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 $D \approx 10^{-10}$ m$^2$/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