Berlin 2012 – wissenschaftliches Programm
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TT: Fachverband Tiefe Temperaturen
TT 2: Superconductivity: Fe-based Superconductors - 1111, LiFeAs & As-free Pnictides
TT 2.1: Vortrag
Montag, 26. März 2012, 09:30–09:45, H 2053
NMR evidence for spin fluctuations in underdoped LaO1−xFxFeAs — •Franziska Hammerath1, Hans-Joachim Grafe1, Guillaume Lang1, Dalibor Paar2, Günter Behr1, Jochen Werner1, and Bernd Büchner1 — 1Leibniz-Institut für Festkörper- & Werkstoffforschung, Dresden, Germany — 2Department of Physics, Faculty of Science, University of Zagreb
We present 75As Nuclear Magnetic Resonance (NMR) measurements on the iron-based superconductor LaO1−xFxFeAs with 0 ≤ x ≤ 0.1, covering a broad range of the phase diagram from magnetically-ordered to optimally-doped superconducting samples [1]. For underdoped samples (x=0.05, x=0.075) the 75As NMR spin-lattice relaxation rate (T1T)−1 shows a Curie-Weiss-like increase at intermediate temperatures, indicating the slowing down of spin fluctuations. However, a simple Curie-Weiss fit fails to describe (T1T)−1(T) above 250 K and the occurrence of a peak in (T1T)−1 slightly above Tc. Instead, the data can be well described by considering a BPP-model for fluctuating magnetic fields in combination with a doping-independent linear temperature dependence at high temperature.
At optimal doping (x=0.1) spin fluctuations are suppressed and only the linear contribution to (T1T)−1 is left. This stands in contrast to other pnictides, such as Ba(Fe1−xCoxAs)2 and Ba(FeAs1−xPx)2.
Our analysis is consistent with charge carrier localization in underdoped LaO1−xFxFeAs as seen by means of resistivity measurements [2].
H. Luetkens et al. Nature Materials 8, 306 (2009).
C. Hess et al., Eur. Phys. Lett. 87, 17005 (2009).