78,182 research outputs found
Atomic Diffusion in the Surface State of Mott Insulator NiS2
We present resistivity measurements of Mott insulator NiS2 single crystals
after heat treatment. We find a strong increase of the low temperature
resistivity that relaxes back towards the pristine behaviour over several days
with a time constant of 45 h at room temperature. The low temperature
resistivity has previously been shown to be dominated by surface conduction (T.
Thio and J. Bennett, PRB 50 10574 1994). Consequently, the changes induced by
heat treatment are attributed to changes to surface states. Our results suggest
the creation of vacancies in the surface that re refilled from the bulk via
atomic diffusion. We estimate a diffusion constant of
m/s at room temperature. We identify sulphur vacancies as the most likely
to form via oxidisation of sulphur forming volatile SO2 during heat treatment.
Our results point towards these sulphur vacancies to be the source of surface
state localisation in NiS2.Comment: 6 pages, 6 figures. Journal of Magnetism and Magnetic Materials
(2015
Molecular cooling in the diffuse interstellar medium
We use a simple one-zone model of the thermal and chemical evolution of
interstellar gas to study whether molecular hydrogen (H2) is ever an important
coolant of the warm, diffuse interstellar medium (ISM). We demonstrate that at
solar metallicity, H2 cooling is unimportant and the thermal evolution of the
ISM is dominated by metal line cooling. At metallicities below 0.1 Z_solar,
however, metal line cooling of low density gas quickly becomes unimportant and
H2 can become the dominant coolant, even though its abundance in the gas
remains small. We investigate the conditions required in order for H2 to
dominate, and show that it provides significant cooling only when the ratio of
the interstellar radiation field strength to the gas density is small. Finally,
we demonstrate that our results are insensitive to changes in the initial
fractional ionization of the gas or to uncertainties in the nature of the dust
present in the low-metallicity ISM.Comment: 13 pages, 6 figures. Minor changes to match version accepted by MNRA
Approximations for modelling CO chemistry in GMCs: a comparison of approaches
We examine several different simplified approaches for modelling the
chemistry of CO in three-dimensional numerical simulations of turbulent
molecular clouds. We compare the different models both by looking at the
behaviour of integrated quantities such as the mean CO fraction or the
cloud-averaged CO-to-H2 conversion factor, and also by studying the detailed
distribution of CO as a function of gas density and visual extinction. In
addition, we examine the extent to which the density and temperature
distributions depend on our choice of chemical model.
We find that all of the models predict the same density PDF and also agree
very well on the form of the temperature PDF for temperatures T > 30 K,
although at lower temperatures, some differences become apparent. All of the
models also predict the same CO-to-H2 conversion factor, to within a factor of
a few. However, when we look more closely at the details of the CO
distribution, we find larger differences. The more complex models tend to
produce less CO and more atomic carbon than the simpler models, suggesting that
the C/CO ratio may be a useful observational tool for determining which model
best fits the observational data. Nevertheless, the fact that these chemical
differences do not appear to have a strong effect on the density or temperature
distributions of the gas suggests that the dynamical behaviour of the molecular
clouds on large scales is not particularly sensitive to how accurately the
small-scale chemistry is modelled.Comment: 18 pages, 10 figures. Minor revisions, including the addition of a
comparison of simulated and observed C/CO ratios. Accepted by MNRA
A Lifecourse Approach to Long-Term Sickness Absence-A Cohort Study
This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited
The First Stellar Cluster
We report results from numerical simulations of star formation in the early
universe that focus on gas at very high densities and very low metallicities.
We argue that the gas in the central regions of protogalactic halos will
fragment as long as it carries sufficient angular momentum. Rotation leads to
the build-up of massive disk-like structures which fragment to form protostars.
At metallicities Z ~ 10^-5 Zsun, dust cooling becomes effective and leads to a
sudden drop of temperature at densities above n = 10^12 cm^-3. This induces
vigorous fragmentation, leading to a very densely-packed cluster of low-mass
stars. This is the first stellar cluster. The mass function of stars peaks
below 1 Msun, similar to what is found in the solar neighborhood, and
comparable to the masses of the very-low metallicity subgiant stars recently
discovered in the halo of our Milky Way. We find that even purely primordial
gas can fragment at densities 10^14 cm^-3 < n < 10^16 cm^-3, although the
resulting mass function contains only a few objects (at least a factor of ten
less than the Z = 10^-5 Zsun mass function), and is biased towards higher
masses. A similar result is found for gas with Z = 10^-6 Zsun. Gas with Z <=
10^-6 Zsun behaves roughly isothermally at these densities (with polytropic
exponent gamma ~ 1.06) and the massive disk-like structures that form due to
angular momentum conservation will be marginally unstable. As fragmentation is
less efficient, we expect stars with Z <= 10^-6 Zsun to be massive, with masses
in excess of several tens of solar masses, consistent with the results from
previous studies.Comment: 9 pages, 6 figures. Accepted by ApJ for publicatio
On the effects of rotation during the formation of population III protostars
It has been suggested that turbulent motions are responsible for the
transport of angular momentum during the formation of Population III stars,
however the exact details of this process have never been studied. We report
the results from three dimensional SPH simulations of a rotating
self-gravitating primordial molecular cloud, in which the initial velocity of
solid-body rotation has been changed. We also examine the build-up of the discs
that form in these idealized calculations.Comment: 4 pages, AIP Conference Proceedings, First Stars IV from Hayashi to
the Future (Kyoto, Japan
Efficient generation of graph states for quantum computation
We present an entanglement generation scheme which allows arbitrary graph
states to be efficiently created in a linear quantum register via an auxiliary
entangling bus. The dynamics of the entangling bus is described by an effective
non-interacting fermionic system undergoing mirror-inversion in which qubits,
encoded as local fermionic modes, become entangled purely by Fermi statistics.
We discuss a possible implementation using two species of neutral atoms stored
in an optical lattice and find that the scheme is realistic in its requirements
even in the presence of noise.Comment: 4 pages, 3 figures, RevTex 4; v2 - Major changes and new result
Interpreting the sub-linear Kennicutt-Schmidt relationship: The case for diffuse molecular gas
Recent statistical analysis of two extragalactic observational surveys
strongly indicate a sublinear Kennicutt-Schmidt (KS) relationship between the
star formation rate (Sigsfr) and molecular gas surface density (Sigmol). Here,
we consider the consequences of these results in the context of common
assumptions, as well as observational support for a linear relationship between
Sigsfr and the surface density of dense gas. If the CO traced gas depletion
time (tau_mol) is constant, and if CO only traces star forming giant molecular
clouds (GMCs), then the physical properties of each GMC must vary, such as the
volume densities or star formation rates. Another possibility is that the
conversion between CO luminosity and Sigmol, the XCO factor, differs from
cloud-to-cloud. A more straightforward explanation is that CO permeates the
hierarchical ISM, including the filaments and lower density regions within
which GMCs are embedded. A number of independent observational results support
this description, with the diffuse gas comprising at least 30% of the total
molecular content. The CO bright diffuse gas can explain the sublinear KS
relationship, and consequently leads to an increasing tau_mol with Sigmol. If
Sigsfr linearly correlates with the dense gas surface density, a sublinear KS
relationship indicates that the fraction of diffuse gas fdiff grows with
Sigmol. In galaxies where Sigmol falls towards the outer disk, this description
suggests that fdiff also decreases radially.Comment: 8 pages, 4 figures, to appear in MNRAS, comments welcom
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