5,425 research outputs found
Interaction Driven Quantum Phase Transitions in Fractional Topological Insulators
We study two species of (or spin-1/2) fermions with short-range intra-species
repulsion in the presence of opposite (effective) magnetic field, each at
Landau level filling factor 1/3. In the absence of inter-species interaction,
the ground state is simply two copies of the 1/3 Laughlin state, with opposite
chirality, representing the fractional topological insulator (FTI) phase. We
show this phase is stable against moderate inter-species interactions. However
strong enough inter-species repulsion leads to phase separation, while strong
enough inter-species attraction drives the system into a superfluid phase. We
obtain the phase diagram through exact diagonalization calculations. The
FTI-superfluid phase transition is shown to be in the (2+1)D XY universality
class, using an appropriate Chern-Simons-Ginsburg-Landau effective field
theory.Comment: 5 pages, 3 figure
Composite fermions in Fock space: Operator algebra, recursion relations, and order parameters
We develop recursion relations, in particle number, for all (unprojected)
Jain composite fermion (CF) wave functions. These recursions generalize a
similar recursion originally written down by Read for Laughlin states, in mixed
first-second quantized notation. In contrast, our approach is purely
second-quantized, giving rise to an algebraic, `pure guiding center' definition
of CF states that de-emphasizes first quantized many-body wave functions. Key
to the construction is a second-quantized representation of the flux attachment
operator that maps any given fermion state to its CF counterpart. An algebra of
generators of edge excitations is identified. In particular, in those cases
where a well-studied parent Hamiltonian exists, its properties can be entirely
understood in the present framework, and the identification of edge state
generators can be understood as an instance of `microscopic bosonization'. The
intimate connection of Read's original recursion with `non-local order
parameters' generalizes to the present situation, and we are able to give
explicit second quantized formulas for non-local order parameters associated
with CF states.Comment: Published version with improved convention
A protected password change protocol
Some protected password change protocols were proposed. However, the previous
protocols were easily vulnerable to several attacks such as denial of service,
password guessing, stolen-verifier and impersonation atacks etc. Recently,
Chang et al. proposed a simple authenticated key agreement and protected
password change protocol for enhancing the security and efficiency. In this
paper, authors shall show that password guessing, denial of service and
known-key attacks can work in their password change protocol. At the same time,
authors shall propose a new password change protocol to withstand all the
threats of security
Construction of a series of new fractional quantum Hall wave functions by conformal field theory
In this paper, a series of fractional quantum Hall wave functions
are constructed from conformal field theory(CFT). They share the same
topological properties with states constructed by Jain's composite fermion
approach. Upon exact lowest Landau level(LLL) projection, some of Jain
composite fermion states would not survive if constraints on Landau level
indices given in the appendices of this paper were not satisfied. By contrast,
states constructed from CFT always stay in LLL. These states are characterized
by different topological shifts and multibody relative angular momenta. As a
by-product, in the appendices we prove the necessary conditions for general composite fermion states to have nonvanishing LLL projection.Comment: 15 pages, 2 figures, minor corrections mad
Numerical Simulation of Multi-phase Flow in Porous Media on Parallel Computers
A parallel reservoir simulator has been developed, which is designed for
large-scale black oil simulations. It handles three phases, including water,
oil and gas, and three components, including water, oil and gas. This simulator
can calculate traditional reservoir models and naturally fractured models.
Various well operations are supported, such as water flooding, gas flooding and
polymer flooding. The operation constraints can be fixed bottom-hole pressure,
a fixed fluid rate, and combinations of them. The simulator is based on our
in-house platform, which provides grids, cell-centred data, linear solvers,
preconditioners and well modeling. The simulator and the platform use MPI for
communications among computation nodes. Our simulator is capable of simulating
giant reservoir models with hundreds of millions of grid cells. Numerical
simulations show that our simulator matches with commercial simulators and it
has excellent scalability
High Performance Monte Carlo Simulation of Ising Model on TPU Clusters
Large-scale deep learning benefits from an emerging class of AI accelerators.
Some of these accelerators' designs are general enough for compute-intensive
applications beyond AI and Cloud TPU is one such example. In this paper, we
demonstrate a novel approach using TensorFlow on Cloud TPU to simulate the
two-dimensional Ising Model. TensorFlow and Cloud TPU framework enable the
simple and readable code to express the complicated distributed algorithm
without compromising the performance. Our code implementation fits into a small
Jupyter Notebook and fully utilizes Cloud TPU's efficient matrix operation and
dedicated high speed inter-chip connection. The performance is highly
competitive: it outperforms the best published benchmarks to our knowledge by
60% in single-core and 250% in multi-core with good linear scaling. When
compared to Tesla V100 GPU, the single-core performance maintains a ~10% gain.
