Similarities between the t−Jt-J and Hubbard models in weakly correlated regimes

We present a comparative study of the Hubbard and $t-J$ models far away from half-filling. We show that, at such fillings the $t-J$ Hamiltonian can be seen as an effective model of the repulsive Hubbard Hamiltonian over the whole range of correlation strength. Indeed, the $|t/U| \in [0,+\infty [$ range of the Hubbard model can be mapped onto the finite range $|J/t| \in [1, 0 ]$ of the $t-J$ model, provided that the effective exchange parameter $J$ is defined variationally as the local singlet-triplet excitation energy. In this picture the uncorrelated limit U=0 is associated with the super-symmetric point $J=-2|t|$ and the infinitely correlated $U=+\infty$ limit with the usual J=0 limit. A numerical comparison between the two models is presented using different macroscopic and microscopic properties such as energies, charge gaps and bond orders on a quarter-filled infinite chain. The usage of the $t-J$ Hamiltonian in low-filled systems can therefore be a good alternative to the Hubbard model in large time-consuming calculations.Comment: To be published in EPJB. 6 pages. 5 figure

Correlation and Dimerization Effects on the Physical Behavior of the NR4[Ni(dmit)2]2NR_4 [Ni(dmit)_2]_2 Charge Transfer Salts : A DMRG Study of the Quarter-Filling t-J Model

The present work studies the quasi one-dimensional $Ni(dmit)_2$-based compounds within a correlated model. More specifically, we focus our attention on the composed influence of the electronic dimerization-factor and the repulsion, on the transport properties and the localization of the electronic density in the ground-state. Those properties are studied through the computation of the charge gaps (difference between the ionization potential and the electro-affinity: IP-EA) and the long- and short-bond orders of an infinite quarter-filled chain within a $t-J(t,U)$ model. The comparison between the computed gaps and the experimental activation energy of the semiconductor $NH_2Me_2 [Ni(dmit)_2]_2$ allows us to estimate the on-site electronic repulsion of the $Ni(dmit)_2$ molecule to $1.16eV$.Comment: 13 pages, 4 figures, RevTe

Benedict XV :a historiographical reading (Part 1)

A man whose voice was greatly ignored during his own lifetime, Benedict XV (1914-1922) seemed to be destined to suffer the same fate even after death. For decades historiography left this pope undisturbed, forgotten in that silence with which death wraps up the fate of most men. Destined to become, with the exception of John Paul I’s short pontificate, the most forgotten pontificate of the twentieth century, few attempts have been made to study this man, his action and legacy, so much so that it has become symptomatic to describe him as the unrecognized,1 indeed the unknown,2 pope. Historiography has even shied away from studying in depth the reasons for such forgetfulness.3 The fact that this relatively short pontificate, only seven years and five months long, was largely marked by the Great War and its aftermath, ending exactly when the rampant nationalistic sentiments so forcefully denounced by Benedict XV were taking over Europe, certainly hindered a positive evaluation of this personality. Just nine months after his death, in Italy, the Marcia su Roma on 28th October 1922, secured power to the Fascists.peer-reviewe

A historiographical reading of the pontificate of Benedict XV (1914-1922) in the last decade

Tragic indeed were the times during which Giacomo della Chiesa is called to the Papacy. His pontificate takes its cue from the tragic moment of a Great War that Europe found itself immersed in, and that would, in many ways, change its face. Although one of the shortest pontificates of the twentieth century, the role of the Holy See at this key moment in history is determinative in forming a Church that is ready to courageously take its place in a world in transformation; an age marked by fluidity, a time of crisis that truly ushers in the contemporary age with its totalitarian claims. Benedict XV died an untimely death on the 22nd January 1922. Aided by his closest collaborators, through his efforts for peace, he regained a respectable place for the Holy See on the international scene, now recognized by many as that moral authority it is called to be, in consonance with its vocation. The aim of this historiographical review continues to be that of bringing together a survey of the way in which different historians have approached the pontificate, life and action, of Benedict XV. Therefore, it remains largely limited to those themes that the authors have presented as being determinative to the Holy See’s political, diplomatic and ecclesial action in this period which sees its re-emergence as an international key player from a situation of dire isolation.peer-reviewe

A historiographical reading of the pontificate of Benedict XV (1914-1922) following the opening of the Vatican Archives

In the quarter century following Spoleto, no substantial developments had been made until the documentation relative to Benedict XV at the Vatican Archives was made available in 1985. Following the enthusiasm that normally accompanies such openings, it remains to be ascertained what use historians have truly made of the now available material. As will be shown the historiographical analysis is now characterized not by the quantity, but rather by the quality of the studies produced. There is a clear continuity with the themes that have already captured the attention of historians in this field. Interest continues to be determined by the nationalistic concerns, especially as regards the diplomatic relationship between the Holy See and the states during the war, and with the newly formed states that now arise from its ashes. Given their specific nature, most studies are published as part of collective works.peer-reviewe

Party finance reform as constitutional engineering? The effectiveness and unintended consequences of party finance reform in France and Britain

In both Britain and France, party funding was traditionally characterized by a laissez faire approach and a conspicuous lack of regulation. In France, this was tantamount to a 'legislative vacuum'. In the last two decades, however, both countries have sought to fundamentally reform their political finance regulation regimes. This prompted, in Britain, the Political Parties, Elections and Referendums Act 2000, and in France a bout of 'legislative incontinence' — profoundly transforming the political finance regime between 1988 and 1995. This article seeks to explore and compare the impacts of the reforms in each country in a bid to explain the unintended consequences of the alternative paths taken and the effectiveness of the new party finance regime in each country. It finds that constitutional engineering through party finance reform is a singularly inexact science, largely due to the imperfect nature of information, the limited predictability of cause and effect, and the constraining influence of non-party actors, such as the Constitutional Council in France, and the Electoral Commission in Britain

SECM study of a chromium-free anticorrosion adhesion primer for aluminum 2024

International audienceHexavalent chromium, largely used for anticorrosion surface treatments of aluminum in aeronautics, will soon be completely banned due to its high toxicity (REACH regulation). Looking for an innovative solution, regarding both anticorrosion and adhesion properties, a chromium free replacement treatment directly inspired by the diazonium chemistry has been developed by the CEA (LICSEN). The evolution of the surface electrochemical properties was followed before and after grafting by means of scanning electrochemical microscopy (SECM). These studies showed that the native aluminum oxide layer is etched during the organic grafting step, which is performed in sulfuric acid. SECM also evidenced that the polymeric grafted film is porous enough to allow the reconstruction of this native aluminum oxide layer after the treatment. On the strength of these results, we decided to extend the thickness of the aluminum oxide layer by an anodization treatment, in the idea that Al$_2$O$_3$ could be formed through the organic layer without altering it. In that purpose, SECM was used to characterize the films and allow the determination of the best parameters for the organic coating (concentration, immersion time, diazonium function, rinsing, cleaning, additives…) and for the anodization process (duration, applied tension…). Thus, bi-functional coatings were obtained, giving very satisfying results both in terms of corrosion protection and adhesion with painting. This simple and low cost process has been patented . It validates standardized tests and could be swiftly industrializable