Cohomology of classes of symbols and classi cation of traces on corresponding classes of operators with non positive order

This thesis is devoted to the classification issue of traces on classical pseudo-differential operators with fixed non positive order on closed manifolds of dimension $n>1$. We describe the space of homogeneous functions on a symplectic cone in terms of Poisson brackets of appropriate homogeneous functions, and we use it to find a representation of a pseudo-differential operator as a sum of commutators. We compute the cohomology groups of certain spaces of classical symbols on the $n$--dimensional Euclidean space with constant coefficients, and we show that any closed linear form on the space of symbols of fixed order can be written either in terms of a leading symbol linear form and the noncommutative residue, or in terms of a leading symbol linear form and the cut-off regularized integral. On the operator level, we infer that any trace on the algebra of classical pseudo-differential operators of order $a\in\Z$ can be written either as a linear combination of a generalized leading symbol trace and the residual trace when $-n+1\leq2a\leq0$, or as a linear combination of a generalized leading symbol trace and any linear map that extends the $L^2$--trace when $2a\leq-n\leq a$. In contrast, for odd class pseudo-differential operators in odd dimensions, any trace can be written as a linear combination of a generalized leading symbol trace and the canonical trace. We derive from these results the classification of determinants on the Fr\'echet Lie group associated to the algebras of classical pseudo-differential operators with non positive integer order

Dark Energy in vector-tensor theories of gravity

We consider a general class of vector-tensor theories of gravity and show that solutions with accelerated expansion and a future type III singularity are a common feature in these models. We also show that there are only six vector-tensor theories with the same small scales behavior as General Relativity and, in addition, only two of them can be made completely free from instabilities. Finally, two particular models as candidates for dark energy are proposed: on one hand, a cosmic vector that allows to alleviate the usual naturalness and coincidence problems and, on the other hand, the electromagnetic field is shown to give rise to an effective cosmological constant on large scales whose value can be explained in terms of inflation at the electroweak scale.Comment: 4 pages, 1 table. Contribution to the proceedings of Spanish Relativity Meeting 2009, Bilbao, Spain, 7-11 September 200

Lagrangians for Massive Dirac Chiral Superfields

A variant for the superspin one-half massive superparticle in $4D$, $\mathcal{N}=1$, based on Dirac superfields, is offered. As opposed to the current known models that use spinor chiral superfields, the propagating fields of the supermultiplet are those of the lowest mass dimensions possible: scalar, Dirac and vector fields. Besides the supersymmetric chiral condition, the Dirac superfields are not further constrained, allowing a very straightforward implementation of the path-integral method. The corresponding superpropagators are presented. In addition, an interaction super Yukawa potential, formed by Dirac and scalar chiral superfields, is given in terms of their component fields. The model is first presented for the case of two superspin one-half superparticles related by the charged conjugation operator, but in order to treat the case of neutral superparticles, the Majorana condition on the Dirac superfields is also studied. We compare our proposal with the known models of spinor superfields for the one-half superparticle and show that it is equivalent to them.Comment: 22 pages. Matches published versio

Short channel effects in graphene-based field effect transistors targeting radio-frequency applications

Channel length scaling in graphene field effect transistors (GFETs) is key in the pursuit of higher performance in radio frequency electronics for both rigid and flexible substrates. Although two-dimensional (2D) materials provide a superior immunity to Short Channel Effects (SCEs) than bulk materials, they could dominate in scaled GFETs. In this work, we have developed a model that calculates electron and hole transport along the graphene channel in a drift-diffusion basis, while considering the 2D electrostatics. Our model obtains the self-consistent solution of the 2D Poisson's equation coupled to the current continuity equation, the latter embedding an appropriate model for drift velocity saturation. We have studied the role played by the electrostatics and the velocity saturation in GFETs with short channel lengths L. Severe scaling results in a high degradation of GFET output conductance. The extrinsic cutoff frequency follows a 1/L^n scaling trend, where the index n fulfills n < 2. The case n = 2 corresponds to long-channel GFETs with low source/drain series resistance, that is, devices where the channel resistance is controlling the drain current. For high series resistance, n decreases down to n= 1, and it degrades to values of n < 1 because of the SCEs, especially at high drain bias. The model predicts high maximum oscillation frequencies above 1 THz for channel lengths below 100 nm, but, in order to obtain these frequencies, it is very important to minimize the gate series resistance. The model shows very good agreement with experimental current voltage curves obtained from short channel GFETs and also reproduces negative differential resistance, which is due to a reduction of diffusion current.Comment: 27-pages manuscript (10 figures) plus 6 pages of supplementary information. European Union Action H2020 (696656) / Department d'Universitats, Recerca i Societat de la Informaci\'o of the Generalitat de Catalunya (2014 SGR 384) / Ministerio de Econom\'ia y Competitividad of Spain (TEC2012-31330 and TEC2015-67462-C2-1-R) / MINECO FEDE

