24,986 research outputs found
A Comparison of Modeling Units in Sequence-to-Sequence Speech Recognition with the Transformer on Mandarin Chinese
The choice of modeling units is critical to automatic speech recognition
(ASR) tasks. Conventional ASR systems typically choose context-dependent states
(CD-states) or context-dependent phonemes (CD-phonemes) as their modeling
units. However, it has been challenged by sequence-to-sequence attention-based
models, which integrate an acoustic, pronunciation and language model into a
single neural network. On English ASR tasks, previous attempts have already
shown that the modeling unit of graphemes can outperform that of phonemes by
sequence-to-sequence attention-based model.
In this paper, we are concerned with modeling units on Mandarin Chinese ASR
tasks using sequence-to-sequence attention-based models with the Transformer.
Five modeling units are explored including context-independent phonemes
(CI-phonemes), syllables, words, sub-words and characters. Experiments on HKUST
datasets demonstrate that the lexicon free modeling units can outperform
lexicon related modeling units in terms of character error rate (CER). Among
five modeling units, character based model performs best and establishes a new
state-of-the-art CER of on HKUST datasets without a hand-designed
lexicon and an extra language model integration, which corresponds to a
relative improvement over the existing best CER of by the joint
CTC-attention based encoder-decoder network.Comment: arXiv admin note: substantial text overlap with arXiv:1804.1075
Floquet topological insulator phase in a Weyl semimetal thin film with disorder
We investigate the effects of periodic fields and disorder on topological
properties of a Weyl-semimetal thin film. The two periodic fields, i.e., a
periodic magnetic field and elliptically polarized light, are discussed
respectively. By use of the Floquet theory, we find that both the two periodic
drives can resonantly induce the topological transitions from normal insulator
(NI) phases to Floquet topological insulator (FTI) phases. The Floquet
topological transitions are characterized by variation of Chern number.
Moreover, we show that the Floquet topological transitions can be explained by
a combination of the quantum well approximation and the rotating wave
approximation. In the disordered Weyl-semimetal thin film model under periodic
fields, we calculate the Bott index to characterize topological phase. It is
found that the FTI phase is robust against weak disorder, and collapses for
strong disorder strength. Interestingly, we find that disorder can also induce
a topological transition from a topological trivial phase to an FTI phase,
establishing the Floquet topological Anderson insulator (FTAI) phase. Finally,
an effective-medium theory based on the Born approximation further confirms the
numerical conclusions
Topological Anderson insulator phase in a Dirac-semimetal thin film
The recently discovered topological Dirac semimetal represents a new exotic
quantum state of matter. Topological Dirac semimetals can be viewed as three
dimensional analogues of graphene, in which the Dirac nodes are protected by
crystalline symmetry. It has been found that quantum confinement effect can gap
out Dirac nodes and convert Dirac semimetal to a band insulator. The band
insulator is either normal insulator or quantum spin Hall insulator depending
on the thin film thickness. We present the study of disorder effects in thin
film of Dirac semimetals. It is found that moderate Anderson disorder strength
can drive a topological phase transition from normal band insulator to
topological Anderson insulator in Dirac semimetal thin film. The numerical
calculation based on the model parameters of Dirac semimetal NaBi shows
that in the topological Anderson insulator phase a quantized conductance
plateau occurs in the bulk gap of band insulator, and the distributions of
local currents further confirm that the quantized conductance plateau arises
from the helical edge states induced by disorder. Finally, an effective medium
theory based on Born approximation fits the numerical data
Disorder-induced topological phase transitions on Lieb lattices
Motivated by the very recent experimental realization of electronic Lieb
lattices and research interest on topological states of matter, we study the
topological phase transitions driven by Anderson disorder on spin-orbit coupled
Lieb lattices in the presence of spin-independent and dependent potentials. By
combining the numerical transport and self-consistent Born approximation
methods, we found that both time-reversal invariant and broken Lieb lattices
can host disorder-induced gapful topological phases, including the quantum spin
Hall insulator (QSHI) and quantum anomalous Hall insulator (QAHI) phases. For
the time-reversal invariant case, this disorder can induce a topological phase
transition directly from normal insulator (NI) to the QSHI. While for the
time-reversal broken case, the disorder can induce either a QAHI-QSHI phase
transition or a NI-QAHI-QSHI phase transition. Remarkably, the time-reversal
broken QSHI phase can be induced by Anderson disorder on the spin-orbit coupled
Lieb lattices without time-reversal symmetry.Comment: accepted for publication in Phys. Rev.
