Determination of positron annihilation lifetime spectroscopy instrument timing resolution function and source terms using standard samples

The extraction of material positron lifetime components from positron annihilation lifetime spectroscopy measurements, performed using conventional unmoderated radionuclide positron sources, requires accurate knowledge of both the spectrometer instrument timing resolution function (IRF) and annihilation events extrinsic to the material, the source correction terms. Here we report the results from study of spectrometer performance made using two reference samples, high purity polycrystalline aluminium, and stainless steel supplied by the National Metrology Institute of Japan (NMIJ RM 5607-a). Both prepared with directly deposited 22NaCl positron sources. The IRFs obtained by fitting spectra from both reference samples were monitored with time to evaluate spectrometer stability and to compare methods of IRF determination. Using the aluminium IRFs the analysis of spectra from the NMIJ stainless steel reference samples yielded a single lifetime component with value 106.9(9) ps

First-Order Reorientation of the Flux-Line Lattice in CaAlSi

The flux line lattice in CaAlSi has been studied by small angle neutron scattering. A well defined hexagonal flux line lattice is seen just above Hc1 in an applied field of only 54 Oe. A 30 degree reorientation of this vortex lattice has been observed in a very low field of 200 Oe. This reorientation transition appears to be of first-order and could be explained by non-local effects. The magnetic field dependence of the form factor is well described by a single penetration depth of 1496(1) angstroms and a single coherence length of 307(1) angstroms at 2 K. At 1.5 K the penetration depth anisotropy is 2.7(1) with the field applied perpendicular to the c axis and agrees with the coherence length anisotropy determined from critical field measurements.Comment: 5 pages including 6 figures, to appear in Physical Review Letter

Anomalous thermal expansion in 1D transition-metal cyanides: what makes the novel trimetallic cyanide Cu1/3Ag1/3Au1/3CN behave differently?

The structural dynamics of a quasi-one-dimensional (1D) mixed-metal cyanide, Cu1/3Ag1/3Au1/3CN, with intriguing thermal properties is explored. All the current known related compounds with straight-chain structures, such as group 11 cyanides CuCN, AgCN, AuCN and bimetallic cyanides MxM’1-xCN (M, M’ = Cu, Ag, Au), exhibit 1D negative thermal expansion (NTE) along the chains and positive thermal expansion (PTE) perpendicular to them. Cu1/3Ag1/3Au1/3CN exhibits similar PTE perpendicular to the chains, however PTE, rather than NTE, is also observed along the chains. In order to understand the origin of this unexpected behavior, inelastic neutron scattering (INS) measurements were carried out, underpinned by lattice-dynamical density-functional-theory (DFT) calculations. Synchrotron-based pair-distribution-function (PDF) analysis and 13C solid-state nuclear-magnetic-resonance (SSNMR) measurements were also performed to build an input structural model for the lattice dynamical study. The results indicate that transverse motions of the metal ions are responsible for the PTE perpendicular to the chains, as is the case for the related group 11 cyanides. However NTE along the chain due to the tension effect of these transverse motions is not observed. As there are different metal-to-cyanide bond lengths in Cu1/3Ag1/3Au1/3CN, the metals in neighboring chains cannot all be truly co-planar in a straight-chain model. For this system, DFT-based phonon calculations predict small PTE along the chain due to low-energy chain-slipping modes induced by a bond-rotation effect on the weak metallophilic bonds. However the observed PTE is greater than that predicted with the straight-chain model. Small bends in the chain to accommodate truly co-planar metals provide an alternative explanation for thermal behavior. These would mitigate the tension effect induced by the transverse motions of the metals and, as temperature increases and the chains move further apart, a straightening could occur resulting in the observed PTE. This hypothesis is further supported by unusual evolution in the phonon spectra, which suggest small changes in local symmetry with temperature

Structural and Magnetic Investigations of Single-Crystals of the Neodymium Zirconate Pyrochlore, Nd2Zr2O7

