Pressure-induced dimerization and valence bond crystal formation in the Kitaev-Heisenberg magnet alpha-RuCl3

Magnetization and high-resolution x-ray diffraction measurements of the Kitaev-Heisenberg material alpha-RuCl3 reveal a pressure-induced crystallographic and magnetic phase transition at a hydrostatic pressure of p=0.2 GPa. This structural transition into a triclinic phase is characterized by a very strong dimerization of the Ru-Ru bonds, accompanied by a collapse of the magnetic susceptibility. Ab initio quantum-chemistry calculations disclose a pressure-induced enhancement of the direct 4d-4d bonding on particular Ru-Ru links, causing a sharp increase of the antiferromagnetic exchange interactions. These combined experimental and computational data show that the Kitaev spin liquid phase in alpha-RuCl3 strongly competes with the crystallization of spin singlets into a valence bond solid

Estudio mediante PIXE y PIGE de huesos arqueofaunísticos de la provincia de Córdoba

Resumen del trabajo presentado a la 98a Reunión Nacional de Física de la Asociación Física Argentina celebrada en San Carlos de Bariloche, Río Negro (Argentina) del 24 al 27 de septiembre de 2013.-- et al.El hueso contiene un 70 por ciento de proteína compleja mineralizada cuyo principal componente es la hidroxiapatita Ca10(PO4)6(OH)2. Actúa como repositorio para una gran cantidad de elementos traza tales como F, Na, Mg, Zn, Sr y Ba y como sumidero de elementos tóxicos tales como Cd y Pb. Debido a su tasa muy lenta de reestructuración, refleja hábitos alimentarios a largo plazo y da indicios de la exposición ambiental. Particularmente, los restos arqueofaunísticos recuperados de los sitios arqueológicos contienen potencial de información sobre las condiciones ambientales y los sistemas culturales en el pasado. En este trabajo se determinó la composición elemental de huesos arqueológicos y actuales de Artiodáctilos entre los que se incluyen Camélidos (llama y guanaco), Cérvidos (corzuela), Roedores y Mustélidos. Todas las muestras proceden del Valle de Ongamira, ubicado al norte de provincia de Córdoba, y los conjuntos arqueológicos se asocian a fechas de unos 3000 años de antigüedad. La caracterización se realizó mediante las técnicas de PIXE y PIGE. Las mediciones se realizaron en la línea de haz externo del Centro Nacional de Aceleradores (Sevilla, España) utilizando un haz de protones de 4.5 MeV. Se cuantificaron los elementos mayoritarios (Ca y P), minoritarios (Na, Si, Cl, K) y trazas (Ti, Mn, Fe, Cu, Zn, Sr, Pb y U) en varias zonas de los huesos (epífisis y diáfisis) y se compararon con los resultados obtenidos para huesos de animales actuales. Además de la presencia de una mayor cantidad de elementos traza, observamos que la relación Sr/Ca es siempre mayor para los huesos arqueológicos comparado con los actuales.Peer Reviewe

Standardless semi-quantitative analysis by PIXE

A method based on the refinement of atomic and experimental parameters developed for the description of PIXE spectra is presented and applied to standardless semi-quantitative PIXE analysis. This method was implemented in the sofware PAMPA (Parameter Assessment Method for PIXE Analysis) and consists in minimizing the quadratic differences between an experimental spectrum and an analytical function proposed to describe it. The first results of PAMPA are presented for the quantification of synthetic and mineral, thin and bulk samples, and they are compared with the results obtained with a commercial software.This work was financially supported by the Secretaría de Ciencia y Técnica (Universidad Nacional de Córdoba). The authors are also grateful to the Centro Atómico Bariloche, Argentina, and to the Centro Nacional de Aceleradores, Sevilla, Spain, where the experimental determinations were performed.Peer reviewe

Characteristics of Patent Foramen Ovale Associated with Cryptogenic Stroke: A Biplane Transesophageal Echocardiography Study

Patent foramen ovale is associated with ischemic stroke in patients without a clearly identifiable etiology for stroke (cryptogenic stroke). Paradoxical embolization is thought to be a potential mechanism. However, patent foramen ovale is also found in patients with known cause of stroke. Therefore, using contrast transesophageal echocardiography, we characterized the patent foramen ovale in cryptogenic stroke patients to assess morphological factors that may contribute to paradoxical embolization. Methods: Contrast transesophageal echocardiographic studies of 74 consecutive patients referred for ischemic stroke were reviewed. Twenty-three patients with patent foramen ovale were identified. These patients were classified as having strokes of determined origin or cryptogenic strokes according to criteria developed for the Stroke Data Bank of the National Institute of Neurological Disorders and Stroke. Separation of septum primum from secundum and the number of microbubbles appearing in left atrium were then quantitated. These parameters were compared between patients with cryptogenic stroke and those with known cause of stroke. Results: The patent foramen ovale dimension was significantly larger in patients with cryptogenic stroke compared with patients with an identifiable cause of stroke (2.1+/-1.7 mm versus 057+/-0.78 mm [mean+/-SD]; P<.01). The number of microbubbles was also greater in patients with cryptogenic stroke compared with patients with an identifiable cause of stroke (13.9+/-10.7 versus 1.62+/-0.8 [mean+/-SD]; P<.0005). Conclusions: Patients with cryptogenic stroke have larger patent foramen ovale with more extensive right-to-left inter-atrial shunting than patients with stroke of determined cause. Transesophageal echocardiographically identifiable characteristics of patent foramen ovale may be important in defining the clinical significance of individual patent foramina