5,081 research outputs found
Heat Capacity study of -FeSi single crystals
Heat Capacity of needle-like [length=5mm, diameter=1 mm] -FeSi
single crystal, grown by chemical vapor transport has been measured. Two
anomalies are found, a broad deviation centered around 160 K and a clear
deviation at a temperature of 255 K approximately. We have attempted to relate
these to the anomalies previously reported in the case of the resistivity data.
The Transient Thermoelectric Effect [TTE] results lead us to the inference that
the system under goes from single carrier system to at least two carrier system
at 220 K-our heat capacity results seem to provide further independent evidence
for this transition in this system.Comment: 8 pages, 5 figure
Drosophila Chitinase 2 is expressed in chitin producing organs for cuticle formation.
The architecture of the outer body wall cuticle is fundamental to protect arthropods against invading pathogens and numerous other harmful stresses. Such robust cuticles are formed by parallel running chitin microfibrils. Molting and also local wounding leads to dynamic assembly and disassembly of the chitin-matrix throughout development. However, the underlying molecular mechanisms that organize proper chitin-matrix formation are poorly known. Recently we identified a key region for cuticle thickening at the apical cell surface, the cuticle assembly zone, where Obstructor-A (Obst-A) coordinates the formation of the chitin-matrix. Obst-A binds chitin and the deacetylase Serpentine (Serp) in a core complex, which is required for chitin-matrix maturation and preservation. Here we present evidence that Chitinase 2 (Cht2) could be essential for this molecular machinery. We show that Cht2 is expressed in the chitin-matrix of epidermis, trachea, and the digestive system. There, Cht2 is enriched at the apical cell surface and the dense chitin-matrix. We further show that in Cht2 knockdown larvae the assembly zone is rudimentary, preventing normal cuticle formation and pore canal organization. As sequence similarities of Cht2 and the core complex proteins indicate evolutionarily conserved molecular mechanisms, our findings suggest that Cht2 is involved in chitin formation also in other insects
Formation of metallic magnetic clusters in a Kondo-lattice metal: Evidence from an optical study
Magnetic materials are usually divided into two classes: those with localised
magnetic moments, and those with itinerant charge carriers. We present a
comprehensive experimental (spectroscopic ellipsomerty) and theoretical study
to demonstrate that these two types of magnetism do not only coexist but
complement each other in the Kondo-lattice metal, Tb2PdSi3. In this material
the itinerant charge carriers interact with large localised magnetic moments of
Tb(4f) states, forming complex magnetic lattices at low temperatures, which we
associate with self-organisation of magnetic clusters. The formation of
magnetic clusters results in low-energy optical spectral weight shifts, which
correspond to opening of the pseudogap in the conduction band of the itinerant
charge carriers and development of the low- and high-spin intersite electronic
transitions. This phenomenon, driven by self-trapping of electrons by magnetic
fluctuations, could be common in correlated metals, including besides
Kondo-lattice metals, Fe-based and cuprate superconductors.Comment: 30 pages, 6 Figure
Spectroscopy of horizontal branch stars in NGC6752 - Anomalous results on atmospheric parameters and masses
We used the ESO VLT-FORS2 facility to collect low-resolution spectra of 51
targets distributed along the Horizontal Branch. We determined atmospheric
parameters by comparison with theoretical models through standard fitting
routines, and masses by basic equations. Results are in general in good
agreement with previous works, although not always with theoretical
expectations for cooler stars (Teff<15000 K). The calculated color excess is
systematically lower than literature values, pointing towards a possible
underestimation of effective temperatures. Moreover, we find two groups of
stars at Teff=14000 K and at Teff=27000$ K that present anomalies with respect
to the general trend and expectations. We suppose that the three peculiar
bright stars at Teff=14000 K are probably affected by an enhanced stellar wind.
