Regensburg 2013 – wissenschaftliches Programm
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TT: Fachverband Tiefe Temperaturen
TT 23: Superconductivity: Fe-based Superconductors - 1111
TT 23.3: Vortrag
Montag, 11. März 2013, 17:15–17:30, H21
Spin Fluctuations in Iron Based Superconducters Probed by NMR Relaxation Rate — •Uwe Gräfe1, Franziska Hammerath1, 2, Tim Kühne1, Sabine Wurmehl1, Guillaume Lang3, Bernd Büchner1, and Hans-Joachim Grafe1 — 1IFW Dresden, Institute for Solid State Research, PF 270116, 01171 Dresden, Germany — 2Department of Physics "A. Volta", University of Pavia-CNISM, I-27100 Pavia, Italy — 33LPEM-UPR5, CNRS, ESPCI Paris Tech, 10 Rue Vauquelin, 75005 Paris, France
We present 75As nuclear magnetic resonance (NMR) results in F doped LaOFeAs iron pnictides. In the underdoped superconducting samples, pronounced spin fluctuations lead to a peak in the NMR spin lattice relaxation rate, (T1T)−1. The peak shows a typical field dependence that indicates a critical slowing of spin fluctuations: it is reduced in height and shifted to higher temperatures. In contrast, a similar peak in the underdoped magnetic samples at the ordering temperature of the spin density wave does not show such a field dependence. Furthermore, the peak is absent in optimally and overdoped samples, suggesting the absence of strong spin fluctuations. Our results indicate a glassy magnetic ordering in the underdoped samples that is in contrast to the often reported Curie Weiss like increase of spin fluctuations towards Tc. Additional measurements of the linewidth and the spin spin relaxation rate are in agreement with such a glassy magnetic ordering that is most likely competing with superconductivity. Our results will be compared to Co doped BaFe2As2, where a similar peak in (T1T)−1 has been observed [Ning, JPSJ 2009].