Yang-Mills instantons in Kaehler spaces with one holomorphic isometry

We consider self-dual Yang-Mills instantons in 4-dimensional Kaehler spaces with one holomorphic isometry and show that they satisfy a generalization of the Bogomol'nyi equation for magnetic monopoles on certain 3-dimensional metrics. We then search for solutions of this equation in 3-dimensional metrics foliated by 2-dimensional spheres, hyperboloids or planes in the case in which the gauge group coincides with the isometry group of the metric (SO(3), SO(1,2) and ISO(2), respectively). Using a generalized hedgehog ansatz the Bogomol'nyi quations reduce to a simple differential equation in the radial variable which admits a universal solution and, in some cases, a particular one, from which one finally recovers instanton solutions in the original Kaehler space. We work out completely a few explicit examples for some Kaehler spaces of interest.Comment: Latex2e file, 16 pages, no figure

One Thousand and One Bubbles

We propose a novel strategy that permits the construction of completely general five-dimensional microstate geometries on a Gibbons-Hawking space. Our scheme is based on two steps. First, we rewrite the bubble equations as a system of linear equations that can be easily solved. Second, we conjecture that the presence or absence of closed timelike curves in the solution can be detected through the evaluation of an algebraic relation. The construction we propose is systematic and covers the whole space of parameters, so it can be applied to find all five-dimensional BPS microstate geometries on a Gibbons-Hawking base. As a first result of this approach, we find that the spectrum of scaling solutions becomes much larger when non-Abelian fields are present. We use our method to describe several smooth horizonless multicenter solutions with the asymptotic charges of three-charge (Abelian and non-Abelian) black holes. In particular, we describe solutions with the centers lying on lines and circles that can be specified with exact precision. We show the power of our method by explicitly constructing a 50-center solution. Moreover, we use it to find the first smooth five-dimensional microstate geometries with arbitrarily small angular momentum.Comment: 33 pages. v2: typos correcte

Holographic studies of Einsteinian cubic gravity

Einsteinian cubic gravity provides a holographic toy model of a nonsupersymmetric CFT in three dimensions, analogous to the one defined by Quasi-topological gravity in four. The theory admits explicit non-hairy AdS$_4$ black holes and allows for numerous exact calculations, fully nonperturbative in the new coupling. We identify several entries of the AdS/CFT dictionary for this theory, and study its thermodynamic phase space, finding interesting new phenomena. We also analyze the dependence of R\'enyi entropies for disk regions on universal quantities characterizing the CFT. In addition, we show that $\eta/s$ is given by a non-analytic function of the ECG coupling, and that the existence of positive-energy black holes strictly forbids violations of the KSS bound. Along the way, we introduce a new method for evaluating Euclidean on-shell actions for general higher-order gravities possessing second-order linearized equations on AdS$_{(d+1)}$. Our generalized action involves the very same Gibbons-Hawking boundary term and counterterms valid for Einstein gravity, which now appear weighted by the universal charge $a^*$ controlling the entanglement entropy across a spherical region in the CFT dual to the corresponding higher-order theory.Comment: 59 pages, 7 figures, 1 table; v4: typos fixe

Regular Stringy Black Holes?

We study the first-order $\alpha'$ corrections to the singular 4-dimensional massless stringy black holes studied in the nineties in the context of the Heterotic Superstring. We show that the $\alpha'$ corrections not only induce a non-vanishing mass and give rise to an event horizon, but also eliminate the singularity giving rise to a regular spacetime whose global structure includes further asymptotically flat regions in which the spacetime's mass is positive or negative. We study the timelike and null geodesics and their effective potential, showing that the spacetime is geodesically complete. We discuss the validity of this solution, arguing that the very interesting and peculiar properties of the solution are associated to the negative energy contributions coming from the terms quadratic in the curvature. As a matter of fact, the 10-dimensional configuration is singular. We extract some general lessons on attempts to eliminate black-hole singularities by introducing terms of higher order in the curvature.Comment: 5 pages, 2 figure

