6,581 research outputs found
Search for Dust Emission from (24) Themis Using the Gemini-North Telescope
We report the results of a search for a dust trail aligned with the orbit
plane of the large main-belt asteroid (24) Themis, which has been reported to
have water ice frost on its surface. Observations were obtained with the GMOS
instrument on the Gemini-North Observatory in imaging mode, where we used a
chip gap to block much of the light from the asteroid, allowing us to take long
exposures while avoiding saturation by the object. No dust trail is detected
within 2' of Themis to a 3-sigma limiting surface brightness magnitude of 29.7
mag/arcsec^2, as measured along the expected direction of the dust trail.
Detailed consideration of dust ejection physics indicates that particles large
enough to form a detectable dust trail were unlikely to be ejected as a result
of sublimation from an object as large as Themis. We nonetheless demonstrate
that our observations would have been capable of detecting faint dust emission
as close as 20" from the object, even in a crowded star field. This approach
could be used to conduct future searches for sublimation-generated dust
emission from Themis or other large asteroids closer to perihelion than was
done in this work. It would also be useful for deep imaging of collisionally
generated dust emission from large asteroids at times when the visibility of
dust features are expected to be maximized, such as during orbit plane
crossings, during close approaches to the Earth, or following detected impact
events.Comment: 11 pages, 4 figures, accepted for publication in PAS
Pair Analysis of Field Galaxies from the Red-Sequence Cluster Survey
We study the evolution of the number of close companions of similar
luminosities per galaxy (Nc) by choosing a volume-limited subset of the
photometric redshift catalog from the Red-Sequence Cluster Survey (RCS-1). The
sample contains over 157,000 objects with a moderate redshift range of 0.25 < z
< 0.8 and absolute magnitude in Rc (M_Rc) < -20. This is the largest sample
used for pair evolution analysis, providing data over 9 redshift bins with
about 17,500 galaxies in each. After applying incompleteness and projection
corrections, Nc shows a clear evolution with redshift. The Nc value for the
whole sample grows with redshift as (1+z)^m, where m = 2.83 +/- 0.33 in good
agreement with N-body simulations in a LCDM cosmology. We also separate the
sample into two different absolute magnitude bins: -25 < M_Rc < -21 and -21 <
M_Rc < -20, and find that the brighter the absolute magnitude, the smaller the
m value. Furthermore, we study the evolution of the pair fraction for different
projected separation bins and different luminosities. We find that the m value
becomes smaller for larger separation, and the pair fraction for the fainter
luminosity bin has stronger evolution. We derive the major merger remnant
fraction f_rem = 0.06, which implies that about 6% of galaxies with -25 < M_Rc
< -20 have undergone major mergers since z = 0.8.Comment: ApJ, in pres
Orbital-assisted metal-insulator transition in VO
We found direct experimental evidence for an orbital switching in the V 3d
states across the metal-insulator transition in VO. We have used
soft-x-ray absorption spectroscopy at the V edges as a sensitive
local probe, and have determined quantitatively the orbital polarizations.
These results strongly suggest that, in going from the metallic to the
insulating state, the orbital occupation changes in a manner that charge
fluctuations and effective band widths are reduced, that the system becomes
more 1-dimensional and more susceptible to a Peierls-like transition, and that
the required massive orbital switching can only be made if the system is close
to a Mott insulating regime
Atomic Interferometer with Amplitude Gratings of Light and its Applications to Atom Based Tests of the Equivalence Principle
We have developed a matter wave interferometer based on the diffraction of
atoms from effective absorption gratings of light. In a setup with cold
rubidium atoms in an atomic fountain the interferometer has been used to carry
out tests of the equivalence principle on an atomic basis. The gravitational
acceleration of the two isotopes 85Rb and 87Rb was compared, yielding a
difference Dg/g =(1.2 +-1.7)x10^{-7}. We also perform a differential free fall
measurement of atoms in two different hyperfine states, and obtained a result
of Dg/g =(0.4 +-1.2)x10^{-7}.Comment: 4 Pages, 4 figures, accepted for Physical Review Letter
Local electronic structure and magnetic properties of LaMn0.5Co0.5O3 studied by x-ray absorption and magnetic circular dichroism spectroscopy
We have studied the local electronic structure of LaMn0.5Co0.5O3 using
soft-x-ray absorption spectroscopy at the Co-L_3,2 and Mn-L_3,2 edges. We found
a high-spin Co^{2+}--Mn^{4+} valence state for samples with the optimal Curie
temperature. We discovered that samples with lower Curie temperatures contain
low-spin nonmagnetic Co^{3+} ions. Using soft-x-ray magnetic circular dichroism
we established that the Co^{2+} and Mn^{4+} ions are ferromagnetically aligned.
