10,408 research outputs found
Ferroelectricity and structure of BaTiO3 grown on YBa2Cu3O7-d thin films
We have investigated the crystal structure and the ferroelectric properties
of BaTiO3 thin films with YBa2Cu3O7-d as the bottom and Au as the top
electrode. Epitaxial heterostructures of YBa2Cu3O7-d and BaTiO3 were prepared
by dc and rf sputtering, respectively. The crystal structure of the films was
characterised by x-ray diffraction. The ferroelectric behaviour of the BaTiO3
films was confirmed by hysteresis loop measurements using a Sawyer Tower
circuit. We obtain a coercive field of 30 kV/cm and a remanent polarisation of
1.25 \muC/cm. At sub-switching fields the capacitance of the films obeys a
relation analogous to the Rayleigh law. This behaviour indicates an interaction
of domain walls with randomly distributed pinning centres. At a field of 5 MV/m
we calculate 3% contribution of irreversible domain wall motion to the total
dielectric constant.Comment: 12 pages and 9 figure
Medium-separation binaries do not affect the first steps of planet formation
The first steps of planet formation are marked by the growth and
crystallization of sub-micrometer-sized dust grains accompanied by dust
settling toward the disk midplane. In this paper we explore whether the first
steps of planet formation are affected by the presence of medium-separation
stellar companions. We selected two large samples of disks around single and
binary T Tauri stars in Taurus that are thought to have only a modest age
spread of a few Myr. The companions of our binary sample are at projected
separations between 10 and 450 AU with masses down to about 0.1 solar masses.
We used the strength and shape of the 10 micron silicate emission feature as a
proxy for grain growth and for crystallization respectively. The degree of dust
settling was evaluated from the ratio of fluxes at two different mid-infrared
wavelengths. We find no statistically significant difference between the
distribution of 10 micron silicate emission features from single and binary
systems. In addition, the distribution of disk flaring is indistinguishable
between the single and binary system samples. These results show that the first
steps of planet formation are not affected by the presence of a companion at
tens of AU.Comment: To appear in the Astrophysical Journa
Microphase separation in Pr0.67Ca0.33MnO3 by small angle neutron scattering
We have evidenced by small angle neutron scattering at low temperature the
coexistence of ferromagnetism (F) and antiferromagnetism (AF) in
Pr0.67Ca0.33MnO3. The results are compared to those obtained in
Pr0.80Ca0.20MnO3 and Pr0.63Ca0.37MnO3, which are F and AF respectively.
Quantitative analysis shows that the small angle scattering is not due to a
mesoscopic mixing but to a nanoscopic electronic and magnetic ''red cabbage''
structure, in which the ferromagnetic phase exists in form of thin layers in
the AF matrix (stripes or 2D ''sheets'').Comment: 4 figure
Impurity induced resonant state in a pseudogap state of a high temperature superconductor
We predict a resonance impurity state generated by the substitution of one Cu
atom with a nonmagnetic atom, such as Zn, in the pseudogap state of a high-T_c
superconductor. The precise microscopic origin of the pseudogap is not
important for this state to be formed, in particular this resonance will be
present even in the absence of superconducting fluctuations in the normal
state. In the presence of superconducting fluctuations, we predict the
existence of a counterpart impurity peak on a symmetric bias.
The nature of impurity resonance is similar to the previously studied
resonance in the d-wave superconducting state.Comment: 4 pages, 2 figure
Small angle neutron scattering study of the step-like magnetic transformation in Pr0.70Ca0.30MnO3
Small angle neutron scattering (SANS) magnetic and electrical transport
measurements were performed to study a single crystal of Pr0.7Ca0.3MnO3, a
colossal magnetoresistive (CMR) material. While the magnetic field induced
transformation of this phase separated compound consisting of an
antiferromagnetic insulating phase (AFI) and a ferromagnetic insulating phase
(FI), is continuous at high temperature (above 5K), at lower temperature a step
like transformation is observed (around 5T at 2K). Macroscopic magnetization
measurements and SANS indicate that this transformation occurs by the formation
of mesoscopic ferromagnetic metallic (FM) domains in the AFI phase, and,
eventually, in the FI phase. Although above 5K this transformation is
continuous, below 5K a magnetization step marks the abrupt transition from a
large scale FI/AFI phase separation to a large scale phase separation between
AFI, FI and FM phases. Our results suggest that relaxation of elastic strains
inherent to the coexistence of these different phases plays a crucial role in
the mechanism of these transformations. The occurrence of magnetization steps
could result from an intrinsic behavior of the AFI phase at low temperature
Field dependence of the electronic phase separation in Pr0.67Ca0.33MnO3 by small angle magnetic neutron scattering
We have studied by small angle neutron scattering the evolution induced by
the application of magnetic field of the coexistence of ferromagnetism (F) and
antiferromagnetism (AF) in a crystal of PrCaMnO. The
results are compared to magnetic measurements which provide the evolution of
the ferromagnetic fraction. These results show that the growth of the
ferromagnetic phase corresponds to an increase of the thickness of the
ferromagnetic ''cabbage'' sheets
Observational Constraints on the Formation and Evolution of Binary Stars
We present a high spatial resolution UV to NIR survey of 44 young binary
stars in Taurus with separations of 10-1000 AU. The primary results include:
(1) The relative ages of binary star components are more similar than the
relative ages of randomly paired single stars, supporting coeval formation. (2)
Only one of the companion masses is substellar, and hence the apparent
overabundance of T Tauri star companions relative to main-sequence star
companions can not be explained by a wealth of substellar secondaries that
would have been missed in main-sequence surveys. (3) Roughly 10% of T Tauri
binary star components have very red NIR colors (K-L > 1.4) and unusually high
mass accretion rates. This phenomenon does not appear to be restricted to
binary systems, however, since a comparable fraction of single T Tauri stars
exhibit the same properties. (4) Although the disk lifetimes of single stars
are roughly equal to their stellar ages, the disk lifetimes of binary stars are
an order of magnitude less than their ages. (5) The accretion rates for both
single and binary T Tauri stars appear to be moderately mass dependent. (6)
Although most classical T Tauri star binaries retain both a circumprimary and a
circumsecondary disk, there are several systems with only a circumprimary disk.
Together with the relative accretion rates, this suggests that circumprimary
disks survive longer, on average, than circumsecondary disks. (7) The disk
lifetimes, mass ratios, and relative accretion signatures of the closest
binaries (10-100 AU) suggest that they are being replenished from a
circumbinary reservoir with low angular momentum. Overall, these results
support fragmentation as the dominant binary star formation mechanism.Comment: 67 pages including 11 figures, LaTeX2e, accepted for publication in
Ap
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