24 research outputs found
Infrared-to-violet tunable optical activity in atomic films of GaSe, InSe, and their heterostructures
Two-dimensional semiconductors - atomic layers of materials with covalent
intra-layer bonding and weak (van der Waals or quadrupole) coupling between the
layers - are a new class of materials with great potential for optoelectronic
applications. Among those, a special position is now being taken by
post-transition metal chalcogenides (PTMC), InSe and GaSe. It has recently been
found that the band gap in 2D crystals of InSe more than doubles in the
monolayer compared to thick multilayer crystals, while the high mobility of
conduction band electrons is promoted by their light in-plane mass. Here, we
use Raman and PL measurements of encapsulated few layer samples, coupled with
accurate atomic force and transmission electron microscope structural
characterisation to reveal new optical properties of atomically thin GaSe
preserved by hBN encapsulation. The band gaps we observe complement the
spectral range provided by InSe films, so that optical activity of these two
almost lattice-matched PTMC films and their heterostructures densely cover the
spectrum of photons from violet to infrared. We demonstrate the realisation of
the latter by the first observation of interlayer excitonic photoluminescence
in few-layer InSe-GaSe heterostructures. The spatially indirect transition is
direct in k-space and therefore is bright, while its energy can be tuned in a
broad range by the number of layers.Comment: 8 pages 4 figure
A review of elliptical and disc galaxy structure, and modern scaling laws
A century ago, in 1911 and 1913, Plummer and then Reynolds introduced their
models to describe the radial distribution of stars in `nebulae'. This article
reviews the progress since then, providing both an historical perspective and a
contemporary review of the stellar structure of bulges, discs and elliptical
galaxies. The quantification of galaxy nuclei, such as central mass deficits
and excess nuclear light, plus the structure of dark matter halos and cD galaxy
envelopes, are discussed. Issues pertaining to spiral galaxies including dust,
bulge-to-disc ratios, bulgeless galaxies, bars and the identification of
pseudobulges are also reviewed. An array of modern scaling relations involving
sizes, luminosities, surface brightnesses and stellar concentrations are
presented, many of which are shown to be curved. These 'redshift zero'
relations not only quantify the behavior and nature of galaxies in the Universe
today, but are the modern benchmark for evolutionary studies of galaxies,
whether based on observations, N-body-simulations or semi-analytical modelling.
For example, it is shown that some of the recently discovered compact
elliptical galaxies at 1.5 < z < 2.5 may be the bulges of modern disc galaxies.Comment: Condensed version (due to Contract) of an invited review article to
appear in "Planets, Stars and Stellar
Systems"(www.springer.com/astronomy/book/978-90-481-8818-5). 500+ references
incl. many somewhat forgotten, pioneer papers. Original submission to
Springer: 07-June-201
Modeling the accretion history of supermassive black holes
There is overwhelming evidence for the presence of supermassive black holes
(SMBHs) in the centers of most nearby galaxies. The mass estimates for these
remnant black holes from the stellar kinematics of local galaxies and the
quasar phenomenon at high redshifts point to the presence of assembled SMBHs.
The accretion history of SMBHs can be reconstructed using observations at high
and low redshifts as model constraints. Observations of galaxies and quasars in
the submillimeter, infrared, optical, and X-ray wavebands are used as
constraints, along with data from the demography of local black holes.
Theoretical modeling of the growth of black hole mass with cosmic time has been
pursued thus far in two distinct directions: a phenomenological approach that
utilizes observations in various wavebands, and a semi-analytic approach that
starts with a theoretical framework and a set of assumptions with a view to
matching observations. Both techniques have been pursued in the context of the
standard paradigm for structure formation in a Cold Dark Matter dominated
universe. Here, we examine the key issues and uncertainties in the theoretical
understanding of the growth of SMBHs.Comment: 19 pages, 4 figures, to appear as Chapter 4 in "Supermassive Black
Holes in the Distant Universe" (2004), ed. A. J. Barger, Kluwer Academic
Publishers, in pres
Shedding Light on the Galaxy Luminosity Function
From as early as the 1930s, astronomers have tried to quantify the
statistical nature of the evolution and large-scale structure of galaxies by
studying their luminosity distribution as a function of redshift - known as the
galaxy luminosity function (LF). Accurately constructing the LF remains a
popular and yet tricky pursuit in modern observational cosmology where the
presence of observational selection effects due to e.g. detection thresholds in
apparent magnitude, colour, surface brightness or some combination thereof can
render any given galaxy survey incomplete and thus introduce bias into the LF.
Over the last seventy years there have been numerous sophisticated
statistical approaches devised to tackle these issues; all have advantages --
but not one is perfect. This review takes a broad historical look at the key
statistical tools that have been developed over this period, discussing their
relative merits and highlighting any significant extensions and modifications.
