889 research outputs found
A NOVEL APPROACH FOR COVERT COMMUNICATION OVER TCP VIA INDUCED CLOCK SKEW
The goal of this thesis is to determine the feasibility and provide a proof of concept for a covert
communications channel based on induced clock skew. Transmission Control Protocol (TCP) timestamps
provide a means for measuring clock skew between two hosts. By intentionally altering timestamps, a host
can induce artificial clock skew as measured by the receiver, thereby providing a means to covertly
communicate. A novel scheme for transforming symbols into skew values is developed in this work, along
with methods for extraction at the receiver. We tested the proposed scheme in a laboratory network
consisting of Dell laptops running Ubuntu 16.04. The results demonstrated a successful implementation of
the proposed covert channel with achieved bit rates as high as 33 bits per second under ideal conditions.
Forward error correction was also successfully employed in the form of a Reed–Solomon code to mitigate
the effects of variation in delay over the Internet.Lieutenant, United States NavyApproved for public release; distribution is unlimited
Magnetic order in Ce0.95Nd0.05CoIn5: the Q-phase at zero magnetic field
We report neutron scattering experiment results revealing the nature of the
magnetic order occurring in the heavy fermion superconductor Ce0.95Nd0.05CoIn5,
a case for which an antiferromagnetic state is stabilized at a temperature
below the superconducting transition one. We evidence an incommensurate order
and its propagation vector is found to be identical to that of the magnetic
field induced antiferromagnetic order occurring in the stoichiometric
superconductor CeCoIn5, the so-called Q-phase. The commonality between these
two cases suggests that superconductivity is a requirement for the formation of
this kind of magnetic order and the proposed mechanism is the enhancement of
nesting condition by d-wave order parameter with nodes in the nesting area.Comment: submitted to Phys. Rev. Lett. on June 30th, 201
Switching of the magnetic order in CeRhInSn in the vicinity of its quantum critical point
We report neutron diffraction experiments performed in the tetragonal
antiferromagnetic heavy fermion system CeRhInSn in its (, )
phase diagram up to the vicinity of the critical concentration
0.40, where long range magnetic order is suppressed. The propagation vector of
the magnetic structure is found to be =(1/2, 1/2, ) with
increasing from =0.298 to =0.410 when increases from =0
to =0.26. Surprisingly, for =0.30, the order has changed drastically and
a commensurate antiferromagnetism with =(1/2, 1/2, 0) is found.
This concentration is located in the proximity of the quantum critical point
where superconductivity is expected.Comment: 5 pages, 5 figures, submitted to Phys. Rev.
Antiferromagnetism and Superconductivity in CeRhIn
We discuss recent results on the heavy fermion superconductor CeRhIn
which presents ideal conditions to study the strong coupling between the
suppression of antiferromagnetic order and the appearance of unconventional
superconductivity. The appearance of superconductivity as function of pressure
is strongly connected to the suppression of the magnetic order. Under magnetic
field, the re-entrance of magnetic order inside the superconducting state shows
that antiferromagnetism nucleates in the vortex cores. The suppression of
antiferromagnetism in CeRhIn by Sn doping is compared to that under
hydrostatic pressure.Comment: 6 pages, 8 figures, to be published in Proc. Int. Conf. Heavy
Electrons (ICHE2010) J. Phys. Soc. Jpn. 80 (2011
Magnetic-field dependence of antiferromagnetic structure in CeRh1-xCoxIn5
We investigated effects of magnetic field H on antiferromagnetic (AF)
structures in CeRh_{1-x}Co_xIn_5 by performing the elastic neutron scattering
measurements. By applying H along the [1,-1,0] direction, the incommensurate AF
state with the propagation vector of q_{h1}=(1/2,1/2,0.297) observed at H=0 is
replaced by the commensurate AF state with the q_{c2} = (1/2, 1/2, 1/4)
modulation above 2 T for x=0.23, while the AF states with the
q_{c1}=(1/2,1/2,1/2) and q_{h2}=(1/2,1/2,0.42) modulations seen at H=0 change
into a single q_{c1}-AF state above ~1.6 T for x=0.7. These results suggest the
different types of AF correlation for Co concentrations of 0.23 and 0.7 in an
applied magnetic field H.Comment: 4 pages, 2 figures, to appear in the proceedings of ICM2009
(Karlsruhe, Germany
Thermoelectric response near a quantum critical point of beta-YbAlB4 and YbRh2Si2: A comparative study
