Pair Correlations in Doped Hubbard Ladders

Hubbard ladders are an important stepping stone to the physics of the two-dimensional Hubbard model. While many of their properties are accessible to numerical and analytical techniques, the question of whether weakly hole-doped Hubbard ladders are dominated by superconducting or charge-density-wave correlations has so far eluded a definitive answer. In particular, previous numerical simulations of Hubbard ladders have seen a much faster decay of superconducting correlations than expected based on analytical arguments. We revisit this question using a state-of-the-art implementation of the density matrix renormalization group algorithm that allows us to simulate larger system sizes with higher accuracy than before. Performing careful extrapolations of the results, we obtain improved estimates for the Luttinger liquid parameter and the correlation functions at long distances. Our results confirm that, as suggested by analytical considerations, superconducting correlations become dominant in the limit of very small doping.Comment: 10 pages, 12 figures, data analysis include

Groups whose character degree graph has diameter three

Let $G$ be a finite group, and let $\Delta(G)$ denote the \emph{prime graph} built on the set of degrees of the irreducible complex characters of $G$. It is well known that, whenever $\Delta(G)$ is connected, the diameter of $\Delta(G)$ is at most $3$. In the present paper, we provide a description of the finite solvable groups for which the diameter of this graph attains the upper bound. This also enables us to confirm a couple of conjectures proposed by M.L. Lewis

Per una rilettura dello "Zibaldone"

Entre lo íntimo y lo filosófico, se estudia el Zibaldoni de Leopardi, en primer lugar, como un trabajo autónomo que profundiza en sus normas internas, las normas que regulan las correspondencias secretas, las formas de su organización cognitiva. En las relaciones autor-lector, en la variedad de registros y ritmos, los elementos constantes de un estoicismo básico brotarán, una metódica suspensión del juicio que permite solamente un método inductivo (el punto de partida es la vida, libros), el derecho a proponer hipótesis sobre la organización cognitiva del mundo. En el punto donde el conocimiento y el sentimiento se dividen, el sentimiento llega a ser una paradoja feliz de una “razón práctica”, un doble punto de vista que va más allá del estancamiento de cualquier falta de conocimiento.Between the intime and the philosophique, Leopardi's Zibaldone is studied, almost for the first time, as an autonomous work in which, its internal laws, the norms that rule its secret correspondences, the forms of its cognitive organization, are delved into. In the author / reader relationship, in the variety of registers and rhythms, the constant elements of a basic Stoicism spring up, a methodical suspension of the judgement that allows only the inductive method (whose starting point is life, books) the right to propound a hypothesis about the cognitive organization of the world. At the point where knowledge and feeling split, the feeling becomes a happy paradox of a 'practical reason`: a double view which reaches beyond the stagnation of any lack of knowledge.notPeerReviewe