114 research outputs found
Effect of annealing on glassy dynamics and non-Fermi liquid behavior in UCu_4Pd
Longitudinal-field muon spin relaxation (LF-muSR) experiments have been
performed in unannealed and annealed samples of the heavy-fermion compound
UCu_4Pd to study the effect of disorder on non-Fermi liquid behavior in this
material. The muon spin relaxation functions G(t,H) obey the time-field scaling
relation G(t,H) = G(t/H^gamma) previously observed in this compound. The
observed scaling exponent gamma = 0.3 pm 0.1, independent of annealing. Fits of
the stretched-exponential relaxation function G(t) = exp[-(Lambda t)^K] to the
data yielded stretching exponentials K < 1 for all samples. Annealed samples
exhibited a reduction of the relaxation rate at low temperatures, indicating
that annealing shifts fluctuation noise power to higher frequencies. There was
no tendency of the inhomogeneous spread in rates to decrease with annealing,
which modifies but does not eliminate the glassy spin dynamics reported
previously in this compound. The correlation with residual resistivity
previously observed for a number of NFL heavy-electron materials is also found
in the present work.Comment: 4 pages, 3 figures, submitted to 10th International Conference on
Muon Spin Rotation, Relaxation, and Resonance, Oxford, UK, August 200
Glassy Spin Dynamics in Non-Fermi-Liquid UCu_{5-x}Pd_x, x = 1.0 and 1.5
Local f-electron spin dynamics in the non-Fermi-liquid heavy-fermion alloys
UCu_{5-x}Pd_x, x = 1.0 and 1.5, have been studied using muon spin-lattice
relaxation. The sample-averaged asymmetry function Gbar(t) indicates strongly
inhomogeneous spin fluctuations, and exhibits the scaling Gbar(t,H) =
Gbar(t/H^\gamma) expected from glassy dynamics. At 0.05 K \gamma(x=1.0) = 0.35
\pm 0.1, but \gamma(x=1.5) = 0.7 \pm 0.1. This is in contrast to inelastic
neutron scattering results, which yield \gamma = 0.33 for both concentrations.
There is no sign of static magnetism \gtrsim 10^{-3} \mu_B/U ion in either
material above 0.05 K. Our results strongy suggest that both alloys are quantum
spin glasses.Comment: 4 pages, 4 figures, to be published in Physical Review Letter
Magnetism and superconductivity in CeRh_{1-x}Ir_xIn_5 heavy fermion materials
We report on zero-field muon spin relaxation studies of cerium based
heavy-fermion materials CeRh_{1-x}Ir_xIn_5. In the superconducting x=0.75 and 1
compositions muon spin relaxation functions were found to be temperature
independent across T_c; no evidence for the presence of electronic magnetic
moments was observed. The x=0.5 material is antiferromagnetic below T_N=3.75 K
and superconducting below T_c=0.8 K. Muon spin realxation spectra show the
gradual onset of damped coherent oscillations characteristic of magnetic order
below T_N. At 1.65 K the total oscillating amplitude accounts for at least 85%
of the sample volume. No change in muon precession frequency or amplitude is
detected on cooling below T_c, indicating the microscopic coexistence of
magnetism and superconductivity in this material.Comment: 6 pages with 3 figures. Revision with corrected axis label (mK) in
Fig.
Critical slowing down in the geometrically frustrated pyrochlore antiferromagnet Gd_2Ti_2O_7
Longitudinal-field muon spin relaxation experiments have been carried out in
the paramagnetic state of single-crystal Gd_2Ti_2O_7 just above the phase
transition at T_m = 1.0 K. At high applied fields the exponential relaxation
time T_1 is proportional to field, whereas T_1 saturates below a crossover
field B_c that is ~2.5 T at 1.5 K and decreases as T_m is approached. At low
fields the relaxation rate increases markedly as the freezing temperature is
approached, as expected for critical slowing down of the spin fluctuations, but
the increase is suppressed by applied field. This behavior is consistent with
the very long autocorrelation function cutoff time implied by the low value of
B_c.Comment: 4 pages, 2 figures, submitted to 10th International Conference on
Muon Spin Rotation, Relaxation, and Resonance, Oxford, UK, August 200
Automated correction for the movement of suspended particulate in microtomographic data
This study reports the development and application of a digital image cross correlation based approach to resolve contiguous microstructural volumes of interest in X-ray microtomography data, collected in fluid suspensions that undergo significant microstructural changes over time, using fresh cementitious pastes as an example. This computational method provides a high precision both for cementitious pastes that sediment only slightly (i.e. are cohesive), and for those that undergo significant sedimentation and/or settlement within the first few minutes of reaction. The normalised cross correlation algorithm presented here enables the observation of an identical volume of interest, i.e., one which contains a contiguous particle group, from the first seconds of observation onwards with excellent accuracy. This method enables segmentation of the same cluster of particles to be almost entirely automated and resolved in large sets of sequentially collected data, therefore enabling particle reaction to be observed directly while removing effects due to sedimentation
Quantum charges and spacetime topology: The emergence of new superselection sectors
In which is developed a new form of superselection sectors of topological
origin. By that it is meant a new investigation that includes several
extensions of the traditional framework of Doplicher, Haag and Roberts in local
quantum theories. At first we generalize the notion of representations of nets
of C*-algebras, then we provide a brand new view on selection criteria by
adopting one with a strong topological flavour. We prove that it is coherent
with the older point of view, hence a clue to a genuine extension. In this
light, we extend Roberts' cohomological analysis to the case where 1--cocycles
bear non trivial unitary representations of the fundamental group of the
spacetime, equivalently of its Cauchy surface in case of global hyperbolicity.
