Exchange effects on electron scattering through a quantum dot embedded in a two-dimensional semiconductor structure

We have developed a theoretical method to study scattering processes of an incident electron through an N-electron quantum dot (QD) embedded in a two-dimensional (2D) semiconductor. The generalized Lippmann-Schwinger equations including the electron-electron exchange interaction in this system are solved for the continuum electron by using the method of continued fractions (MCF) combined with 2D partial-wave expansion technique. The method is applied to a one-electron QD case. Cross-sections are obtained for both the singlet and triplet couplings between the incident electron and the QD electron during the scattering. The total elastic cross-sections as well as the spin-flip scattering cross-sections resulting from the exchange potential are presented. Furthermore, inelastic scattering processes are also studied using a multichannel formalism of the MCF.Comment: 11 pages, 4 figure

Dephasing of Electrons on Helium by Collisions with Gas Atoms

The damping of quantum effects in the transport properties of electrons deposited on a surface of liquid helium is studied. It is found that due to vertical motion of the helium vapour atoms the interference of paths of duration $t$ is damped by a factor $\exp - (t/\tau_v)^3$. An expression is derived for the weak-localization lineshape in the case that damping occurs by a combination of processes with this type of cubic exponential damping and processes with a simple exponential damping factor.Comment: 7 pages, 2 figures, Revte

Multiple Transitions to Chaos in a Damped Parametrically Forced Pendulum

We study bifurcations associated with stability of the lowest stationary point (SP) of a damped parametrically forced pendulum by varying $\omega_0$ (the natural frequency of the pendulum) and $A$ (the amplitude of the external driving force). As $A$ is increased, the SP will restabilize after its instability, destabilize again, and so {\it ad infinitum} for any given $\omega_0$. Its destabilizations (restabilizations) occur via alternating supercritical (subcritical) period-doubling bifurcations (PDB's) and pitchfork bifurcations, except the first destabilization at which a supercritical or subcritical bifurcation takes place depending on the value of $\omega_0$. For each case of the supercritical destabilizations, an infinite sequence of PDB's follows and leads to chaos. Consequently, an infinite series of period-doubling transitions to chaos appears with increasing $A$. The critical behaviors at the transition points are also discussed.Comment: 20 pages + 7 figures (available upon request), RevTex 3.

Exact Absorption Probability in the Extremal Six-Dimensional Dyonic String Background

We show that the minimally coupled massless scalar wave equation in the background of an six-dimensional extremal dyonic string (or D1-D5 brane intersection) is exactly solvable, in terms of Mathieu functions. Using this fact, we calculate absorption probabilities for these scalar waves, and present the explicit results for the first few low energy corrections to the leading-order expressions. For a specific tuning of the dyonic charges one can reach a domain where the low energy absorption probability goes to zero with inverse powers of the logarithm of the energy. This is a dividing domain between the regime where the low energy absorption probability approaches zero with positive powers of energy and the regime where the probability is an oscillatory function of the logarithm of the energy. By the conjectured AdS/CFT correspondence, these results shed novel light on the strongly coupled two-dimensional field theory away from its infrared conformally invariant fixed point (the strongly coupled ``non-critical'' string).Comment: Latex (3 times), 23 page

Early diagenetic vivianite [Fe-3(PO4)(2) center dot 8H(2)O] in a contaminated freshwater sediment and insights into zinc uptake: a mu-EXAFS, mu-XANES and Raman study

The sediments in the Salford Quays, a heavily-modified urban water body, contain high levels of organic matter, Fe, Zn and nutrients as a result of past contaminant inputs. Vivianite [Fe3(PO4)2 · 8H2O] has been observed to have precipitated within these sediments during early diagenesis as a result of the release of Fe and P to porewaters. These mineral grains are small (<100 μm) and micron-scale analysis techniques (SEM, electron microprobe, μ-EXAFS, μ-XANES and Raman) have been applied in this study to obtain information upon the structure of this vivianite and the nature of Zn uptake in the mineral. Petrographic observations, and elemental, X-ray diffraction and Raman spectroscopic analysis confirms the presence of vivianite. EXAFS model fitting of the FeK-edge spectra for individual vivianite grains produces Fe–O and Fe–P co-ordination numbers and bond lengths consistent with previous structural studies of vivianite (4O atoms at 1.99–2.05 Å; 2P atoms at 3.17–3.25 Å). One analysed grain displays evidence of a significant Fe3+ component, which is interpreted to have resulted from oxidation during sample handling and/or analysis. EXAFS modelling of the Zn K-edge data, together with linear combination XANES fitting of model compounds, indicates that Zn may be incorporated into the crystal structure of vivianite (4O atoms at 1.97 Å; 2P atoms at 3.17 Å). Low levels of Zn sulphate or Zn-sorbed goethite are also indicated from linear combination XANES fitting and to a limited extent, the EXAFS fitting, the origin of which may either be an oxidation artifact or the inclusion of Zn sulphate into the vivianite grains during precipitation. This study confirms that early diagenetic vivianite may act as a sink for Zn, and potentially other contaminants (e.g. As) during its formation and, therefore, forms an important component of metal cycling in contaminated sediments and waters. Furthermore, for the case of Zn, the EXAFS fits for Zn phosphate suggest this uptake is structural and not via surface adsorption

