634 research outputs found
Growth of a smooth CaF 2 layer on NdFeAsO thin film
We studied the method to grow a smooth and flat CaF 2 layer on NdFeAsO thin films since CaF 2 is a promising candidate material for the barrier layer of a superconducting junction. When the CaF 2 layer was grown at 800°C, the surface was very rough because {111} facets had grown preferentially. However, when CaF 2 was grown at lower temperatures and post-annealed in situ at 800°C for 30 min the facets were eliminated and a CaF 2 layer with a smooth surface was obtained. Fluorine diffusing from CaF 2 into NdFeAsO was observed when CaF 2 was grown at high temperatures, but the diffusion was suppressed by lowering the growth temperature to 400°C
Reduction of the Superfluid Density in the Vortex-Liquid Phase of Bi2Sr2CaCu2Oy
In-plane complex surface impedance of a Bi2Sr2CaCu2Oy single crystal was
measured in the mixed state at 40.8 GHz.The surface reactance, which is
proportional to the real part of the effective penetration depth, increased
rapidly just above the first-order vortex-lattice melting transition field and
the second magnetization peak field.This increase is ascribed to the decrease
in the superfluid density rather than the loss of pinning.This result indicates
that the vortex melting transition changes the electronic structure as well as
the vortex structure.Comment: 4 pages, 4 figures, accepted for publication in Phys. Rev. Let
Free flux flow resistivity in strongly overdoped high-T_c cuprate; purely viscous motion of the vortices in semiclassical d-wave superconductor
We report the free flux flow (FFF) resistivity associated with a purely
viscous motion of the vortices in moderately clean d-wave superconductor
Bi:2201 in the strongly overdoped regime (T_c=16K) for a wide range of the
magnetic field in the vortex state. The FFF resistivity is obtained by
measuring the microwave surface impedance at different microwave frequencies.
It is found that the FFF resistivity is remarkably different from that of
conventional s-wave superconductors. At low fields (H<0.2H_c2) the FFF
resistivity increases linearly with H with a coefficient which is far larger
than that found in conventional s-wave superconductors. At higher fields, the
FFF resistivity increases in proportion to \sqrt H up to H_c2. Based on these
results, the energy dissipation mechanism associated with the viscous vortex
motion in "semiclassical" d-wave superconductors with gap nodes is discussed.
Two possible scenarios are put forth for these field dependence; the
enhancement of the quasiparticle relaxation rate and the reduction of the
number of the quasiparticles participating the energy dissipation in d-wave
vortex state.Comment: 9 pages 7 figures, to appear in Phys. Rev.
Chemical Evolution of the Galaxy Based on the Oscillatory Star Formation History
We model the star formation history (SFH) and the chemical evolution of the
Galactic disk by combining an infall model and a limit-cycle model of the
interstellar medium (ISM). Recent observations have shown that the SFH of the
Galactic disk violently variates or oscillates. We model the oscillatory SFH
based on the limit-cycle behavior of the fractional masses of three components
of the ISM. The observed period of the oscillation ( Gyr) is reproduced
within the natural parameter range. This means that we can interpret the
oscillatory SFH as the limit-cycle behavior of the ISM. We then test the
chemical evolution of stars and gas in the framework of the limit-cycle model,
since the oscillatory behavior of the SFH may cause an oscillatory evolution of
the metallicity. We find however that the oscillatory behavior of metallicity
is not prominent because the metallicity reflects the past integrated SFH. This
indicates that the metallicity cannot be used to distinguish an oscillatory SFH
from one without oscillations.Comment: 21 pages LaTeX, to appear in Ap
Self-enrichment in globular clusters. I. An analytic approach
By means of analytical calculations, we explore the self-enrichment scenario
for Globular Cluster formation. According to this scenario, an initial burst of
star formation occurs inside the core radius of the initial gaseous
distribution. The outward-propagating shock wave sweeps up a shell in which
gravitational instabilities may arise, leading to the formation of a second,
metal-enriched, population of stars. We find a minimum mass of the
proto-globular cluster of the order of 10^6 solar masses. We also find that the
observed spread in the Magnitude-Metallicity relation can be explained assuming
cluster-to-cluster variations of some parameters like the thermalization
efficiency, the mixing efficiency and the Initial Mass Function, as well as
variations of the external pressure.Comment: 12 pages, 13 figures, accepted for publication in A&
Selective mass enhancement close to the quantum critical point in BaFeâ(Asâââ Pâ )â
A quantum critical point (QCP) is currently being conjectured for the BaFeâ(Asâââ Pâ )â system at the
critical value xc â 0.3. In the proximity of a QCP, all thermodynamic and transport properties are
expected to scale with a single characteristic energy, given by the quantum fluctuations. Such a
universal behavior has not, however, been found in the superconducting upper critical field Hc2. Here
we report Hc2 data for epitaxial thin films extracted from the electrical resistance measured in very
high magnetic fields up to 67 Tesla. Using a multi-band analysis we find that Hcâ is sensitive to the QCP,
implying a significant charge carrier effective mass enhancement at the doping-induced QCP that is
essentially band-dependent. Our results point to two qualitatively different groups of electrons in
BaFeâ(Asâââ Pâ )â . The first one (possibly associated to hot spots or whole Fermi sheets) has a strong mass
enhancement at the QCP, and the second one is insensitive to the QCP. The observed duality could also
be present in many other quantum critical systems
Charged Vortices in High Temperature Superconductors Probed by NMR
We report a first experimental evidence that a vortex in the high temperature
superconductors (HTSC) traps a finite electric charge from the high resolution
measurements of the nuclear quadrupole frequencies. In slightly overdoped
YBa_2Cu_3O_7 the vortex is negatively charged by trapping electrons, while in
underdoped YBa_2Cu_4O_8 it is positively charged by expelling electrons. The
sign of the trapped charge is opposite to the sign predicted by the
conventional BCS theory. Moreover, in both materials, the deviation of the
magnitude of the charge from the theory is also significant. These unexpected
features can be attributed to the novel electronic structure of the vortex in
HTSC.Comment: 6 pages, 7 figures, to be published in Phys Rev.
Full-Fledged Dwarf Irregular Galaxy Leo A
We have studied Leo A - the isolated and extremely gas rich dwarf irregular
galaxy of very low stellar mass and metallicity. Ages of the stellar
populations in Leo A are ranging from ~10 Myr to ~10 Gyr. Here we report the
discovery of an old stellar halo and a sharp stellar edge. Also we find the
distribution of stars extending beyond the gaseous envelope of the galaxy.
Therefore, Leo A by its structure as well as stellar and gaseous content is
found to resemble massive disk galaxies. This implies that galaxies of very low
stellar mass are also able to develop complex structures, challenging
contemporary understanding of galaxy evolution.Comment: 9 pages, 3 figures, submitted to ApJ
- âŠ