14,213 research outputs found
The impact of spatial fluctuations in the ultra-violet background on intergalactic carbon and silicon
Spatial inhomogeneities in the spectral shape of the ultra-violet background
(UVB) at the tail-end of HeII reionisation are thought to be the primary cause
of the large fluctuations observed in the HeII to HI Ly-a forest optical depth
ratio, tau_HeII/tau_HI, at z~2-3. These spectral hardness fluctuations will
also influence the ionisation balance of intergalactic metals; we extract
realistic quasar absorption spectra from a large hydrodynamical simulation to
examine their impact on intergalactic SiIV and CIV absorbers. Using a variety
of toy UVB models, we find that while the predicted spatial inhomogeneities in
spectral hardness have a significant impact on tau_HeII/tau_HI, the longer mean
free path for photons with frequencies above and below the HeII ionisation edge
means these fluctuations have less effect on the SiIV and CIV ionisation
balance. Furthermore, UVB models which produce the largest fluctuations in
specific intensity at the HeII ionisation edge also have the softest ionising
spectra, and thus result in photo-ionisation rates which are too low to produce
significant fluctuations in the observed tau_SiIV/tau_CIV. Instead, we find
spatial variations in the IGM metallicity will dominate any scatter in
tau_SiIV/tau_CIV. Our results suggest that observational evidence for
homogeneity in the observed tau_SiIV/tau_CIV distribution does not rule out the
possibility of significant fluctuations in the UVB spectral shape at z~2-3. On
the other hand, the scatter in metallicity inferred from observations of
intergalactic CIV and SiIV absorption at z~2-3 using spatially uniform
ionisation corrections is likely intrinsic, and therefore provides a valuable
constraint on intergalactic metal enrichment scenarios at these redshifts.Comment: 13 pages, 7 figures, accepted to MNRA
Orbital currents, anapoles, and magnetic quadrupoles in CuO
We show that orbital currents in a CuO2 plane, if present, should be
described by two independent parity and time-reversal odd order parameters, a
toroidal dipole (anapole) and a magnetic quadrupole. Based on this, we derive
the resonant X-ray diffraction cross-section for monoclinic CuO at the
antiferromagnetic wavevector and show that the two order parameters can be
disentangled. From our analysis, we examine a recent claim of detecting
anapoles in CuO.Comment: 7 pages, 5 figure
The magnetic ground state of Sr2IrO4 and implications for second-harmonic generation
The currently accepted magnetic ground state of Sr2IrO4 (the -++- state)
preserves inversion symmetry. This is at odds, though, with recent experiments
that indicate a magnetoelectric ground state, leading to the speculation that
orbital currents or more exotic magnetic multipoles might exist in this
material. Here, we analyze various magnetic configurations and demonstrate that
two of them, the magnetoelectric -+-+ state and the non-magnetoelectric ++++
state, can explain these recent second-harmonic generation (SHG) experiments,
obviating the need to invoke orbital currents. The SHG-probed magnetic order
parameter has the symmetry of a parity-breaking multipole in the -+-+ state and
of a parity-preserving multipole in the ++++ state. We speculate that either
might have been created by the laser pump used in the experiments. An
alternative is that the observed magnetic SHG signal is a surface effect. We
suggest experiments that could be performed to test these various
possibilities, and also address the important issue of the suppression of the
RXS intensity at the L2 edge.Comment: 28 pages, 8 figures, v3 - an expanded discussion of the origin of the
SHG signa
A Face Can Launch a Thousand Shares—and an 0.80% Abnormal Return
In this paper we examine the market reaction—price and volume—to the appearance of a firm in the Who’s News column of The Wall Street Journal. We differentiate between those firms whose articles are accompanied by a picture of an executive and a control set of firms whose articles on the same day are not accompanied by a picture. The results show a more pronounced market reaction to the “cum picture” articles, consistent with the incomplete information theory of Merton [1987] and the heuristic-based familiarity hypothesis. There is no evidence of significant long-run abnormal performance for the sample firms
The nature of the tensor order in Cd2Re2O7
The pyrochlore metal Cd2Re2O7 has been recently investigated by
second-harmonic generation (SHG) reflectivity. In this paper, we develop a
general formalism that allows for the identification of the relevant tensor
components of the SHG from azimuthal scans. We demonstrate that the secondary
order parameter identified by SHG at the structural phase transition is the
x2-y2 component of the axial toroidal quadrupole. This differs from the 3z2-r2
symmetry of the atomic displacements associated with the I-4m2 crystal
structure that was previously thought to be its origin. Within the same
formalism, we suggest that the primary order parameter detected in the SHG
experiment is the 3z2-r2 component of the magnetic quadrupole. We discuss the
general mechanism driving the phase transition in our proposed framework, and
suggest experiments, particularly resonant X-ray scattering ones, that could
clarify this issue.Comment: some additions and clarifications adde
On the nature of the magnetic ground-state wave function of V_2O_3
After a brief historical introduction, we dwell on two recent experiments in
the low-temperature, monoclinic phase of V_2O_3: K-edge resonant x-ray
scattering and non-reciprocal linear dichroism, whose interpretations are in
conflict, as they require incompatible magnetic space groups. Such a conflict
is critically reviewed, in the light of the present literature, and new
experimental tests are suggested, in order to determine unambiguously the
magnetic group. We then focus on the correlated, non-local nature of the
ground-state wave function, that is at the basis of some drawbacks of the LDA+U
approach: we singled out the physical mechanism that makes LDA+U unreliable,
and indicate the way out for a possible remedy. Finally we explain, by means of
a symmetry argument related to the molecular wave function, why the magnetic
moment lies in the glide plane, even in the absence of any local symmetry at
vanadium sites.Comment: 7 pages, 1 figur
X-ray Dichroism and the Pseudogap Phase of Cuprates
A recent polarized x-ray absorption experiment on the high temperature
cuprate superconductor Bi2Sr2CaCu2O8 indicates the presence of broken parity
symmetry below the temperature, T*, where a pseudogap appears in photoemission.
We critically analyze the x-ray data, and conclude that a parity-breaking
signal of the kind suggested is unlikely based on the crystal structures
reported in the literature. Possible other origins of the observed dichroism
signal are discussed. We propose x-ray scattering experiments that can be done
in order to determine whether such alternative interpretations are valid or
not.Comment: final version to be published in Phys Rev B: some calculational
details added, clarification of XNLD contamination and biaxiality, more
discussion on possible space groups and previous optics result
- …