Continuum model for chiral induced spin selectivity in helical molecules

A minimal model is exactly solved for electron spin transport on a helix. Electron transport is assumed to be supported by well oriented $p_z$ type orbitals on base molecules forming a staircase of definite chirality. In a tight binding interpretation, the SOC opens up an effective $\pi_z-\pi_z$ coupling via interbase $p_{x,y}-p_z$ hopping, introducing spin coupled transport. The resulting continuum model spectrum shows two Kramers doublet transport channels with a gap proportional to the SOC. Each doubly degenerate channel satisfies time reversal symmetry, nevertheless, a bias chooses a transport direction and thus selects for spin orientation. The model predicts which spin orientation is selected depending on chirality and bias, changes in spin preference as a function of input Fermi level and scattering suppression protected by the SO gap. We compute the spin current with a definite helicity and find it to be proportional to the torsion of the chiral structure and the non-adiabatic Aharonov- Anandan phase. To describe room temperature transport we assume that the total transmission is the result of a product of coherent steps limited by the coherence length

A mesoscopic ring as a XNOR gate: An exact result

We describe XNOR gate response in a mesoscopic ring threaded by a magnetic flux $\phi$. The ring is attached symmetrically to two semi-infinite one-dimensional metallic electrodes and two gate voltages, viz, $V_a$ and $V_b$, are applied in one arm of the ring which are treated as the inputs of the XNOR gate. The calculations are based on the tight-binding model and the Green's function method, which numerically compute the conductance-energy and current-voltage characteristics as functions of the ring-to-electrode coupling strength, magnetic flux and gate voltages. Our theoretical study shows that, for a particular value of $\phi$ ($=\phi_0/2$) ($\phi_0=ch/e$, the elementary flux-quantum), a high output current (1) (in the logical sense) appears if both the two inputs to the gate are the same, while if one but not both inputs are high (1), a low output current (0) results. It clearly exhibits the XNOR gate behavior and this aspect may be utilized in designing an electronic logic gate.Comment: 8 pages, 5 figure

Low-energy tetrahedral polymorphs of carbon, silicon, and germanium

Searches for low-energy tetrahedral polymorphs of carbon and silicon have been performed using density functional theory computations and the ab initio random structure searching (AIRSS) ap- proach. Several of the hypothetical phases obtained in our searches have enthalpies that are lower or comparable to those of other polymorphs of group 14 elements that have either been experimentally synthesized or recently proposed as the structure of unknown phases obtained in experiments, and should thus be considered as particularly interesting candidates. A structure of P bam symmetry with 24 atoms in the unit cell was found to be a low energy, low-density metastable polymorph in carbon, silicon, and germanium. In silicon, Pbam is found to have a direct band gap at the zone center with an estimated value of 1.4 eV, which suggests applications as a photovoltaic material. We have also found a low-energy chiral framework structure of P 41 21 2 symmetry with 20 atoms per cell containing fivefold spirals of atoms, whose projected topology is that of the so-called Cairo-type two- dimensional pentagonal tiling. We suggest that P41 21 2 is a likely candidate for the structure of the unknown phase XIII of silicon. We discuss Pbam and P41 21 2 in detail, contrasting their energetics and structures with those of other group 14 elements, particularly the recently proposed P42 /ncm structure, for which we also provide a detailed interpretation as a network of tilted diamond-like tetrahedra.AM acknowledges the financial support of the Ministerio de Educaci´on, Cultura y Deporte (MECD, Spain) through its Programa de Movilidad de Recursos Humanos (Plan Nacional de I+D+i), grant PRX12/00335, and of project MAT2010-21270-C04-03. Access to the MALTA computer cluster (Universidad de Oviedo, Project CSD2007-00045) and the High Performance Computing Service of the University of Cambridge are gratefully acknowledged. RJN and CJP were supported by the Engineering and Physical Sciences Research Council (EPSRC) of the UK.We thank Keith Refson for useful discussions.This is the author accepted manuscript. The final version is available from APS at http://journals.aps.org/prb/abstract/10.1103/PhysRevB.91.214104

Phoenix, 2009-01-22

The Phoenix is a student newspaper published at Governors State University since August 2002. The newspaper features student reporting, opinions, news, and photos

Análisis de prefactibilidad de un proyecto de turismo rural en la ciudad de Balcarce, provincia de Buenos Aires, Argentina

