1,992 research outputs found
The multi-frequency angular power spectrum of the epoch of reionization 21 cm signal
Observations of redshifted 21cm radiation from HI at high redshifts is an
important future probe of reionization. We consider the Multi-frequency Angular
Power Spectrum (MAPS) to quantify the statistics of the HI signal as a joint
function of the angular multipole l and frequency separation \Delta\nu. The
signal at two different frequencies is expected to get decorrelated as
\Delta\nu is increased, and quantifying this decorrelation is particularly
important in deciding the frequency resolution for future HI observations. This
is also expected to play a very crucial role in extracting the signal from
foregrounds as the signal is expected to decorrelate much faster than the
foregrounds (which are largely continuum sources) with increasing \Delta\nu. In
this paper we develop formulae relating the MAPS to different components of the
three dimensional HI power spectrum taking into account HI peculiar velocities.
We show that the flat-sky approximation provides a very good representation
over the angular scales of interest, and a final expression which is very
simple to calculate and interpret. We present results considering two models
for the HI distribution, namely, (i) DM: where the HI traces the dark matter
and (ii) PR: where the effects of patchy reionization are incorporated through
two parameters. We find that while the DM signal is largely featureless, the PR
signal peaks at the angular scales of the individual bubbles, and the signal is
considerably enhanced for large bubble size. For most cases of interest at l
\sim 100 the signal is uncorrelated beyond \Delta\nu \sim 1 MHz or even less,
whereas it occurs around \sim 0.1 MHz at l \sim 10^3. The \Delta\nu dependence
also carries an imprint of the bubble size and the bias, and is expected to be
an important probe of the reionization scenario (abridged).Comment: Accepted for publication in MNRAS. Revised to match the accepted
versio
The CMBR ISW and HI 21-cm Cross-correlation Angular Power Spectrum
The late-time growth of large scale structures (LSS) is imprinted in the CMBR
anisotropy through the Integrated Sachs Wolfe (ISW) effect. This is perceived
to be a very important observational probe of dark energy. Future observations
of redshifted 21-cm radiation from the cosmological neutral hydrogen (HI)
distribution hold the potential of probing the LSS over a large redshift range.
We have investigated the possibility of detecting the ISW through
cross-correlations between the CMBR anisotropies and redshifted 21-cm
observations. Assuming that the HI traces the dark matter, we find that the
ISW-HI cross-correlation angular power spectrum at an angular multipole l is
proportional to the dark matter power spectrum evaluated at the comoving wave
number l/r, where r is the comoving distance to the redshift from which the HI
signal originated. The amplitude of the cross-correlation signal depends on
parameters related to the HI distribution and the growth of cosmological
perturbations. However the cross-correlation is extremely weak as compared to
the CMBR anisotropies and the predicted HI signal. As a consequence the
cross-correlation signal is smaller than the cosmic variance, and a
statistically significant detection is not very likely.Comment: 13 pages, 4 eps figures, submitte
Supercell Altermagnets
Altermagnets are compensated magnets with unconvetional , and -wave
spin-channel order in reciprocal space. So far the search for new altermagnetic
candidates has been focused on materials in which the magnetic unit cell is
identical to the non-magnetic one, i.e. magnetic structures with zero
propagation vector. Here, we substantially broaden the family of altermagnetic
candidates by predicting supercell altermagnets. Their magnetic unit cell is
constructed by enlarging the paramagnetic primitive unit cell, resulting in a
non-zero propagation vector for the magnetic structure. This connection of the
magnetic configuration to the ordering of sublattices gives an extra degree of
freedom to supercell altermagnets, which can allow for the control over the
order parameter spatial orientation. We identify realistic candidates MnSe
with a -wave order, and RbCoBr, CsCoCr, and BaMnO with -wave
order. We demonstrate the reorientation of the order parameter in MnSe,
which has two different magnetic configurations, whose energy difference is
only 5 meV, opening the possibility of controlling the orientation of the
altermagnetic order parameter by external perturbations.Comment: 10 pages, 4 figure
Evidence for Filamentarity in the Las Campanas Redshift Survey
We apply Shapefinders, statistical measures of `shape' constructed from two
dimensional partial Minkowski functionals, to study the degree of filamentarity
in the Las Campanas Redshift Survey (LCRS). In two dimensions, three Minkowski
functionals characterise the morphology of an object, they are: its perimeter
(L), area (S), and genus. Out of L and S a single dimensionless Shapefinder
Statistic, F can be constructed (0 <=F <=1). F acquires extreme values on a
circle (F = 0) and a filament (F = 1). Using F, we quantify the extent of
filamentarity in the LCRS by comparing our results with a Poisson distribution
with similar geometrical properties and having the same selection function as
the survey. Our results unambiguously demonstrate that the LCRS displays a high
degree of filamentarity both in the Northern and Southern galactic sections a
result that is in general agreement with the visual appearance of the
catalogue. It is well known that gravitational clustering from Gaussian initial
conditions gives rise to the development of non-Gaussianity reflected in the
formation of a network-like filamentary structure on supercluster scales.
