16,608 research outputs found
Bacterial chemotaxis without gradient-sensing
Models for chemotaxis are based on gradient sensing of individual organisms.
The key contribution of Keller and Segel is showing that erratic movements of
individuals may result in an accurate chemotaxis phenomenon as a group. In this
paper we provide another option to understand chemotactic behavior when
individuals do not sense the gradient of chemical concentration by any means.
We show that, if individuals increase their motility to find food when they are
hungry, an accurate chemotactic behavior may obtained without sensing the
gradient. Such a random dispersal has been suggested by Cho and Kim and is
called starvation driven diffusion. This model is surprisingly similar to the
original derivation of Keller-Segel model. A comprehensive picture of traveling
band and front solutions is provided with numerical simulations.Comment: 19 pages, 4 figure
Statistical Self-Similar Properties of Complex Networks
It has been shown that many complex networks shared distinctive features,
which differ in many ways from the random and the regular networks. Although
these features capture important characteristics of complex networks, their
applicability depends on the type of networks. To unravel ubiquitous
characteristics that complex networks may have in common, we adopt the
clustering coefficient as the probability measure, and present a systematic
analysis of various types of complex networks from the perspective of
statistical self-similarity. We find that the probability distribution of the
clustering coefficient is best characterized by the multifractal; moreover, the
support of the measure had a fractal dimension. These two features enable us to
describe complex networks in a unified way; at the same time, offer unforeseen
possibilities to comprehend complex networks.Comment: 11 pages, 4 figure
Opportunistic Interference Mitigation Achieves Optimal Degrees-of-Freedom in Wireless Multi-cell Uplink Networks
We introduce an opportunistic interference mitigation (OIM) protocol, where a
user scheduling strategy is utilized in -cell uplink networks with
time-invariant channel coefficients and base stations (BSs) having
antennas. Each BS opportunistically selects a set of users who generate the
minimum interference to the other BSs. Two OIM protocols are shown according to
the number of simultaneously transmitting users per cell: opportunistic
interference nulling (OIN) and opportunistic interference alignment (OIA).
Then, their performance is analyzed in terms of degrees-of-freedom (DoFs). As
our main result, it is shown that DoFs are achievable under the OIN
protocol with selected users per cell, if the total number of users in
a cell scales at least as . Similarly, it turns out that
the OIA scheme with () selected users achieves DoFs, if scales
faster than . These results indicate that there exists a
trade-off between the achievable DoFs and the minimum required . By deriving
the corresponding upper bound on the DoFs, it is shown that the OIN scheme is
DoF optimal. Finally, numerical evaluation, a two-step scheduling method, and
the extension to multi-carrier scenarios are shown.Comment: 18 pages, 3 figures, Submitted to IEEE Transactions on Communication
Topological insulators and metal-insulator transition in the pyrochlore iridates
The possible existence of topological insulators in cubic pyrochlore iridates
AIrO (A = Y or rare-earth elements) is investigated by taking
into account the strong spin-orbit coupling and trigonal crystal field effect.
It is found that the trigonal crystal field effect, which is always present in
real systems, may destabilize the topological insulator proposed for the ideal
cubic crystal field, leading to a metallic ground state. Thus the trigonal
crystal field is an important control parameter for the metal-insulator
changeover. We propose that this could be one of the reasons why distinct low
temperature ground states may arise for the pyrochlore iridates with different
A-site ions. On the other hand, examining the electron-lattice coupling, we
find that softening of the =0 modes corresponding to trigonal or
tetragonal distortions of the Ir pyrochlore lattice leads to the resurrection
of the strong topological insulator. Thus, in principle, a finite temperature
transition to a low-temperature topological insulator can occur via structural
changes. We also suggest that the application of the external pressure along
[111] or its equivalent directions would be the most efficient way of
generating strong topological insulators in pyrochlore iridates.Comment: 10 pages, 11 figures, 2 table
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