7,419 research outputs found
Topological Band Theory for Non-Hermitian Hamiltonians
We develop the topological band theory for systems described by non-Hermitian
Hamiltonians, whose energy spectra are generally complex. After generalizing
the notion of gapped band structures to the non-Hermitian case, we classify
"gapped" bands in one and two dimensions by explicitly finding their
topological invariants. We find nontrivial generalizations of the Chern number
in two dimensions, and a new classification in one dimension, whose topology is
determined by the energy dispersion rather than the energy eigenstates. We then
study the bulk-edge correspondence and the topological phase transition in two
dimensions. Different from the Hermitian case, the transition generically
involves an extended intermediate phase with complex-energy band degeneracies
at isolated "exceptional points" in momentum space. We also systematically
classify all types of band degeneracies.Comment: 6 pages, 3 figures + 6 pages of supplemental materia
Totally Corrective Multiclass Boosting with Binary Weak Learners
In this work, we propose a new optimization framework for multiclass boosting
learning. In the literature, AdaBoost.MO and AdaBoost.ECC are the two
successful multiclass boosting algorithms, which can use binary weak learners.
We explicitly derive these two algorithms' Lagrange dual problems based on
their regularized loss functions. We show that the Lagrange dual formulations
enable us to design totally-corrective multiclass algorithms by using the
primal-dual optimization technique. Experiments on benchmark data sets suggest
that our multiclass boosting can achieve a comparable generalization capability
with state-of-the-art, but the convergence speed is much faster than stage-wise
gradient descent boosting. In other words, the new totally corrective
algorithms can maximize the margin more aggressively.Comment: 11 page
A QoS-Aware Scheduling Algorithm for High-Speed Railway Communication System
With the rapid development of high-speed railway (HSR), how to provide the
passengers with multimedia services has attracted increasing attention. A key
issue is to develop an effective scheduling algorithm for multiple services
with different quality of service (QoS) requirements. In this paper, we
investigate the downlink service scheduling problem in HSR network taking
account of end-to-end deadline constraints and successfully packet delivery
ratio requirements. Firstly, by exploiting the deterministic high-speed train
trajectory, we present a time-distance mapping in order to obtain the highly
dynamic link capacity effectively. Next, a novel service model is developed for
deadline constrained services with delivery ratio requirements, which enables
us to turn the delivery ratio requirement into a single queue stability
problem. Based on the Lyapunov drift, the optimal scheduling problem is
formulated and the corresponding scheduling service algorithm is proposed by
stochastic network optimization approach. Simulation results show that the
proposed algorithm outperforms the conventional schemes in terms of QoS
requirements.Comment: 6 pages, 3 figures, accepted by IEEE ICC 2014 conferenc
Linear magnetoconductivity in an intrinsic topological Weyl semimetal
Searching for the signature of the violation of chiral charge conservation in
solids has inspired a growing passion on the magneto-transport in topological
semimetals. One of the open questions is how the conductivity depends on
magnetic fields in a semimetal phase when the Fermi energy crosses the Weyl
nodes. Here, we study both the longitudinal and transverse magnetoconductivity
of a topological Weyl semimetal near the Weyl nodes with the help of a two-node
model that includes all the topological semimetal properties. In the semimetal
phase, the Fermi energy crosses only the 0th Landau bands in magnetic fields.
For a finite potential range of impurities, it is found that both the
longitudinal and transverse magnetoconductivity are positive and linear at the
Weyl nodes, leading to an anisotropic and negative magnetoresistivity. The
longitudinal magnetoconductivity depends on the potential range of impurities.
The longitudinal conductivity remains finite at zero field, even though the
density of states vanishes at the Weyl nodes. This work establishes a relation
between the linear magnetoconductivity and the intrinsic topological Weyl
semimetal phase.Comment: An extended version accepted by New. J. Phys. with 15 pages and 3
figure
Edge states and integer quantum Hall effect in topological insulator thin films
The integer quantum Hall effect is a topological state of quantum matter in
two dimensions, and has recently been observed in three-dimensional topological
insulator thin films. Here we study the Landau levels and edge states of
surface Dirac fermions in topological insulators under strong magnetic field.
We examine the formation of the quantum plateaux of the Hall conductance and
find two different patterns, in one pattern the filling number covers all
integers while only odd integers in the other. We focus on the quantum plateau
closest to zero energy and demonstrate the breakdown of the quantum spin Hall
effect resulting from structure inversion asymmetry. The phase diagrams of the
quantum Hall states are presented as functions of magnetic field, gate voltage
and chemical potential. This work establishes an intuitive picture of the edge
states to understand the integer quantum Hall effect for Dirac electrons in
topological insulator thin films.Comment: 10 pages, 5 figure
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