El turismo constituye una actividad económica dinamizadora, y dentro del mismo, el turismo rural representa para el sector agropecuario y para las comunidades locales una oportunidad para la generación de empleo directo e indirecto; una alternativa para diversificar y ampliar los ingresos, y una forma de difundir y revalorizar las culturas, tradiciones, prácticas y saberes locales. De esta manera, el presente trabajo se propone analizar la prefactibilidad de un proyecto de inversión para brindar un servicio de turismo rural en un terreno propio en el Partido de Balcarce. Para ello se realizó una caracterización del predio, se llevo a cabo un análisis Fortalezas-Oportunidades-Debilidades-Amenazas (FODA); se estimó el monto de la inversión inicial, los ingresos y los costos de prestación del servicio; se elaboró un flujo de fondos para estimar el Valor Actual Neto (VAN) y, finalmente, un análisis de sensibilidad con 9 escenarios posibles, donde se obtuvieron resultados negativos y positivos que oscilaron entre -1,38 y 1,74 millones de pesos.Tourism is a dynamic economic activity, and rural tourism represents for the agriculture sector and for the local communities an opportunity to create direct and indirect employment, and an alternative strategy to spread out incomes. It is also a way to increase de value of cultures, traditions, practices and local knowledges. Therefore, the aim of this report is to analyze the feasibility of a Project Investment to offer services in rural tourism in an own field in Balcarce. To this end, it was conducted a characterization and analysis of the fiel's natural resources, and it was further elaborated a DAFO analysis. In adittion, it was estimated the amount of the initial investment, as well as the cost of providing the service and the estimated incomes. Morever, it was developed a model to estimate the project's cash flow and the Net Present Value (NPV). Finally, it was made a sensitivity analysis in respect of nine scenarios, in which were obtained negative and positive scores ranging from -1.38 through to 1.74 million pesos.Fil: Rodríguez, Julieta A. Universidad Nacional de Mar del Plata. Facultad de Ciencias Económicas y Sociales; Argentina.Fil: Mujica, Guillermina. Universidad Nacional de Mar del Plata. Facultad de Ciencias Agrarias; Argentina.Fil: Molpeceres, Celeste. Universidad Nacional de Mar del Plata. Facultad de Ciencias Agrarias; Argentina

Chiral molecular films as electron polarizers and polarization modulators

Recent experiments on electron scattering through molecular films have shown that chiral molecules can be efficient sources of polarized electrons even in the absence of heavy nuclei as source of a strong spin-orbit interaction. We show that self-assembled monolayers (SAMs) of chiral molecules are strong electron polarizers due to the high density effect of the monolayers and explicitly compute the scattering amplitude off a helical molecular model of carbon atoms. Longitudinal polarization is shown to be the signature of chiral scattering. For elastic scattering, we find that at least double scattering events must take place for longitudinal polarization to arise. We predict energy windows for strong polarization, determined by the energy dependences of spin-orbit strength and multiple scattering probability. An incoherent mechanism for polarization amplification is proposed, that increases the polarization linearly with the number of helix turns, consistent with recent experiments on DNA SAMs.Comment: 5 Pages, 4 figure

Stated preferences for anti-malarial drug characteristics in Zomba, a malaria endemic area of Malawi

Published onlineJournal ArticlePublished open access article.BACKGROUND: The evidence on determinants of individuals' choices for anti-malarial drug treatments is scarce. This study sought to measure the strength of preference for adult antimalarial drug treatment attributes of heads of urban, rural and peri-urban households in a resource-limited malaria-endemic area of sub-Saharan Africa. METHODS: Discrete choice experiments were conducted with 508 heads of household interviewed face-to-face for a household population survey of health-seeking behavior in Zomba District, Malawi. The interviews were held in Chichewa and the choice experiment questions were presented with cartoon aids. The anti-malarial drug attributes included in the stated preference experiment were: speed of fever resolution, side effects (pruritus) risk, protection (duration of prophylactic effect), price, duration of treatment course and recommendation by a health professional. Sixteen treatment profiles from a fractional factorial design by orthogonal array were paired into choice scenarios, and scenarios were randomly assigned to participants so that each participant was presented with a series of eight pairwise choice scenarios. Respondents had the option to state indifference between the two profiles or decline to choose. Data were analysed in a mixed logit model, with normally distributed coefficients for all six attributes. RESULTS: The sex ratio was balanced in urban areas, whereas 63% of participants in rural areas were male. The proportion of individuals with no education was considerably higher in the rural group (25%) than in the urban (5%) and peri-urban (6%) groups. All attributes investigated had the expected influence, and traded-off in most respondents' choices. There were heterogeneous effects of price, pruritus risk, treatment recommendation by a professional, and duration of prophylaxis across respondents, only partly explained by their differences in education, household per capita expenditure, sex and age. Individuals' demand elasticity (simulated median, inter-quartile range) was highest (most responsive) to speed of symptom resolution (0.88, 0.80-0.89) and pruritus risk (0.25, 0.08-0.62). CONCLUSIONS: Most adult antimalarial users are willing to use treatments without recommendation from health professional, and may be influenced by price. Future studies should investigate the magnitude of differences in price and treatment attribute sensitivity between adult anti-malarial drug users in rural, peri-urban and urban areas in order to determine optimal price subsidies