7,452 research outputs found
Electron-Phonon Coupling Origin of the resistivity in YNi_{2}B_{2}C Single Crystals
Resistivity measurements from 4.2 K up to 300 K were made on YNi_{2}B_{2}C
single crystals with Tc=15.5 K. The resulting rho(T) curve shows a perfect
Bloch-Grueneisen (BG) behavior, with a very small residual resistivity which
indicates the low impurity content and the high cristallographic quality of the
samples. The value lambda_{tr}=0.53 for the transport electron-phonon coupling
constant was obtained by using the high-temperature constant value of d(rho)/dT
and the plasma frequency reported in literature. The BG expression for the
phononic part of the resistivity rho_{ph}(T) was then used to fit the data in
the whole temperature range, by approximating alpha^{2}_{tr}F(Omega) with the
experimental phonon spectral density G(Omega) multiplied by a two-step
weighting function to be determined by the fit. The resulting fitting curve
perfectly agrees with the experimental points. We also solved the real-axis
Eliashberg equations in both s- and d-wave symmetries under the approximation
alpha^{2}F(Omega)= alpha^{2}_{tr}F(Omega). We found that the value of
lambda_{tr} here determined in single-band approximation is quite compatible
with Tc and the gap Delta experimentally observed. Finally, we calculated the
normalized tunneling conductance, whose comparison with break-junction tunnel
data gives indication of the possible s-wave symmetry for the order parameter
in YNi_{2}B_{2}C.Comment: 6 pages, 5 figures. Proceedings of SATT10 Conference, to be published
in Int. J. Mod. Phys.
D-branes and matrix factorisations in supersymmetric coset models
Matrix factorisations describe B-type boundary conditions in N=2
supersymmetric Landau-Ginzburg models. At the infrared fixed point, they
correspond to superconformal boundary states. We investigate the relation
between boundary states and matrix factorisations in the Grassmannian
Kazama-Suzuki coset models. For the first non-minimal series, i.e. for the
models of type SU(3)_k/U(2), we identify matrix factorisations for a subset of
the maximally symmetric boundary states. This set provides a basis for the RR
charge lattice, and can be used to generate (presumably all) other boundary
states by tachyon condensation.Comment: 63 pages, 2 figure
Derivation of a dynamic model of the kinetics of nitrogen uptake throughout the growth of lettuce : calibration and validation
A kinetic model of nitrogen (N) uptake throughout growth was developed for lettuce
cultivated in nutrient solution under varying natural light conditions. The model couples
nitrogen uptake with dry matter accumulation using a two-compartment mechanistic
approach, incorporating structural and non-structural pools. Maximum nitrogen uptake
rates are assumed to decline with shoot dry weight, to allow for the effects of plant
age. The model was parameterized using data from the literature, and calibrated for
differences in light intensity using an optimization algorithm utilizing data from three
experiments in different growing seasons. The calibrated model was validated against
the data from two independent experiments conducted under different light conditions.
Results showed that the model made good predictions of nitrogen uptake by plants from
seedlings to maturity under fluctuating light levels in a glasshouse. Plants grown at a
higher light intensity showed larger maximum nitrogen uptake rates, but the effect of
light intensity declined towards plant maturity
Pr magnetism and its interplay with the Fe spin density wave in PrFeAsO
We have studied the magnetism of the Pr3+ ions in PrFeAsO_1-xF_x (x = 0;
0.15) and its interaction with the Fe magnetic order (for x = 0). Specific heat
data confirm the presence of a first excited crystal electric field (CEF) level
around 3.5 meV in the undoped compound PrFeAsO. This finding is in agreement
with recent neutron scattering experiments. The doped compound is found to have
a much lower first CEF splitting of about 2.0 meV. The Pr ordering in PrFeAsO
gives rise to large anomalies in the specific heat and the thermal expansion
coefficient. In addition, a field-induced transition is found at low
temperatures that is most pronounced for the magnetostriction coefficient. This
transition, which is absent in the doped compound, is attributed to a reversal
of the Fe spin canting as the antiferromagnetic Pr order is destroyed by the
external magnetic field.Comment: 8 pages, 6 figure
Field-induced gapless electron pocket in the superconducting vortex phase of YNi2B2C as probed by magnetoacoustic quantum oscillations
By use of ultrasound studies we resolved magneto-acoustic quantum oscillation
deep into the mixed state of the multiband nonmagnetic superconductor YNi2B2C.
Below the upper critical field, only a very weak additional damping appears
that can be well explained by the field inhomogeneity caused by the flux-line
lattice in the mixed state. This is clear evidence for no or a vanishingly
small gap for one of the bands, namely, the spheroidal alpha band. This
contrasts de Haas--van Alphen data obtained by use of torque magnetometry for
the same sample, with a rapidly vanishing oscillation signal in the mixed
state. This points to a strongly distorted flux-line lattice in the latter case
that, in general, can hamper a reliable extraction of gap parameters by use of
such techniques.Comment: 6 pages, 6 figure
Rotations and Abundances of Blue Horizontal-Branch Stars in Globular Cluster M15
High-resolution optical spectra of eighteen blue horizontal-branch (BHB)
stars in the globular cluster M15 indicate that their stellar rotation rates
and photospheric compositions vary strongly as a function of effective
temperature. Among the cooler stars in the sample, at Teff ~ 8500 K, metal
abundances are in rough agreement with the canonical cluster metallicity, and
the v sin i rotations appear to have a bimodal distribution, with eight stars
at v sin i < 15 km/s and two stars at v sin i ~ 35 km/s. Most of the stars at
Teff > 10000 K, however, are slowly rotating, v sin i < 7 km/s, and their iron
and titanium are enhanced by a factor of 300 to solar abundance levels.
Magnesium maintains a nearly constant abundance over the entire range of Teff,
and helium is depleted by factors of 10 to 30 in three of the hotter stars.
Diffusion effects in the stellar atmospheres are the most likely explanation
for these large differences in composition. Our results are qualitatively very
similar to those previously reported for M13 and NGC 6752, but with even larger
enhancement amplitudes, presumably due to the increased efficiency of radiative
levitation at lower intrinsic [Fe/H]. We also see evidence for faster stellar
rotation explicitly preventing the onset of the diffusion mechanisms among a
subset of the hotter stars.Comment: 11 pages, 1 figure, 1 table, accepted to ApJ
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