Competing effects of Mn and Y doping on the low-energy excitations and phase diagram ofLa1−yYyFe1−xMnx

Abstract

Muon Spin Rotation (µSR ) and19 F Nuclear Magnetic Resonance (NMR) measurements were performed to investigate the effect of Mn for Fe substitutions in La1−yYyFe1−xMnxAsO0.89F0.11 superconductors. While for y = 0 a very low critical concentration of Mn (x = 0.2%) is needed to quench superconductivity, as y increases the negative chemical pressure introduced by Y for La substitution stabilizes superconductivity and for y = 20% it is suppressed at Mn contents an order of magnitude larger. A magnetic phase arises once superconductivity is suppressed both for y=0 and for y = 20%. Low-energy spin fluctuations give rise to a peak in 19 F NMR 1/T1 with an onset well above the superconducting transition temperature and whose magnitude increases with x. Also the static magnetic correlations probed by 19 F NMR linewidth measurements show a marked increase with Mn content. The disruption of superconductivity and the onset of the magnetic ground-state are discussed in the light of the proximity of LaFeAsO0.89F0.11 to a quantum critical point.

Role of magnetic dopants in the phase diagram of Sm 1111 pnictides: The case of Mn

“…14,15 Interestingly, the partial substitution of La with smaller Y ions drives the system away from quantum criticality, implying that a higher chemical pressure reduces the effects of Mn magnetic correlations. 16 It is conceivable that the same mechanism, i.e., a higher chemical pressure, reflecting the smaller size of Sm ions, can explain why T c decreases more slowly in the Sm-1111 than in La-1111 compounds for nominally equal Mn contents. 6 Although a higher chemical pressure implies weaker electronic correlations, these still persist and have been shown to enhance the inter-impurity Ruderman-Kittel-Kasuya-Yosida (RKKY) exchange interaction, responsible for the competition between the magnetically-ordered and the superconducting phase.…”

Fast recovery of the stripe magnetic order by Mn/Fe substitution in F-doped LaFeAsO superconductors

“…In fact, while in most the IBS compounds the T c suppression rate (dT c /dx) is well below 10 K/% Mn, in LaFeAsO 0.89 F 0.11 just 0.2 -0.3% of Mn impurities suppress superconductivity from the optimal T c 27 K (dT c /dx ∼ 110 K/% Mn) and then, at higher Mn doping levels, a magnetic order develops (see Fig. 1b) [32][33][34]. The understanding of why such a dramatic effect is present, what type of magnetic order is developing and how to describe these materials at the microscopic level are presently subject of debate [35][36][37].…”

“…Previous experimental results have shown that this system can be driven away from the QCP via the total substitution of La with Nd or by the partial substitution with Y [33,34], which shrink the structure and cause a reduction of the electronic correlations [27]. Hence, the Ln1111 compound can be considered as a formidable example of how the electronic properties of strongly correlated systems can be significantly affected by fine-tuning the correlation strength with impurities and chemical pressure.…”

Unconventional Disorder Effects in Correlated Superconductors

“…Recently it was shown that Y-substitution for La can similarly shift OD La-1111 away from the QCP and remove the poisoning effect. [27] Non-magnetic disorder. We now turn to the discussion of non-magnetic disorder, and again motivate the study by a set of puzzling experimental findings from FeSCs summarized in Figs.…”