Charge Nematicity and Electronic Raman Scattering in Iron-based Superconductors

We review the recent developments in electronic Raman scattering measurements of charge nematic fluctuations in iron-based superconductors. A simple theoretical framework of a $d$-wave Pomeranchuk transition is proposed in order to capture the salient features of the spectra. We discuss the available Raman data in the normal state of 122 iron-based systems, particularly Co doped BaFe$_2$As$_2$, and we show that the low energy quasi-elastic peak, the extracted nematic susceptibility and the scattering rates are consistent with an electronic driven structural phase transition. In the superconducting state with a full gap the quasi-elastic peak transforms into a finite frequency nematic resonance, evidences for which are particularly strong in the electron doped systems. A crucial feature of the analysis is the fact that the electronic Raman signal is unaffected by the acoustic phonons. This makes Raman spectroscopy a unique probe of electronic nematicity.Comment: Invited review to a special issue in Iron-based superconductor

Sign refinement for combinatorial link Floer homology

Link Floer homology is an invariant for links which has recently been described entirely in a combinatorial way. Originally constructed with mod 2 coefficients, it was generalized to integer coefficients thanks to a sign refinement. In this paper, thanks to the spin extension of the permutation group we give an alternative construction of the combinatorial link Floer chain complex associated to a grid diagram with integer coefficients. We prove that the filtered homology of this complex is an invariant for the link and that it gives the previous sign refinement by means of a 2-cohomological class corresponding to the spin extension of the permutation group.Comment: 17 pages, 10 figures. correction of the Alexander grading and of the formula of lemma 5.2 of the sign refinemen

Resonant Raman scattering in mercurate single crystals

We report resonant electronic Raman scattering in optimally doped single layer HgBa$_2$CuO$_{4+\delta}$ (Hg-1201) and trilayer HgBa$_2$Ca$_2$Cu$_3$O$_{8+\delta}$ (Hg-1223) single crystals. Analysis of the $B_{1g}$ and B$_{2g}$ channels in the superconducting state of Hg-1201 advocates for a gap having d-wave symmetry. In addition a resonant study $B_{1g}$ pair-breaking peak and the $A_{1g}$ peak suggests that the $A_{1g}$ peak is not directly related to the d-wave superconducting gap amplitude. Comparison with trilayer Hg-1223 demonstrates the universal behavior of this two energy scales in optimally doped cuprates.Comment: 4 pages, 4 figures, M2S-Rio (invited paper). to appear in Physica

Near-infrared mapping of spiral barred galaxies

In external galaxies, near-infrared emission originates from stellar populations, hot dust, free-free emission from H+ regions, gaseous emission, non-thermal nucleus if any. Because of the low extinction compared to the visible, infrared wavelengths are useful to probe regions obscured by dust such as central parts where starburst phenomena can occur because of the large quantity of matter. The results presented were obtained with a 32 x 32 InSb charge injection device (CID) array cooled at 4K, at the f/36 cassegrain focus of the 3m60 Canada-France-Hawaii telescope with a spatial resolution of 0.5 inches per pixel. The objects presented are spiral barred galaxies mapped at J(1.25 microns), H(1.65 microns) and K(2.2 microns). The non-axisymetric potential due to the presence of a bar induces dynamical processes leading to the confinement of matter and peculiar morphologies. Infrared imaging is used to study the link between various components. Correlations with other wavelengths ranges and 2-colors diagrams ((J-H), (H-K)) lead to the identification of star forming regions, nucleus. Maps show structures connected to the central core. The question is, are they flowing away or toward the nucleus. Observations of M83 lead to several conclusions. The star forming region, detected in the visible and the infrared cannot be very compact and must extend to the edge of the matter concentration. The general shape of the near-infrared emission and the location of radio and 10 micron peaks suggest the confinement of matter between the inner Linblad resonances localized from CO measurements about 100 and 400 pc. The distribution of color indices in the arc from southern part to the star forming region suggests an increasing amount of gas and a time evolution eventually triggered by supernova explosions. Close to the direction of the bar, a bridge-like structure connects the arc to the nucleus with peculiar color indices. Perhaps, this structure can be linked to a height velocity component seen in UV and we can attribute it to a jet and/or a matter flow along the bar toward the nucleus, fuelling it. NGC 1068 is the nearest Seyfert 2 galaxy. It has been a subject of many studies at all wavelengths. This object was mapped at J, H, K, L and M, and in polaro-imagery. Results are given

Une classe de patrimoine a Carnac

En el marc de l'agermanament del Col·legi amb el Museu de Prehistòria, semblava útil estendre a un públic tan ampli com fos possible el benefici de les nostres experiències i de les nostres reflexions mitjantçant una classe de patrimoni. Associant sense treva el procés de descoberta i d'observació del medi real i d'aprofundiment més teòric, en aquest tipus de classe es proposa igualment els tallers de pràctica i experimentació, que són la prolongació indispensable de l'estudi tractat, de forma complementària i variada, durant tota una setmana.En el marco del hermanamiento del Colegio con el Museo de Prehistoria, parecía útil hacer extensivo a un público tan amplio como fuera posible el beneficio de nuestras experiencias y de nuestras reflexiones mediante una clase de patrimonio. Asociando sin tregua el proceso de descubrimiento y de observación del medio real y de profundización más teórica, en este tipo de clase se propone de la misma manera los talleres de práctica y experimentación, que son la prolongación indispensable del estudio tratado, de forma complementaria y variada, durante toda una semana.Dans le cadre du jumelage du Collège avec le Musée de Préhistoire, il paraissait utile d'étendre à un public aussi large que possible le bénéfice de nos expériences et de nos réflexions par le biais d'une classe de patrimoine. Associant sans cesse la démarche de découverte et d'observation du milieu réel et d'approfondissement plus théorique, ce type de classe propose également des ateliers de pratique et d'expérimentation qui sont l'indispensable prolongement de l'étude menée, de façon complémentaire et variée, durant toute une semaine

The necessity to correct hedge fund returns: empirical evidence and correction method

We study two principal mechanisms suggested in the literature to correct the serial correlationin hedge fund returns and the impact of this correction on financial characteristics of their returnsas well as on their risk level and on their performances. The methods of Geltner (1993), its extensionby Okunev & White (2003) and of Getmansky, Lo & Makarov (2004) are realized on a sampleof 54 hedge fund indexes. The results show that the unsmoothing leaves the mean unchangedbut increases significantly the risk level of hedge funds, whether the risk is measured in terms ofthe return standard-deviation or the modified Value-At-Risk. Funds' performances, measured bytraditional Sharpe ratio and Omega index decline considerably. By contrast, funds' rankings afterthe unsmoothing unexpectedly change slightly. However, some notable modifications in ranks ofseveral funds are observed. The necessary transparency of the management practice requires thatsuch a correction must be systematically done.hedge funds; smoothed returns; performance evaluation; Sharpe ratio; Omega index