A crucial tool is a notion of group von Neumann algebras generated by the
1-cocycles evaluated on loops over fixed regions. One proves that these group
von Neumann algebras are localized at the bounded region where loops start and
end and to be factorial of finite type I. All that amounts to a new invariant,
in a topological sense, which can be defined as the dimension of the factor. We
prove that any 1-cocycle can be factorized into a part that contains only the
charge content and another where only the topological information is stored.
This second part resembles much what in literature are known as geometric
phases. Indeed, by the very geometrical origin of the 1-cocycles that we
discuss in the paper, they are essential tools in the theory of net bundles,
and the topological part is related to their holonomy content. At the end we
prove the existence of net representations
The pore structure and water absorption in Portland/slag blended hardened cement paste determined by synchrotron X-ray microtomography and neutron radiography
The pore structures of hardened Portland/slag cement pastes (>75 wt% slag content), and the initial capillary absorption of moisture through these pores, were monitored using ex situ synchrotron X-ray computerised microtomography and in situ quantitative neutron radiography. The pore structure becomes more constricted as the cement hydrates and its microstructure develops. This mechanism was effective even at a slag content as high as 90 wt% in the cementitious blend, where the lowest total porosity and a significant pore refinement were identified at extended curing ages (360 d). By combining this information with neutron radiographic imaging, and directly quantifying both depth and mass of water uptake, it was observed that 90 wt% slag cement outperformed the 75 wt% slag blend at 90 days in terms of resistance to capillary water uptake, although the higher-slag blend had not yet developed such a refined microstructure at 28 days of curing. The assumptions associated with the “sharp front model” for water ingress do not hold true for highly substituted slag cement pastes. Testing transport properties at 28 days may not give a true indication of the performance of these materials in service in the long term
Diacol Guadalupe, Diacol Caribe y Diacol Sumapaz, tres nuevas variedades de papa
En trabajos desarrollados en el Centro Nacional de Investigaciones Agropecuarias Tibaitatá y otras estaciones informan de las tres nuevas variedades de papa de buena calidad y alto rendimiento. En la descripciĂłn de cada una de estas variedades se anotan sus caracterĂsticas más importantes. Diacol-Guadalupe corresponde al hĂbrido No. 50-44-87 obtenido en 1950, mediante el cruzamiento de la variedad Branca cascuda (S. tuberosum) del brasil y la variedad pajarita careta (S. andigenum) de Colombia. Diacol-Capiro hibrido No. 53-110-13 obtenida en 1953 por cruzamiento entre lineas Escocesas (S. tuberosum) y la variedad tuquerreña (S. andigenum) de Colombia. Diacol Sumapaz, hĂbrido No. 54-699-2 obtenida en 1954 por cruzamiento de la linea 101 y la variedad algodona (S. andigenum) de Colombia. Todas son de buena producciĂłn (de 35 a 50 t/ha), resistentes a la gota y las aspersiones se pueden reducir a la mitadPapa-Solanum tuberosu
Superselection Sectors and General Covariance.I
This paper is devoted to the analysis of charged superselection sectors in
the framework of the locally covariant quantum field theories. We shall analize
sharply localizable charges, and use net-cohomology of J.E. Roberts as a main
tool. We show that to any 4-dimensional globally hyperbolic spacetime it is
attached a unique, up to equivalence, symmetric tensor \Crm^*-category with
conjugates (in case of finite statistics); to any embedding between different
spacetimes, the corresponding categories can be embedded, contravariantly, in
such a way that all the charged quantum numbers of sectors are preserved. This
entails that to any spacetime is associated a unique gauge group, up to
isomorphisms, and that to any embedding between two spacetimes there
corresponds a group morphism between the related gauge groups. This form of
covariance between sectors also brings to light the issue whether local and
global sectors are the same. We conjecture this holds that at least on simply
connected spacetimes. It is argued that the possible failure might be related
to the presence of topological charges. Our analysis seems to describe theories
which have a well defined short-distance asymptotic behaviour.Comment: 66 page
Defibrotide inhibits donor leucocyte-endothelial interactions and protects against acute graft-versus-host disease
Altres ajuts: This study has been funded by Josep Carreras Leukaemia Foundation (Grant 11R/2016 and 03R/2019).Allogeneic hematopoietic stem cell transplantation (allo-HCT) is an effective therapy for the treatment of high-risk haematological malignant disorders and other life-threatening haematological and genetic diseases. Acute graft-versus-host disease (aGvHD) remains the most frequent cause of non-relapse mortality following allo-HCT and limits its extensive clinical application. Current pharmacologic agents used for prophylaxis and treatment of aGvHD are not uniformly successful and have serious secondary side effects. Therefore, more effective and safe prophylaxis and therapy for aGvHD are an unmet clinical need. Defibrotide is a multi-target drug successfully employed for prophylaxis and treatment of veno-occlusive disease/sinusoidal obstruction syndrome. Recent preliminary clinical data have suggested some efficacy of defibrotide in the prevention of aGvHD after allo-HCT. Using a fully MHC-mismatched murine model of allo-HCT, we report here that defibrotide, either in prophylaxis or treatment, is effective in preventing T cell and neutrophil infiltration and aGvHD-associated tissue injury, thus reducing aGvHD incidence and severity, with significantly improved survival after allo-HCT. Moreover, we performed in vitro mechanistic studies using human cells revealing that defibrotide inhibits leucocyte-endothelial interactions by down-regulating expression of key endothelial adhesion molecules involved in leucocyte trafficking. Together, these findings provide evidence that defibrotide may represent an effective and safe clinical alternative for both prophylaxis and treatment of aGvHD after allo-HCT, paving the way for new therapeutic approaches
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