9,097 research outputs found
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
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
Long term memories of developed and emerging markets: using the scaling analysis to characterize their stage of development
The scaling properties encompass in a simple analysis many of the volatility
characteristics of financial markets. That is why we use them to probe the
different degree of markets development. We empirically study the scaling
properties of daily Foreign Exchange rates, Stock Market indices and fixed
income instruments by using the generalized Hurst approach. We show that the
scaling exponents are associated with characteristics of the specific markets
and can be used to differentiate markets in their stage of development. The
robustness of the results is tested by both Monte-Carlo studies and a
computation of the scaling in the frequency-domain.Comment: 46 pages, 7 figures, accepted for publication in Journal of Banking &
Financ
A study of the personal income distribution in Australia
We analyze the data on personal income distribution from the Australian
Bureau of Statistics. We compare fits of the data to the exponential,
log-normal, and gamma distributions. The exponential function gives a good
(albeit not perfect) description of 98% of the population in the lower part of
the distribution. The log-normal and gamma functions do not improve the fit
significantly, despite having more parameters, and mimic the exponential
function. We find that the probability density at zero income is not zero,
which contradicts the log-normal and gamma distributions, but is consistent
with the exponential one. The high-resolution histogram of the probability
density shows a very sharp and narrow peak at low incomes, which we interpret
as the result of a government policy on income redistribution.Comment: 7 pages, 4 figures, Proceedings of the Econophysics Colloquium,
Canberra, 14-18 November 200
Quantifying stellar radial migration in an N-body simulation: blurring, churning, and the outer regions of galaxy discs
Radial stellar migration in galactic discs has received much attention in
studies of galactic dynamics and chemical evolution, but remains a dynamical
phenomenon that needs to be fully quantified. In this work, using a Tree-SPH
simulation of an Sb-type disc galaxy, we quantify the effects of blurring
(epicyclic excursions) and churning (change of guiding radius). We quantify
migration (either blurring or churning) both in terms of flux (the number of
migrators passing at a given radius), and by estimating the population of
migrators at a given radius at the end of the simulation compared to
non-migrators, but also by giving the distance over which the migration is
effective at all radii. We confirm that the corotation of the bar is the main
source of migrators by churning in a bar-dominated galaxy, its intensity being
directly linked to the episode of a strong bar, in the first 1-3 Gyr of the
simulation. We show that within the outer Lindblad resonance (OLR), migration
is strongly dominated by churning, while blurring gains progressively more
importance towards the outer disc and at later times. Most importantly, we show
that the OLR limits the exchange of angular momentum, separating the disc in
two distinct parts with minimal or null exchange, except in the transition
zone, which is delimited by the position of the OLR at the epoch of the
formation of the bar, and at the final epoch. We discuss the consequences of
these findings for our understanding of the structure of the Milky Way disc.
Because the Sun is situated slightly outside the OLR, we suggest that the solar
vicinity may have experienced very limited churning from the inner disc.Comment: Accepted for publication in Astronomy and Astrophysics (acceptance
date: 27/04/15), 24 pages, 24 figure
Hiding its age: the case for a younger bulge
The determination of the age of the bulge has led to two contradictory
results. On the one side, the color-magnitude diagrams in different bulge
fields seem to indicate a uniformly old (10 Gyr) population. On the other
side, individual ages derived from dwarfs observed through microlensing events
seem to indicate a large spread, from 2 to 13 Gyr. Because the
bulge is now recognised as being mainly a boxy peanut-shaped bar, it is
suggested that disk stars are one of its main constituents, and therefore also
stars with ages significantly younger than 10 Gyr. Other arguments as well
point to the fact that the bulge cannot be exclusively old, and in particular
cannot be a burst population, as it is usually expected if the bulge was the
fossil remnant of a merger phase in the early Galaxy. In the present study, we
show that given the range of metallicities observed in the bulge, a uniformly
old population would be reflected into a significant spread in color at the
turn-off which is not observed. Inversely, we demonstrate that the correlation
between age and metallicity expected to hold for the inner disk would conspire
to form a color-magnitude diagram with a remarkably small spread in color, thus
mimicking the color-magnitude diagram of a uniformly old population. If stars
younger than 10 Gyr are part of the bulge, as must be the case if the bulge has
been mainly formed through dynamical instabilities in the disk, then a very
small spread at the turn-off is expected, as seen in the observations.Comment: 11 pages, 11 figures. Accepted for publication in A&
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
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