Impacto físico-psico-social del Practicum de Enfermería: estudio del estrés derivado de las prácticas clínicas sobre el estado de salud de los alumnos

A research project has been designed to highlight the imnportance of preparing Nursing students before clinical practice. This is based on the assertion that the stressful nature of this learning activity can affect both the quality of the training process and the students' state of health, which is well documented in the relevant literature. Previous research is reviewed and several working hypotheses are established. An exploratory and transversal methodology is used. Existing research material and instruments are adapted, and other specific ones are created. Information is gathered about 136 variables of interest from the data supplied by a sample of 357 students from the University School of Nursing students is confirmed. There is empirical evidence that this stressful experience has negative impact on the students' state of health, on a physical, psychological and behavioural level with predominance of psychosocial over physiological symptoms. These and other results obtained are discussed and it is suggested that this innovative experiment should be extended to other Health Science courses, particularly Medicine.Se ha diseñado un proyecto de investigación para poner de relieve la importancia que tiene el preparar a los alumnos de Enfermería antes de iniciar su periodo de prácticas clínicas sobre la base, bien fundamentada en la literatura pertinente, de que el carácter estresante de dicha actividad de aprendizaje puede afectar tanto a la calidad del proceso formativo como al estado de salud de los alumnos. Se revisan las investigaciones previas. Se establecen diversas hipótesis de trabajo. Se utiliza una metodología exploratoria y transversal. Se adaptan materiales e instrumentos de investigación y se crean otros específicos. Se recoge información sobre 136 variables de interés a partir de los datos suministrados por una muestra de 357 estudiantes de la E.U. de Enfermería (71 % de la población). Se confirma la hipótesis de que las prácticas clínicas constituyen una experiencia estresante para los estudiantes de Enfermería. Existe evidencia empírica de que dicha experiencia estresante tiene un impacto negativo sobre el estado de salud de los alumnos, a nivel físico, psíquico y de comportamiento, con un predominio de los síntomas psico-sociales sobre los fisiológicos. Se discuten estos y otros resultados obtenidos y se sugiere la conveniencia de extender esta experiencia innovadora a otras titulaciones de Ciencias de la Salud, particularmente

Análisis y evaluación de la acción tutorial en Enfermería: confirmación empírica de una sospecha

The purpose of this work is contributing empirical evidence about the tutorial action in the Nursing School of The Cordoba University. Two samples (27 Professors and 103 students) performed both booklets collecting information on outstanding aspects within the tutorial field. The results confirm the preliminary hypothesis: the tutorial activities in the Nursing School are minimal. This opinion is equally unanimous both between Professors and students, although students' valuation probed to be more critical and negative. These and others findings are discussed and some proposal to improve evaluation strategies and practical activities within the tutorial field are formulated.El propósito del presente trabajo es aportar evidencia empírica acerca de la acción tutorial en la E.U. de Enfermería de la Universidad de Córdoba. Dos muestras de sujetos (27 profesores 103 alumnos) han cumplimentado sendos protocolos donde se recoge información diferenciada para cada muestra acerca de aspectos relevantes en el ámbito de la tutoría. Los resultados han confirmado la hipótesis de partida, a saber, la sospecha de la praxis de la actividad tutorial en la E.U. de Enfermería está bajo mínimos. Esta opinión es igualmente unánime en el profesorado y alumnado, aunque este último se muestra mucho más crítico y negativo en sus valoraciones. Se discuten estos y otros resultados y se formulan propuestas para mejorar la evaluación y la praxis de la acción tutorial

A cosmic vector for dark energy

In this work we show that the presence of a vector field on cosmological scales could explain the present phase of accelerated expansion of the universe. The proposed theory contains no dimensional parameters nor potential terms and does not require unnatural initial conditions in the early universe, thus avoiding the so called cosmic coincidence problem. In addition, it fits the data from high-redshift supernovae with excellent precision, making definite predictions for cosmological parameters. Upcoming observations will be able to clearly discriminate this model from standard cosmology with cosmological constant.Comment: 5 pages, 3 figures, 1 table. New comments and references included. Final version to appear in Phys. Rev.