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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 Hammerath^{1, 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 T_c. 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].