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2009 METALS, SUPERCONDUCTORS...Transport measurements of H c2 and its anisotropy in FeSe 1−x Te x single crystalsExploring the temperature dependence of H c2 and itsanisotropy in the new Fe superconductors is an importantexperimental tool to help revealing the mechanism of superconductivity.For example in LaFeAsO 0.89 F 0.11 , the upturnin curvature for H c2 was observed, which suggeststwo-band superconductivity [Hunte et al, Nature 453, 903(2008)]. The high transition temperatures imply high valuesof H c2 , meaning that it is not easy to obtain the full curvedown to low temperature, and many low temperature valuesof H c2 are in fact extrapolations. However, in order to comparethe real behavior to theoretical models, measurementsdown to low temperature, and thus at very high field, arenecessary.A serious limitation at present is that the homogeneity ofmany samples that are studied is highly questionable, andthe highest T c values are obtained on doped, probably inhomogeneous,systems. From this aspect the recent discoveryof superconductivity in FeSe and FeTe 1−x Se x suggestsa promising new direction as these compounds are close tobeing stoichiometric and have a relatively simple structure.We have recently grown single crystals of FeSe [Braithwaiteet al, J. Phys.: Condens. Matter 21, 232202 (2009)],which so far are not homogeneous, and FeTe 1−x Se x whichshow homogeneous bulk superconductivity, with T c around14 K, characterized by specific heat measurements (figure78), and seem suitable for more detailed studies.In the LNCMI-Grenoble, we have measured the anisotropyof H c2 on our FeTe 1−x Se x single crystals up to 28 T. Thisgives a very precise determination of a large part of thecurve. These measurements have been completed by measurementsin pulsed field in LNCMI-Toulouse where thelow temperature part of the curve was obtained, showingthat H c2 reaches almost 50 T at low temperature (figure 79).Analysis of the data is now in progress but a preliminarystudy suggests that although H c2 significantly exceeds thePauli limit, some Pauli limiting is occurring, and this isquite anisotropic. Refinements of the analysis and preparationof a publication are in progress.Figure 78: Specific heat of the single crystal of FeSe 1−x Te xgrown in CEA/Grenoble used in these measurements. Inset showsthe electronic part of C/T after subtraction of the phonon contribution.Figure 79: Upper critical field of a FeSe 1−x Te x single crystalgrown in CEA/Grenoble, combining DC field measurements upto 9 T made in CEA/Grenoble, DC field measurements up to 28T made in LNCMI/Grenoble, and pulsed field measurements upto 55 T made in LNCMI/Toulouse. Lines show fits made using aBCS weak coupling model.I. Sheikin, W. KnafoD. Braithwaite , G. Lapertot, C. Marin (CEA/Grenoble, Grenoble, France),59