For the eight Extreme Horizontal Branch stars at Teff=27000 K which show
unusually high masses we find no plausible explanation. While most of our
results agree well with the predictions of standard horizontal branch
evolution, we still have problems with the low masses we derive in certain
temperature ranges. We believe that Kurucz ATLAS9 LTE model atmospheres with
solar-scaled abundances are probably inadequate for these temperature ranges.
Concerning the group of anomalous stars at Teff=27000 K, a Kolmogorov-Smirnov
test indicates that there is only an 8.4% probability that these stars are
randomly drawn from the general distribution in the color-magnitude diagram.
This is not conclusive but points out that these stars could be both (and
independently) spectroscopically and photometrically peculiar with respect to
the general Extreme Horizontal Branch population.Comment: 12 pages, 10 figures, accepted for pubblication in A&A. Replaced for
typos and better LaTeX outpu
Superconductivity of epsilon-Fe: complete resistive transition
Last year, iron was reported to become superconducting at temperatures below
2K and pressures between 15 and 30 GPa. The evidence presented was a weak
resistivity drop, suppressed by a magnetic field above 0.2 T, and a small
Meissner signal. However, a compelling demonstration, such as the occurrence of
zero resistance, was lacking. Here we report the measurement of a complete
resistive transition at 22.2 GPa with an onset slightly above 2 K in two very
pure samples of iron, of different origins. The superconductivity appears
unusually sensitive to disorder, developing only when the electronic mean free
path is above a threshold value, while the normal state resistivity is
characteristic of a nearly ferromagnetic metal.Comment: 4 pages, 4 figures. To be published in Physics Letters
Preliminary studies for anapole moment measurements in rubidium and francium
Preparations for the anapole measurement in Fr indicate the possibility of
performing a similar measurement in a chain of Rb. The sensitivity analysis
based on a single nucleon model shows the potential for placing strong limits
on the nucleon weak interaction parameters. There are values of the magnetic
fields at much lower values than found before that are insensitive to first
order changes in the field. The anapole moment effect in Rb corresponds to an
equivalent electric field that is eighty times smaller than Fr, but the
stability of the isotopes and the current performance of the dipole trap in the
apparatus, presented here, are encouraging for pursuing the measurment.Comment: 16 pages, 6 figures. Accepted for publication in the J. Phys.
Universal linear-temperature dependence of static magnetic susceptibility in iron-pnictides
A universal linear-temperature dependence of the uniform magnetic
susceptibility has been observed in the nonmagnetic normal state of
iron-pnictides. This non-Pauli and non-Curie-Weiss-like paramagnetic behavior
cannot be understood within a pure itinerant picture. We argue that it results
from the existence of a wide antiferromagnetic fluctuation window in which the
local spin-density-wave correlations exist but the global directional order has
not been established yet.Comment: 4 pages, 2 figure
Upper critical field measurements up to 60 T in arsenic-deficient LaO_(0.9)F_(0.1)FeAs_(1-delta): Pauli limiting behaviour at high fields vs improved superconductivity at low fields
We report resistivity and upper critical field B_c2(T) data for As deficient
LaO_(0.9)F_(0.1)FeAs_(1-delta) in a wide temperature and high field range up to
60 T. These disordered samples exhibit a slightly enhanced superconducting
transition at T_c = 29 K and a significantly enlarged slope dB_(c2))/dT = -5.4
T/K near T_c which contrasts with a flattening of B_(c2)(T) starting near 23 K
above 30 T. This flattening is interpreted as Pauli limiting behaviour (PLB)
with B_(c2)(0) approx 63 T. We compare our results with B_(c2)(T)-data reported
in the literature for clean and disordered samples. Whereas clean samples show
no PLB for fields below 60 to 70 T, the hitherto unexplained flattening of
B_(c2)(T) for applied fields H || ab observed for several disordered closely
related systems is interpreted also as a manifestation of PLB. Consequences of
our results are discussed in terms of disorder effects within the frame of
conventional and unconventional superconductivity.Comment: 4 pages, 3 figures, submitted to RHMF09 (9th International Conference
on the Research in High Magnetic Fields), Dresden, July 22-25, 200
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