Models of Holographic superconductivity

We construct general models for holographic superconductivity parametrized by three couplings which are functions of a real scalar field and show that under general assumptions they describe superconducting phase transitions. While some features are universal and model independent, important aspects of the quantum critical behavior strongly depend on the choice of couplings, such as the order of the phase transition and critical exponents of second-order phase transitions. In particular, we study a one-parameter model where the phase transition changes from second to first order above some critical value of the parameter and a model with tunable critical exponents.Comment: 15 pages, 6 figure

Two Dimensional Quantum Dilaton Gravity and the Positivity of Energy

Using an argument due to Regge and Teitelboim, an expression for the ADM mass of 2d quantum dilaton gravity is obtained. By evaluating this expression we establish that the quantum theories which can be written as a Liouville-like theory, have a lower bound to energy, provided there is no critical boundary. This fact is then reconciled with the observation made earlier that the Hawking radiation does not appear to stop. The physical picture that emerges is that of a black hole in a bath of quantum radiation. We also evaluate the ADM mass for the models with RST boundary conditions and find that negative values are allowed. The Bondi mass of these models goes to zero for large retarded times, but becomes negative at intermediate times in a manner that is consistent with the thunderpop of RST.Comment: 16 pages, phyzzx, COLO-HEP-309. (Confusing points in previous version clarified, discussion of ADM and Bondi masses in RST case added.

Quantum Black Holes in Two Dimensions

We show that a whole class of quantum actions for dilaton-gravity, which reduce to the CGHS theory in the classical limit, can be written as a Liouville-like theory. In a sub-class of this, the field space singularity observed by several authors is absent, regardless of the number of matter fields, and in addition it is such that the dilaton-gravity functional integration range (the real line) transforms into itself for the Liouville theory fields. We also discuss some problems associated with the usual calculation of Hawking radiation, which stem from the neglect of back reaction. We give an alternative argument incorporating back reaction but find that the rate is still asymptotically constant. The latter is due to the fact that the quantum theory does not seem to have a lower bound in energy and Hawking radiation takes positive Bondi (or ADM) mass solutions to arbitrarily large negative mass.Comment: 28 pages, phyzzx, revised discussion of Hawking radiatio

AMOA, Associazione Medici Oculisti per l'Africa, un esempio di solidariet\ue0 istituzionalizzata. La missione Zimbabwe.

AMOA (Associazione Medici Oculisti per l\u2019Africa), \ue8 un\u2019associazione di volontariato legalmente costituita, con sede a Bologna, aconfessionale che non persegue fini di lucro. L\u2019associazione, improntata al volontariato, svolge metodicamente e con continuit\ue0 attivit\ue0 di cooperazione allo sviluppo in favore delle popolazioni del Terzo Mondo ed in particolare di quelle africane. A tale scopo l\u2019associazione stabilisce con le Autorit\ue0 nazionali dei Paesi in cui opera, con le istituzioni nazionali ed internazionali operanti localmente e le organizzazioni sociali, una fattiva collaborazione nel rispetto delle sovranit\ue0 nazionali e secondo i principi della cooperazione internazionale definiti dalla Carta delle Nazioni Unite. Essa opera per mezzo di medici e paramedici volontari che offrono gratuitamente una parte del loro tempo per servire ai bisogni sanitari delle popolazioni di questi Paesi che si trovano in uno stadio di sottosviluppo sanitario in campo oculistico per mancanza di strutture, risorse economiche e umane. Tra le varie missioni effettuate possiamo menzionare quelle in Senegal, Togo, Etiopia, Madagascar, Rwanda, Cameroun, Zimbabwe. L\u2019analisi del presente lavoro verter\ue0 sull\u2019operato della\u2019associazione in Zimbabwe evidenziando: l\u2019epidemiologia delle patologie oculari della zona di intervento; il trattamento delle cause pi\uf9 frequenti di cecit\ue0: cataratta, glaucoma, malposizione palpebrale da tracoma, i traumi del bulbo oculare; lo screening delle deficienze visive nell\u2019et\ue0 scolare: ipermetropia, miopia, astigmatismo, ambliopia; la correzione della presbiopia che riduce la capacit\ue0 lavorativa dopo i 40 anni; la promozione dell\u2019educazione sanitaria nelle scuole e nei dispensari per la prevenzione primaria e secondaria delle carenze visive; l\u2019addestramento del personale scolastico locale a misurare la capacit\ue0 visiva dei bambini a partire dal periodo prescolare; l\u2019aggiornamento del personale medico alla chirurgia mediante soggiorni sia in loco che in Italia ed infine le ricadute sulle condizioni di salute e benessere dei soggetti coinvolti