Topological Superconductivity Intertwined with Broken Symmetries
Recently the superconductor and topological semimetal PbTaSe was
experimentally found to exhibit surface-only lattice rotational symmetry
breaking below . We exploit the Ginzburg-Landau free energy and propose a
microscopic two-channel model to study possible superconducting states on the
surface of PbTaSe. We identify two types of topological superconducting
states. One is time-reversal invariant and preserves the lattice hexagonal
symmetry while the other breaks both symmetries. We find that such
time-reversal symmetry breaking is unavoidable for a superconducting state in a
two dimensional irreducible representation of crystal point group in a system
where the spatial inversion symmetry is broken and the strong spin-orbit
coupling is present. Our findings will guide the search for topological chiral
superconductors.Comment: 4+5 pages, 5 figure
Improving Action Localization by Progressive Cross-stream Cooperation
Spatio-temporal action localization consists of three levels of tasks:
spatial localization, action classification, and temporal segmentation. In this
work, we propose a new Progressive Cross-stream Cooperation (PCSC) framework to
use both region proposals and features from one stream (i.e. Flow/RGB) to help
another stream (i.e. RGB/Flow) to iteratively improve action localization
results and generate better bounding boxes in an iterative fashion.
Specifically, we first generate a larger set of region proposals by combining
the latest region proposals from both streams, from which we can readily obtain
a larger set of labelled training samples to help learn better action detection
models. Second, we also propose a new message passing approach to pass
information from one stream to another stream in order to learn better
representations, which also leads to better action detection models. As a
result, our iterative framework progressively improves action localization
results at the frame level. To improve action localization results at the video
level, we additionally propose a new strategy to train class-specific
actionness detectors for better temporal segmentation, which can be readily
learnt by focusing on "confusing" samples from the same action class.
Comprehensive experiments on two benchmark datasets UCF-101-24 and J-HMDB
demonstrate the effectiveness of our newly proposed approaches for
spatio-temporal action localization in realistic scenarios.Comment: CVPR201
Design and optimization of resistive anode for a two-dimensional imaging triple-GEM detector
The optimization of resistive anode for two dimensional imaging detectors
which consists of a series of high resistive square pads surrounding by low
resistive strips has been studied by both numerical simulations and
experimental tests. It has been found that to obtain good detector performance,
the resistance ratio of the pad to the strip should be larger than 5, the
nonuniformity of the pad surface resistivity had better be less than , a
smaller pad width leads to a smaller spatial resolution and when the pad width
is , the spatial resolution () can reach about . Based
on the study results, a 2-D GEM detector prototype with the optimized resistive
anode is constructed and a good imaging performance is achieved.Comment: 6 pages,11 figure
Finite-size effects in non-Hermitian topological systems
We systematically investigate the finite-size effects in non-Hermitian
one-dimensional (1D) Su-Schrieffer-Heeger (SSH) and two-dimensional (2D) Chern
insulator models. Using a combination of analytical and numerical calculations,
we show that the non-Hermitian intra-cell hoppings in the SSH model can modify
the localization lengths of bulk and end states, giving rise to a complex
finite-size energy gap that exhibits an oscillating exponential decay as the
chain length grows. However, the imaginary staggered on-site potentials in the
SSH model only change the end-state energy, leaving the localization lengths of
the system unchanged. In this case, the finite-size energy gap can undergo a
transition from real values to imaginary values. We observed similar phenomena
for the finite-size effect in 2D Chern insulator systems.Comment: 12 pages, 12 figures. Accepted by Physical Review
From Nodal Ring Topological Superfluids to Spiral Majorana Modes in Cold Atomic Systems
In this work, we consider a 3D cubic optical lattice composed of coupled 1D
wires with 1D spin-orbit coupling. When the s-wave pairing is induced through
Feshbach resonance, the system becomes a topological superfluid with ring
nodes, which are the ring nodal degeneracies in the bulk, and supports a large
number of surface Majorana zero energy modes. The large number of surface
Majorana modes remain at zero energy even in the presence of disorder due to
the protection from a chiral symmetry. When the chiral symmetry is broken, the
system becomes a Weyl topological superfluid with Majorana arcs. With 3D
spin-orbit coupling, the Weyl superfluid becomes a novel gapless phase with
spiral Majorana modes on the surface. The spatial resolved radio frequency
spectroscopy is suggested to detect this novel nodal ring topological
superfluid phase.Comment: 5 pages, 4 figures. Comments are welcom
The effect of in-plane magnetic field and applied strain in quantum spin Hall systems: application to InAs/GaSb quantum wells
Motivated by the recent discovery of quantized spin Hall effect in InAs/GaSb
quantum wells\cite{du2013}\cite{xu2014}, we theoretically study the effects
of in-plane magnetic field and strain effect to the quantization of charge
conductance by using Landauer-Butikker formalism. Our theory predicts a
robustness of the conductance quantization against the magnetic field up to a
very high field of 20 tesla. We use a disordered hopping term to model the
strain and show that the strain may help the quantization of the conductance.
Relevance to the experiments will be discussed.Comment: 8 pages, 10 figures. Comments are welcome
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