Progressive slowing down of spin fluctuations in underdoped LaFeAsO${}_{1-x}$F${}_x$

Abstract

The evolution of low-energy spin dynamics in the iron-based superconductor LaFeAsO${}_{1-x}$F${}_x$ was studied over a broad doping, temperature, and magnetic field range (x = 0 - 0.15, T up to 480K, H up to 30T) by means of As nuclear magnetic resonance (NMR). An enhanced spin-lattice relaxation rate divided by temperature, 1/T1T, in underdoped superconducting samples (x = 0.045, 0.05 and 0.075) suggests the presence of antiferromagnetic spin fluctuations, which are strongly reduced in optimally-doped (x = 0.10) and completely absent in overdoped (x = 0.15) samples. In contrast to previous analysis, Curie-Weiss fits are shown to be insufficient to describe the data over the whole temperature range. Instead, a BPP-type model is used to describe the occurrence of a peak in 1/T1T clearly above the superconducting transition, reflecting a progressive slowing down of the spin fluctuations down to the superconducting phase transition.

10 pages, 10 figures

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• Enhancement of Superconducting Transition Temperature Due to Antiferromagnetic Spin Fluctuations in Iron-pnictides LaFe(As_{1-x}P_x)(O_{1-y}F_y) : 31P-NMR Studies
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• Superconductivity of underdoped PrFeAs(O,F) investigated via point-contact spectroscopy and nuclear magnetic resonance
• Experimental evidence of $T_c$ enhancement without the influence of spin fluctuations: NMR study on LaFeAsO_{1-x}H_x under a pressure of 3.0 GPa
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• Freezing of molecular rotation in a paramagnetic crystal studied by ${}^{31}$P NMR
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• Structural Phase Transition, Antiferromagnetism and Two Superconducting Domes in LaFeAsO${}_{1-x}$F${}_x$ (0 $<x\le$ 0.75)
• Random-strain-induced correlations in materials with intertwined nematic and magnetic orders
• Mapping out the spin fluctuations in Co-doped LaFeAsO single crystals by NMR