We show how chiral type I models whose tadpole conditions have no supersymmetric solution can be consistently defined introducing antibranes with non-supersymmetric world volumes. At tree level, the resulting stable non-BPS configurations correspond to tachyon-free spectra, where supersymmetry is broken at the string scale on some (anti)branes but is exact in the bulk, and can be further deformed by the addition of brane-antibrane pairs of the same type. As a result, a scalar potential is generated, that can stabilize some radii of the compact space. This setting has the novel virtue of linking supersymmetry breaking to the consistency requirements of an underlying fundamental theory.Comment: 45 pages. Late

We study Einstein-Maxwell-dilaton gravity models in four-dimensional antide\ud Sitter (AdS) spacetime which admit the Reissner-Nordstrom (RN) black hole solution.\ud We show that below a critical temperature the AdS-RN solution becomes unstable against\ud scalar perturbations and the gravitational system undergoes a phase transition. We show\ud using numerical calculations that the new phase is a charged dilatonic black hole. Using\ud the AdS/CFT correspondence we discuss the phase transition in the dual field theory both\ud for non-vanishing temperatures and in the extremal limit. The extremal solution has a\ud Lifshitz scaling symmetry. We discuss the optical conductivity in the new dual phase\ud and find interesting behavior at low frequencies where it shows a “Drude peak”. The\ud resistivity varies with temperature in a non-monotonic way and displays a minimum at low\ud temperatures which is reminiscent of the celebrated Kondo effect

We study total and partial supersymmetry breaking by freely acting orbifolds, or equivalently by Scherk-Schwarz compactifications, in type I string theory. In particular, we describe a four-dimensional chiral compactification with spontaneously broken N = 1 supersymmetry, some models with partial N = 4 → N = 2 and N = 4 → N = 1 supersymmetry breaking and their heterotic and M-theory duals. A generic feature of these models is that in the gravitational sector and in the spectrum of D-branes parallel to the breaking coordinate, all mass splittings are proportional to the compactification scale, while global (extended) supersymmetry remains unbroken at tree level for the massless excitations of D-branes transverse to the breaking direction.

We extend previous techniques for calculations of transplanckian-energy stringstring collisions to the high-energy scattering of massless closed strings from a stack of N Dp-branes in Minkowski spacetime. We show that an effective non-trivial metric emerges from the string scattering amplitudes by comparing them against the semiclassical dynamics of high-energy strings in the extremal p-brane background. By changing the energy, impact parameter and effective open string coupling λ = gN , we are able to explore various interesting regimes and to reproduce classical expectations, including tidal-force excitations, even beyond the leading-eikonal approximation.