Anderson localization in Hubbard ladders

The effect of a weak random potential on two-leg Hubbard ladders is investigated. The random potential is shown to induce Anderson localization except for attractive enough interactions, supressing completely d-wave superconductivity. These localization effects remain very strong even for many ladders coupled by Josephson coupling. Both dc and ac conductivities and localization lengths are obtained. Consequences for the superconducting ladder compound Sr$_x$Ca$_{14-x}$Cu$_{24}$O$_{41+\delta}$ are discussed.Comment: 2 Pages, 2 Figures, Uses espcrc2.sty (included); Proceedings of the SCES98 conference, July 1998 Paris, France; To be published in Physica

Declaraciones de enfermeras hospitalarias de la década del sesenta: ayuda para la comprensión de la enfermería actual

This study is part of a more extensive project that proposes to recover significant aspects related to the evolution of nursing care from 1950's to 1990's. This report is made through the technique of oral declaration by active and retired registered nurses, in the context of an University-hospital from the interior of São Paulo State. The present study emphasizes the outcomes related 1960s. The results show the effort undertaken by nurses in the struggle for profession's recognition and prestige; intense and deep changes related to nurse's new roles as leaders of the nursing staff and members of the medical team.Este estudio es parte de un proyecto más ámplio que apunta rescatar aspestos significativos relacionados a la evolución de la asistencia de enfermería en las décadas del 50 al 90. Este rescate es hecho a través de la técnica de declaraciones de enfermeros en ejercicio y pensionados, en el contexto de un Hospital Escuela del interior Paulista. El presente estudio particulariza los resultados obtenidos, referentes a la década del 60. Como resultado se evidencia el esfuerzo empreendido por los enfermeros en la lucha por elreconocimiento y prestigio de la prefesión, las transformaciones intensas y profundas a los nuevos papeles de enfermería, como líder del grupo de enfermería y miembro del equipo médico.Este estudo é parte de um projeto mais amplo que visa resgatar aspectos significativos relacionados à evolução da assistência de enfermagem nas décadas de 50 a 90. Este resgate é feito através da técnica de depoimentos orais de enfermeiros em exercício e aposentados, no contexto de um Hospital Escola do interior Paulista. O presente estudo particulariza os resultados obtidos, referentes à década de 60. Como resultado evidencia-se o esforço empreendido pelos enfermeiros na luta pelo reconhecimento e prestígio da profissão, as transformações intensa e profundas aos novos papéis da enfermeira enquanto líder da equipe e membro da equipe médica

Effects of disorder on two strongly correlated coupled chains

We study the effects of disorder on a system of two coupled chain of strongly correlated fermions (ladder system), using renormalization group. The stability of the phases of the pure system is investigated as a function of interactions both for fermions with spin and spinless fermions. For spinless fermions the repulsive side is strongly localized whereas the system with attractive interactions is stable with respect to disorder, at variance with the single chain case. For fermions with spins, the repulsive side is also localized, and in particular the d-wave superconducting phase found for the pure system is totally destroyed by an arbitrarily small amount of disorder. On the other hand the attractive side is again remarkably stable with respect to localization. We have also computed the charge stiffness, the localization length and the temperature dependence of the conductivity for the various phases. In the range of parameter where d-wave superconductivity would occur for the pure system the conductivity is found to decrease monotonically with temperature, even at high temperature, and we discuss this surprising result. For a model with one site repulsion and nearest neighbor attraction, the most stable phase is an orbital antiferromagnet . Although this phase has no divergent superconducting fluctuation it can have a divergent conductivity at low temperature. We argue based on our results that the superconductivity observed in some two chain compounds cannot be a simple stabilization of the d-wave phase found for a pure single ladder. Applications to quantum wires are discussed.Comment: 47 pages, ReVTeX , 8 eps figures submitted to PR

Thermal transport in one-dimensional spin gap systems

We study thermal transport in one dimensional spin systems both in the presence and absence of impurities. In the absence of disorder, all these spin systems display a temperature dependent Drude peak in the thermal conductivity. In gapless systems, the low temperature Drude weight is proportional to temperature and to the central charge which characterizes the conformal field theory that describes the system at low energies. On the other hand, the low temperature Drude weight of spin gap systems shows an activated behavior modulated by a power law. For temperatures higher than the spin gap, one recovers the linear T behavior akin to gapless systems. For temperatures larger than the exchange coupling, the Drude weight decays as 1/T^2. We argue that this behavior is a generic feature of quasi one dimensional spin gap systems with a relativistic-like low energy dispersion. We also consider the effect of a magnetic field on the Drude weight with emphasis on the commensurate-incommensurate transition induced by it. We then study the effect of nonmagnetic impurities on the thermal conductivity of the dimerized XY chain and the spin-1/2 two leg ladder. Impurities destroy the Drude peak and the thermal conductivity exhibits a purely activated behavior at low temperature, with an activation gap renormalized by disorder. The relevance of these results for experiments is briefly discussed.Comment: 13 pages, 6 eps figures, RevTeX