1,493 research outputs found
Chandra X-ray Sources in the LALA Cetus Field
The 174 ks Chandra Advanced CCD Imaging Spectrometer exposure of the Large
Area Lyman Alpha Survey (LALA) Cetus field is the second of the two deep
Chandra images on LALA fields. In this paper we present the Chandra X-ray
sources detected in the Cetus field, along with an analysis of X-ray source
counts, stacked X-ray spectrum, and optical identifications. A total of 188
X-ray sources were detected: 174 in the 0.5-7.0 keV band, 154 in the 0.5-2.0
keV band, and 113 in the 2.0-7.0 keV band. The X-ray source counts were derived
and compared with LALA Bootes field (172 ks exposure). Interestingly, we find
consistent hard band X-ray source density, but 36+-12% higher soft band X-ray
source density in Cetus field. The weighted stacked spectrum of the detected
X-ray sources can be fitted by a powerlaw with photon index Gamma = 1.55. Based
on the weighted stacked spectrum, we find that the resolved fraction of the
X-ray background drops from 72+-1% at 0.5-1.0 keV to 63+-4% at 6.0-8.0 keV. The
unresolved spectrum can be fitted by a powerlaw over the range 0.5-7 keV, with
a photon index Gamma = 1.22. We also present optical counterparts for 154 of
the X-ray sources, down to a limiting magnitude of r' = 25.9 (Vega), using a
deep r' band image obtained with the MMT.Comment: 21 pages, including 6 figures, 1 table, ApJ accepte
Quantum field theory on a growing lattice
We construct the classical and canonically quantized theories of a massless
scalar field on a background lattice in which the number of points--and hence
the number of modes--may grow in time. To obtain a well-defined theory certain
restrictions must be imposed on the lattice. Growth-induced particle creation
is studied in a two-dimensional example. The results suggest that local mode
birth of this sort injects too much energy into the vacuum to be a viable model
of cosmological mode birth.Comment: 28 pages, 2 figures; v.2: added comments on defining energy, and
reference
Constraining the Lyα escape fraction with far-infrared observations of Lyα emitters
We study the far-infrared properties of 498 Lyα emitters (LAEs) at z = 2.8, 3.1, and 4.5 in the Extended Chandra Deep Field-South, using 250, 350, and 500μm data from the Herschel Multi-tiered Extragalactic Survey and 870μm data from the LABOCA ECDFS Submillimeter Survey. None of the 126, 280, or 92 LAEs at z = 2.8, 3.1, and 4.5, respectively, are individually detected in the far-infrared data. We use stacking to probe the average emission to deeper flux limits, reaching 1σ depths of ∼0.1 to 0.4 mJy. The LAEs are also undetected at ?3σ in the stacks, although a 2.5σ signal is observed at 870μm for the z = 2.8 sources. We consider a wide range of far-infrared spectral energy distributions (SEDs), including an M82 and an Sd galaxy template, to determine upper limits on the far-infrared luminosities and far-infrared-derived star formation rates of the LAEs. These star formation rates are then combined with those inferred from the Lyα and UV emission to determine lower limits on the LAEs’ Lyα escape fraction (f esc (Lyα)). For the Sd SED template, the inferred LAEs f esc (Lyα) are ?30% (1σ) at z = 2.8, 3.1, and 4.5, which are all significantly higher than the global f esc (Lyα) at these redshifts. Thus, if the LAEs f esc (Lyα) follows the global evolution, then they have warmer far-infrared SEDs than the Sd galaxy template. The average and M82 SEDs produce lower limits on the LAE f esc (Lyα) of ∼10%–20% (1σ), all of which are slightly higher than the global evolution of f esc (Lyα), but consistent with it at the 2σ–3σ level
Effect of 3d-doping on the electronic structure of BaFe2As2
The electronic structure of BaFe2As2 doped with Co, Ni, and Cu has been
studied by a variety of experimental and theoretical methods, but a clear
picture of the dopant 3d states has not yet emerged. Herein we provide
experimental evidence of the distribution of Co, Ni, and Cu 3d states in the
valence band. We conclude that the Co and Ni 3d states provide additional free
carriers to the Fermi level, while the Cu 3d states are found at the bottom of
the valence band in a localized 3d10 shell. These findings help shed light on
why superconductivity can occur in BaFe2As2 doped with Co and Ni but not Cu.Comment: 18 pages, 8 figure
Correlation effects in Ni 3d states of LaNiPO
The electronic structure of the new superconducting material LaNiPO
experimentally probed by soft X-ray spectroscopy and theoretically calculated
by the combination of local density approximation with Dynamical Mean-Field
Theory (LDA+DMFT) are compared herein. We have measured the Ni L2,3 X-ray
emission (XES) and absorption (XAS) spectra which probe the occupied and
