Negative-U properties for substitutional Au in Si

The isolated substitutional gold impurity in bulk silicon is studied in detail using electronic structure calculations based on density-functional theory. The defect system is found to be a non-spin-polarized negative-U centre, thus providing a simple solution to the long-standing debate over the electron paramagnetic resonance signal for gold in silicon. There is an excellent agreement (within 0.03 eV) between the well-established experimental donor and acceptor levels and the predicted stable charge state transition levels, allowing for the unambiguous assignment of the two experimental levels to the (1+/1-) and (1-/3-) transitions, respectively, in contrast to previously held assumptions about the system.Comment: 6 pages, 5 figure

Eikonalized mini-jet cross-sections in γγ\gamma \gamma collisions

In this note we assess the validity and uncertainties in the predictions of the eikonalised mini-jet model for $\sigma^{inel}_{\gamma \gamma}$. We are able to find a choice of parameters where the predictions are compatible with the current data. Even for this restricted range of parameters the predictions at the high c.m. energies, which can be reached at the TeV energy $e^+e^-$ colliders, differ by about $\pm 25\%$. LEP 2 data can help pinpoint these parameters and hence reduce the uncertainties in the predictions.Comment: 5 pages, latex, requires epsfig.sty, a4wide.sty, full ps file available at http://hpteor.lnf.infn.it/pancheri/lc2000.p

Photon-Photon total inelastic cross-section

We discuss predictions for the total inelastic gamma-gamma cross-section and their model dependence on the input parameters. We compare results from a simple extension of the Regge Pomeron exchange model as well as predictions from the eikonalized mini-jet model with recent LEP data.Comment: 7 pages, LateX, 2 eps figures. Talk presented at Photon'97, Egmond aan Zee, May 199

Productivity Spillovers, Terms of Trade and the "Home Market Effect"

This paper analyzes the welfare implications of international spillovers related to productivity gains, changes in market size, or government spending. We introduce trade costs and endogenous varieties in a two-country general-equilibrium model with monopolistic competition, drawing a distinction between productivity gains that enhance manufacturing efficiency, and gains that lower the cost of firms' entry and product differentiation. Our model suggests that countries with lower manufacturing costs have higher GDP but supply a smaller number of goods at a lower international price. Countries with lower entry and differentiation costs also have higher GDP, but supply a larger array of goods at improved terms of trade. The sign of the international welfare spillovers depends on terms of trade, but also on consumers' taste for variety. Higher domestic demand has macroeconomic implications that are similar to those of a reduction in firms' entry costs.

Twist-angle dependence of electron correlations in moir\'e graphene bilayers

Motivated by the recent observation of correlated insulator states and unconventional superconductivity in twisted bilayer graphene, we study the dependence of electron correlations on the twist angle and reveal the existence of strong correlations over a narrow range of twist-angles near the magic angle. Specifically, we determine the on-site and extended Hubbard parameters of the low-energy Wannier states using an atomistic quantum-mechanical approach. The ratio of the on-site Hubbard parameter and the width of the flat bands, which is an indicator of the strength of electron correlations, depends sensitively on the screening by the semiconducting substrate and the metallic gates. Including the effect of long-ranged Coulomb interactions significantly reduces electron correlations and explains the experimentally observed sensitivity of strong correlation phenomena on twist angle.Comment: 17 pages, 6 figure

A Center-Periphery Model of Monetary Coordination and Exchange Rate Crises

The paper analyzes the modalities and consequences of a breakdown of cooperation between the monetary authorities of inflation-prone Periphery Countries that use an exchange rate peg as an anti- inflationary device, when the Center is hit by an aggregate demand shock. Cooperation in the Periphery is constrained to be symmetric: costs and benefits must be equal for all. Our model suggests that there are at least two ways in which a generalized crisis of the exchange rate system may emerge. The first is when the constrained cooperative response of the Periphery is a moderate common devaluation while the non-cooperative equilibrium has large devaluations by a few countries. An exchange rate crisis emerges if Periphery countries give in to their individual incentives to renege on the cooperative agreement. In the second case, the Center shock is not large enough to trigger a general devaluation in the constrained cooperative equilibrium; yet some of the Periphery countries would devalue in the Nash equilibrium, making the monetary stance in the system more expansionary. In this case, reversion to Nash is collectively rational. We offer this model as a useful parable for interpreting the collapse of the EMR in 1992-93.

