Phonon-induced resistance oscillations of two-dimensional electron systems drifting with supersonic velocities

We present a theory of the phonon-assisted nonlinear dc transport of 2D electrons in high Landau levels. The nonlinear dissipative resistivity displays quantum magneto-oscillations governed by two parameters which are proportional to the Hall drift velocity $v_H$ of electrons in electric field and the speed of sound $s$. In the subsonic regime, $v_H<s$, the theory quantitatively reproduces the oscillation pattern observed in recent experiments. We also find the $\pi/2$ phase change of oscillations across the sound barrier $v_H=s$. In the supersonic regime, $v_H>s$, the amplitude of oscillations saturates with lowering temperature, while the subsonic region displays exponential suppression of the phonon-assisted oscillations with temperature.Comment: 4 pages, 3 figure

A new mechanism of mass protection for fermions

We present a way of protecting a Dirac fermion interacting with a scalar (Higgs) field from getting a mass from the vacuum. It is obtained through an implementation of translational symmetry when the theory is formulated with a momentum cutoff, which forbids the usual Yukawa term. We consider that this mechanism can help to understand the smallness of neutrino masses without a tuning of the Yukawa coupling. The prohibition of the Yukawa term for the neutrino forbids at the same time a gauge coupling between the right-handed electron and neutrino. We prove that this mechanism can be implemented on the lattice.Comment: LATTICE99(Higgs,Yukawa,SUSY), 3 page

The Problem of Confirmation in the Everett Interpretation

I argue that the Oxford school Everett interpretation is internally incoherent, because we cannot claim that in an Everettian universe the kinds of reasoning we have used to arrive at our beliefs about quantum mechanics would lead us to form true beliefs. I show that in an Everettian context, the experimental evidence that we have available could not provide empirical confirmation for quantum mechanics, and moreover that we would not even be able to establish reference to the theoretical entities of quantum mechanics. I then consider a range of existing Everettian approaches to the probability problem and show that they do not succeed in overcoming this incoherence

Instanton Induced Neutrino Majorana Masses in CFT Orientifolds with MSSM-like spectra

Recently it has been shown that string instanton effects may give rise to neutrino Majorana masses in certain classes of semi-realistic string compactifications. In this paper we make a systematic search for supersymmetric MSSM-like Type II Gepner orientifold constructions admitting boundary states associated with instantons giving rise to neutrino Majorana masses and other L- and/or B-violating operators. We analyze the zero mode structure of D-brane instantons on general type II orientifold compactifications, and show that only instantons with O(1) symmetry can have just the two zero modes required to contribute to the 4d superpotential. We however discuss how the addition of fluxes and/or possible non-perturbative extensions of the orientifold compactifications would allow also instantons with $Sp(2)$ and U(1) symmetries to generate such superpotentials. In the context of Gepner orientifolds with MSSM-like spectra, we find no models with O(1) instantons with just the required zero modes to generate a neutrino mass superpotential. On the other hand we find a number of models in one particular orientifold of the Gepner model $(2,4,22,22)$ with $Sp(2)$ instantons with a few extra uncharged non-chiral zero modes which could be easily lifted by the mentioned effects. A few more orientifold examples are also found under less stringent constraints on the zero modes. This class of $Sp(2)$ instantons have the interesting property that R-parity conservation is automatic and the flavour structure of the neutrino Majorana mass matrices has a simple factorized form.Comment: 68 pages, 2 figures; v2. typos corrected, refs adde

Neutrino Majorana Masses from String Theory Instanton Effects

Finding a plausible origin for right-handed neutrino Majorana masses in semirealistic compactifications of string theory remains one of the most difficult problems in string phenomenology. We argue that right-handed neutrino Majorana masses are induced by non-perturbative instanton effects in certain classes of string compactifications in which the $U(1)_{B-L}$ gauge boson has a St\"uckelberg mass. The induced operators are of the form $e^{-U}\nu_R\nu_R$ where $U$ is a closed string modulus whose imaginary part transforms appropriately under $B-L$. This mass term may be quite large since this is not a gauge instanton and $Re U$ is not directly related to SM gauge couplings. Thus the size of the induced right-handed neutrino masses could be a few orders of magnitude below the string scale, as phenomenologically required. It is also argued that this origin for neutrino masses would predict the existence of R-parity in SUSY versions of the SM. Finally we comment on other phenomenological applications of similar instanton effects, like the generation of a $\mu$-term, or of Yukawa couplings forbidden in perturbation theory.Comment: 40 pages, 4 figures (v2: added references, small corrections)(v3: minor corrections

Nonlinear Constitutive Model and Finite Element Analysis of Damage Processes in Ferroelectrics

AbstractIn this paper a damage model for ferroelectric materials is presented. It is implemented in terms of a user element in a commercial FEM-code Abaqus. The model is based on micromechanical considerations of domain switching and its interaction with microc- rack growth and coalescence. The influence of damage evolution on the effective material properties is demonstrated. Further, a finite element analysis of a multilayer actuator is performed, showing damage and crack patterns

Propuesta de uso de la tierra con fines de recarga hídrica, en 144 fincas de los municipios de San Juan de Limay, Estelí y Murra, Nueva Segovia, Nicaragua

El presente estudio se llevó acabo en los municipios de San Juan de Murra, Nueva Segovia y San Juan de Limay, Estelí; con el fin de proponer alternativas de usos de la tierra, en base a las características identificadas en cada parcela y en conformidad con los productores, incrementando la cobertura vegetal, para aumentar la recarga del acuífero. Dentro de la metología se realizó un levantamiento de línea base en 246 parcelas ditribuidas en 144 fincas, ubicadas dentro del área de recarga de las fuentes de agua(nanocuencas):6 en San Juan de Limay y 6 en San Juan de Murra. Se visitó a cada productor , se acordó en que parcelas de sus fincas realizarían u n cambio en el uso de la tierra. Tomando en cuenta la información necesaria sobre el uso actual y condiciones del terreno, se propusieron sistemas agroforestales en conjunto con obras de conservación de suelo y agua en cada parcela. El resultado del levantamiento indica que el uso de la tierra en el total de las parcelas de San Juan de Murra está dividido en: café con sombra 21%, bosque 44%, pasto 26%, cultivo anual 7%, tacotal 2%, y el de San Juan de Limay se divididó en: bosque 53%, pasto 23%, cultivo anual 6 %, regeneración natural 3%, café con sombra 12%, tacotal 3 %. Los usos propuestos fueron las siguientes: Regeneración natural con enriquecimiento, Sistemas silvopastoril: árboles dispersos, Sistema quesungual, SAF labranza mín ima con cercas vivas, SAF árboles dispersos cortinas rompe vientos, SAF cultivo en callejón, Regeneración natural con rondas corta fuegos y SAF perennes con árboles y zanjas de infiltración. En ambos municipios los porcentajes de cobertura vegetal son variables, las pendientes bastante pronunciadas, se diferenciarón 11 usos actules: con estos datos se definieron 16 alternativas de uso de la tierra