3,226 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
Reducing Randomness via Irrational Numbers
We propose a general methodology for testing whether a given polynomial with
integer coefficients is identically zero. The methodology evaluates the
polynomial at efficiently computable approximations of suitable irrational
points. In contrast to the classical technique of DeMillo, Lipton, Schwartz,
and Zippel, this methodology can decrease the error probability by increasing
the precision of the approximations instead of using more random bits.
Consequently, randomized algorithms that use the classical technique can
generally be improved using the new methodology. To demonstrate the
methodology, we discuss two nontrivial applications. The first is to decide
whether a graph has a perfect matching in parallel. Our new NC algorithm uses
fewer random bits while doing less work than the previously best NC algorithm
by Chari, Rohatgi, and Srinivasan. The second application is to test the
equality of two multisets of integers. Our new algorithm improves upon the
previously best algorithms by Blum and Kannan and can speed up their checking
algorithm for sorting programs on a large range of inputs
Transverse Shift in Andreev Reflection
An incoming electron is reflected back as a hole at a
normal-metal-superconductor interface, a process known as Andreev reflection.
We predict that there exists a universal transverse shift in this process due
to the effect of spin-orbit coupling in the normal metal. Particularly, using
both the scattering approach and the argument of angular momentum conservation,
we demonstrate that the shifts are pronounced for lightly-doped Weyl
semimetals, and are opposite for incoming electrons with different chirality,
generating a chirality-dependent Hall effect for the reflected holes. The
predicted shift is not limited to Weyl systems, but exists for a general
three-dimensional spin-orbit- coupled metal interfaced with a superconductor.Comment: 5 pages, 2 figure
Common-Face Embeddings of Planar Graphs
Given a planar graph G and a sequence C_1,...,C_q, where each C_i is a family
of vertex subsets of G, we wish to find a plane embedding of G, if any exists,
such that for each i in {1,...,q}, there is a face F_i in the embedding whose
boundary contains at least one vertex from each set in C_i. This problem has
applications to the recovery of topological information from geographical data
and the design of constrained layouts in VLSI. Let I be the input size, i.e.,
the total number of vertices and edges in G and the families C_i, counting
multiplicity. We show that this problem is NP-complete in general. We also show
that it is solvable in O(I log I) time for the special case where for each
input family C_i, each set in C_i induces a connected subgraph of the input
graph G. Note that the classical problem of simply finding a planar embedding
is a further special case of this case with q=0. Therefore, the processing of
the additional constraints C_1,...,C_q only incurs a logarithmic factor of
overhead.Comment: A preliminary version appeared in the Proceedings of the 10th Annual
ACM-SIAM Symposium on Discrete Algorithms, 1999, pp. 195-20
Predicted Unusual Magnetoresponse in Type-II Weyl Semimetals
We show several distinct signatures in the magneto-response of type-II Weyl
semimetals. The energy tilt tends to squeeze the Landau levels (LLs), and for a
type-II Weyl node, there always exists a critical angle between the B-fileld
and the tilt, at which the LL spectrum collapses, regardless of the fileld
strength. Before collapse, signatures also appear in the magneto-optical
spectrum, including the invariable presence of intraband peaks, the absence of
absorption tails, and the special anisotropic fileld dependence
Task-space coordinated tracking of multiple heterogeneous manipulators via controller-estimator approaches
This paper studies the task-space coordinated tracking of a time-varying
leader for multiple heterogeneous manipulators (MHMs), containing redundant
manipulators and nonredundant ones. Different from the traditional coordinated
control, distributed controller-estimator algorithms (DCEA), which consist of
local algorithms and networked algorithms, are developed for MHMs with
parametric uncertainties and input disturbances. By invoking differential
inclusions, nonsmooth analysis, and input-to-state stability, some conditions
(including sufficient conditions, necessary and sufficient conditions) on the
asymptotic stability of the task-space tracking errors and the subtask errors
are developed. Simulation results are given to show the effectiveness of the
presented DCEA.Comment: 17 pages, 7 figures, Journal of the Franklin Institut
Strong light illumination on gain-switched semiconductor lasers helps the eavesdropper in practical quantum key distribution systems
The temperature of the semiconductor diode increases under strong light
illumination whether thermoelectric cooler is installed or not, which changes
the output wavelength of the laser (Lee M. S. et al., 2017). However, other
characteristics also vary as temperature increases. These variations may help
the eavesdropper in practical quantum key distribution systems. We study the
effects of temperature increase on gain-switched semiconductor lasers by
simulating temperature dependent rate equations. The results show that
temperature increase may cause large intensity fluctuation, decrease the output
intensity and lead the signal state and decoy state distinguishable. We also
propose a modified photon number splitting attack by exploiting the effects of
temperature increase. Countermeasures are also proposed.Comment: Accepted by Optics Communications. 7 Figure
A hybrid approach for cooperative output regulation with sampled compensator
This work investigates the cooperative output regulation problem of linear
multi-agent systems with hybrid sampled data control. Due to the limited data
sensing and communication, in many practical situations, only sampled data are
available for the cooperation of multi-agent systems. To overcome this problem,
a distributed hybrid controller is presented for the cooperative output
regulation, and cooperative output regulation is achieved by well designed
state feedback law. Then it proposed a method for the designing of sampled data
controller to solve the cooperative output regulation problem with continuous
linear systems and discrete-time communication data. Finally, numerical
simulation example for cooperative tracking and a simulation example for
optimal control of micro-grids are proposed to illustrate the result of the
sampled data control law
Unconventional pairing induced anomalous transverse shift in Andreev reflection
Superconductors with unconventional pairings have been a fascinating subject
of research, for which a central issue is to explore effects that can be used
to characterize the pairing. The process of Andreev reflection--the reflection
of an electron as a hole at a normal-mental-superconductor interface by
transferring a Cooper pair into the superconductor--offers a basic mechanism to
probe the pairing through transport. Here we predict that in Andreev reflection
from unconventional superconductors, the reflected hole acquires an anomalous
spatial shift normal to the plane of incidence, arising from the unconventional
pairing. The transverse shift is sensitive to the superconducting gap
structure, exhibiting characteristic features for each pairing type, and can be
detected as voltage signals. Our work not only unveils a fundamentally new
effect but also suggests a powerful new technique capable of probing the
structure of unconventional pairings.Comment: 4 pages, 4 figure
Type-II topological metals
Topological metals (TMs) are a kind of special metallic materials, which
feature nontrivial band crossings near the Fermi energy, giving rise to
peculiar quasiparticle excitations. TMs can be classified based on the
characteristics of these band crossings. For example, according to the
dimensionality of the crossing, TMs can be classified into nodal-point,
nodal-line, and nodal-surface metals. Another important property is the type of
dispersion. According to degree of the tilt of the local dispersion around the
crossing, we have type-I and type-II dispersions. This leads to significant
distinctions in the physical properties of the materials, owing to their
contrasting Fermi surface topologies. In this article, we briefly review the
recent advances in this research direction, focusing on the concepts, the
physical properties, and the material realizations of the type-II nodal-point
and nodal-line TMs.Comment: 12 pages, 16 figure
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