3,658 research outputs found
Fe-spin reorientation in PrFeAsO : Evidences from resistivity and specific heat studies
We report the magnetic field dependence of resistivity () and specific
heat () for the non-superconducting PrFeAsO compound. Our study shows a
hitherto unobserved anomaly at in the resistivity and specific heat
data which arises as a result of the interplay of antiferromagnetic (AFM) Pr
and Fe sublattices. Below the AFM transition temperature (), Pr
moment orders along the crystallographic c axis and its effect on the iron
subsystem causes a reorientation of the ordered inplane Fe moments in a
direction out of the plane. Application of magnetic field introduces
disorder in the AFM Pr sublattice, which, in turn, reduces the out-of-plane
Pr-Fe exchange interaction responsible for Fe spin reorientation. Both in
() and curves, the peak at broadens with the
increase of due to the introduction of the disorder in the AFM Pr
sublattice by magnetic field. In () curve, the peak shifts towards
lower temperature with and disappears above 6 T while in curve
the peak remains visible up to 14 T. The broadening of the anomaly at
in with increasing further confirms that magnetic
field induces disorder in the AFM Pr sublattice.Comment: 8 pages, 10 Figure
Quantm Magnetoresistance of the PrFeAsO oxypnictides
We report the observation of an unusual dependence of transverse
magnetoresistance (MR) in the PrFeAsO, one of the parent compound of pnictide
superconductors. Below the spin density wave transition, MR is large, positive
and increases with decreasing temperature. At low temperatures, MR increases
linearly with up to 14 T. For 40 K, MR vs curve develops a
weak curvature in the low-field region which indicates a crossover from
linear to dependence as 0. The linear MR originates
from the Dirac cone states and has been explained by the quantum mechanical
model proposed by Abrikosov.Comment: accepted for publication in Appl. Phys. Let
Excess compressibility and excess volume studies in the binary mixtures of methyl and ethyl acetate in O-chlorophenol at different temperatures
Ultrasonic velocity and density measurements have been carried out in the binary mixtures of methyl acetate (MA) and ethyl acetate (EA) in O-chlorophenol (OCP) at 303.15, 308.15, 313.15, 318.15 and 323.15 K. From the experimental data, the excess thermodynamical parameters such as excess adiabatic compressibility (β), excess intermolecular free length (L) and excess molar volume (VE) have been calculated. The results have been interpreted in terms of intermolecular interactions leading to complex formation through the formation of hydrogen bonds between the component molecules of the mixture
Performance of distributed mechanisms for flow admission in wireless adhoc networks
Given a wireless network where some pairs of communication links interfere
with each other, we study sufficient conditions for determining whether a given
set of minimum bandwidth quality-of-service (QoS) requirements can be
satisfied. We are especially interested in algorithms which have low
communication overhead and low processing complexity. The interference in the
network is modeled using a conflict graph whose vertices correspond to the
communication links in the network. Two links are adjacent in this graph if and
only if they interfere with each other due to being in the same vicinity and
hence cannot be simultaneously active. The problem of scheduling the
transmission of the various links is then essentially a fractional, weighted
vertex coloring problem, for which upper bounds on the fractional chromatic
number are sought using only localized information. We recall some distributed
algorithms for this problem, and then assess their worst-case performance. Our
results on this fundamental problem imply that for some well known classes of
networks and interference models, the performance of these distributed
algorithms is within a bounded factor away from that of an optimal, centralized
algorithm. The performance bounds are simple expressions in terms of graph
invariants. It is seen that the induced star number of a network plays an
important role in the design and performance of such networks.Comment: 21 pages, submitted. Journal version of arXiv:0906.378
Evaluating the LRFD Factor for Cold-formed Steel Compression Members
This paper summarizes recent work to determine if the LRFD resistance factor for cold-formed steel compression member s can be increased above its current value of φ c =0.85. An experimental database of 675 concentrically loaded columns with plain and lipped C-sections, plain and lipped Z-sections, hat sections and angle sections, including members with holes was compiled. The predicted strength of each specimen was calculated with the AISI-S100-07 Main Specification and Direct Strength Method (DSM). Test-to-predicted strength statistics were employed with the first order second moment reliability approach in AISI-S100-07 Chapter F to calculate the resistance factors. The observed trends demonstrate that DSM is a more accurate strength predictor than the current Main Specifica tion, especially for columns with partially effective cross sections. Serious consideration should be given to replacing the Main Specification with DSM, which would provide improved prediction accuracy and a viable rationale for increasing the resistance factor. The test-to-predicted strength ratios for columns with plain and lipped angle cross-sections exhibit a high coefficient of variation and b ecome increasingly conservative with increasing global slenderness. Fundamen tal research on the mechanics of angle compression members is needed to improve existing design methods
Anomalous thermal expansion of SbTe topological insulator
We have investigated the temperature dependence of the linear thermal
expansion along the hexagonal c axis (), in-plane resistivity
(), and specific heat () of the topological insulator SbTe
single crystal. exhibits a clear anomaly in the temperature region
204-236 K. The coefficient of linear thermal expansion decreases
rapidly above 204 K, passes through a deep minimum at around 225 K and then
increases abruptly in the region 225-236 K. is negative in the
interval 221-228 K. The temperature dependence of both and can
be described well by the Debye model from 2 to 290 K, excluding the region
around the anomaly in
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