First-principles calculation of positron lifetimes and affinities in perfect and imperfect transition-metal carbides and nitrides

First-principles electronic structure and positron-state calculations for transition-metal carbides and nitrides are performed. Perfect NaCl structures as well as structures with metal or carbon/nitrogen vacancies are considered. The positron affinities and lifetimes are determined. The trends are discussed and the results are compared with recent positron lifetime measurements for group-IV and -V refractory metal carbides. The present analysis suggests, contradictory to an earlier interpretation, that positrons are trapped and annihilated at both carbon and metal vacancies. The concentration of metal vacancies detected by positron annihilation methods is probably very low, below the sensitivity limit of other experimental methods.Peer reviewe

Evaluation of some basic positron-related characteristics of SiC

First-principles electronic structure and positron-state calculations for perfect and defected 3C- and 6H-SiC polytypes of SiC have been performed. Monovacancies and divacancies have been treated; the influence of lattice position and nitrogen impurities have been considered in the former case. Positron affinities and binding energies have been calculated; trends are discussed, and the results compared with recent atomic superposition method calculations. Experimental determination of the electron and positron work functions of the same 6H-SiC allows an assessment of the accuracy of the present first-principles calculations, and to suggest further improvements. © 1996 The American Physical Society.Peer reviewe

Effects of anharmonic strain on phase stability of epitaxial films and superlattices: applications to noble metals

Epitaxial strain energies of epitaxial films and bulk superlattices are studied via first-principles total energy calculations using the local-density approximation. Anharmonic effects due to large lattice mismatch, beyond the reach of the harmonic elasticity theory, are found to be very important in Cu/Au (lattice mismatch 12%), Cu/Ag (12%) and Ni/Au (15%). We find that is the elastically soft direction for biaxial expansion of Cu and Ni, but it is for large biaxial compression of Cu, Ag, and Au. The stability of superlattices is discussed in terms of the coherency strain and interfacial energies. We find that in phase-separating systems such as Cu-Ag the superlattice formation energies decrease with superlattice period, and the interfacial energy is positive. Superlattices are formed easiest on (001) and hardest on (111) substrates. For ordering systems, such as Cu-Au and Ag-Au, the formation energy of superlattices increases with period, and interfacial energies are negative. These superlattices are formed easiest on (001) or (110) and hardest on (111) substrates. For Ni-Au we find a hybrid behavior: superlattices along and like in phase-separating systems, while for they behave like in ordering systems. Finally, recent experimental results on epitaxial stabilization of disordered Ni-Au and Cu-Ag alloys, immiscible in the bulk form, are explained in terms of destabilization of the phase separated state due to lattice mismatch between the substrate and constituents.Comment: RevTeX galley format, 16 pages, includes 9 EPS figures, to appear in Physical Review

Ab initio Investigation of Elasticity and Stability of Metal Aluminum

On the basis of the pseudopotential plane-wave(PP-PW) method in combination with the local-density-functional theory(LDFT), complete stress-strain curves for the uniaxial loading and uniaxial deformation along the [001] and [111] directions, and the biaxial proportional extension along [010] and [001] of aluminium are obtained. During the uniaxial loading, certain general behaviors of energy versus stretch and the load versus the stretch are confirmed; in each acse, there exist three special unstressed structures: f.c.c., b.c.c. and f.c.t. for [001]; f.c.c., s.c. and b.c.c. for [111]. Using stability criteria, we find that all of these state are unstable, and always occur together with shear instability, except the natural f.c.c. structure. A Bain transformation from the stable f.c.c. structure to the stable b.c.c. configuration cannot be obtained by uniaxial compression along any equivalent [001] and [111] direction. The tensile strength are similar for the two directions. For the higher energy barrier of [111] direction, the compressive strength is greater than that for the [001] direction. With increase in the ratio of the biaxial proportional extension, the stress and tensile strength increase; however, the critical strain does not change significantly. Our results add to the existing ab initio database for use in fitting and testing interatomic potentials.Comment: 9 Pages in Revtex and 11 Eps figure

'Surf's up!':A call to take english soccer fan interactions on the internet more seriously