The thermoelectric coefficients have been measured on the Yb-based heavy
fermion compounds beta-YbAlB4 and YbRh2Si2 down to a very low temperature. We
observe a striking difference in the behavior of the Seebeck coefficient, S in
the vicinity of the Quantum Critical Point (QCP) in the two systems. As the
critical field is approached, S/T enhances in beta-YbAlB4 but is drastically
reduced in YbRh2Si2. While in the former system, the ratio of
thermopower-to-specific heat remains constant, it drastically drops near the
QCP in YbRh2Si2. In both systems, on the other hand, the Nernst coefficient
shows a diverging behavior near the QCP. The results provide a new window to
the way various energy scales of the system behave and eventually vanish near a
QCP
Bose-Einstein condensation and entanglement in magnetic systems
We present a study of magnetic field induced quantum phase transitions in
insulating systems. A generalized scaling theory is used to obtain the
temperature dependence of several physical quantities along the quantum
critical trajectory (, ) where is a longitudinal external
magnetic field and the critical value at which the transition occurs.
We consider transitions from a spin liquid at a critical field and
from a fully polarized paramagnet, at , into phases with long range
order in the transverse components. The transitions at and
can be viewed as Bose-Einstein condensations of magnons which however belong to
different universality classes since they have different values of the dynamic
critical exponent . Finally, we use that the magnetic susceptibility is an
entanglement witness to discuss how this type of correlation sets in as the
system approaches the quantum critical point along the critical trajectory,
, .Comment: 7 pages, 1 Table; accepted version; changes in text and new
reference
Representation of Sound Objects within Early-Stage Auditory Areas: A Repetition Effect Study Using 7T fMRI.
Environmental sounds are highly complex stimuli whose recognition depends on the interaction of top-down and bottom-up processes in the brain. Their semantic representations were shown to yield repetition suppression effects, i. e. a decrease in activity during exposure to a sound that is perceived as belonging to the same source as a preceding sound. Making use of the high spatial resolution of 7T fMRI we have investigated the representations of sound objects within early-stage auditory areas on the supratemporal plane. The primary auditory cortex was identified by means of tonotopic mapping and the non-primary areas by comparison with previous histological studies. Repeated presentations of different exemplars of the same sound source, as compared to the presentation of different sound sources, yielded significant repetition suppression effects within a subset of early-stage areas. This effect was found within the right hemisphere in primary areas A1 and R as well as two non-primary areas on the antero-medial part of the planum temporale, and within the left hemisphere in A1 and a non-primary area on the medial part of Heschl's gyrus. Thus, several, but not all early-stage auditory areas encode the meaning of environmental sounds
Quantum replica approach to the under-screened Kondo model
We extend the Schwinger boson large N treatment of the underscreened Kondo
model in a way that correctly captures the finite elastic phase shift in the
singular Fermi liquid. The new feature of the approach, is the introduction of
a flavor quantum number with K possible values, associated with the Schwinger
boson representation. The large N limit is taken maintaining the ratio k=K/N
fixed. This approach differs from previous approaches, in that we do not
explicitly enforce a constraint on the spin representation of the Schwinger
bosons. Instead, the energetics of the Kondo model cause the bosonic degrees of
freedom to ``self assemble'' into a ground-state in which the spins of K bosons
and N-K conduction electrons are antisymmetrically arranged into a Kondo
singlet. With this device, the large N limit can be taken, in such a way that a
fraction K/N of the Abrikosov Suhl resonance is immersed inside the Fermi sea.
We show how this method can be used to model the full energy dependence of the
singular Abrikosov Suhl resonance in the underscreened Kondo model and the
field-dependent magnetization.Comment: Revised draft, with plots explicitly showing logarithmic scaling of
inverse coupling constant. Small corrections prior to submission to journa
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