223 research outputs found
Vortex Softening: Origin of the second peak effect in BiSrCaCuO
Transverse ac permeability measurements in BiSrCaCuO single crystals at low fields and temperatures in a vortex configuration
free of external forces show that the decrease of the critical current as
measured by magnetization loops at the second peak effect is an artifact due to
creep. On the other hand, the increase of critical current at the second peak
is due to a genuine softening of the tilting elastic properties of vortices in
the individual pinning regime that precedes the transition to a disorder state.Comment: 4 pages, 5 figures, RevTex, two column versio
B fields in OB stars (BOB): The discovery of a magnetic field in a multiple system in the Trifid Nebula, one of the youngest star forming regions
Recent magnetic field surveys in O- and B-type stars revealed that about 10%
of the core-hydrogen-burning massive stars host large-scale magnetic fields.
The physical origin of these fields is highly debated. To identify and model
the physical processes responsible for the generation of magnetic fields in
massive stars, it is important to establish whether magnetic massive stars are
found in very young star-forming regions or whether they are formed in close
interacting binary systems.
In the framework of our ESO Large Program, we carried out low-resolution
spectropolarimetric observations with FORS2 in 2013 April of the three most
massive central stars in the Trifid nebula, HD164492A, HD164492C, and
HD164492D. These observations indicated a strong longitudinal magnetic field of
about 500-600G in the poorly studied component HD164492C. To confirm this
detection, we used HARPS in spectropolarimetric mode on two consecutive nights
in 2013 June.
Our HARPS observations confirmed the longitudinal magnetic field in
HD164492C. Furthermore, the HARPS observations revealed that HD164492C cannot
be considered as a single star as it possesses one or two companions. The
spectral appearance indicates that the primary is most likely of spectral type
B1-B1.5V. Since in both observing nights most spectral lines appear blended, it
is currently unclear which components are magnetic. Long-term monitoring using
high-resolution spectropolarimetry is necessary to separate the contribution of
each component to the magnetic signal. Given the location of the system
HD164492C in one of the youngest star formation regions, this system can be
considered as a Rosetta Stone for our understanding of the origin of magnetic
fields in massive stars.Comment: Five pages, six figures, accepted for publication in A&
Role of twin boundaries on the vortex dynamics in YBaCuO
By means of a novel technique of rotating the applied current we have
directly measured the influence of twin boundaries on the vortex motion in a
YBaCuO single crystal. The results indicate that the effect of twin
planes on the vortex dynamics starts to develop below a certain temperature,
being responsible for an anisotropic viscosity in the vortex liquid state and a
guided motion in the solid state.Comment: 4 pages, 4 figure
Suppression of matching field effects by splay and pinning energy dispersion in YBa_2Cu_3O_7 with columnar defects
We report measurements of the irreversible magnetization M_i of a large
number of YBa_2Cu_3O_7 single crystals with columnar defects (CD). Some of them
exhibit a maximum in M_i when the density of vortices equals the density of
tracks, at temperatures above 40K. We show that the observation of these
matching field effects is constrained to those crystals where the orientational
and pinning energy dispersion of the CD system lies below a certain threshold.
The amount of such dispersion is determined by the mass and energy of the
irradiation ions, and by the crystal thickness. Time relaxation measurements
show that the matching effects are associated with a reduction of the creep
rate, and occur deep into the collective pinning regime.Comment: 7 pages, 5 figures, submitted to Phys. Rev.