Consequently the fact that the smoothed LCRS catalogue shows properties
consistent with those of a Gaussian random field (Colley 1997) whereas the
unsmoothed catalogue demonstrates the presence of filamentarity lends strong
support to the conjecture that the large scale clustering of galaxies is driven
by gravitational instability.Comment: Accepted for publication in Ap
HI Fluctuations at Large Redshifts: I--Visibility correlation
We investigate the possibility of probing the large scale structure in the
universe at large redshifts by studying fluctuations in the redshifted 1420 MHz
emission from the neutral hydrogen (HI) at early epochs. The neutral hydrogen
content of the universe is known from absorption studies for z<4.5. The HI
distribution is expected to be inhomogeneous in the gravitational instability
picture and this inhomogeneity leads to anisotropy in the redshifted HI
emission. The best hope of detecting this anisotropy is by using a large
low-frequency interferometric instrument like the Giant Meter-Wave Radio
Telescope (GMRT). We calculate the visibility correlation function <V_nu(u)
V_nu'(u)> at two frequencies nu and nu' of the redshifted HI emission for an
interferometric observation. In particular we give numerical results for the
two GMRT channels centered around nu =325 and 610 MHz from density
inhomogeneity and peculiar velocity of the HI distribution. The visibility
correlation is ~10^-9 to 10^-10 Jy^2. We calculate the signal-to-noise for
detecting the correlation signal in the presence of system noise and show that
the GMRT might detect the signal for integration times ~ 100 hrs. We argue that
the measurement of visibility correlation allows optimal use of the
uncorrelated nature of the system noise across baselines and frequency
channels.Comment: 17 pages, 2 figures, Submitted to JA
Fast high fidelity quantum non-demolition qubit readout via a non-perturbative cross-Kerr coupling
Qubit readout is an indispensable element of any quantum information
processor. In this work, we experimentally demonstrate a non-perturbative
cross-Kerr coupling between a transmon and a polariton mode which enables an
improved quantum non-demolition (QND) readout for superconducting qubits. The
new mechanism uses the same experimental techniques as the standard QND qubit
readout in the dispersive approximation, but due to its non-perturbative
nature, it maximizes the speed, the single-shot fidelity and the QND properties
of the readout. In addition, it minimizes the effect of unwanted decay channels
such as the Purcell effect. We observed a single-shot readout fidelity of 97.4%
for short 50 ns pulses, and we quantified a QND-ness of 99% for long
measurement pulses with repeated single-shot readouts
Strain control of band topology and surface states in antiferromagnetic EuCdAs
Topological semimetal antiferromagnets provide a rich source of exotic
topological states which can be controlled by manipulating the orientation of
the N\'eel vector, or by modulating the lattice parameters through strain. We
investigate via density functional theory calculations, the
effects of shear strain on the bulk and surface states n two antiferromagnetic
EuCdAs phases with out-of-plane and in-plane spin configurations. When
magnetic moments are along the -axis, a longitudinal or
diagonal shear strain can tune the Dirac semimetal phase to an axion insulator
phase, characterized by the parity-based invariant . For an
in-plane magnetic order, the axion insulator phase remains robust under all
shear strains. We further find that for both magnetic orders, the bulk gap
increases and a surface gap opens on the (001) surface up to 16 meV. Because of
a nonzero index and gapped states on the (001) surface, hinge modes
are expected to happen on the side surface states between those gapped surface
states. This result can provide a valuable insight in the realization of the
long-sought axion states.Comment: 5 pages, 4 figure
Using HI to probe large scale structures at z ~ 3
The redshifted 1420 MHz emission from the HI in unresolved damped
Lyman-\alpha clouds at high z will appear as a background radiation in low
frequency radio observations. This holds the possibility of a new tool for
studying the universe at high-z, using the mean brightness temperature to probe
the HI content and its fluctuations to probe the power spectrum. Existing
estimates of the HI density at z~3 imply a mean brightness temperature of 1 mK
at 320 Mhz. The cross-correlation between the temperature fluctuations across
different frequencies and sight lines is predicted to vary from 10^{-7} K^2 to
10^{-8} K^2 over intervals corresponding to spatial scales from 10 Mpc to 40
Mpc for some of the currently favoured cosmological models. Comparing this with
the expected sensitivity of the GMRT, we find that this can be detected with
\~10 hrs of integration, provided we can distinguish it from the galactic and
extragalactic foregrounds which will swamp this signal. We discuss a strategy
based on the very distinct spectral properties of the foregrounds as against
the HI emission, possibly allowing the removal of the foregrounds from the
observed maps.Comment: 16 pages, includes 6 figures, accepted in JAA (minor revisions,
references added
HI Fluctuations at Large Redshifts: III - Simulating the Signal Expected at GMRT
We simulate the distribution of neutral hydrogen (HI) at the redshifts z=1.3
and 3.4 using a cosmological N-body simulation along with a prescription for
assigning HI masses to the particles. The HI is distributed in clouds whose
properties are consistent with those of the damped Lyman-\alpha absorption
systems (DLAs) seen in quasar spectra. The clustering properties of these
clouds are identical to those of the dark matter. We use this to simulate the
redshifted HI emission expected at 610 MHz and 325 MHz, two of the observing
bands a the GMRT. These are used to predict the correlations expected between
the complex visibilities measured at different baselines and frequencies in
radio-interferometric observations with the GMRT. The visibility correlations
directly probe the power spectrum of HI fluctuations at the epoch when the HI
emission originated, and this holds the possibility of using HI observations to
study large-scale structures at high z.Comment: Submitted to JApA, 12 Latex pages including 6 figure
- …