Dresden 2009 – scientific programme

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TT: Fachverband Tiefe Temperaturen

TT 40: Superconductivity: Ferropnictides 1

TT 40.18: Talk

Thursday, March 26, 2009, 18:45–19:00, HSZ 03

ESR spectroscopy on (Gd,La)O_1−xF_xFeAs superconductors — •A. Alfonsov, F. Murányi, V. Kataev, N. Leps, R. Klingeler, A. Kondrat, C. Hess, A. Köhler, J. Werner, G. Behr, and B. Büchner — IFW Dresden, Institute for Solid State Research, D-01171 Dresden, Germany

We present results on electron spin resonance (ESR) spectroscopy of polycrystalline samples of the (Gd,La)O_1−xF_xFeAs superconductor with different levels of fluorine and gadolinium doping. The ESR signal of a small amount of Gd spins doped to the parent compound LaOFeAs is sensitive to the structural and in particular to the magnetic phase transition occurring in this material at temperatures ∼ 130 − 150 K. Fluorine doping suppresses both transitions and leads to superconductivity. Correspondingly, the Gd ESR response shows no signatures of the magnetic order in the FeAs planes of samples with a superconducting ground state. In the concentrated compound GdOFeAs the Gd ESR response is sensitive to the magnetism of the FeAs planes, too. Doping of this material with ∼ 15 % of fluorine yields superconductivity with T_c≈ 21 K. Surprisingly, Gd ESR gives clear indications of the enhancement of (quasi)-static magnetic correlations in the superconducting samples which set in below ∼ 80 K and continue to develop even in the superconducting state. We compare ESR data with results of thermodynamic and transport measurements on these samples and discuss a possible role of magnetic rare-earths for the magnetism of the FeAs-planes in which the superconductivity evolves upon the fluorine doping.