13,752 research outputs found
Generalized effective-potential Landau theory for the two-dimensional extended Bose-Hubbard model
We analytically study the quantum phase diagrams of ultracold dipolar Bose
gases in an optical square lattice at zero temperature by using the generalized
effective-potential Landau theory (GEPLT). For a weak nearest-neighbor
repulsion, our analytical results are better than the third-order
strong-coupling expansion theory calculation [M. Iskin et al.,
\textcolor[rgb]{0.00,0.00,1.00}{ Phys. Rev. A \textbf{79}, 053634 (2009)}]. In
contrast to a previous quantum Monte Carlo (QMC) simulation [T. Ohgoe et al.,
\textcolor[rgb]{0.00,0.00,1.00}{Phys. Rev. B \textbf{86}, 054520 (2012)}], we
analytically calculate phase transition boundaries up to the third-order
hopping, which are in excellent agreement with QMC simulations for second-order
phase transition
Feature-Rich Electronic Properties in Graphene Ripples
Graphene ripples possess peculiar essential properties owing to the strong
chemical bonds, as an investigation using first principle calculations clearly
revealed. Various charge distributions, bond lengths, energy bands, and
densities of states strongly depend on the corrugation structures, ripple
curvatures and periods. Armchair ripples belonging to a zero-gap semiconductor
display split middle-energy states, while the zigzag ripples exhibit highly
anisotropic energy bands, semi-metallic behavior implicated by the destruction
of the Dirac cone, and the newly created critical points. Their density of
states exhibit many low-lying prominent peaks and can explain the experimental
measurements. There exist certain important similarities and differences
between graphene ripples and carbon nanotubes.Comment: 16 pages, 7 figure
SpecWatch: A Framework for Adversarial Spectrum Monitoring with Unknown Statistics
In cognitive radio networks (CRNs), dynamic spectrum access has been proposed
to improve the spectrum utilization, but it also generates spectrum misuse
problems. One common solution to these problems is to deploy monitors to detect
misbehaviors on certain channel. However, in multi-channel CRNs, it is very
costly to deploy monitors on every channel. With a limited number of monitors,
we have to decide which channels to monitor. In addition, we need to determine
how long to monitor each channel and in which order to monitor, because
switching channels incurs costs. Moreover, the information about the misuse
behavior is not available a priori. To answer those questions, we model the
spectrum monitoring problem as an adversarial multi-armed bandit problem with
switching costs (MAB-SC), propose an effective framework, and design two online
algorithms, SpecWatch-II and SpecWatch-III, based on the same framework. To
evaluate the algorithms, we use weak regret, i.e., the performance difference
between the solution of our algorithm and optimal (fixed) solution in
hindsight, as the metric. We prove that the expected weak regret of
SpecWatch-II is O(T^{2/3}), where T is the time horizon. Whereas, the actual
weak regret of SpecWatch-III is O(T^{2/3}) with probability 1 - {\delta}, for
any {\delta} in (0, 1). Both algorithms guarantee the upper bounds matching the
lower bound of the general adversarial MAB- SC problem. Therefore, they are all
asymptotically optimal
Configuration- and concentration-dependent electronic properties of hydrogenated graphene
The electronic properties of hydrogenated graphenes are investigated with the
first-principles calculations. Geometric structures, energy bands, charge
distributions, and density of states (DOS) strongly depend on the different
configurations and concentrations of hydrogen adatoms. Among three types of
optimized periodical configurations, only in the zigzag systems the band gaps
can be remarkably modulated by H-concentrations. There exist middle-gap
semiconductors, narrow-gap semiconductors, and gapless systems. The band
structures exhibit the rich features, including the destruction or recovery of
the Dirac-cone structure, newly formed critical points, weakly dispersive
bands, and (C,H)-related partially flat bands. The orbital-projected DOS are
evidenced by the low-energy prominent peaks, delta-function-like peaks,
discontinuous shoulders, and logarithmically divergent peaks. The DOS and
spatial charge distributions clearly indicate that the critical bondings in C-C
and C-H is responsible for the diversified properties
H-Si bonding-induced unusual electronic properties of silicene: a method to identify hydrogen concentration
Hydrogenated silicenes possess peculiar properties owing to the strong H-Si
bonds, as revealed by an investigation using first principles calculations. The
various charge distributions, bond lengths, energy bands, and densities of
states strongly depend on different hydrogen configurations and concentrations.
The competition of strong H-Si bondings and weak sp3 hybridization dominate the
electronic properties. Chair configurations belong to semiconductors, while the
top configurations show a nearly dispersionless energy band at the Fermi level.