unoccupied the Ni 3d states, respectively. In LaNiPO, the Ni 3d states are
strongly renormalized by dynamical correlations and shifted about 1.5 eV lower
in the valence band than the corresponding Fe 3d states in LaFeAsO. We further
obtain a lower Hubbard band at -9 eV below the Fermi level in LaNiPO which
bears striking resemblance to the lower Hubbard band in the correlated oxide
NiO, while no such band is observed in LaFeAsO. These results are also
supported by the intensity ratio between the transition metal L2 and L3 bands
measured experimentally to be higher in LaNiPO than in LaFeAsO, indicating the
presence of the stronger electron correlations in the Ni 3d states in LaNiPO in
comparison with the Fe 3d states in LaFeAsO. These findings are in accordance
with resonantly excited transition metal L3 X-ray emission spectra which probe
occupied metal 3d-states and show the appearance of the lower Hubbard band in
LaNiPO and NiO and its absence in LaFeAsO.Comment: 6 pages, 5 figure
Charged exctions in the fractional quantum Hall regime
We study the photoluminescence spectrum of a low density ()
two-dimensional electron gas at high magnetic fields and low temperatures. We
find that the spectrum in the fractional quantum Hall regime can be understood
in terms of singlet and triplet charged-excitons. We show that these spectral
lines are sensitive probes for the electrons compressibility. We identify the
dark triplet charged-exciton and show that it is visible at the spectrum at
K. We find that its binding energy scales like , where is
the magnetic length, and it crosses the singlet slightly above 15 T.Comment: 10 pages, 5 figure
Excitons and charged excitons in semiconductor quantum wells
A variational calculation of the ground-state energy of neutral excitons and
of positively and negatively charged excitons (trions) confined in a
single-quantum well is presented. We study the dependence of the correlation
energy and of the binding energy on the well width and on the hole mass. The
conditional probability distribution for positively and negatively charged
excitons is obtained, providing information on the correlation and the charge
distribution in the system. A comparison is made with available experimental
data on trion binding energies in GaAs-, ZnSe-, and CdTe-based quantum well
structures, which indicates that trions become localized with decreasing
quantum well width.Comment: 9 pages, 11 figure
Superconductivity from doping a spin liquid insulator: a simple one-dimensional example
We study the phase diagram of a one-dimensional Hubbard model where, in
addition to the standard nearest neighbor hopping , we also include a
next-to-nearest neighbor hopping . For strong enough on-site repulsion,
this model has a transition at half filling from a magnetic insulator with
gapless spin excitations at small to a dimerized insulator with a spin
gap at larger . We show that upon doping this model exhibits quite
interesting features, which include the presence of a metallic phase with a
spin gap and dominant superconducting fluctuations, in spite of the repulsive
interaction. More interestingly, we find that this superconducting phase can be
reached upon hole doping the magnetic insulator. The connections between this
model and the two chain models, recently object of intensive investigations,
are also discussed.Comment: 19 pages, plain LaTex using RevTex, 7 postscript figures Modified
version which excludes some LaTex commands giving problems for the previous
versio
Theory of Exciton Recombination from the Magnetically Induced Wigner Crystal
We study the theory of itinerant-hole photoluminescence of two-dimensional
electron systems in the regime of the magnetically induced Wigner crystal. We
show that the exciton recombination transition develops structure related to
the presence of the Wigner crystal. The form of this structure depends strongly
on the separation between the photo-excited hole and the plane of the
two-dimensional electron gas. When is small compared to the magnetic
length, additional peaks appear in the spectrum due to the recombination of
exciton states with wavevectors equal to the reciprocal lattice vectors of the
crystal. For larger than the magnetic length, the exciton becomes strongly
confined to an interstitial site of the lattice, and the structure in the
spectrum reflects the short-range correlations of the Wigner crystal. We derive
expressions for the energies and the radiative lifetimes of the states
contributing to photoluminescence, and discuss how the results of our analysis
compare with experimental observations.Comment: 10 pages, no figures, uses Revtex and multicol.st
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