Exact solution of the 1D Hubbard model in the atomic limit with inter-site magnetic coupling

In this paper we present for the first time the exact solution in the narrow-band limit of the 1D extended Hubbard model with nearest-neighbour spin-spin interactions described by an exchange constant J. An external magnetic field h is also taken into account. This result has been obtained in the framework of the Green's functions formalism, using the Composite Operator Method. By means of this theoretical background, we have studied some relevant features such as double occupancy, magnetization, spin-spin and charge-charge correlation functions and derived a phase diagram for both ferro (J>0) and anti-ferro (J<0) coupling in the limit of zero temperature. We also report a study on density of states, specific heat, charge and spin susceptibilities. In the limit of zero temperature, we show that the model exhibits a very rich phase diagram characterized by different magnetic orders and by the coexistence of charge and spin orderings at commensurate filling. Moreover, our analysis at finite temperature of density of states and response functions shows the presence of low-temperature charge and spin excitations near the phase boundaries.Comment: 20 pages, 32 figure

Lesiones por accidentes : traumatismos cráneo-cerebrales y espinomedulares

Fil: Sica, Roberto E. Universidad de Buenos Aires. Facultad de Medicina. Secretaría de Ciencia y Técnica; Argentina.Las tres principales causas del traumatismo cráneo-cerebral y raqui-medular son los accidentes de\ntránsito, las caídas y la violencia física. En todo el mundo, el accidente de tránsito constituye la razón del\n45 % de las internaciones hospitalarias o institucionales ocasionadas por este tipo de traumatismos. La\nmayor parte de estas víctimas habitan en países en desarrollo y están constituidas por peatones, ciclistas\ny pasajeros de transporte público, en la más alta proporción. La cantidad de muertes infantiles es, en los\npaíses en desarrollo, seis veces mayor que la que se ve en los países desarrollados. Sin embargo, en\nestos últimos ha habido un incremento sostenido de esta causa de muerte en los años recientes

Annoyance due to railway vibration at different times of the day

The time of day when vibration occurs is considered as a factor influencing the human response to vibration. The aim of the present paper is to identify the times of day during which railway vibration causes the greatest annoyance, to measure the differences between annoyance responses for different time periods and to obtain estimates of the time of day penalties. This was achieved using data from case studies comprised of face-to-face interviews and internal vibration measurements (N=755). Results indicate that vibration annoyance differs with time of day and that separate time of day weights can be applied when considering exposure–response relationships from railway vibration in residential environments

Encefalitis chagásica pseudotumoral en pacientes con SIDA: presentación atípica en uno de ellos e historia de la enfermedad en una pequeña serie de casos

Chagas' disease is an intracellular parasitic infection owed to a protozoarium, the Trypanosoma cruzi1, affecting a large population in Latinamerica. Within the region 15 to 16 million people are infected2. The worldwide pandemia, due to the infection of the HIV 1 virus, also affects Latinamerican countries. The number of patients with this condition in Central and South Americas amounts to 1.6 million persons3,4. Therefore, both illnesses overlap in a broad geographical area and may coincide in the same patient. The HIV infection, which causes the AIDS syndrome, impairs the immunological system and predisposes to the appearance of opportunistic infections, which may have been hosted unnoticed by the patient until then. Therefore, Chagas' disease, which is a dormant infection in most patients5, may reactivate if the immunological surveillance wanes off as the consequence of the viral insult. Along the last years we6,7 and others8-10 found patients afflicted by AIDS, who developed brain lesions yielded by the Trypanosoma cruzi. The present communication describes three further patients with this condition; one of them is unique because his clinical, radiological and immunological findings differ from those previously reported in the literature.Fil: Sica, Roberto E. P.. Universidad de Buenos Aires. Facultad de Medicina; Argentina. Gobierno de la Ciudad de Buenos Aires. Hospital General de Agudos "Ramos Mejía"; ArgentinaFil: Gargiulo Monachelli, Gisella Mariana. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Biología y Medicina Experimental. Fundación de Instituto de Biología y Medicina Experimental. Instituto de Biología y Medicina Experimental; Argentina. Gobierno de la Ciudad de Buenos Aires. Hospital General de Agudos "Ramos Mejía"; ArgentinaFil: Papayanis, Cristina. Universidad de Buenos Aires. Facultad de Medicina; Argentina. Gobierno de la Ciudad de Buenos Aires. Hospital General de Agudos "Ramos Mejía"; Argentin