Evidence for vortex staircases in the whole angular range due to competing correlated pinning mechanisms
We analyze the angular dependence of the irreversible magnetization of
YBaCuO crystals with columnar defects inclined from the c-axis. At
high fields a sharp maximum centered at the tracks' direction is observed. At
low fields we identify a lock-in phase characterized by an angle-independent
pinning strength and observe an angular shift of the peak towards the c-axis
that originates in the material anisotropy. The interplay among columnar
defects, twins and ab-planes generates a variety of staircase structures. We
show that correlated pinning dominates for all field orientations.Comment: 9 figures, 4 figure
Iron abundances of B-type post-Asymptotic Giant Branch stars in globular clusters: Barnard 29 in M 13 and ROA 5701 in omega Cen
High resolution optical and ultraviolet spectra of two B-type post-Asymptotic
Giant Branch (post-AGB) stars in globular clusters, Barnard 29 in M 13 and ROA
5701 in omega Cen, have been analysed using model atmosphere techniques. The
optical spectra have been obtained with FEROS on the ESO 2.2-m telescope and
the 2d-Coud\'e spectrograph on the 2.7-m McDonald telescope, while the
ultraviolet observations are from the GHRS on the HST. Abundances of light
elements (C, N, O, Mg, Al and S) plus Fe have been determined from the optical
spectra, while the ultraviolet data provide additional Fe abundance estimates
from Fe III absorption lines in the 1875-1900 {\AA} wavelength region. A
general metal underabundance relative to young B-type stars is found for both
Barnard 29 and ROA 5701. These results are consistent with the metallicities of
the respective clusters, as well as with previous studies of the objects. The
derived abundance patterns suggest that the stars have not undergone a gas-dust
separation, contrary to previous suggestions, although they may have evolved
from the AGB before the onset of the third dredge-up. However, the Fe
abundances derived from the HST spectra are lower than those expected from the
metallicities of the respective clusters, by 0.5 dex for Barnard 29 and 0.8 dex
for ROA 5701. A similar systematic underabundance is also found for other
B-type stars in environments of known metallicity, such as the Magellanic
Clouds. These results indicate that the Fe III ultraviolet lines may yield
abundance values which are systematically too low by typically 0.6 dex and
hence such estimates should be treated with caution.Comment: 15 pages, 3 figures. Accepted for publication in MNRA
On the stability of 2 \sqrt{2} x 2 \sqrt{2} oxygen ordered superstructures in YBa2Cu3O6+x
We have compared the ground-state energy of several observed or proposed " 2
\sqrt{2} x 2 \sqrt{2} oxygen (O) ordered superstructures " (from now on HS),
with those of "chain superstructures" (CS) (in which the O atoms of the basal
plane are ordered in chains), for different compositions x in YBa2Cu3O6+x. The
model Hamiltonian contains i) the Madelung energy, ii) a term linear in the
difference between Cu and O hole occupancies which controls charge transfer,
and iii) covalency effects based on known results for models in one and
two dimensions. The optimum distribution of charge is determined minimizing the
total energy, and depends on two parameters which are determined from known
results for x=1 and x=0.5. We obtain that on the O lean side, only CS are
stable, while for x=7/8, a HS with regularly spaced O vacancies added to the
x=1 structure is more stable than the corresponding CS for the same x. We find
that the detailed positions of the atoms in the structure, and long-range
Coulomb interactions, are crucial for the electronic structure, the mechanism
of charge transfer, the stability of the different phases, and the possibility
of phase separation.Comment: 24 text pages, Latex, one fig. included as ps file, to be publisheb
in Phys. Rev.
Tuning ultrafast electron thermalization pathways in a van der Waals heterostructure
Ultrafast electron thermalization - the process leading to Auger
recombination, carrier multiplication via impact ionization and hot carrier
luminescence - occurs when optically excited electrons in a material undergo
rapid electron-electron scattering to redistribute excess energy and reach
electronic thermal equilibrium. Due to extremely short time and length scales,
the measurement and manipulation of electron thermalization in nanoscale
devices remains challenging even with the most advanced ultrafast laser
techniques. Here, we overcome this challenge by leveraging the atomic thinness
of two-dimensional van der Waals (vdW) materials in order to introduce a highly
tunable electron transfer pathway that directly competes with electron
thermalization. We realize this scheme in a graphene-boron nitride-graphene
(G-BN-G) vdW heterostructure, through which optically excited carriers are
transported from one graphene layer to the other. By applying an interlayer
bias voltage or varying the excitation photon energy, interlayer carrier
transport can be controlled to occur faster or slower than the intralayer
scattering events, thus effectively tuning the electron thermalization pathways
in graphene. Our findings, which demonstrate a novel means to probe and
directly modulate electron energy transport in nanoscale materials, represent
an important step toward designing and implementing novel optoelectronic and
energy-harvesting devices with tailored microscopic properties.Comment: Accepted to Nature Physic
Multiple populations in globular clusters. Lessons learned from the Milky Way globular clusters
Recent progress in studies of globular clusters has shown that they are not
simple stellar populations, being rather made of multiple generations. Evidence
stems both from photometry and spectroscopy. A new paradigm is then arising for
the formation of massive star clusters, which includes several episodes of star
formation. While this provides an explanation for several features of globular
clusters, including the second parameter problem, it also opens new
perspectives about the relation between globular clusters and the halo of our
Galaxy, and by extension of all populations with a high specific frequency of
globular clusters, such as, e.g., giant elliptical galaxies. We review progress
in this area, focusing on the most recent studies. Several points remain to be
properly understood, in particular those concerning the nature of the polluters
producing the abundance pattern in the clusters and the typical timescale, the
range of cluster masses where this phenomenon is active, and the relation
between globular clusters and other satellites of our Galaxy.Comment: In press (The Astronomy and Astrophysics Review
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