F19NMR study of the coupling between4fand itinerant electrons in the pnictide superconductors

Giacomo Prando

¹

,

P. Carretta

²

,

A. Rigamonti

³

et al.

Abstract

F NMR measurements in SmFeAsO 1−x F x , for 0.15Յ x Յ 0.2, are presented. The nuclear spin-lattice relaxation rate 1 / T 1 increases upon cooling with a trend analogous to the one already observed in CeCu 5.2 Au 0.8 , a quasi-two-dimensional heavy-fermion intermetallic compound with an antiferromagnetic ground state. In particular, the behavior of the relaxation rate either in SmFeAsO 1−x F x or in CeCu 5.2 Au 0.8 can be described in the framework of the self-consistent renormalization theory for weakly itinerant electron systems. Remarkably, no effect of the superconducting transition on 19 F 1/ T 1 is detected, a phenomenon which can hardly be explained within a single band model.

Poisoning effect of Mn inLaFe1−xMnxAsO0.89F0.11: Unveiling a quantum critical point in

“…Its temperature dependence can be used for a double-check of the assumption of antiferromagnetic spin fluctuations in the proximity to a QCP, since in that case the in plane correlation length should scale as ξ ∝ T 0 /T for T ≪ T 0 . 35,37 This is indeed what we find [see Fig. 8(c)] and confirms that the 75 As-NQR (T 1 T ) −1 is determined by 2D antiferromagnetic spin fluctuations.…”

“…This is very alike to what has been observed in SmFeAsO 1−x F x . 35 In this compound, a similar increase of 19 F-NMR (T 1 T ) −1 has been observed due to a non-neglible coupling between f electrons and conduction electrons and has been analyzed within the framework of the self-consistent renormalization (SCR) theory. This justifies to analyze also our data in the framework of the SCR theory, which is usually used to describe spin fluctuations in weakly itinerant systems near a QCP.…”

Common effect of chemical and external pressures on the magnetic properties ofRCoPO (R=La,Pr,Nd,Sm).

“…In the case of RFeAsO superconductors, it is well known that R strongly influences the optimal-doping T c value, which is more than doubled from ∼ 25 K in the case of R = La to ∼ 55 K for R = Sm. While it is excluded that this enhancement is associated with 4f orbitals and with the localized magnetic moment at the R site, 20 no theory can still fully account for the experimental observations. The structural origin (i. e., chemical pressure) seems to be a more adequate explanation for the observed effects, 21 however in this case it would be naively expected that external pressure in optimally-doped systems could enhance T c as well, which is not the case experimentally.…”

Effect of external pressure on the magnetic properties of R CoAsO ( R =La, Pr, Sm): a μSR study

“…In this respect, it should be stressed that the chemical substitution of R ions also shrinks the lattice and sizeably affects crystallographic properties like the tetrahedral angle and the pnictogen height, two quantities which are strictly correlated with the value of the superconducting transition temperature T c [14]. Accordingly, only small R ions like Sm allow to obtain the maximum T c value of ∼ 55 K -incidentally, the highest value known up to now for pnictide materials [15,16,17].…”