5,829 research outputs found
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
Chemical and kinematical properties of BSSs and HB stars in NGC 6397
We used three sets of high-resolution spectra acquired with the multifiber
facility FLAMES at the Very Large Telescope of the European Southern
Observatory to investigate the chemical and kinematical properties of a sample
of 42 horizontal branch (HB) stars, 18 Blue Straggler Stars (BSSs) and 86 main
sequence turn-off and sub-giant branch stars in the nearby globular cluster NGC
6397. We measured rotational velocities and Fe, O and Mg abundances. All the
unevolved stars in our sample turn out to have low rotational velocites (v sin
i< 10\kms), while HB stars and BSSs show a broad distribution, with values
ranging from 0 to 70 \kms. For HB stars with T<10500 K there is a clear
temperature-oxygen anti-correlation, that can be understood if the star
position along the HB is mainly determined by the He content. The hottest BSSs
and HB stars (with temperatures T>8200 K and T> 10500 K, respectively) also
show significant deviations in their iron abundance with respect to the cluster
metallicity (as traced by the unevolved stars, [Fe/H]=-2.12). While similar
chemical patterns have been already observed in other hot HB stars, this is the
first evidence ever collected for BSSs. We interprete these abundance anomalies
as due to the metal radiative levitation, occurring in stars with shallow or no
convective envelopes
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.
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
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