Coupling between 4f and itinerant electrons in SmFeAsO1-xFx (0.15 < x < 0.2) superconductors: an NMR study

$^{19}$F NMR measurements in SmFeAsO$_{1-x}$F$_x$, for $0.15\leq x\leq 0.2$, are presented. The nuclear spin-lattice relaxation rate $1/T_1$ increases upon cooling with a trend analogous to the one already observed in CeCu$_{5.2}$Au$_{0.8}$, a quasi two-dimensional heavy-fermion intermetallic compound with an antiferromagnetic ground-state. In particular, the behaviour of the relaxation rate either in SmFeAsO$_{1-x}$F$_x$ or in CeCu$_{5.2}$Au$_{0.8}$ can be described in the framework of the self-consistent renormalization theory for weakly itinerant electron systems. Remarkably, no effect of the superconducting transition on $^{19}$F $1/T_1$ is detected, a phenomenon which can hardly be explained within a single band model.Comment: 4 figure

Superconducting phase fluctuations in SmFeAsO0.8_{0.8}F0.2_{0.2} from diamagnetism at low magnetic field above TcT_{c}

Superconducting fluctuations (SF) in SmFeAsO$_{0.8}$F$_{0.2}$ (characterized by superconducting transition temperature $T_{c} \simeq 52.3$ K) are investigated by means of isothermal high-resolution dc magnetization measurements. The diamagnetic response to magnetic fields up to 1 T above $T_{c}$ is similar to what previously reported for underdoped cuprate superconductors and it can be justified in terms of metastable superconducting islands at non-zero order parameter lacking of long-range coherence because of strong phase fluctuations. In the high-field regime ($H \gtrsim 1.5$ T) scaling arguments predicted on the basis of the Ginzburg-Landau theory of conventional SF are found to be applicable, at variance with what observed in the low-field regime. This fact enlightens that two different phenomena are simultaneously present in the fluctuating diamagnetism, namely the phase SF of novel character and the conventional SF. High magnetic fields (1.5 T $\lesssim H \ll H_{c2}$) are found to suppress the former while leaving unaltered the latter one.Comment: 7 pages, 5 figure

Experimental confirmation of the low B isotope coefficient in MgB2

Recent investigations have shown that the first proposed explanations of the disagreement between experimental and theoretical value of isotope coefficient in MgB2 need to be reconsidered. Considering that in samples with residual resistivity of few mu-Ohm cm critical temperature variations produced by disorder effects can be comparable with variations due to the isotopic effect, we adopt a procedure in evaluating the B isotope coefficient which take account of these effects, obtaining a value which is in agreement with previous results and then confirming that there is something still unclear in the physics of MgB2.Comment: 8 pages, 3 figures Title has been changed A statement has been added in page 7 of the pdf file "Finally we would..." Reference 21 has been added Figure 1 anf Figure 2 have been change

Some Remarks on the Seismic Design of Multipropped Retaining Walls

The behavior under seismic condition of embedded retaining structures is quite complex. When the geometry (prop levels) prevents the formation of kinematic mechanisms and the structural elements do not achieve yield strength conditions, permanent displacements are expected to be relatively low and, therefore, seismic actions may cause significant increases of the forces acting on the structures: these forces are dependent on a number of factors such as the characteristics of the ground motion, the problem geometry, the mechanical behavior of the soil and the soil-structure relative stiffness. In the present study, the results of several dynamic numerical analyses of a multi-propped retaining wall in a dry coarse soil are presented and discussed. The results of the analyses indicate that large structural stresses (bending moments in walls and axial loads on props) develop as consequence of seismic actions. Post seismic stresses remain significantly large as compared to the static condition. The maximum ground acceleration in the free-field seems not to be an effective parameter in order to evaluate the seismic performance of this kind of retaining structures

Tc=21K in epitaxial FeSe0.5Te0.5 thin films with biaxial compressive strain

High purity epitaxial FeSe0.5Te0.5 thin films with different thickness were grown by Pulsed Laser Ablation on different substrates. By varying the film thickness, Tc up to 21K were observed, significantly larger than the bulk value. Structural analyses indicated that the a axis changes significantly with the film thickness and is linearly related to the Tc. The latter result indicates the important role of the compressive strain in enhancing Tc. Tc is also related to both the Fe-(Se,Te) bond length and angle, suggesting the possibility of further enhancement

2D equivalent linear analysis for the seismic vulnerability evaluation of multi-propped retaining structures

The evaluation of the seismic behaviour of underground structures represents one of the most actual seismic geotechnical and structural engineering research topics about the study of the complex phenomena of soil-structural interaction. In the last decades, different types of simplified and numerical approaches have been developed for the correct analysis of the seismic vulnerability of these important infrastructures and a series of laboratory tests for the seismic behaviour characterization of the soils (resonant column test, etc.) and of the coupled soil-structure system (centrifuge test, etc.) have been conducted, especially after the recent strong earthquakes where the underground structures have been subjected to significant damages. In the same way, in the last few years, the International Codes are beginning to pay attention to the concepts of the seismic design of these structures. Despite the significant development of knowledge, described above, still remain open several uncertainties of the correct reproduction of the underground structures behaviour under seismic load. In this paper, the evaluation of the seismic behaviour of a multi-propped retaining structure was conducted, considering the soil-structure interaction effects. The results of the 2D equivalent linear analysis are analysed in terms of bending moment acting on the concrete retaining walls.L‘évaluation du comportement sismique des structures souterraines représentent un des sujets de recherche les plus courants sismique, géotechnique et de construction, qui concerne l‘étude du phénomène complexe de l‘interaction sol-structure. Pendant les dernières décennies, différents types d‘approches simples et numeriques ont été développés pour une analyse exacte de la vulnérabilité sismique de ces infrastructures importantes et encore une série de tests de laboratoire pour la caractérisation du comportement sismique du sols (test de colonne résonnante etc.) et du système couplé du sol-structure (test de centrifugation etc.) ont été menées, après le fort tremblement de terre où les structures souterraines ont subi des dommages importants. De la même manière, pendant les dèrnieres années, les Codes Internationales ont commencé a prêter plus d‘attention aux concepts de désign sismique de ces structures. Malgré la considèrable connaissance, décrit ci-dessus, il y a quand même de l‘incertitude sur la correcte reproduction du comportement des structures sous charge sismique. Dans cet article, il a été mené l‘evaluation du comportement sismique des structures de soutenement multi-etayé, considérant que les effets de l‘interaction sol-structure. Les resultats de l‘analyse linéaire 2D equivalente sont analysées en termes de moment de flexion agissant sur les murs de soutènement en béton