Mini-jet Total Cross-sections and Overlap Functions through Bloch-Nordsieck Summation

Predictions for total inelastic cross-sections for photon induced processes are discussed in the context of the QCD-inspired minijet model. Large theoretical uncertainties exist, some of them related to the parton distributions of hadrons in impact parameter space. A model for such distribution is presented, based on soft gluon summation. This model incorporates (the salient features of distributions obtained from) the intrinsic transverse momentum behaviour of hadrons. Under the assumption that the intrinsic behaviour is dominated by soft gluon emission stimulated by the scattering process, the b-spectrum becomes softer and softer as the scattering energy increases. In minijet models for the inclusive cross-sections, this will counter the increase from $\sigma_{jet}$ .Comment: 11 pages, two postscript figures, latex, uses epsfig.sty, a4wide.sty full postscript file of the paper is available at http://hpteor.lnf.infn.it/pancheri/starelesna.p

Articulating an ethic of care: the moral narratives and practices of working lone mothers in South Wales

This thesis explores the experiences of Working Lone Mothers (WLMs) as they attempt to negotiate the complex articulation of public/private spaces whilst caring for their children and themselves. The focus of the research is set on WLMs “understanding and practice of care within their „relational network” of family and friends and against the contextual backdrop of the community in which they live. More specifically, it seeks to ascertain the type and quality of care currently articulated in the private domain against the assumption that changing gender roles may produce a 'care deficit' or caring 'gaps' within the family context of 21st century Britain. With this objective in mind, I have carried out 35 in-depth interviews of WLMs living in the South Wales area between the Rhondda Valley and Cardiff. The research is conceptually placed within the framework of an „ethic of care‟, which, as a moral theory, is only a few decades old and, as such, is still in the process of being formulated. With a relative small number of care ethicists currently publishing in the international and national arena, this thesis seeks to enhance the value and importance of care both as a private and as a public virtue and practice against the cultural and political dominance of an ethic of work. From this particular standpoint, if an ethic of work seems to be increasingly hijacked by consumer capitalism and shaped to respond primarily to “privatised” and “marketised” self-interest, an ethic of care appears to have been relegated to the private concerns and considerations of women in the domestic sphere of life. Against this backdrop, whilst the understanding of the public/private articulation has remained primarily a matter of theoretical discussion, the important dimension of care has not yet been thoroughly investigated within the empirical framework of these conflicting environments. This thesis intends to offer an empirical investigation of these issues by evaluating the inter-dynamic nature of WLMs “paid work in relationship to their caring commitments. In the process, the embedded tensions of both environments will be exposed and analysed. I will argue that whilst “enabling flexible arrangements” at work are conducive to a better management of caring work and promote a better balance between the two domains, “disabling flexible arrangements”are not. Given that the latter have emerged as the most common form of work management, many WLMs do struggle to find the time to care. Yet, under these constraints, WLMs have devised strategies that allow them to carry out their caring responsibilities by ways of maximising their limited resources of time, space and energy whilst minimising their overall investment. This process appears to be so clearly widespread amongst the respondents as to have generated definable patterns. Namely, under apparently normalised and routinised conditions, the cumulative effect of transferring and converting commodified values and practices from the public sphere and the workplace into the world of informal care has been increased and intensified to such an extent as to affect and change the type and quality of care that WLMs are able to experience and practice in relation to their children, to themselves and to some extent, to their "relational networks"