Open Strings in IIB Orientifold Reductions

We consider type IIB compactifications on a general 4D group manifold with different types of possible spacetime filling O-planes and the corresponding D-branes parallel to them. Once fluxes allowed by the associated orientifold projection are included, a 6D $\mathcal{N}=(1,1)$ gauged supergravity is obtained. In this paper we show how the consistent coupling to dynamical open strings living on the spacetime filling D-branes may be captured by the inclusion of extra vector multiplets and extra embedding tensor deformations on the gauged supergravity side. As a result, the quadratic constraints on the embedding tensor consistently reproduce the source corrected 10D Bianchi identities. Furthermore, the field strength modifications induced by the open string sector could potentially be understood as U-dual versions of the Green-Schwarz terms. Finally, the entire scalar potential of the theory exactly matches the one obtained from reduction of the bulk action plus the source contributions.Comment: 45 pages, 1 figur

Beyond the near-horizon limit: Stringy corrections to Heterotic Black Holes

We study the first-order in $\alpha'$ corrections to 4-charge black holes (with the Reissner-Nordstr\"om black hole as a particular example) beyond the near-horizon limit in the Heterotic Superstring effective action framework. The higher-curvature terms behave as delocalized sources in the equations of motion and in the Bianchi identity of the 3-form. For some charges, this introduces a shift between their values measured at the horizon and asymptotically. Some of these corrections and their associated charge shifts, but not all of them, can be canceled using appropriate SU$(2)$ instantons for the heterotic gauge fields. The entropy, computed using Wald's formula, is in agreement with the result obtained via microstate counting when the delocalized sources are properly taken into account.Comment: 46 page

How cyclic chain topology can reduce the crystallization rate of Poly(3- hexylthiophene) and promote the formation of liquid crystalline phases in comparison with linear analogue chains

We have studied how cyclic topology affects the crystallization and morphology of π-conjugated poly(3- hexylthiophene) (P3HT) molecules by comparing linear and cyclic analogues for the first time, with three different chain lengths. We employ a range of experimental techniques including wide-angle X-ray scattering (WAXS), polarized light optical microscopy (PLOM), differential scanning calorimetry (DSC) and a combination of Polarized Light Optical Microscopy (PLOM) and Spectroscopy (S). The crystallization and melting points, melting/crystallization enthalpies, and overall crystallization kinetics of cyclic P3HTs are found to be substantially lower than their linear counterparts. The results are explained by the higher rigidity of cyclic molecules, as predicted by density functional theory (DFT) calculations, in the low molecular weight range explored here, in comparison with linear P3HT chains. Additionally, we have found that cyclic P3HT can form liquid crystalline phases above their crystalline melting points and a new crystal form at lower temperatures. PLOM, WAXS, DSC and PLOM/S results indicate that cyclic P3HTs display bipolar and concentric nematic textures that disappear at a well-defined nematic-isotropic transition temperature. Such liquid crystalline textures are completely absent in the linear P3HTs analogues studied here. We conclude that the cyclic topology induces the formation of liquid crystalline phases as the rigid P3HT oligomers can self-assemble above their crystalline melting temperatures.J.M. acknowledges support from the Provincial Council of Gipuzkoa under the program Fellow Gipuzkoa and “Fomento San Sebastián” in the framework program “Retorno del Talento Local” Donostia up! 2016. We acknowledge the help of Dr. Jaime Martín and Ms. Sara Marina with the combined Microscopy/Spectroscopy measurements. J.M. and A.J.M. acknowledge funding by Mineco MAT2017- 83014-C2-1-P project. N.D. and O.C. are grateful to the Science Policy Office of the Belgian Federal Government (PAI 7/5) and to the Belgian FRFC-FNRS (n°2.4508.12). O.C. is Research Associates of the F.R.S.-FNRS. G.L. is grateful to the Youth Innovation Promotion Association of the Chinese Academy of Sciences (2015026). D.W. and A.J.M acknowledge the support from the National Natural Science Foundation of China (51820105005). All authors gratefully acknowledge the support of the EU through the H2020-MSCA-RISE-2017-778092 BIODEST project. Technical and human support provided by IZO-SGI, SGIker (UPV/EHU, MICINN, GV/EJ, ERDF and ESF) is gratefully acknowledged for assistance and generous allocation of computational resources