698 research outputs found
Direction and symmetry transition of the vector order parameter in topological superconductors CuxBi2Se3
Topological superconductors have attracted wide-spreading interests for the bright application perspectives to quantum computing. Cu0.3Bi2Se3 is a rare bulk topological superconductor with an odd-parity wave function, but the details of the vector order parameter d and its pinning mechanism are still unclear. Here, we succeed in growing CuxBi2Se3 single crystals with unprecedented high doping levels. For samples with x = 0.28, 0.36 and 0.37 with similar carrier density as evidenced by the Knight shift, the in-plane upper critical field Hc2 shows a two-fold symmetry. However, the angle at which the Hc2 becomes minimal is different by 90° among them, which indicates that the d-vector direction is different for each crystal likely due to a different local environment. The carrier density for x = 0.46 and 0.54 increases substantially compared to x ≤ 0.37. Surprisingly, the in-plane Hc2 anisotropy disappears, indicating that the gap symmetry undergoes a transition from nematic to isotropic (possibly chiral) as carrier increases
Phase transitions in crowd dynamics of resource allocation
We define and study a class of resources allocation processes where
agents, by repeatedly visiting resources, try to converge to optimal
configuration where each resource is occupied by at most one agent. The process
exhibits a phase transition, as the density of agents grows, from an
absorbing to an active phase. In the latter, even if the number of resources is
in principle enough for all agents (), the system never settles to a
frozen configuration. We recast these processes in terms of zero-range
interacting particles, studying analytically the mean field dynamics and
investigating numerically the phase transition in finite dimensions. We find a
good agreement with the critical exponents of the stochastic fixed-energy
sandpile. The lack of coordination in the active phase also leads to a
non-trivial faster-is-slower effect.Comment: 7 pages, 7 fig
Wide-Field Infrared Imaging Polarimetry of the NGC 6334 Region: A Nest of Infrared Reflection Nebulae
We report the detection of eighteen infrared reflection nebulae (IRNe) in the
, , & linear polarimetric observations of the NGC 6334 massive
star-formation complex, of which 16 IRNe are new discoveries. Our images cover
180 square arcminutes, one of the widest near-infrared polarization data
in star-formation regions so far. These IRNe are most likely associated with
embedded young OB stars at different evolutionary phases, showing a variety of
sizes, morphologies, and polarization properties, which can be divided into
four categories. We argue the different nebula characteristics to be a possible
evolutionary sequence of circumstellar structures around young massive stars.Comment: 4 pages, 1 figur
N2H+ Observations of Molecular Cloud Cores in Taurus
N2H+ observations of molecular cloud cores in Taurus with the Nobeyama 45 m
radio telescope are reported. We compare ``cores with young stars'' with
``cores without young stars''. The differences in core radius, linewidth, and
core mass are small. Linewidth is dominated by thermal motions in both cases.
N2H+ maps show that the intensity distribution does not differ much between
cores without stars and those with stars. This is in contrast to the result
previously obtained in H13CO+ toward Taurus molecular cloud cores. Larger
degree of depletion of H13CO+ in starless cores will be one possible
explanation for this difference. We studied the physical state of molecular
cloud cores in terms of ``critical pressure'' for the surface (external)
pressure. There is no systematic difference between starless cores and cores
with stars in this analysis. Both are not far from the critical state for
pressure equilibrium. We suggest that molecular cloud cores in which thermal
support is dominated evolve toward star formation by keeping close to the
critical state. This result is in contrast with that obtained in the
intermediate-mass star forming region OMC-2/3, where molecular cloud cores
evolve by decreasing the critical pressure appreciably. We investigate the
radial distribution of the integrated intensity. Cores with stars are found to
have shallow (-1.8 to -1.6) power-law density profiles.Comment: 19 pages, 5 figure
Orbital characterization of GJ1108A system, and comparison of dynamical mass with model-derived mass for resolved binaries
We report an orbital characterization of GJ1108Aab that is a low-mass binary
system in pre-main-sequence phase. Via the combination of astrometry using
adaptive optics and radial velocity measurements, an eccentric orbital solution
of =0.63 is obtained, which might be induced by the Kozai-Lidov mechanism
with a widely separated GJ1108B system. Combined with several observed
properties, we confirm the system is indeed young. Columba is the most probable
moving group, to which the GJ1108A system belongs, although its membership to
the group has not been established. If the age of Columba is assumed for
GJ1108A, the dynamical masses of both GJ1108Aa and GJ1108Ab ( and ) are more massive than what an
evolutionary model predicts based on the age and luminosities. We consider the
discrepancy in mass comparison can attribute to an age uncertainty; the system
is likely older than stars in Columba, and effects that are not implemented in
classical models such as accretion history and magnetic activity are not
preferred to explain the mass discrepancy. We also discuss the performance of
the evolutionary model by compiling similar low-mass objects in evolutionary
state based on the literature. Consequently, it is suggested that the current
model on average reproduces the mass of resolved low-mass binaries without any
significant offsets.Comment: Accepted in Ap
Direct Imaging of Fine Structures in Giant Planet Forming Regions of the Protoplanetary Disk around AB Aurigae
We report high-resolution 1.6 \micron polarized intensity () images of
the circumstellar disk around the Herbig Ae star AB Aur at a radial distance of
22 AU () up to 554 AU (3.85), which have been obtained by the
high-contrast instrument HiCIAO with the dual-beam polarimetry. We revealed
complicated and asymmetrical structures in the inner part (140 AU) of
the disk, while confirming the previously reported outer ( 200 AU)
spiral structure. We have imaged a double ring structure at 40 and
100 AU and a ring-like gap between the two. We found a significant
discrepancy of inclination angles between two rings, which may indicate that
the disk of AB Aur is warped. Furthermore, we found seven dips (the typical
size is 45 AU or less) within two rings as well as three prominent
peaks at 40 AU. The observed structures, including a bumpy double ring, a
ring-like gap, and a warped disk in the innermost regions, provide essential
information for understanding the formation mechanism of recently detected
wide-orbit ( 20 AU) planets.Comment: 12 pages, 3 figure
SEEDS direct imaging of the RV-detected companion to V450 Andromedae, and characterization of the system
We report the direct imaging detection of a low-mass companion to a young,
moderately active star V450 And, that was previously identified with the radial
velocity method. The companion was found in high-contrast images obtained with
the Subaru Telescope equipped with the HiCIAO camera and AO188 adaptive optics
system. From the public ELODIE and SOPHIE archives we extracted available
high-resolution spectra and radial velocity (RV) measurements, along with RVs
from the Lick planet search program. We combined our multi-epoch astrometry
with these archival, partially unpublished RVs, and found that the companion is
a low-mass star, not a brown dwarf, as previously suggested. We found the
best-fitting dynamical masses to be and
M. We also performed spectral analysis of
the SOPHIE spectra with the iSpec code. The Hipparcos time-series photometry
shows a periodicity of d, which is also seen in SOPHIE spectra as an
RV modulation of the star A. We interpret it as being caused by spots on the
stellar surface, and the star to be rotating with the given period. From the
rotation and level of activity, we found that the system is
Myr old, consistent with an isochrone analysis ( Myr). This
work may serve as a test case for future studies of low-mass stars, brown
dwarfs and exoplanets by combination of RV and direct imaging data.Comment: 15 pages, 9 figures, 7 tables, to appear in Ap
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