Progressive slowing down of spin fluctuations in underdoped LaFeAsO1−xFx

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 480 K, μ 0 H 30 T) by means of 75 As nuclear magnetic resonance. An enhanced spin-lattice relaxation rate divided by temperature (T 1 T ) −1 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 Bloembergen-Purcell-Pound (BPP) model is used to describe the occurrence of a peak in (T 1 T ) −1 clearly above the superconducting transition, reflecting a progressive slowing down of the spin fluctuations down to the superconducting phase transition.

Persistence of slow fluctuations in the overdoped regime of Ba(Fe_1-xRh_x)₂As₂ superconductors

“…Analogous activated behavior with similar U values have been reported in LaFeAsO 1−x F x [8,9]. However, in contrast to LaFeAsO 1−x F x , here the energy barrier is significantly doping dependent, with a marked decrease upon increasing the Rh content (Fig.…”

“…These slow dynamics are also evidenced by a field dependent hump in 1/T 1 (Fig. 7) [9], while in the cuprates 1/T 1 is dominated by high frequency correlated spin dynamics yielding only a weak, if any, magnetic field dependence of 1/T 1 at T > T c [47,48]. The only case in cuprates that bears some resemblance to our data is the increase of 1/T 2 triggered below the onset of charge order [49][50][51].…”

“…The latter slightly deviates from linearity above T c , evidencing the presence of weak magnetic correlations [41,42]. Additionally, the optimally doped and weakly overdoped compounds display a hump in the spin-lattice relaxation rate, above T c [6,9], when the magnetic field is oriented in-plane (H ⊥ c). If we assume that the spin diffusion is associated with random fluctuating local fields, which can be described by a correlation function g(t) = h 0 e −t/τ D , the spectral density of spin fluctuations at the Larmor frequency ω L [23] leads to:…”

“…More recent NMR and neutron scattering studies [4,5] gave further support for the presence of a cluster spin-glass phase coexisting with superconductivity at low electron doping. Remarkably, even when long-range magnetic order and cluster spin-glass phases vanish, enhanced low-frequency fluctuations (MHz range) persist in the normal phase of different families of iron-based superconductors [3,[6][7][8][9][10]. The origin of these slow dynamics still remains unsettled.…”

Antiferromagnetic Spin Fluctuations Enhancing Superconducting Transition Temperature in LaFeAsO-Based High-Tc Superconductors

“…In the previous studies on Fe-pnictide compounds such as Ba(Fe,Co) 2 As 2 [13], Fe(Se,Te) [14,18], BaFe 2 (As,P) 2 [19], and Na(Fe,Co)As [20], it has been well established that T c exhibits a maximum around the AFM phase in which AFMSFs are critically enhanced. On the other hand, for LaFeAs(O 1−y F y ) series, the maximum T c emerges at y=0.1 without any development of AFMSFs upon cooling down to T c [11,21], although AFMSFs can be observed in the vicinity of the AFM ordered phase with a lower T c , i.e., in the range of 0.04 < y < 0.08 [22][23][24]. Thus, we emphasize that the present studies of the LaFe(As,P)(O,F) compounds series provide clear evidence that the development of AFMSFs enhances the T c even if the present La1111 compounds are far away from the AFM ordered phase and optimal lattice parameters.…”