Both two systems display H-related partially flat bands at middle energy, and
recovery of low-lying \pi bands during the reduction of concentration. Their
densities of states exhibit prominent peaks at middle energy, and the top
systems have a delta-funtion-like peak at E=0. The intensity of these peaks are
gradually weakened as the concentration decreases, providing an effective
method to identify the H-concentration in scanning tunneling spectroscopy
experiments
Chemical Bondings Induced Rich Electronic Properties of Oxygen Absorbed Few-layer Graphenes
Electronic properties of graphene oxides enriched by the strong chemical
bondings are investigated using first-principle calculations. They are very
sensitive to the changes in the number of graphene layer, stacking
configuration, and distribution of oxygen. The feature-rich electronic
structures exhibit the destruction or distortion of Dirac cone, opening of band
gap, anisotropic energy dispersions, O- and (C,O)-dominated energy dispersions,
and extra critical points. All the few-layer graphene oxides are semi-metals
except for the semiconducting monolayer ones. For the former, the distorted
Dirac-cone structures and the O-dominated energy bands near the Fermi level are
revealed simultaneously. The orbital-projected density of states (DOS) have
many special structures mainly coming from a composite energy band, the
parabolic and partially flat ones. The DOS and spatial charge distributions
clearly indicate the critical bondings in O-O, C-O and C-C bonds, being
responsible for the diversified properties
DIHARD II is Still Hard: Experimental Results and Discussions from the DKU-LENOVO Team
In this paper, we present the submitted system for the second DIHARD Speech
Diarization Challenge from the DKULENOVO team. Our diarization system includes
multiple modules, namely voice activity detection (VAD), segmentation, speaker
embedding extraction, similarity scoring, clustering, resegmentation and
overlap detection. For each module, we explore different techniques to enhance
performance. Our final submission employs the ResNet-LSTM based VAD, the Deep
ResNet based speaker embedding, the LSTM based similarity scoring and spectral
clustering. Variational Bayes (VB) diarization is applied in the resegmentation
stage and overlap detection also brings slight improvement. Our proposed system
achieves 18.84% DER in Track1 and 27.90% DER in Track2. Although our systems
have reduced the DERs by 27.5% and 31.7% relatively against the official
baselines, we believe that the diarization task is still very difficult.Comment: Submitted to Odyssesy 202
Geometric, magnetic and electronic properties of folded graphene nanoribbons
Geometric and electronic properties of folded graphene nanoribbons (FGNRs)
are investigated by first-principles calculations. These properties are mainly
dominated by the competition or cooperation among stacking, curvature and edge
effects. For the zigzag FGNRs, the more stable structures are revealed to be AB
stackings, while for the armchair types, AA" stackings are more stable. The
interlayer interactions and hybridization of four orbitals lead to smaller
energy gaps, anti-crossing bands, and more band-edge states. Specifically, the
broken mirror symmetry in the odd-AB stacked zigzag FGNRs is responsible for
the spin-up and spin-down splitting subbands. All FGNRs are direct-gap
semiconductors except that the edge-edge interactions cause the even-AA stacked
zigzag FGNRs to exhibit a pair of metallic linear bands. The width-dependent
energy gaps in the armchair FGNRs can be classified into six groups.
Furthermore, there exist rich features in density of states, including the
form, number, intensity and energy of the special structures
-bonding-dominated energy gaps in graphene oxides
Chemical bondings of graphene oxides with oxygen concentration from 1\% to
50\% are investigated using first-principle calculations. Energy gaps are
mainly determined by the competition of orbital hybridizations in C-C, O-O, and
C-O bonds. They are very sensitive to the changes in oxygen concentration and
distributions. There exists five types of bondings during the variation
from the full to vanishing adsorptions, namely the complete termination, the
partial suppression, the 1D bonding, the deformed planar bonding, and the
well-behaved one. They can account for the finite and gapless characteristics,
corresponding to the O-concentrations of 25\% and 3\%, respectively. The
feature-rich chemical bondings dominate band structures and density of states,
leading to diverse electronic properties.Comment: 19 pages, 4 figure
New iron-based multiferroics with improper ferroelectricity
In this contribution to the special issue on magnetoelectrics and their
applications, we focus on some single phase multiferroics theoretically
predicted and/or experimentally discovered by the authors in recent years. In
these materials, iron is the common core element. However, these materials are
conceptually different from the mostly-studied BiFeO, since their
ferroelectricity is improper. Our reviewed materials are not simply repeating
one magnetoelectric mechanism, but cover multiple branches of improper
ferroelectricity, including the magnetism-driven ferroelectrics, geometric
ferroelectric, as well as electronic ferroelectric driven by charge ordering.
In this sense, these iron-based improper ferroelectrics can be an encyclopaedic
playground to explore the comprehensive physics of multiferroics and
magnetoelectricity. Furthermore, the unique characteristics of iron's
orbitals make some of their magnetoelectric properties quite prominent,
comparing with the extensively-studied Mn-based improper multiferroics. In
addition, these materials establish the crossover between multiferroics and
other fields of functional materials, which enlarges the application scope of
multiferroics.Comment: 25 pages, 12 figures. A topical review for JP
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