We revealed also that the Co^{2+} ions have a large orbital moment:
m_orb/m_spin ~ 0.47. Together with model calculations, this suggests the
presence of a large magnetocrystalline anisotropy in the material and predicts
a non-trivial temperature dependence for the magnetic susceptibility.Comment: 8 pages, 7 figure
A topological insulator surface under strong Coulomb, magnetic and disorder perturbations
Three dimensional topological insulators embody a newly discovered state of
matter characterized by conducting spin-momentum locked surface states that
span the bulk band gap as demonstrated via spin-resolved ARPES measurements .
This highly unusual surface environment provides a rich ground for the
discovery of novel physical phenomena. Here we present the first controlled
study of the topological insulator surfaces under strong Coulomb, magnetic and
disorder perturbations. We have used interaction of iron, with a large Coulomb
state and significant magnetic moment as a probe to \textit{systematically test
the robustness} of the topological surface states of the model topological
insulator BiSe. We observe that strong perturbation leads to the
creation of odd multiples of Dirac fermions and that magnetic interactions
break time reversal symmetry in the presence of band hybridization. We also
present a theoretical model to account for the altered surface of BiSe.
Taken collectively, these results are a critical guide in manipulating
topological surfaces for probing fundamental physics or developing device
applications.Comment: 14 pages, 4 Figures. arXiv admin note: substantial text overlap with
arXiv:1009.621
Valence, spin, and orbital state of the Co ions in the one-dimensional Ca3Co2O6: an x-ray absorption and magnetic circular dichroism study
We have investigated the valence, spin, and orbital state of the Co ions in
the one-dimensional cobaltate Ca3Co2O6 using x-ray absorption and x-ray
magnetic circular dichroism at the Co-L2,3 edges. The Co ions at both the
octahedral Co_oct and trigonal Co_trig sites are found to be in a 3+ state.
From the analysis of the dichroism we established a low-spin state for the
Co_oct and a high-spin state with an anomalously large orbital moment of 1.7
muB at the Co3+ trig ions. This large orbital moment along the c-axis chain and
the unusually large magnetocrystalline anisotropy can be traced back to the
double occupancy of the d2 orbital in trigonal crystal field.Comment: 5 pages, 4 figure
Theory for measuring bivalent surface binding kinetics using total internal reflection with fluorescence photobleaching recovery
Total internal reflection with fluorescence photobleaching recovery (TIR-FPR) is a method for experimentally examining coupled diffusion and reaction kinetics at surfaces. In a previous work (Thompson et al. 1981. Biophys. J. 33:435-454), a theoretical basis for interpreting TIR-FPR data was described for monovalent ligands that undergo a reversible reaction with monovalent surface sites in a single step. Here, the theory for TIR-FPR has been extended to two different surface binding mechanisms that involve sequential, bivalent surface attachment. Methods for obtaining the intrinsic surface association and dissociation kinetic rates from measured fluorescence photobleaching recovery curves are described. The new theory should be applicable to the association of bivalent protein ligands such as antibodies with supported planar model membranes
Determination of the orbital moment and crystal field splitting in LaTiO
Utilizing a sum-rule in a spin-resolved photoelectron spectroscopic
experiment with circularly polarized light, we show that the orbital moment in
LaTiO is strongly reduced both below and above the N\'{e}el temperature.
Using Ti x-ray absorption spectroscopy as a local probe, we found
that the crystal field splitting in the subshell is about 0.12-0.30
eV. This large splitting does not facilitate the formation of an orbital
liquid
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