1,365 research outputs found
Evolution of the Fermi surface in phase fluctuating d-wave superconductors
One of the most puzzling aspects of the high superconductors is the
appearance of Fermi arcs in the normal state of the underdoped cuprate
materials. These are loci of low energy excitations covering part of the fermi
surface, that suddenly appear above instead of the nodal quasiparticles.
Based on a semiclassical theory, we argue that partial Fermi surfaces arise
naturally in a d-wave superconductor that is destroyed by thermal phase
fluctuations. Specifically, we show that the electron spectral function
develops a square root singularity at low frequencies for wave-vectors
positioned on the bare Fermi surface. We predict a temperature dependence of
the arc length that can partially account for results of recent angle resolved
photo emission (ARPES) experiments.Comment: Journal ref. adde
The Fermi Problem in Discrete Systems
The Fermi two-atom problem illustrates an apparent causality violation in
Quantum Field Theory which has to do with the nature of the built in
correlations in the vacuum. It has been a constant subject of theoretical
debate and discussions during the last few decades. Nevertheless, although the
issues at hand could in principle be tested experimentally, the smallness of
such apparent violations of causality in Quantum Electrodynamics prevented the
observation of the predicted effect. In the present paper we show that the
problem can be simulated within the framework of discrete systems that can be
manifested, for instance, by trapped atoms in optical lattices or trapped ions.
Unlike the original continuum case, the causal structure is no longer sharp.
Nevertheless, as we show, it is possible to distinguish between "trivial"
effects due to "direct" causality violations, and the effects associated with
Fermi's problem, even in such discrete settings. The ability to control
externally the strength of the atom-field interactions, enables us also to
study both the original Fermi problem with "bare atoms", as well as correction
in the scenario that involves "dressed" atoms. Finally, we show that in
principle, the Fermi effect can be detected using trapped ions.Comment: Second version - minor change
Possible way out of the Hawking paradox: Erasing the information at the horizon
We show that small deviations from spherical symmetry, described by means of
exact solutions to Einstein equations, provide a mechanism to "bleach" the
information about the collapsing body as it falls through the aparent horizon,
thereby resolving the information loss paradox. The resulting picture and its
implication related to the Landauer's principle in the presence of a
gravitational field, is discussed.Comment: 11 pages, Latex. Some comments added to answer to some raised
questions. Typos corected. Final version, to appear in Int. J. Modern. Phys.
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MEF: Malicious Email Filter: A UNIX Mail Filter that Detects Malicious Windows Executables
We present Malicious Email Filter, MEF, a freely distributed malicious binary filter incorporated into Procmail that can detect malicious Windows attachments by integrating with a UNIX mail server. The system has three capabilities: detection of known and unknown malicious attachments, tracking the propagation of malicious attachments and efficient model update algorithms. The system filters multiple malicious attachments in an email by using detection models obtained from data mining over known malicious attachments. It leverages preliminary research in data mining applied to malicious executables which allows the detection of previously unseen, malicious attachments. In addition, the system provides a method for monitoring and measurement of the spread of malicious attachments. Finally, the system also allows for the efficient propagation of detection models from a central server. These updated models can be downloaded by a system administrator and easily incorporated into the current model. The system will be released under GPL in June 2001
The ground states of the two-component order parameter superconductor
We show that in presence of an applied external field the two-component order
parameter superconductor falls in two categories of ground states, namely, in
the traditional Abrikosov ground state or in a new ground state fitted to
describe a superconducting layer with texture, that is, patched regions
separated by a phase difference of . The existence of these two kinds of
ground states follows from the sole assumption that the total supercurrent is
the sum of the two individual supercurrents and is independent of any
consideration about the free energy expansion. Uniquely defined relations
between the current density and the superfluid density hold for these two
ground states, which also determine the magnetization in terms of average
values of the order parameters. Because these ground state conditions are also
Bogomolny equations we construct the free energy for the two-component
superconductor which admits the Bogomolny solution at a special coupling value.Comment: 5 page
Stealthy Deception Attacks Against SCADA Systems
SCADA protocols for Industrial Control Systems (ICS) are vulnerable to
network attacks such as session hijacking. Hence, research focuses on network
anomaly detection based on meta--data (message sizes, timing, command
sequence), or on the state values of the physical process. In this work we
present a class of semantic network-based attacks against SCADA systems that
are undetectable by the above mentioned anomaly detection. After hijacking the
communication channels between the Human Machine Interface (HMI) and
Programmable Logic Controllers (PLCs), our attacks cause the HMI to present a
fake view of the industrial process, deceiving the human operator into taking
manual actions. Our most advanced attack also manipulates the messages
generated by the operator's actions, reversing their semantic meaning while
causing the HMI to present a view that is consistent with the attempted human
actions. The attacks are totaly stealthy because the message sizes and timing,
the command sequences, and the data values of the ICS's state all remain
legitimate.
We implemented and tested several attack scenarios in the test lab of our
local electric company, against a real HMI and real PLCs, separated by a
commercial-grade firewall. We developed a real-time security assessment tool,
that can simultaneously manipulate the communication to multiple PLCs and cause
the HMI to display a coherent system--wide fake view. Our tool is configured
with message-manipulating rules written in an ICS Attack Markup Language (IAML)
we designed, which may be of independent interest. Our semantic attacks all
successfully fooled the operator and brought the system to states of blackout
and possible equipment damage
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