On the Prior Sensitivity of Thompson Sampling

The empirically successful Thompson Sampling algorithm for stochastic bandits has drawn much interest in understanding its theoretical properties. One important benefit of the algorithm is that it allows domain knowledge to be conveniently encoded as a prior distribution to balance exploration and exploitation more effectively. While it is generally believed that the algorithm's regret is low (high) when the prior is good (bad), little is known about the exact dependence. In this paper, we fully characterize the algorithm's worst-case dependence of regret on the choice of prior, focusing on a special yet representative case. These results also provide insights into the general sensitivity of the algorithm to the choice of priors. In particular, with $p$ being the prior probability mass of the true reward-generating model, we prove $O(\sqrt{T/p})$ and $O(\sqrt{(1-p)T})$ regret upper bounds for the bad- and good-prior cases, respectively, as well as \emph{matching} lower bounds. Our proofs rely on the discovery of a fundamental property of Thompson Sampling and make heavy use of martingale theory, both of which appear novel in the literature, to the best of our knowledge.Comment: Appears in the 27th International Conference on Algorithmic Learning Theory (ALT), 201

D0-D4 system and QCD_{3+1}

We consider a $(3+1)$-dimensional QCD model using a dual supergravity description with a non-extremal $D0$-$D4$ brane background. We calculate the spectrum of glueball masses and Wilson loops in the background. The mass spectrum is shown to coincide with one in non-extremal $D4$-brane systems, and an area low of spatial Wilson loops is established. We show that there is a region that Kaluza-Klein modes of the Euclidean time direction are decoupled without decoupling glueball masses.Comment: 10 pages, REVTeX; typos correcte

Correlation effects in the density of states of annealed GaMnAs

We report on an experimental study of low temperature tunnelling in hybrid NbTiN/GaMnAs structures. The conductance measurements display a root mean square V dependence, consistent with the opening of a correlation gap in the density of states of GaMnAs. Our experiment shows that low temperature annealing is a direct empirical tool that modifies the correlation gap and thus the electron-electron interaction. Consistent with previous results on boron-doped silicon we find, as a function of voltage, a transition across the phase boundary delimiting the direct and exchange correlation regime.Comment: Replaced with revised version. To appear in Phys. Rev.

Supersymmetry and Positive Energy in Classical and Quantum Two-Dimensional Dilaton Gravity

An $N = 1$ supersymmetric version of two dimensional dilaton gravity coupled to matter is considered. It is shown that the linear dilaton vacuum spontaneously breaks half the supersymmetries, leaving broken a linear combination of left and right supersymmetries which squares to time translations. Supersymmetry suggests a spinorial expression for the ADM energy $M$, as found by Witten in four-dimensional general relativity. Using this expression it is proven that ${M}$ is non-negative for smooth initial data asymptotic (in both directions) to the linear dilaton vacuum, provided that the (not necessarily supersymmetric) matter stress tensor obeys the dominant energy condition. A {\it quantum} positive energy theorem is also proven for the semiclassical large-$N$ equations, despite the indefiniteness of the quantum stress tensor. For black hole spacetimes, it is shown that $M$ is bounded from below by $e^{- 2 \phi_H}$, where $\phi_H$ is the value of the dilaton at the apparent horizon, provided only that the stress tensor is positive outside the apparent horizon. This is the two-dimensional analogue of an unproven conjecture due to Penrose. Finally, supersymmetry is used to prove positive energy theorems for a large class of generalizations of dilaton gravity which arise in consideration of the quantum theory.Comment: 21 page