In addition, the more generalised methods that have emerged within the last few
years are examined. These methods propose a more rigorous statistical framework
within which to determine the LF compared to some of the more traditional
methods. I also look at how photometric redshift estimations are being
incorporated into the LF methodology as well as considering the construction of
bivariate LFs. Finally, I review the ongoing development of completeness
estimators which test some of the fundamental assumptions going into LF
estimators and can be powerful probes of any residual systematic effects
inherent magnitude-redshift data.Comment: 95 pages, 23 figures, 3 tables. Now published in The Astronomy &
Astrophysics Review. This version: bring in line with A&AR format
requirements, also minor typo corrections made, additional citations and
higher rez images adde
Relativistic Dynamics and Extreme Mass Ratio Inspirals
It is now well-established that a dark, compact object (DCO), very likely a
massive black hole (MBH) of around four million solar masses is lurking at the
centre of the Milky Way. While a consensus is emerging about the origin and
growth of supermassive black holes (with masses larger than a billion solar
masses), MBHs with smaller masses, such as the one in our galactic centre,
remain understudied and enigmatic. The key to understanding these holes - how
some of them grow by orders of magnitude in mass - lies in understanding the
dynamics of the stars in the galactic neighbourhood. Stars interact with the
central MBH primarily through their gradual inspiral due to the emission of
gravitational radiation. Also stars produce gases which will subsequently be
accreted by the MBH through collisions and disruptions brought about by the
strong central tidal field. Such processes can contribute significantly to the
mass of the MBH and progress in understanding them requires theoretical work in
preparation for future gravitational radiation millihertz missions and X-ray
observatories. In particular, a unique probe of these regions is the
gravitational radiation that is emitted by some compact stars very close to the
black holes and which could be surveyed by a millihertz gravitational wave
interferometer scrutinizing the range of masses fundamental to understanding
the origin and growth of supermassive black holes. By extracting the
information carried by the gravitational radiation, we can determine the mass
and spin of the central MBH with unprecedented precision and we can determine
how the holes "eat" stars that happen to be near them.Comment: Update from the first version, 151 pages, accepted for publication @
Living Reviews in Relativit
Mass profiles of elliptical galaxies
I apply constant-anisotropy spherical dynamical models to a sample of 18 round ellipticals to look for evidence of dark halos. All 18 lie along a Tully-Fisher relation parallel to that for spirals, but fainter at given upsilon(c) by about 1 mag. By constructing more general, flattened models, I show there is a degeneracy between anisotropy and flattening, and discuss its implications
Turbulent plumes from a glacier terminus melting in a stratified ocean
The melting of submerged faces of marine-terminating glaciers is a key contributor to the glacial mass budget via direct thermodynamic ablation and the impact of ablation on calving. This study considers the behavior of turbulent plumes of buoyant meltwater in a stratified ocean, generated by melting of either near-vertical calving faces or sloping ice shelves. We build insight by applying a turbulent plume model to describe melting of a locally planar region of ice face in a linearly stratified ocean, in a regime where subglacial discharge is insignificant. The plumes rise until becoming neutrally buoyant, before intruding into the ocean background. For strong stratifications, we obtain leading-order scaling laws for the flow including the height reached by the plume before intrusion, and the melt rate, expressed in terms of the background ocean temperature and salinity stratifications. These scaling laws provide a new perspective for parameterizing glacial melting in response to a piecewise-linear discretization of the ocean stratification
Polarization memory in the nonpolar magnetic ground state of multiferroic CuFeO2
We investigate polarization memory effects in single-crystal CuFeO2, which has a magnetically induced ferroelectric phase at low temperatures and applied B fields between 7.5 and 13 T. Following electrical poling of the ferroelectric phase, we find that the nonpolar collinear antiferromagnetic ground state at B=0 T retains a strong memory of the polarization magnitude and direction, such that upon reentering the ferroelectric phase a net polarization of comparable magnitude to the initial polarization is recovered in the absence of external bias. This memory effect is very robust: in pulsed-magnetic-field measurements, several pulses into the ferroelectric phase with reverse bias are required to switch the polarization direction, with significant switching only seen after the system is driven out of the ferroelectric phase and ground state either magnetically (by application of B>13 T) or thermally. The memory effect is also largely insensitive to the magnetoelastic domain composition, since no change in the memory effect is observed for a sample driven into a single-domain state by application of stress in the [110] direction. On the basis of Monte Carlo simulations of the ground-state spin configurations, we propose that the memory effect is due to the existence of helical domain walls within the nonpolar collinear antiferromagnetic ground state, which would retain the helicity of the polar phase for certain magnetothermal histories