88 research outputs found
Band Topology and Linking Structure of Nodal Line Semimetals with Z2 Monopole Charges
We study the band topology and the associated linking structure of
topological semimetals with nodal lines carrying monopole charges,
which can be realized in three-dimensional systems invariant under the
combination of inversion and time reversal when spin-orbit coupling is
negligible. In contrast to the well-known -symmetric nodal lines protected
only by Berry phase in which a single nodal line can exist, the nodal
lines with monopole charges should always exist in pairs. We show that
a pair of nodal lines with monopole charges is created by a {\it double
band inversion} (DBI) process, and that the resulting nodal lines are always
{\it linked by another nodal line} formed between the two topmost occupied
bands. It is shown that both the linking structure and the monopole
charge are the manifestation of the nontrivial band topology characterized by
the {\it second Stiefel-Whitney class}, which can be read off from the Wilson
loop spectrum. We show that the second Stiefel-Whitney class can serve as a
well-defined topological invariant of a -invariant two-dimensional (2D)
insulator in the absence of Berry phase. Based on this, we propose that pair
creation and annihilation of nodal lines with monopole charges can
mediate a topological phase transition between a normal insulator and a
three-dimensional weak Stiefel-Whitney insulator (3D weak SWI). Moreover, using
first-principles calculations, we predict ABC-stacked graphdiyne as a nodal
line semimetal (NLSM) with monopole charges having the linking
structure. Finally, we develop a formula for computing the second
Stiefel-Whitney class based on parity eigenvalues at inversion invariant
momenta, which is used to prove the quantized bulk magnetoelectric response of
NLSMs with monopole charges under a -breaking perturbation.Comment: 4+28 pages, 3+17 figure
Pilot Beam Sequence Design for Channel Estimation in Millimeter-Wave MIMO Systems: A POMDP Framework
In this paper, adaptive pilot beam sequence design for channel estimation in
large millimeter-wave (mmWave) MIMO systems is considered. By exploiting the
sparsity of mmWave MIMO channels with the virtual channel representation and
imposing a Markovian random walk assumption on the physical movement of the
line-of-sight (LOS) and reflection clusters, it is shown that the sparse
channel estimation problem in large mmWave MIMO systems reduces to a sequential
detection problem that finds the locations and values of the non-zero-valued
bins in a two-dimensional rectangular grid, and the optimal adaptive pilot
design problem can be cast into the framework of a partially observable Markov
decision process (POMDP). Under the POMDP framework, an optimal adaptive pilot
beam sequence design method is obtained to maximize the accumulated
transmission data rate for a given period of time. Numerical results are
provided to validate our pilot signal design method and they show that the
proposed method yields good performance.Comment: 6 pages, 6 figures, submitted to IEEE ICC 201
Two-dimensional higher-order topology in monolayer graphdiyne
Based on first-principles calculations and tight-binding model analysis, we
propose monolayer graphdiyne as a candidate material for a two-dimensional
higher-order topological insulator protected by inversion symmetry. Despite the
absence of chiral symmetry, the higher-order topology of monolayer graphdiyne
is manifested in the filling anomaly and charge accumulation at two corners.
Although its low energy band structure can be properly described by the
tight-binding Hamiltonian constructed by using only the orbital of each
atom, the corresponding bulk band topology is trivial. The nontrivial bulk
topology can be correctly captured only when the contribution from the core
levels derived from and orbitals are included, which is further
confirmed by the Wilson loop calculations. We also show that the higher-order
band topology of a monolayer graphdyine gives rise to the nontrivial band
topology of the corresponding three-dimensional material, ABC-stacked
graphdiyne, which hosts monopole nodal lines and hinge states.Comment: 19 pages, 4 figures, new titl
BIRP: Bitcoin Information Retrieval Prediction Model Based on Multimodal Pattern Matching
Financial time series have historically been assumed to be a martingale
process under the Random Walk hypothesis. Instead of making investment
decisions using the raw prices alone, various multimodal pattern matching
algorithms have been developed to help detect subtly hidden repeatable patterns
within the financial market. Many of the chart-based pattern matching tools
only retrieve similar past chart (PC) patterns given the current chart (CC)
pattern, and leaves the entire interpretive and predictive analysis, thus
ultimately the final investment decision, to the investors. In this paper, we
propose an approach of ranking similar PC movements given the CC information
and show that exploiting this as additional features improves the directional
prediction capacity of our model. We apply our ranking and directional
prediction modeling methodologies on Bitcoin due to its highly volatile prices
that make it challenging to predict its future movements.Comment: 5 pages, 2 figures, KDD 2023 Machine Learning in Finance worksho
Structured Co-reference Graph Attention for Video-grounded Dialogue
A video-grounded dialogue system referred to as the Structured Co-reference
Graph Attention (SCGA) is presented for decoding the answer sequence to a
question regarding a given video while keeping track of the dialogue context.
Although recent efforts have made great strides in improving the quality of the
response, performance is still far from satisfactory. The two main challenging
issues are as follows: (1) how to deduce co-reference among multiple modalities
and (2) how to reason on the rich underlying semantic structure of video with
complex spatial and temporal dynamics. To this end, SCGA is based on (1)
Structured Co-reference Resolver that performs dereferencing via building a
structured graph over multiple modalities, (2) Spatio-temporal Video Reasoner
that captures local-to-global dynamics of video via gradually neighboring graph
attention. SCGA makes use of pointer network to dynamically replicate parts of
the question for decoding the answer sequence. The validity of the proposed
SCGA is demonstrated on AVSD@DSTC7 and AVSD@DSTC8 datasets, a challenging
video-grounded dialogue benchmarks, and TVQA dataset, a large-scale videoQA
benchmark. Our empirical results show that SCGA outperforms other
state-of-the-art dialogue systems on both benchmarks, while extensive ablation
study and qualitative analysis reveal performance gain and improved
interpretability.Comment: Accepted to AAAI202
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