368 research outputs found
Proactive Caching for Energy-Efficiency in Wireless Networks: A Markov Decision Process Approach
Content caching in wireless networks provides a substantial opportunity to
trade off low cost memory storage with energy consumption, yet finding the
optimal causal policy with low computational complexity remains a challenge.
This paper models the Joint Pushing and Caching (JPC) problem as a Markov
Decision Process (MDP) and provides a solution to determine the optimal
randomized policy. A novel approach to decouple the influence from buffer
occupancy and user requests is proposed to turn the high-dimensional
optimization problem into three low-dimensional ones. Furthermore, a
non-iterative algorithm to solve one of the sub-problems is presented,
exploiting a structural property we found as \textit{generalized monotonicity},
and hence significantly reduces the computational complexity. The result
attains close performance in comparison with theoretical bounds from
non-practical policies, while benefiting from higher time efficiency than the
unadapted MDP solution.Comment: 6 pages, 6 figures, submitted to IEEE International Conference on
Communications 201
Nonequilibrium generation of charge defects in kagome spin ice under slow cooling
Kagome spin ice is one of the canonical examples of highly frustrated
magnets. The effective magnetic degrees of freedom in kagome spin ice are Ising
spins residing on a two-dimensional network of corner-sharing triangles. Due to
strong geometrical frustration, nearest-neighbor antiferromagnetic interactions
on the kagome lattice give rise to a macroscopic number of degenerate classical
ground states characterized by ice rules. Elementary excitations at low
temperatures are defect-triangles that violate the ice rules and carry an
additional net magnetic charge relative to the background. We perform
large-scale Glauber dynamics simulations to study the nonequilibrium dynamics
of kagome ice under slow cooling. We show that the density of residual charge
defects exhibits a power law dependence on the quench rate for the class of
algebraic cooling protocols. The numerical results are well captured by the
rate equation for the charge defects based on the reaction kinetics theory. As
the relaxation time of the kagome ice phase remains finite, there is no
dynamical freezing as in the Kibble-Zurek scenario. Instead, we show that the
power-law behavior originates from the a thermal excitation that decay
algebraically with time at the late stage of the cooling schedule. Similarities
and differences in quench dynamics of other spin ice systems are also
discussed.Comment: 8 pages, 4 figure
Kibble-Zurek Mechanism for Nonequilibrium Generation of Magnetic Monopoles in Spin Ices
The proliferation of topological defects is a common out-of-equilibrium
phenomenon when a system is driven into a phase of broken symmetry. The
Kibble-Zurek mechanism (KZM) provides a theoretical framework for the critical
dynamics and generation of topological defects in such scenarios. One of the
early applications of KZM is the estimation of heavy magnetic monopoles left
behind by the cosmological phase transitions in the early universe. The
scarcity of such relic monopoles, which contradicts the prediction of KZM, is
one of the main motivations for cosmological inflationary theories. On the
other hand, magnetic monopoles as emergent quasi-particles have been observed
in spin ices, a peculiar class of frustrated magnets that remain disordered at
temperatures well below the energy scale of exchange interaction. Here we study
the annihilation dynamics of magnetic monopoles when spin ice is cooled to zero
temperature in a finite time. Through extensive Glauber dynamics simulations,
we find that the density of residual monopole follows a power law dependence on
the annealing rate. A kinetic reaction theory that precisely captures the
annihilation process from Monte Carlo simulations is developed. We further show
that the KZM can be generalized to describe the critical dynamics of spin ice,
where the exponent of the power-law behavior is determined by the dynamic
critical exponent and the cooling protocol.Comment: 13 pages, 7 figure
Facile synthesis of α-Fe2O3 micro-ellipsoids by surfactant-free hydrothermal method for sub-ppm level H2S detection
The α-Fe2O3 micro-ellipsoids were prepared using a facile hydrothermal process without any surfactant or template, and their morphological, structural and H2S sensing properties were investigated. The α-Fe2O3 showed uniform micro-ellipsoids with a long axis diameter of 1.7 μm and a short axis diameter of 1.2 μm. Detailed structural analysis confirmed that the synthesized α-Fe2O3 micro-ellipsoids were compact particles with a hexagonal structure. Gas sensor base on the α-Fe2O3 micro-ellipsoids showed excellent response, short response/recovery time (< 90 s and 30 s, respectively), low detection concentration (~ 0.5 ppm), good long-term stability and excellent selectivity towards H2S gas at the optimized operating temperature of 350 °C. The sensing mechanism of the sensor based on the α-Fe2O3 micro-ellipsoids towards H2S was discussed
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