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Magnetism and Superconductivity in the Iron-based Superconductor La1-xFxFeAs as Probed by Nuclear Magnetic Resonance

13/03/2012 Franziska Hammerath, IFW, Dresden, Germany

In my talk I will give an overview of NMR measurements on LaO1-xFxFeAs with 0 ≤ x ≤ 0:1, covering a broad range of the phase diagram from magnetically-ordered to optimally-doped superconducting samples [1]. Considering the normal state properties, I will mainly focus on the role of spin fluctuations in pnictides. For underdoped superconducting samples (x = 0.045, x = 0.05 and x = 0.075) the 75As NMR spin-lattice relaxation rate (T1T)-1 shows a Curie-Weiss-like increase at intermediate temperatures, indicating the presence of spin fluctuations. However, a simple Curie-Weiss fit fails to describe (T1T)-1 (T) above 250K and the occurrence of a peak in (T1T)-1 clearly above Tc. Instead, the data can be described by considering a BPP-model for fluctuating magnetic fields in combination with a doping independent linear temperature dependence at high temperature. Our analysis is consistent with charge carrier localization in underdoped LaO1-xFxFeAs as seen by means of resistivity measurements [2]. At optimal doping (x = 0.1), spin fluctuations are suppressed and only the linear contribution to (T1T)-1 is left. I will discuss our findings in comparison to NMR measurements on other pnictides.
In the superconducting state a drastic change of the temperature dependence of T1-1(T) below Tc from a T3-law for optimally-doped LaFeAsO0.:9F0.1 to a T5-law for As-deficient LaFeAs1-δO0:9F0:1 will be presented and discussed in terms of different possible superconducting gap symmetries and the effect of As deficiencies as a special kind of impurities [3,4].
[1] H. Luetkens et al. Nature Materials 8, 306 (2009).
[2] C. Hess et al., Eur. Phys. Lett. 87, 17005 (2009).
[3] G. Fuchs et al., Phys. Rev. Lett. 101, 237003 (2008).

[4] F. Hammerath et al., Phys. Rev. B 81, 140504(R) (2010)