377 research outputs found
Current rectification in a single molecule diode: the role of electrode coupling
We demonstrate large rectification ratios (> 100) in single-molecule
junctions based on a metal-oxide cluster (polyoxometalate), using a scanning
tunneling microscope (STM) both at ambient conditions and at low temperature.
These rectification ratios are the largest ever observed in a single-molecule
junction, and in addition these junctions sustain current densities larger than
10^5 A/cm^2. By following the variation of the I-V characteristics with
tip-molecule separation we demonstrate unambiguously that rectification is due
to asymmetric coupling to the electrodes of a molecule with an asymmetric level
structure. This mechanism can be implemented in other type of molecular
junctions using both organic and inorganic molecules and provides a simple
strategy for the rational design of molecular diodes
Numerical simulation of strongly nonlinear and dispersive waves using a Green-Naghdi model
We investigate here the ability of a Green-Naghdi model to reproduce strongly
nonlinear and dispersive wave propagation. We test in particular the behavior
of the new hybrid finite-volume and finite-difference splitting approach
recently developed by the authors and collaborators on the challenging
benchmark of waves propagating over a submerged bar. Such a configuration
requires a model with very good dispersive properties, because of the
high-order harmonics generated by topography-induced nonlinear interactions. We
thus depart from the aforementioned work and choose to use a new Green-Naghdi
system with improved frequency dispersion characteristics. The absence of dry
areas also allows us to improve the treatment of the hyperbolic part of the
equations. This leads to very satisfying results for the demanding benchmarks
under consideration
Profilaxis antibiótica en cirugía
Existe una abundantísima documentación en la literatura acerca de la eficacia de los
antibióticos utilizados profilácticamente en cirugía (1-10). Probablemente lo más útil
para el trabajo diario de un hospital sea la adopción de pautas concretas que permitan
ser evaluadas periódicamente a través de la colaboración, entre otros, de anestesistas,
cirujanos y microbiólogos (11,12). En esta línea hemos revisado, de una manera
intencionadamente esquemática, diversas facetas relativas a la profilaxis antibiótica en
cirugía. Para ello comentaremos algunos aspectos generales de profilaxis de la
infección postquirúrgica, unas breves normas para la utilización profiláctica de
antibióticos y resumiremos determinadas indicaciones en función del tipo de
intervención
Comparación de la eficacia y seguridad diagnóstica entre el estudio isotópico hepatobiliar y la ecografía en la colecistitis aguda
Hemos estudiado en 67 pacientes con el diagnóstico de colecistitis aguda (CA) confirmado mediante intervención quirúrgica precoz, la sensibilidad diagnóstica del estudio isotópico con HIDA comparándola con la de la ecografía. El estudio se llevó a cabo de forma retrospectiva. La sensibilidad del estudio isotópico (97%) ha sido claramente superior a la de la ecografía (82%), aunque esta última ofrece ventajas en cuanto a la rapidez y economía de valor indudable. Asimismo hemos intentado establecer la seguridad de ambas técnicas en el diagnóstico de la coledocolitiasis asociada a la CA, que en nuestro estudio ha mostrado una incidencia alta (26%), siendo mala para ambas técnicas; la sensibilidad en este caso para la ecografía ha sido del 50% mientras que para el HIDA ha sido del 12,5%. Por tanto creemos conveniente realizar como primer estudio diagnóstico para la confirmación de la sospecha clínica de CA una ecografía abdominal, reservando la escintigrafía para los casos, por otra parte numéricamente apreciables, en que persista la duda diagnóstica. Asimismo recomendamos la realización de colangiografía intraoperatoria debido a la deficiente fiabilidad que ambas exploraciones han demostrado en la detección de coledocolitiasis en los pacientes con C
Influence of tumor characteristics on the outcome of liver transplantation among
Hepatocellular carcinoma (HCC) may recur after liver transplantation (LT), mainly
in patients with multinodular and large tumors. However, factors predictive of
outcome after LT in patients with small tumors remain ill defined. We
investigated which factors were related to mortality or tumor recurrence among 47
liver transplant recipients with liver cirrhosis and HCC and compared them with
107 patients with liver cirrhosis without tumor who underwent LT in the same
period. Patients with HCC were older (P <.001), more frequently had cirrhosis of
a viral origin (P <.001), and had lower Child-Pugh scores (P <.001) than patients
without tumor. Survival of patients with and without tumor was not significantly
different (P =.20). Among patients with HCC, those with lower recurrence-free
survival rates had liver cirrhosis of a viral origin, vascular invasion, bilobar
disease, and tumor-node-metastasis (TNM) stage IV. At multivariate analysis, the
only factor associated with mortality or recurrence was TNM stage IV (P =.02).
Our results suggest that in patients with HCC and TNM stage IV, LT might be
contraindicate
Switching the Magnetic Vortex Core in a Single Nanoparticle
Imaging and manipulating the spin structure of nano- and mesoscale magnetic systems is a challenging topic in magnetism, yielding a wide range of spin phenomena such as skyrmions, hedgehog-like spin structures, or vortices. A key example has been provided by the vortex spin texture, which can be addressed in four independent states of magnetization, enabling the development of multibit magnetic storage media. Most of the works devoted to the study of the magnetization reversal mechanisms of the magnetic vortices have been focused on micrometer-size magnetic platelets. Here we report the experimental observation of the vortex state formation and annihilation in individual 25 nm molecular-based magnetic nanoparticles measured by low-temperature variable-field magnetic force microscopy. Interestingly, in these nanoparticles the switching of the vortex core can be induced with very small values of the applied static magnetic field
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