Maximising transparency in a doctoral thesis: The complexities of writing about the use of QSR*NVIVO within a grounded theory study

This paper discusses the challenges of how to provide a transparent account of the use of the software programme QSR*NVIVO (QSR 2000) within a Grounded Theory framework (Glaser and Strauss 1967; Strauss and Corbin 1998). Psychology students are increasingly pursuing qualitative research projects such to the extent that the UK Economic and Social Research Council (ESRC) advise that students should have skill in the use of computer assisted qualitative data analysis software (CAQDAS) (Economic and Social Research Council 2001). Unlike quantitative studies, rigid formulae do not exist for writing-up qualitative projects for doctoral theses. Most authors, however, agree that transparency is essential when communicating the findings of qualitative research. Sparkes (2001) recommends that evaluative criteria for qualitative research should be commensurable with the aims, objectives, and epistemological assumptions of the research project. Likewise, the use of CAQDAS should vary according to the research methodology followed, and thus researchers should include a discussion of how CAQDAS was used. This paper describes how the evolving process of coding data, writing memos, categorising, and theorising were integrated into the written thesis. The structure of the written document is described including considerations about restructuring and the difficulties of writing about an iterative process within a linear document

Fermions scattering in a three dimensional extreme black hole background

The absorption cross section for scattering of fermions off an extreme BTZ black hole is calculated. It is shown that, as in the case of scalar particles, an extreme BTZ black hole exhibits a vanishing absorption cross section, which is consistent with the vanishing entropy of such object. Additionally, we give a general argument to prove that the particle flux near the horizon is zero. Finally we show that the {\it reciprocal space} introduced previously in \cite{gm} gives rise to the same result and, therefore, it could be considered as the space where the scattering process takes place in an AdS spacetime.Comment: 15 pages, RevTex4. Revised version. To be published in Class. Quantum. Gra

Implications of invariance of the Hamiltonian under canonical transformations in phase space

We observe that, within the effective generating function formalism for the implementation of canonical transformations within wave mechanics, non-trivial canonical transformations which leave invariant the form of the Hamilton function of the classical analogue of a quantum system manifest themselves in an integral equation for its stationary state eigenfunctions. We restrict ourselves to that subclass of these dynamical symmetries for which the corresponding effective generating functions are necessaarily free of quantum corrections. We demonstrate that infinite families of such transformations exist for a variety of familiar conservative systems of one degree of freedom. We show how the geometry of the canonical transformations and the symmetry of the effective generating function can be exploited to pin down the precise form of the integral equations for stationary state eigenfunctions. We recover several integral equations found in the literature on standard special functions of mathematical physics. We end with a brief discussion (relevant to string theory) of the generalization to scalar field theories in 1+1 dimensions.Comment: REVTeX v3.1, 13 page

Social Value of Marine and Coastal Protected Areas in England and Wales.

The U.K. government is committed to establishing a coherent network of marine protected areas by 2012 and the recentMarine and Coastal Access Act, 2009 will designate marine conservation zones and provide wider access rights to the coast. To fulfill these goals, this article argues the need for a clearer, shared understanding of the social value of protected areas in creating new designations and managing existing ones. Although marine and coastal environments attract many people and are vitally important in terms of realized and potential social value, the majority of the public in the United Kingdom lacks understanding and awareness regarding them. Combined with this, the social value of marine and coastal protected areas (MCPAs) have been largely ignored relative to conservation and economics, with the latter invariably taking precedence in environmental policymaking. Social value reflects the complex, individual responses that people experience in a given place. Many reasons determine why one area is valued above another, and this research investigates the social value of MCPAs from a practitioner’s perspective through a series of interviews. Understanding why we “socially” value MCPAs will ultimately equip managers with an informed understanding of these spaces, influence management decisions, and, potentially, policymaking. This article defines social value in the context of MCPAs in England and Wales from a practitioner perspective, explores key concepts, and suggests possible improvements in decision-making

Various Models for Pion Probability Distributions from Heavy-Ion Collisions

Various models for pion multiplicity distributions produced in relativistic heavy ion collisions are discussed. The models include a relativistic hydrodynamic model, a thermodynamic description, an emitting source pion laser model, and a description which generates a negative binomial description. The approach developed can be used to discuss other cases which will be mentioned. The pion probability distributions for these various cases are compared. Comparison of the pion laser model and Bose-Einstein condensation in a laser trap and with the thermal model are made. The thermal model and hydrodynamic model are also used to illustrate why the number of pions never diverges and why the Bose-Einstein correction effects are relatively small. The pion emission strength $\eta$ of a Poisson emitter and a critical density $\eta_c$ are connected in a thermal model by $\eta/n_c = e^{-m/T} < 1$, and this fact reduces any Bose-Einstein correction effects in the number and number fluctuation of pions. Fluctuations can be much larger than Poisson in the pion laser model and for a negative binomial description. The clan representation of the negative binomial distribution due to Van Hove and Giovannini is discussed using the present description. Applications to CERN/NA44 and CERN/NA49 data are discussed in terms of the relativistic hydrodynamic model.Comment: 12 pages, incl. 3 figures and 4 tables. You can also download a PostScript file of the manuscript from http://p2hp2.lanl.gov/people/schlei/eprint.htm