Caesium on Si(100) Studied by Biassed Secondary Electron Microscopy

An ultra-high vacuum scanning electron microscope (UHV-SEM) has been used to study sub-monolayers of Cs on Si(100) surface. Cs adsorption on the surface causes a considerable change in the work function. Coverages below 1/2 monolayer (ML) have been estimated by correlating the work function changes with the secondary electron (SE) signal. It has been found that this signal is sensitive down to ~ 0.005 ML when the sample is biassed to a few hundred volts. Electron trajectories from a biassed sample have been simulated for electrons originating from different areas with different work functions across the sample. This indicates that variations in coverage can be determined by secondary electron imaging provided these coverages are less than 1/2 ML. The diffusion of Cs (\u3c 1/2 ML) above room temperature has been studied using the biassed-SE imaging technique. Observed diffusion profiles have unusual features including two linear regions. These can be explained by a model which contains two competing adsorption sites, and includes blocking of the diffusion paths by other Cs atoms

Asymptotic Capture-Number and Island-Size Distributions for One-Dimensional Irreversible Submonolayer Growth

Using a set of evolution equations [J.G. Amar {\it et al}, Phys. Rev. Lett. {\bf 86}, 3092 (2001)] for the average gap-size between islands, we calculate analytically the asymptotic scaled capture-number distribution (CND) for one-dimensional irreversible submonolayer growth of point islands. The predicted asymptotic CND is in reasonably good agreement with kinetic Monte-Carlo (KMC) results and leads to a \textit{non-divergent asymptotic} scaled island-size distribution (ISD). We then show that a slight modification of our analytical form leads to an analytic expression for the asymptotic CND and a resulting asymptotic ISD which are in excellent agreement with KMC simulations. We also show that in the asymptotic limit the self-averaging property of the capture zones holds exactly while the asymptotic scaled gap distribution is equal to the scaled CND.Comment: 4 pages, 1 figure, submitted to Phys. Rev.

Spatio-temporal distribution of nucleation events during crystal growth

We consider irreversible second-layer nucleation that occurs when two adatoms on a terrace meet. We solve the problem analytically in one dimension for zero and infinite step-edge barriers, and numerically for any value of the barriers in one and two dimensions. For large barriers, the spatial distribution of nucleation events strongly differs from $\rho^2$, where $\rho$ is the stationary adatom density in the presence of a constant flux. The probability $Q(t)$ that nucleation occurs at time $t$ after the deposition of the second adatom, decays for short time as a power law [$Q(t)\sim t^{-1/2}$] in $d=1$ and logarithmically [$Q(t)\sim 1/\ln(t/t_0)$] in $d=2$; for long time it decays exponentially. Theories of the nucleation rate $\omega$ based on the assumption that it is proportional to $\rho^2$ are shown to overestimate $\omega$ by a factor proportional to the number of times an adatom diffusing on the terrace visits an already visited lattice site.Comment: 4 pages, 3 figures; accepted for publication on PR

Bronchiectasis: a model for chronic bacterial infection inducing autoimmunity in rheumatoid arthritis.

ArticleCopyright © 2015 The Authors. Arthritis & Rheumatology is published by Wiley Periodicals, Inc. on behalf of the American College of Rheumatology.Objective: Bronchiectasis (BR) is a risk factor for rheumatoid arthritis (RA). Here we examine the potential of BR in generating rheumatoid factors (RFs) and anti-citrullinated peptide antibodies (ACPA) in patients with BR alone and in patients with BR and RA (BRRA). Methods: We studied 122 patients with BR alone, 50 BRRA, 50 RA without lung disease, with 87 asthma and 79 healthy subjects as controls. RFs were measured by an automated analyzer, and ACPA using CCP2. Fine specificities to citrullinated α-enolase (CEP-1), citrullinated vimentin (cVim) and fibrinogen (cFib) with their arginine control peptides (REP-1, Vim and Fib) measured by ELISA. Results: In the BR patients 39% were ever smokers compared to 42% of the controls. Serum samples from BR patients had an increased frequency of RF (25%; p< 0.05) and 5% to CCP2, 7% to CEP-1, 7% to cVIM (all p=ns) and 12% cFib (p <0.05). There was also a corresponding increase in antibodies to the arginine-containing control peptides in the BR patients; REP-1, 19% (p< 0.01) and Vim, 16% (p<0.05), demonstrating that the ACPA response in BR is not citrulline-specific. Lack of citrulline specificity was further confirmed by absorption studies. In BRRA all ACPA specificities were highly citrulline-specific. Conclusion: Bronchiectasis is an unusual but potent model for the induction of autoimmunity in RA by bacterial infection in the lung. Our study suggests that in the early stages of tolerance breakdown, the ACPA response is not citrulline-specific, but becomes more so in those patients with BR that develop BRRA.Arthritis Research UKEuropean UnionIMI project BTCure7th Framework Programme project Gums and Joint

Epidemiology of basal and cutaneous squamous cell carcinoma in the U.K. 2013–15:a cohort study