We also demonstrate that using low precision arithmetic---bfloat16---does not
compromise the correctness of the simulation results.Comment: 26 pages, 8 figures, 7 tables. Results (excluding new results in 7.2)
published in ACM SC2019 Proceedings: ACM ISBN 978-1-4503-6229-0/19/11.
https://doi.org/10.1145/3295500.335614
A Multi-Agent Reinforcement Learning Method for Impression Allocation in Online Display Advertising
In online display advertising, guaranteed contracts and real-time bidding
(RTB) are two major ways to sell impressions for a publisher. Despite the
increasing popularity of RTB, there is still half of online display advertising
revenue generated from guaranteed contracts. Therefore, simultaneously selling
impressions through both guaranteed contracts and RTB is a straightforward
choice for a publisher to maximize its yield. However, deriving the optimal
strategy to allocate impressions is not a trivial task, especially when the
environment is unstable in real-world applications. In this paper, we formulate
the impression allocation problem as an auction problem where each contract can
submit virtual bids for individual impressions. With this formulation, we
derive the optimal impression allocation strategy by solving the optimal
bidding functions for contracts. Since the bids from contracts are decided by
the publisher, we propose a multi-agent reinforcement learning (MARL) approach
to derive cooperative policies for the publisher to maximize its yield in an
unstable environment. The proposed approach also resolves the common challenges
in MARL such as input dimension explosion, reward credit assignment, and
non-stationary environment. Experimental evaluations on large-scale real
datasets demonstrate the effectiveness of our approach
Reconstruction-Aware Imaging System Ranking by use of a Sparsity-Driven Numerical Observer Enabled by Variational Bayesian Inference
It is widely accepted that optimization of imaging system performance should
be guided by task-based measures of image quality (IQ). It has been advocated
that imaging hardware or data-acquisition designs should be optimized by use of
an ideal observer (IO) that exploits full statistical knowledge of the
measurement noise and class of objects to be imaged, without consideration of
the reconstruction method. In practice, accurate and tractable models of the
complete object statistics are often difficult to determine. Moreover, in
imaging systems that employ compressive sensing concepts, imaging hardware and
sparse image reconstruction are innately coupled technologies. In this work, a
sparsity-driven observer (SDO) that can be employed to optimize hardware by use
of a stochastic object model describing object sparsity is described and
investigated. The SDO and sparse reconstruction method can therefore be
"matched" in the sense that they both utilize the same statistical information
regarding the class of objects to be imaged. To efficiently compute the SDO
test statistic, computational tools developed recently for variational Bayesian
inference with sparse linear models are adopted. The use of the SDO to rank
data-acquisition designs in a stylized example as motivated by magnetic
resonance imaging (MRI) is demonstrated. This study reveals that the SDO can
produce rankings that are consistent with visual assessments of the
reconstructed images but different from those produced by use of the
traditionally employed Hotelling observer (HO).Comment: IEEE transactions on medical imaging (2018
Thermal conductivity of graphene kirigami: ultralow and strain robustness
Kirigami structure, from the macro- to the nanoscale, exhibits distinct and
tunable properties from original 2-dimensional sheet by tailoring. In present
work, the extreme reduction of the thermal conductivity by tailoring sizes in
graphene nanoribbon kirigami (GNR-k) is demonstrated using nonequilibrium
molecular dynamics simulations. The results show that the thermal conductivity
of GNR-k (around 5.1 Wm-1K-1) is about two orders of magnitude lower than that
of the pristine graphene nanoribbon (GNR) (around 151.6 Wm-1K-1), while the
minimum value is expected to be approaching zero in extreme case from our
theoretical model. To explore the origin of the reduction of the thermal
conductivity, the micro-heat flux on each atoms of GNR-k has been further
studied. The results attribute the reduction of the thermal conductivity to
three main sources as: the elongation of real heat flux path, the
overestimation of real heat flux area and the phonon scattering at the vacancy
of the edge. Moreover, the strain engineering effect on the thermal
conductivity of GNR-k and a thermal robustness property has been investigated.
Our results provide physical insights into the origins of the ultralow and
robust thermal conductivity of GNR-k, which also suggests that the GNR-k can be
used for nanaoscale heat management and thermoelectric application.Comment: 15 pages, 10 figure
Photonic Spin Hall Effect in Waveguides Composed of Two Types of Single-Negative Metamaterials
The polarization controlled optical signal routing has many important
applications in photonics such as polarization beam splitter. By using
two-dimensional transmission lines with lumped elements, we experimentally
demonstrate the selective excitation of guided modes in waveguides composed of
two kinds of single-negative metamaterials. A localized, circularly polarized
emitter placed near the interface of the two kinds of single-negative
metamaterials only couples with one guided mode with a specific propagating
direction determined by the polarization handedness of the source. Moreover,
this optical spin-orbit locking phenomenon, also called the photonic spin Hall
effect, is robust against interface fluctuations, which may be very useful in
the manipulation of electromagnetic signals.Comment: 6 figures, published in Scientific Reports, See
https://www.nature.com/articles/s41598-017-08171-
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