of diagonalization of the small quantum
spin clusters which in most cases
are feasible and further calculation of the eigenvalues of classical transfer matrix
which is also straightforward. A
various spin systems with Ising and Heisenberg bonds have been considered
recently, e.g. diamond-chain, alternating
linear chain, chains of triangle quantum cluster, sawtooth chain, orthogonal-
dimer chain, e.t.c. Here we would like
to emphasize one particular phenomena which is inherent to such a systems
where left and right side neighboring of
each cluster of quantum spins are non equivalent to each other. On the example
of two systems, the sawtooth chain
with quantum clusters of two spins and orthogonal-dimer chain with quantum
cluster of three spins in triangular
topology, we demonstrate the appearance of magnetization plateau at M=Msat =
1=4 which is connected with
breaking of translational symetry of the lattice, more precisely with the doubling
of unit cell. Solving the problem
of partition function calculation exactly we also obtain analytic expressions for
free energy and all thermodynamic
function, presenting the plots of magnetization processes for ¯nite temperature
displaying magnetization plateaus
at M=Msat = 0; 1=4; 1=2. Analyzing ground states properties we obtain T = 0
ground states phase diagrams with
exact description of all transition lines and triple points.
Prof. PROTOGENOV, Alexander:
A chain of strongly correlated $SU(2)_{4}$ anyons
L. Martina, A. Protogenov, V. Verbus
One-dimensional lattice model of $SU(2)_{4}$ anyons
containing a transition into
the topological ordered phase state is considered. An
effective low-energy Hamiltonian
is found for half-integer and integer indices of the type of
strongly correlated non-Abelian
anyons. The Hilbert state space properties of the considered
modular tensor category are studied.
Mr. QAIUMZADEH, Alireza:
Mr. RAHIMI, Mojtaba:
Prof. RAO, Sumathi:
Dr. RUTKEVICH, Sergei:
Kink confinement in the $1d$-quantum ferromagnet
$\mathrm{Co}\mathrm{Nb}_2\mathrm{O}_6$.
Calculation of bound-state energies and intensities.
Recently Coldea {\it et al} (2010 Science {\bf 327} 177)
reported observation
of the weak confinement of kinks in the Ising spin chain
ferromagnet
$\mathrm{Co}\mathrm{Nb}_2\mathrm{O}_6$
at low temperatures. To interpret the entire spectra of
magnetic excitations measured via neutron scattering,
they introduce a phenomenological model, which takes into
account only the two-kink configurations of the
spin chain.
We describe the exact solution of this model. The explicit
expressions for the two-kink bound-state energy
spectra
and for the relative intensities of neutron scattering on
these magnetic modes are derived in terms of the
Bessel function.
Dr. SALOMOV, Uktam:
Dr. SATO, Masahiro:
Ferromagnetic transition in one-dimensional spin-1/2 Bose
and Fermi gases
Internal degrees of freedom in many-body systems
generally
provide a variety of phenomena, as electron spins in solids
generate
various magnetic structures. As the simplest system with
internal degrees
of freedom we study one-dimensional two-component
(pseudospin-1/2)
Bose/Fermi gas, which could be realized by using ultra-cold
atoms. If
a strong repulsion is introduced between two components, a
spontaneous
population imbalance (i.e., ferromagnetism) is expected to
appear.
However, it is known that the existent weak- and strong-
coupling
theories cannot capture the nature of the imbalanced phase
and the ordering.
We have thoroughly studied them [1] by combining
numerical approaches
(exact diagonalization and infinite time-evolving block
decimation) with
some analytic methods in an efficient manner. It is shown
that (i)
the universality class of the ferromagnetic transition
drastically changes
from first order to an Ising type when an inter-component
hopping is
introduced, and (ii) the imbalanced phase has a gapless
charge mode
and a gapped spin one.
[1] S.Takayoshi, M.Sato and S.Furukawa, PRA81, 053606
(2010).
Dr. SEDLMAYR, Nicholas:
Non-Collinear Ferromagnetic Luttinger Liquids
The presence of the electron-electron interaction in one
dimension profoundly changes the properties of a system.
The separation of charge and spin degrees of freedom is just
one example. Though such an effect does not survive in the
ferromagnetic case, a diagonal basis can still be found for
the appropriate quasi-particles. This will have consequences
for spin-transfer-torque effects. We consider what happens
when a ferromagnetic one dimensional interacting system
(more specifically a Luttinger liquid) has a region of non-
collinearity present, \emph{i.e.}~a domain wall. The
presence of the domain wall introduces a spin dependent
scatterer into the problem, an idea which has already
received some attention in the limit of weak magnetization.
In this work we would like to go further and look at how
this affects the transport properties of the Luttinger liquid,
and also how the magnetization dynamics of the domain
wall are modified for a Luttinger liquid.
SOLUYANOV, Alexey:
Dr. TEZUKA, Masaki:
Effect of disorder on 1D ultracold Fermi atomic gases
Cold atom experiments have made it possible to realize
quantum degenerate matter with controlled disorder and
interactions
with unprecedented precision. This presents an ideal setting
for the
study of novel phases of quantum matter and quantum
phase transitions,
such as the FFLO phase in harmonically trapped 1D system
of
population-imbalanced Fermi gases [1]. Specifically,
introduction of a
multichromatic potential [2] to a system of 2D array of 1D
Fermi gases
[3] would allow the study of the effect of disorder on
fermionic
superfluidity. While true long-range pairing order is
forbidden in 1D,
superfluidity is possible in a finite-size system.
We have studied a 1D Fermi gas with attractive short-
ranged
interactions in a multichromatic potential by the density-
matrix
renormalization group (DMRG) method [4]. We have
identified a region
of parameters where the disorder enhances pairing. When
the disorder
is further increased, global superfluidity eventually breaks
down. The
separation between the disorder strengths at superfluidity
breakdown
and the insulating transition suggests that the intermediate
“pseudo-gap” phase is characterized by strong pairing
without quasi
long-range order.
[1] M. Tezuka and M. Ueda, New J. Phys. 12, 055029
(2010) and
references therein.
[2] G. Roati et al., Nature 453, 895 (2008).
[3] Y. A. Liao et al., arXiv:0912.0092v2 (preprint).
[4] M. Tezuka and A.M. García-García, arXiv:0912.2263
(preprint).
Dr. TOTH, Anna:
