1,101 research outputs found
OH(1720 MHz) Masers As Signposts of Molecular Shocks
We present observations of molecular gas made with the 15-m James Clark
Maxwell Telescope toward the sites of OH(1720 MHz) masers in three supernova
remnants: W28, W44 and 3C391. Maps made in the 12CO J=3-2 line reveal that the
OH masers are preferentially located along the edges of thin filaments or
clumps of molecular gas. There is a strong correlation between the morphology
of the molecular gas and the relativistic gas traced by synchrotron emission at
centimeter wavelengths. Broad CO line widths (dV=30-50 km/s) are seen along
these gaseous ridges, while narrow lines are seen off the ridges. The ratio of
H2CO line strengths is used to determine temperatures in the broad-line gas of
80 K, and the 13CO J=3-2 column density suggests densities of 10^4-10^5 cm{-3}.
These observations support the hypothesis that the OH(1720 MHz) masers
originate in post-shock gas, heated by the passage of a supernova remnant shock
through dense molecular gas. From the observational constraints on the density,
velocity and magnetic field we examine the physical properties of the shock and
discuss the shock-production of OH. These OH(1720 MHz) masers are useful
``signposts'', which point to the most promising locations to study supernova
remnant/molecular cloud interactions.Comment: ApJ (in press
Symmetry Analysis of Second Harmonic Generation at Surfaces of Antiferromagnets
Using group theory we classify the nonlinear magneto-optical response at
low-index surfaces of fcc antiferromagnets, such as NiO. Structures consisting
of one atomic layer are discussed in detail. We find that optical second
harmonic generation is sensitive to surface antiferromagnetism in many cases.
We discuss the influence of a second type of magnetic atoms, and also of a
possible oxygen sublattice distortion on the output signal. Finally, our
symmetry analysis yields the possibility of antiferromagnetic surface domain
imaging even in the presence of magnetic unit-cell doubling.Comment: 23 pages, 10 figures incorporated. Accepted to Phys. Rev. B,
scheduled for July'9
Residual meson-meson interaction from lattice gauge simulation in a simple QED model
The residual interaction for a meson-meson system is computed utilizing the
cumulant, or cluster, expansion of the momentum-space time correlation matrix.
The cumulant expansion serves to define asymptotic, or free, meson-meson
operators. The definition of an effective interaction is then based on a
comparison of the full (interacting) and the free (noninteracting) time
correlation matrices. The proposed method, which may straightforwardly be
transcribed to other hadron-hadron systems, here is applied to a simple 2+1
dimensional U(1) lattice gauge model tuned such that it is confining. Fermions
are treated in the staggered scheme. The effective interaction exhibits a
repulsive core and attraction at intermediate relative distances. These
findings are consistent with an earlier study of the same model utilizing
L\"{u}scher's method where scattering phase shifts are obtained directly.Comment: 28 pages, compressed postscript fil
We have to go back: A Historic IP Attribution Service for Network Measurement
Researchers and practitioners often face the issue of having to attribute anIP address to an organization. For current data this is comparably easy, usingservices like whois or other databases. Similarly, for historic data, severalentities like the RIPE NCC provide websites that provide access to historicrecords. For large-scale network measurement work, though, researchers oftenhave to attribute millions of addresses. For current data, Team Cymru providesa bulk whois service which allows bulk address attribution. However, at thetime of writing, there is no service available that allows historic bulkattribution of IP addresses. Hence, in this paper, we introduce and evaluateour `Back-to-the-Future whois' service, allowing historic bulk attribution ofIP addresses on a daily granularity based on CAIDA Routeviews aggregates. Weprovide this service to the community for free, and also share ourimplementation so researchers can run instances themselves.<br
{SoK}: {An} Analysis of Protocol Design: Avoiding Traps for Implementation and Deployment
Today's Internet utilizes a multitude of different protocols. While some of these protocols were first implemented and used and later documented, other were first specified and then implemented. Regardless of how protocols came to be, their definitions can contain traps that lead to insecure implementations or deployments. A classical example is insufficiently strict authentication requirements in a protocol specification. The resulting Misconfigurations, i.e., not enabling strong authentication, are common root causes for Internet security incidents. Indeed, Internet protocols have been commonly designed without security in mind which leads to a multitude of misconfiguration traps. While this is slowly changing, to strict security considerations can have a similarly bad effect. Due to complex implementations and insufficient documentation, security features may remain unused, leaving deployments vulnerable. In this paper we provide a systematization of the security traps found in common Internet protocols. By separating protocols in four classes we identify major factors that lead to common security traps. These insights together with observations about end-user centric usability and security by default are then used to derive recommendations for improving existing and designing new protocols---without such security sensitive traps for operators, implementors and users
{SoK}: {An} Analysis of Protocol Design: Avoiding Traps for Implementation and Deployment
Today's Internet utilizes a multitude of different protocols. While some of these protocols were first implemented and used and later documented, other were first specified and then implemented. Regardless of how protocols came to be, their definitions can contain traps that lead to insecure implementations or deployments. A classical example is insufficiently strict authentication requirements in a protocol specification. The resulting Misconfigurations, i.e., not enabling strong authentication, are common root causes for Internet security incidents. Indeed, Internet protocols have been commonly designed without security in mind which leads to a multitude of misconfiguration traps. While this is slowly changing, to strict security considerations can have a similarly bad effect. Due to complex implementations and insufficient documentation, security features may remain unused, leaving deployments vulnerable. In this paper we provide a systematization of the security traps found in common Internet protocols. By separating protocols in four classes we identify major factors that lead to common security traps. These insights together with observations about end-user centric usability and security by default are then used to derive recommendations for improving existing and designing new protocols---without such security sensitive traps for operators, implementors and users
Charge density waves enhance the electronic noise of manganites
The transport and noise properties of Pr_{0.7}Ca_{0.3}MnO_{3} epitaxial thin
films in the temperature range from room temperature to 160 K are reported. It
is shown that both the broadband 1/f noise properties and the dependence of
resistance on electric field are consistent with the idea of a collective
electrical transport, as in the classical model of sliding charge density
waves. On the other hand, the observations cannot be reconciled with standard
models of charge ordering and charge melting. Methodologically, it is proposed
to consider noise-spectra analysis as a unique tool for the identification of
the transport mechanism in such highly correlated systems. On the basis of the
results, the electrical transport is envisaged as one of the most effective
ways to understand the nature of the insulating, charge-modulated ground states
in manganites.Comment: 6 two-column pages, 5 figure
The complex multiferroic phase diagram of MnCoWO
The complete magnetic and multiferroic phase diagram of
MnCoWO single crystals is investigated by means of magnetic,
heat capacity, and polarization experiments. We show that the ferroelectric
polarization in the multiferroic state abruptly changes
its direction twice upon increasing Co content, x. At x=0.075,
rotates from the axis into the plane and at
x=0.15 it flips back to the axis. The origin of the multiple
polarization flops is identified as an effect of the Co anisotropy on the
orientation and shape of the spin helix leading to thermodynamic instabilities
caused by the decrease of the magnitude of the polarization in the
corresponding phases. A qualitative description of the ferroelectric
polarization is derived by taking into account the intrachain (axis) as
well as the interchain (axis) exchange pathways connecting the magnetic
ions. In a narrow Co concentration range (0.1x0.15), an
intermediate phase, sandwiched between the collinear high-temperature and the
helical low-temperature phases, is discovered. The new phase exhibits a
collinear and commensurate spin modulation similar to the low-temperature
magnetic structure of MnWO.Comment: 18 pages, 6 figure
Magnetodielectric effect and optic soft mode behaviour in quantum paraelectric EuTiO3 ceramics
Infrared reflectivity and time-domain terahertz transmission spectra of
EuTiO3 ceramics revealed a polar optic phonon at 6 - 300K, whose softening is
fully responsible for the recently observed quantum paraelectric behaviour.
Even if our EuTiO3 ceramics show lower permittivity than the single crystal due
to a reduced density and/or small amount of secondary pyrochlore Eu2Ti2O7
phase, we confirmed the magnetic field dependence of the permittivity, also
slightly smaller than in single crystal. Attempt to reveal the soft phonon
dependence at 1.8K on the magnetic field up to 13T remained below the accuracy
of our infrared reflectivity experiment
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