Soccer fandom practices in England have been significantly impacted by globalization. The creation of the Premier League in 1992, and the way in which satellite television company BSkyB dominated coverage of this, together with other developments, have led to changes in how fans consume top-level English soccer. Whilst such global transformations are well documented in the sociology of soccer literature, the implications of the rise of the most advanced global form of communication - the Internet - on the practices of fans of English soccer clubs, have not been fully taken into account by academics. As such, the significance of the Internet as a site for fans to interact remains under-investigated. This article argues that online interactions between fans of English clubs need to be taken more seriously by academics if they are to more fully understand how soccer contributes to the maintenance of social identities in contemporary England

Exploring adults’ experiences of sedentary behaviour and participation in nonworkplace interventions designed to reduce sedentary behaviour: a thematic synthesis of qualitative studies

Background: Sedentary behaviour is any waking behaviour characterised by an energy expenditure of ≤1.5 metabolic equivalent of task while in a sitting or reclining posture. Prolonged bouts of sedentary behaviour have been associated with negative health outcomes in all age groups. We examined qualitative research investigating perceptions and experiences of sedentary behaviour and of participation in non-workplace interventions designed to reduce sedentary behaviour in adult populations. Method: A systematic search of seven databases (MEDLINE, AMED, Cochrane, PsychINFO, SPORTDiscus, CINAHL and Web of Science) was conducted in September 2017. Studies were assessed for methodological quality and a thematic synthesis was conducted. Prospero database ID: CRD42017083436. Results: Thirty individual studies capturing the experiences of 918 individuals were included. Eleven studies examined experiences and/or perceptions of sedentary behaviour in older adults (typically ≥60 years); ten studies focused on sedentary behaviour in people experiencing a clinical condition, four explored influences on sedentary behaviour in adults living in socio-economically disadvantaged communities, two examined university students’ experiences of sedentary behaviour, two on those of working-age adults, and one focused on cultural influences on sedentary behaviour. Three analytical themes were identified: 1) the impact of different life stages on sedentary behaviour 2) lifestyle factors influencing sedentary behaviour and 3) barriers and facilitators to changing sedentary behaviour. Conclusions: Sedentary behaviour is multifaceted and influenced by a complex interaction between individual, environmental and socio-cultural factors. Micro and macro pressures are experienced at different life stages and in the context of illness; these shape individuals’ beliefs and behaviour related to sedentariness. Knowledge of sedentary behaviour and the associated health consequences appears limited in adult populations, therefore there is a need for provision of accessible information about ways in which sedentary behaviour reduction can be integrated in people’s daily lives. Interventions targeting a reduction in sedentary behaviour need to consider the multiple influences on sedentariness when designing and implementing interventions

Archeologisch Bureauonderzoek en Inventariserend Veldonderzoek door middel van grondboringen 'Plangebied Handelskade', Vreeswijk, Gemeente Nieuwegein

In het kader van de vergunningverlening ten behoeve van Plangebied Handelskade te Vreeswijk (Gemeente Nieuwegein) werd een Archeologisch Bureauonderzoek en een Inventariserend Veldonderzoek door middel van grondboringen uitgevoerd

Archeologische Opgraving, variant Archeologische Begeleiding Plangebied Dijksestraat 5 en 7, Didam, Gemeente Montferland

In het westen werden in de top van de cultuurlaag 24 archeologische sporen en ook archeologische vondsten aangetroffen. Het betrof twintig paalkuilen, een waterput en een leidingsleuf. Al deze sporen dateren uit de periode van circa 1900 - 1950. In het oosten werden in de top van het dekzand twintig archeologische sporen en ook archeologische vondsten aangetroffen. Het betrof tien paalkuilen en een kuil uit de IJzertijd. Op basis van de sporen en het vondstmateriaal kan worden geconcludeerd dat er ter plaatse van het onderzoeksgebied en de directe omgeving daarvan een nederzettingsterrein uit de IJzertijd is gelegen. Er konden geen structuren worden gereconstrueerd, vooral als gevolg van de beperkte omvang van de Archeologische Begeleiding evenals de aanwezigheid van omvangrijke recente verstoringen in het vlak en natuurlijke verstoringen in de top van het dekzand. Wel valt een viertal sporen op dat in een rij was gelegen

Archeologisch Bureauonderzoek Voorbereiding Aanleg Ecologische Verbindingszone Oude Aa, Vlierden en Helmond, Gemeente Deurne en Gemeente Helmond

In het kader van de vergunningverlening ten behoeve van de aanleg van de Ecologische Verbindingszone Oude Aa te Vlierden en Helmond (Gemeente Deurne en Gemeente Helmond) werd een Archeologisch Bureauonderzoek uitgevoerd