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1. High field performances in iron-based superconductorsGregory S. Boebinger, Florida State University, DMR 0654118Applied Superconductivity Center Upper critical field, Hc2, of Ba1-xKxFe2As2 single crystals with three doping levels: the optimal doped Ba-1 shows an Hc2 strongly exceeding 85 T above 13K, the maximum field available nowadays, in both field configurations (H//ab and H//c, full and empty symbols respectively) . The superconducting properties of two classes of iron-based materials have been studied in high fields up to 85 T. The critical temperature Tc, the critical field Hc2 and the critical current density Jc define the applications limit of any superconductor. We found that Tc and Hc2 of Ba1-xKxFe2As2 can be easily tuned by changing the doping: Hc2 can exceed 110 Tesla at 0K, almost twice as large as any other material with similar Tc. In FeSe0.5Te0.5 thin films Tc and Hc2 have been enhanced by stress induced by the substrate. In single crystals Tc is 15K and the Hc2 slope reaches 40T/K. In stressed films Tc increases to more than 18K and the extraordinarily slope of 500T/K is achieved, a value never before reached in any material. This work has been collaborative with Chinese Academy of Sciences and CNR-SPIN (Italy). Upper critical field, Hc2, of FeSe0.5Te0.5 thin films with field parallel (a) and perpendicular (b) to the c-axis. Panel (a) inset shows the huge slope obtained close to Tc. Panel (b) inset reveals the small transition shift upon applied field. C. Tarantini, S. Lee, Y. Zhang, J. Jiang, C.W. Bark, J.D. Weiss, A. Polyanskii, C.T. Nelson, H.W. Jang, C.M. Folkman, S.H. Baek, X.Q. Pan, A. Gurevich, E.E. Hellstrom, C.B. Eom, and D.C. Larbalestier, Appl. Phys. Lett. 96, 142510 (2010).; Y. Zhang, C.T. Nelson, S. Lee, J. Jiang, C.W. Bark, J.D. Weiss, C. Tarantini, C.M. Folkman, S.H. Baek, E.E. Hellstrom, D.C. Larbalestier, C.B. Eom, and X.Q. Pan,Appl.Phys Lett. 98, 042509 (2011)

2. High field performances in iron-based superconductorsGregory S. Boebinger, Florida State University, DMR 0654118Applied Superconductivity Center We studied the critical current of Ba(Fe1-xCox)2As2 (Ba122) thin films in collaboration with University of Wisconsin. We found that the properties of this material, unlike other high temperature superconductors, are highly tunable via the introduction of 5nm-wide columnar defects. Such defects are ideal to improve the in-field behavior and enhance Jc. In fact, whereas the clean Ba122 has a larger Jc when H//ab, we were able to strongly enhance the H//c behavior inverting the natural material anisotropy. The in-field improvement in Ba122 is more efficient than what is obtainable in YBa2Cu3O7-x because of the larger defects density. The high flexibility of Ba122 makes it a possible choice for future applications. Planar view and cross-sectional TEM of a Ba(Fe1-xCox)2As2 thin film revealing a high density of columnar defects (left panel). The higher magnification shows no buckling around the defects (right panel). C. Tarantini, S. Lee, Y. Zhang, J. Jiang, C.W. Bark, J.D. Weiss, A. Polyanskii, C.T. Nelson, H.W. Jang, C.M. Folkman, S.H. Baek, X.Q. Pan, A. Gurevich, E.E. Hellstrom, C.B. Eom, and D.C. Larbalestier, Appl. Phys. Lett. 96, 142510 (2010).; Y. Zhang, C.T. Nelson, S. Lee, J. Jiang, C.W. Bark, J.D. Weiss, C. Tarantini, C.M. Folkman, S.H. Baek, E.E. Hellstrom, D.C. Larbalestier, C.B. Eom, and X.Q. Pan,Appl.Phys Lett. 98, 042509 (2011) Critical current density Jc of a Ba(Fe1-xCox)2As2 thin film showing a weak in-field behavior and a strong Jc enhancement when the field is along the c-axis, so parallel to the columnar defects.