11,976 research outputs found
Spin wave approach to the two-magnon Raman scattering in an J1x-J1y-J2-Jc antiferromagnetic Heisenberg model
We study the two-magnon non-resonant Raman scattering in the (pi,pi) and
(pi,0) ordered antiferromagnetic phases of a J1x-J1y-J2-Jc Heisenberg model on
the tetragonal lattice within the framework of the spin-wave theory. We discuss
the effects of various tuning factors to the two-magnon Raman spectra. We find
that both the magnetic frustration J2/J1 and the interlayer exchange coupling
Jc may significantly affect the spectra in both the B1g and A1g' channels in
the (pi,pi) Neel ordered phase. Moreover, we find a splitting of the two-magnon
peak in the (pi,0) antiferromagnetic phase. We further discuss the implications
of our results to the BaMnBi2 and iron pnictide systems.Comment: 11 pages, 10 figures, 5 table
Semiclassical ground-state phase diagram and multi-Q phase of a spin-orbit coupled model on triangular lattice
Motivated by recent experiments on the frustrated quantum magnetic compound
YbMgGaO4, we study an effective spin model on triangular lattice taking into
account the effects of the spin-orbit coupling. We determine the classical
ground-state phase diagram of this model, which includes a 120 degree Neel and
two collinear antiferromagnetic phases. In the vicinity of the phase boundary
between the Neel and collinear phases, we identify three intermediate
non-collinear antiferromagnetic phases. In each of them the magnetic moments
are ordered at multiple incommensurate wave vector Q values. We further study
the effects of quantum fluctuations in this model via a linear spin-wave
theory. We find that the spin excitation gap of the non-collinear multi-Q
antiferromagnetic state is finite but can be vanishingly small, and this state
is unstable to a spin liquid phase under strong quantum fluctuations in some
large J_{z+-} regime.Comment: 11 pages, 5 figure
Fermions tunnelling with quantum gravity correction
Quantum gravity correction is truly important to study tunnelling process of
black hole. Base on the generalized uncertainty principle, we investigate the
influence of quantum gravity and the result tell us that the quantum gravity
correction accelerates the evaporation of black hole. Using corrected Dirac
equation in curved spacetime and Hamilton-Jacobi method, we address the
tunnelling of fermions in a 4-dimensional Schwarzschild spacetime.
After solving the equation of motion of the spin 1/2 field, we obtain the
corrected Hawking temperature. It turns out that the correction depends not
only on the mass of black hole but aslo on the mass of emitted fermions. In our
calculation, the quantum gravity correction accelerates the increasing of
Hawking temperature during the radiation explicitly. This correction leads to
the increasing of the evaporation of black hole.Comment: 5page
Galaxy alignment as a probe of large-scale filaments
The orientations of the red galaxies in a filament are aligned with the
orientation of the filament. We thus develop a location-alignment-method (LAM)
of detecting filaments around clusters of galaxies, which uses both the
alignments of red galaxies and their distributions in two-dimensional images.
For the first time, the orientations of red galaxies are used as probes of
filaments. We apply LAM to the environment of Coma cluster, and find four
filaments (two filaments are located in sheets) in two selected regions, which
are compared with the filaments detected with the method of \cite{Falco14}. We
find that LAM can effectively detect the filaments around a cluster, even with
confidence level, and clearly reveal the number and overall
orientations of the detected filaments. LAM is independent of the redshifts of
galaxies, and thus can be applied at relatively high redshifts and to the
samples of red galaxies without the information of redshifts. We also find that
the images of background galaxies (interlopers) which are lensed by the gravity
of foreground filaments are amplifiers to probe the filaments.Comment: 11 pages, 11 figures. Accepted to MNRAS after moderate expansion to
further strengthen the main conlusions of the previous versio
On the Acceleration of L-BFGS with Second-Order Information and Stochastic Batches
This paper proposes a framework of L-BFGS based on the (approximate)
second-order information with stochastic batches, as a novel approach to the
finite-sum minimization problems. Different from the classical L-BFGS where
stochastic batches lead to instability, we use a smooth estimate for the
evaluations of the gradient differences while achieving acceleration by
well-scaling the initial Hessians. We provide theoretical analyses for both
convex and nonconvex cases. In addition, we demonstrate that within the popular
applications of least-square and cross-entropy losses, the algorithm admits a
simple implementation in the distributed environment. Numerical experiments
support the efficiency of our algorithms
Singlet States Preparation for Three -type Atoms with Rydberg Blockade Mechanism
A proposal for the generation of singlet states of three -type
Rydberg atoms is presented. The singlet state is prepared through the
combination of a Rydberg state and an EPR pair, and the scheme relies on the
Rydberg blockade effect which prevents the simultaneous excitation of the two
atoms to a Rydberg state. In addition, some frequency detuning between lasers
and atomic transitions is set to eliminate the degenerate of the two ground
states. And finally, a series of numerical simulations are made to show the
feasibility of the scheme.Comment: 8 pages,5 figure
Securing Cognitive Radio Networks against Primary User Emulation Attacks
Cognitive Radio (CR) is a promising technology for next-generation wireless
networks in order to efficiently utilize the limited spectrum resources and
satisfy the rapidly increasing demand for wireless applications and services.
Security is a very important but not well addressed issue in CR networks. In
this paper we focus on security problems arising from Primary User Emulation
(PUE) attacks in CR networks. We present a comprehensive introduction to PUE
attacks, from the attacking rationale and its impact on CR networks, to
detection and defense approaches. In order to secure CR networks against PUE
attacks, a two-level database-assisted detection approach is proposed to detect
such attacks. Energy detection and location verification are combined for fast
and reliable detection. An admission control based defense approach is proposed
to mitigate the performance degradation of a CR network under a PUE attack.
Illustrative results are presented to demonstrate the effectiveness of the
proposed detection and defense approaches.Comment: 9 pages, 6 figure
Anomaly analysis of Hawking radiation from 2+1 dimensional spinning black hole
Considering gravitational and gauge anomalies at the horizon, a new
successful method that to derive Hawking radiations from black holes has been
developed recently by Wilczek et al.. By using the dimensional reduction
technique, we apply this method to a non-vacuum solution, the 2+1 dimensional
spinning black hole. The Hawking temperature and angular velocity on the
horizon are obtained. The results may partially imply that this method is
independent of the gravity theory, the dimension of spacetime and the
topological structure of the event horizon.Comment: v2: 5 pages, typos corrected, references adde
Adiabatic Generation of N-quNit Singlet States with Cavity QED
A theoretical scheme is presented for the adiabatic generation of N-quNit
singlet states with , which may be more feasible than previous
ones in a cavity. In this proposal, the system may be robust both parameter
fluctuations and dissipation along a dark state. In addition, quantum
information is only stored in atomic ground states and there is no energy
exchanged between atoms and photons in a cavity so as to reduce the influence
of atomic spontaneous emission and cavity decays.Comment: 7 pages,5 figure
Physical States and BRST Operators for Higher-spin Strings
In this paper, we mainly investigate the
system, in which the matter and the Liouville subsystems generate
and algebras respectively. We first give a brief discussion of the
physical states for corresponding stings. The lower states are given by
freezing the spin-2 and spin- currents. Then, introducing two pairs of
ghost-like fields, we give the realizations of algebras. Based on
these linear realizations, BRST operators for algebras are obtained.
Finally, we construct new BRST charges of Liouville system for
strings at the specific values of central charges : for
algebra, for algebra and
for algebra, at which the corresponding algebras are
singular.Comment: 18 pages, 2 tables, no figure
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