École normale supérieure de Cachan

About: École normale supérieure de Cachan is a education organization based out in Cachan, Île-de-France, France. It is known for research contribution in the topics: Decidability & Finite element method. The organization has 2717 authors who have published 5585 publications receiving 175925 citations.

Room-Temperature Optical Tunability and Inhomogeneous Broadening in 2D-Layered Organic-Inorganic Perovskite Pseudobinary Alloys.

Abstract: We focus here our attention on a particular family of 2D-layered and 3D hybrid perovskite molecular crystals, the mixed perovskites (C6H5–C2H4–NH3)2PbZ4(1–x)Y4x and (CH3–NH3)PbZ3(1–x)Y3x, where Z and Y are halogen ions such as I, Br, and Cl. Studying experimentally the disorder-induced effects on the optical properties of the 2D mixed layered materials, we demonstrate that they can be considered as pseudobinary alloys, exactly like Ga1–xAlxAs, Cd1–xHgxTe inorganic semiconductors, or previously reported 3D mixed hybrid perovskite compounds. 2D-layered and 3D hybrid perovskites afford similar continuous optical tunability at room temperature. Our theoretical analysis allows one to describe the influence of alloying on the excitonic properties of 2D-layered perovskite molecular crystals. This model is further refined by considering different Bohr radii for pure compounds. This study confirms that despite a large binding energy of several 100 meV, the 2D excitons present a Wannier character rather than a Fren...

Guarded negation

Abstract: We consider restrictions of first-order logic and of fixpoint logic in which all occurrences of negation are required to be guarded by an atomic predicate. In terms of expressive power, the logics in question, called GNFO and GNFP, extend the guarded fragment of first-order logic and guarded least fixpoint logic, respectively. They also extend the recently introduced unary negation fragments of first-order logic and of least fixpoint logic. We show that the satisfiability problem for GNFO and for GNFP is 2ExpTime- complete, both on arbitrary structures and on finite structures. We also study the complexity of the associated model checking problems. Finally, we show that GNFO and GNFP are not only computationally well behaved, but also model theoretically: we show that GNFO and GNFP have the tree-like model property and that GNFO has the finite model property, and we characterize the expressive power of GNFO in terms of invariance for an appropriate notion of bisimulation.