Continuous Family of Einstein-Yang-Mills Wormholes

It is shown that for some particular value of the cosmological constant depending on the gauge coupling constant a continuous one-parameter family of Einstein-Yang-Mills wormholes exists which interpolates between the instanton and the gravitating meron solutions. In contradistinction with the previously known solutions the topological charge of these wormholes is not quantized. For all of them the contribution of gravity to the action exactly cancels that of the gauge field.Comment: 15 page

Infinite chain of N different deltas: a simple model for a Quantum Wire

We present the exact diagonalization of the Schrodinger operator corresponding to a periodic potential with N deltas of different couplings, for arbitrary N. This basic structure can repeat itself an infinite number of times. Calculations of band structure can be performed with a high degree of accuracy for an infinite chain and of the correspondent eigenlevels in the case of a random chain. The main physical motivation is to modelate quantum wire band structure and the calculation of the associated density of states. These quantities show the fundamental properties we expect for periodic structures although for low energy the band gaps follow unpredictable patterns. In the case of random chains we find Anderson localization; we analize also the role of the eigenstates in the localization patterns and find clear signals of fractality in the conductance. In spite of the simplicity of the model many of the salient features expected in a quantum wire are well reproduced.Comment: 28 pages, LaTeX, 13 eps figures (3 color

Responses of substantia gelatinosa neurons to putative neurotransmitters in an in vitro preparation of the adult rat spinal cord

Extracellular recordings were performed from neurons of the substantia gelatinosa (SG) in an in vitro preparation obtained from the spinal cord of adult rats. About 40% of neurons were spontaneously active. They could be synaptically influenced by low and high threshold fiber input entering the spinal cord through dorsal and ventral and ventral roots. Repetitive low threshold stimulation led to a transient increase in activity of a number of these neurons, whereas high intensity stimulation predominantly reduced excitability. The majority of non-spontaneously active neurons responded to an increase of stimulus intensity covariantly with an increase in firing rate. The excitatory effect of phoretically administeredl-glutamate as well as synaptically induced and spontaneous activity was reduced or abolished by phoretically administered GABA, glycine or the enkephalin-analogued-Ala2-d-Leu5-enkephalin. The actions of the enkephalin analogue were blocked by phoretically applied naloxone. The findings are consistent with the notion from in vivo investigations of a structurally and functionally heterogeneous population of neurons which display a responsiveness to microtopically applied putative neurotransmitters resembling dorsal horn neurons in deeper layers

Simple model for a Quantum Wire II. Correlations

In a previous paper (Eur. Phys. J. B 30, 239-251 (2002)) we have presented the main features and properties of a simple model which -in spite of its simplicity- describes quite accurately the qualitative behaviour of a quantum wire. The model was composed of N distinct deltas each one carrying a different coupling. We were able to diagonalize the Hamiltonian in the periodic case and yield a complete and analytic description of the subsequent band structure. Furthermore the random case was also analyzed and we were able to describe Anderson localization and fractal structure of the conductance. In the present paper we go one step further and show how to introduce correlations among the sites of the wire. The presence of a correlated disorder manifests itself by altering the distribution of states and the localization of the electrons within the systemComment: RevTex, 7 pages, 9 figures (3 greyscale, 6 coloured

Vacuum Radiation and Symmetry Breaking in Conformally Invariant Quantum Field Theory

The underlying reasons for the difficulty of unitarily implementing the whole conformal group $SO(4,2)$ in a massless Quantum Field Theory (QFT) are investigated in this paper. Firstly, we demonstrate that the singular action of the subgroup of special conformal transformations (SCT), on the standard Minkowski space $M$, cannot be primarily associated with the vacuum radiation problems, the reason being more profound and related to the dynamical breakdown of part of the conformal symmetry (the SCT subgroup, to be more precise) when representations of null mass are selected inside the representations of the whole conformal group. Then we show how the vacuum of the massless QFT radiates under the action of SCT (usually interpreted as transitions to a uniformly accelerated frame) and we calculate exactly the spectrum of the outgoing particles, which proves to be a generalization of the Planckian one, this recovered as a given limit.Comment: 29 pages, Latex, 1 figure, to appear in Commun. Math. Phy

Effects of Light and Commuter Rail Transit on Land Prices: Experiences in San Diego County

Using hedonic price models, appreciable land-value premiums were found for multiple land uses in different rail corridors of San Diego County. The most appreciable benefits were for condominiums and single-family housing near commuter-rail stations in the north county, multifamily housing near light-rail stations, and commercial properties near downtown commuterrail stations and light-rail stops in the Mission Valley. Elsewhere, commercial properties accrued small or even negative capitalization benefits. Pro-development policies, worsening traffic congestion, and a generally healthy economy are thought to have generally boosted land values in San Diego County, though impacts are corridor- and land-use specific

The Built Environment and Travel

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Morfodinàmica de platges de graves. Elements per al disseny de platges artificials

L'estudi de les platges de grava és un aspecte de la dinàmica de costes on encara hi ha una manca de dedicació, si comparem amb les platges de sorra. Donat el seu interès i la seva possible utilitat com a efectiu mètode de protecció de costes, en aquesta tesina s'ha intentat ampliar el coneixement sobre aquest tipus de platges mitjançant una experimentació al laboratori i una posterior validació dels models vigents en l'actualitat