engleski

NMR evidence for spin fluctuations in underdoped LaO1−xFxFeAs

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. 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.

75As; NMR; pnictides

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