44,653 research outputs found
Shot Noise in Magnetic Tunnel Junctions: Evidence for Sequential Tunneling
We report the experimental observation of sub-Poissonian shot noise in single
magnetic tunnel junctions, indicating the importance of tunneling via impurity
levels inside the tunnel barrier. For junctions with weak zero-bias anomaly in
conductance, the Fano factor (normalized shot noise) depends on the magnetic
configuration being enhanced for antiparallel alignment of the ferromagnetic
electrodes. We propose a model of sequential tunneling through nonmagnetic and
paramagnetic impurity levels inside the tunnel barrier to qualitatively explain
the observations.Comment: 5 pages, 5 figure
Emergence of Hierarchy on a Network of Complementary Agents
Complementarity is one of the main features underlying the interactions in
biological and biochemical systems. Inspired by those systems we propose a
model for the dynamical evolution of a system composed by agents that interact
due to their complementary attributes rather than their similarities. Each
agent is represented by a bit-string and has an activity associated to it; the
coupling among complementary peers depends on their activity. The connectivity
of the system changes in time respecting the constraint of complementarity. We
observe the formation of a network of active agents whose stability depends on
the rate at which activity diffuses in the system. The model exhibits a
non-equilibrium phase transition between the ordered phase, where a stable
network is generated, and a disordered phase characterized by the absence of
correlation among the agents. The ordered phase exhibits multi-modal
distributions of connectivity and activity, indicating a hierarchy of
interaction among different populations characterized by different degrees of
activity. This model may be used to study the hierarchy observed in social
organizations as well as in business and other networks.Comment: 13 pages, 4 figures, submitte
Equivalence between the Lovelock-Cartan action and a constrained gauge theory
We show that the four-dimensional Lovelock-Cartan action can be derived from
a massless gauge theory for the group with an additional BRST trivial
part. The model is originally composed by a topological sector and a BRST exact
piece and has no explicit dependence on the metric, the vierbein or a mass
parameter. The vierbein is introduced together with a mass parameter through
some BRST trivial constraints. The effect of the constraints is to identify the
vierbein with some of the additional fields, transforming the original action
into the Lovelock-Cartan one. In this scenario, the mass parameter is
identified with Newton's constant while the gauge field is identified with the
spin-connection. The symmetries of the model are also explored. Moreover, the
extension of the model to a quantum version is qualitatively discussed.Comment: 17 pages. No figures. Final version accepted for publication at the
EPJ
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