We report structural and magnetic properties studies of large high quality single-crystals of the frustrated magnet, Nd$_2$Zr$_2$O$_7$. Powder x-ray diffraction analysis confirms that Nd$_2$Zr$_2$O$_7$ adopts the pyrochlore structure. Room-temperature x-ray diffraction and time-of-flight neutron scattering experiments show that the crystals are stoichiometric in composition with no measurable site disorder. The temperature dependence of the magnetic susceptibility shows no magnetic ordering at temperatures down to 0.5 K. Fits to the magnetic susceptibility data using a Curie-Weiss law reveal a ferromagnetic coupling between the Nd moments. Magnetization versus field measurements show a local Ising anisotropy along the axes of the Nd$^{3+}$ ions in the ground state. Specific heat versus temperature measurements in zero applied magnetic field indicate the presence of a thermal anomaly below $T\sim7$ K, but no evidence of magnetic ordering is observed down to 0.5 K. The experimental temperature dependence of the single-crystal bulk dc susceptibility and isothermal magnetization are analyzed using crystal field theory and the crystal field parameters and exchange coupling constants determined.Comment: 10 pages, 6 figures, 4 tables. Accepted for publication in Physical Review

Operation Moshtarak and the manufacture of credible, “heroic” warfare

Richard Lance Keeble argues that Fleet Street’s coverage of the Afghan conflict has served largely to promote the interests of the military/industrial/media complex – and marginalise the views of the public who have consistently appealed in polls for the troops to be brought back hom

Menopausal/Post-Menopausal Women and Maternal Career Disruption

This chapter seeks to consider the possibility of a blindness to considering the impact of mothering and post mothering (menopause) upon the workforce in terms of career development models. There has been a continuing ‘onward and upward’ tournament (Malhotra et al., 2010) in the UK which suggests workers should continue strive to change jobs to seek higher pay and status moving around the country throughout their worklife with little regards for their familial responsibilities. Despite a discourse which suggests the economic position of women has been heralded as positively changed as there is no longer a ‘glass ceiling’ for female employees who are willing to put work before their employment (Whiteside and Harding, 2013), this provides a hollow victory. It fails to recognise that within the labour market, there will be parental years. Legislation in the UK and even Europe largely deals with ‘baby years’ but despite the inclusion of some ‘family friendly work-life balance’ initiatives, the experience for many parents is that the workplace represents a cause for work family conflict. In part this is because mothering is still stereotypically seen as a women’s role and increasing legislation provides an illusion of resolving discrimination (TUC, 2014). As such there is little consideration for the development of mothers during their ‘career‘ lifetime

Employment mobility in high-technology agglomerations: the cases of Oxfordshire and Cambridgeshire

This paper examines labour market behaviour of the highly skilled in high-tech local economies, taking the UK examples of Oxfordshire and Cambridgeshire as case studies. It reports on data from a survey of members of three scientific institutes to compare rates of employee mobility in the two locations and considers the likely explanations and implications of those patterns

Characterization of vacancy defects in Cu(In,Ga)Se2 by positron annihilation spectroscopy

The photovoltaic performance of Cu(In1-x,Gax)Se2 (CIGS) materials is commonly assumed to be degraded by the presence of vacancy-related defects. However, experimental identification of specific vacancy defects remains challenging. In this work we report positron lifetime measurements on CIGS crystals with x = 0, and x = 0.05, saturation trapping to two dominant vacancy defect types, in both types of crystal, is observed and found to be independent of temperature between 15–300 K. Atomic superposition method calculations of the positron lifetimes for a range of vacancy defects in CIS and CGS are reported. The calculated lifetimes support the assignment of the first experimental lifetime component to monovacancy or divacancy defects, and the second to trivacancies, or possibly the large In-Se divacancy. Further, the calculated positron parameters obtained here provide evidence that positron annihilation spectroscopy has the capability to identify specific vacancy-related defects in the Cu(In1-x,Gax)Se2 chalcogenides

Measuring regional creative capacity: A literature review for rural-specific approaches

Recent theories on regional creative capacity often focus on urban regions without taking into account rural regions. In addition, the application of such analyses to rural regions may lead to misrepresentation or misunderstanding of rural creative capacity. Against this background, the aim of the present study is to integrate the existing literature on different components of creative capacity, namely, knowledge, innovation, entrepreneurship and networks, in order to build a more comprehensive framework for rural creative capacity and its evaluation. In the light of the perspective from the empirical literature review on the evaluation of creative capacity in rural regions, various empirical measurements seem to misrepresent or underestimate the creative capacity of rural regions. Therefore, there is a clear need to use the locality in relation to its dynamics, i.e. tacit knowledge, cultural heritage and social and physical environment as the main and basic measurement unit for creative capacity analysis. 2010 Taylor & Francis