BACKGROUND: Basal cell carcinoma (BCC) and cutaneous squamous cell carcinoma (cSCC), together known as keratinocyte cancers (KCs), are the commonest cancer in white ethnic populations. Recent improvements to registry data collection in England has allowed more accurate analysis of the epidemiology of BCC and cSCC and for the first time we are able to provide an accurate (representative) tumour burden for KC in the U.K. OBJECTIVES: To estimate the incidence of BCC and cSCC in the U.K. METHODS: A cohort of patients with KCs between 2013 and 2015 were identified using linkage to diagnostic codes derived from pathology reports collected into the national cancer registry. Data from England's cancer registry were combined with data from Scotland, Northern Ireland and Wales. European age-standardized incidence rates (EASRs) of the first BCC and cSCC per patient per annum (PPPA) were calculated. RESULTS: In the U.K, the EASR of the first BCC and cSCC PPPA in 2013-15 were 285 and 77 per 100 000 person years, respectively (211 120 KCs total in 2015). The mean annual percentage increase was 5% between 2013 and 2015 for both BCC and cSCC. By counting the first KC PPPA, we include an additional 51% KCs compared with the previous reporting technique which counts only the first BCC and cSCC in a patient's lifetime, yet it represents a probable underestimation of 5-11% of the true tumour count. CONCLUSIONS: Based on an improved methodology, a more representative incidence of KC is presented, which is essential to healthcare planning and will lead to improved understanding of the epidemiology of KC. What's already known about this topic? Keratinocyte cancers (KCs) are the most common cancers affecting white ethnic populations. The incidence of basal cell carcinoma (BCC) and cutaneous squamous cell carcinoma (cSCC) is increasing worldwide including the U.K., most commonly in elderly male Caucasian patients. These cancers are traditionally substantially underreported and frequently excluded from national cancer statistics. What does this study add? Using improved data collection methods in England and validated tumour-reporting techniques, we report the most accurate BCC and cSCC incidence data for the U.K. ever published. Identifying the first BCC and cSCC per patient per annum, the incidence of BCC and cSCC in the U.K. (excluding Wales) was 285 and 77 per 100 000 person years, respectively, between 2013 and 2015, with more than 210 000 KCs in the U.K. in 2015

The process of irreversible nucleation in multilayer growth. I. Failure of the mean-field approach

The formation of stable dimers on top of terraces during epitaxial growth is investigated in detail. In this paper we focus on mean-field theory, the standard approach to study nucleation. Such theory is shown to be unsuitable for the present problem, because it is equivalent to considering adatoms as independent diffusing particles. This leads to an overestimate of the correct nucleation rate by a factor N, which has a direct physical meaning: in average, a visited lattice site is visited N times by a diffusing adatom. The dependence of N on the size of the terrace and on the strength of step-edge barriers is derived from well known results for random walks. The spatial distribution of nucleation events is shown to be different from the mean-field prediction, for the same physical reason. In the following paper we develop an exact treatment of the problem.Comment: 19 pages, 3 figures. To appear in Phys. Rev.

The effect of monomer evaporation on a simple model of submonolayer growth

We present a model for thin film growth by particle deposition that takes into account the possible evaporation of the particles deposited on the surface. Our model focuses on the formation of two-dimensional structures. We find that the presence of evaporation can dramatically affect the growth kinetics of the film, and can give rise to regimes characterized by different ``growth'' exponents and island size distributions. Our results are obtained by extensive computer simulations as well as through a simple scaling approach and the analysis of rate equations describing the system. We carefully discuss the relationship of our model with previous studies by Venables and Stoyanov of the same physical situation, and we show that our analysis is more general.Comment: 41 pages including figures, Revtex, to be published in Physical Review

Formation of a stable deacagonal quasicrystalline Al-Pd-Mn surface layer

We report the in situ formation of an ordered equilibrium decagonal Al-Pd-Mn quasicrystal overlayer on the 5-fold symmetric surface of an icosahedral Al-Pd-Mn monograin. The decagonal structure of the epilayer is evidenced by x-ray photoelectron diffraction, low-energy electron diffraction and electron backscatter diffraction. This overlayer is also characterized by a reduced density of states near the Fermi edge as expected for quasicrystals. This is the first time that a millimeter-size surface of the stable decagonal Al-Pd-Mn is obtained, studied and compared to its icosahedral counterpart.Comment: Submitted to Phys. Ref. Lett. (18 July 2001

Estimating Nuisance Parameters in Inverse Problems

Many inverse problems include nuisance parameters which, while not of direct interest, are required to recover primary parameters. Structure present in these problems allows efficient optimization strategies - a well known example is variable projection, where nonlinear least squares problems which are linear in some parameters can be very efficiently optimized. In this paper, we extend the idea of projecting out a subset over the variables to a broad class of maximum likelihood (ML) and maximum a posteriori likelihood (MAP) problems with nuisance parameters, such as variance or degrees of freedom. As a result, we are able to incorporate nuisance parameter estimation into large-scale constrained and unconstrained inverse problem formulations. We apply the approach to a variety of problems, including estimation of unknown variance parameters in the Gaussian model, degree of freedom (d.o.f.) parameter estimation in the context of robust inverse problems, automatic calibration, and optimal experimental design. Using numerical examples, we demonstrate improvement in recovery of primary parameters for several large- scale inverse problems. The proposed approach is compatible with a wide variety of algorithms and formulations, and its implementation requires only minor modifications to existing algorithms.Comment: 16 pages, 5 figure

Influence of adatom interactions on second layer nucleation

We develop a theory for the inclusion of adatom interactions in second layer nucleation occurring in epitaxial growth. The interactions considered are due to ring barriers between pairs of adatoms and binding energies of unstable clusters. The theory is based on a master equation, which describes the time development of microscopic states that are specified by cluster configurations on top of an island. The transition rates are derived by scaling arguments and tested against kinetic Monte-Carlo simulations. As an application we reanalyze experiments to determine the step edge barrier for Ag/Pt(111).Comment: 4 pages, 4 figure