Evidence of unconventional low-frequency dynamics in the normal phase of Ba(Fe1-xRhx)2As2 iron-based supercondutors

This work presents 75As NMR spin echo decay rate (1/T2) measurements in Ba(Fe1-xRhx)2As2 superconductors, for 0.041 < x < 0.094. It is shown that 1/T2 increases upon cooling, in the normal phase, suggesting the onset of an unconventional very low-frequency activated dynamic. The correlation times of the fluctuations and their energy barriers are derived. The motion is favored at large Rh content, while it is hindered by the application of a magnetic field perpendicular to the FeAs layers. The same dynamic is observed in the spin-lattice relaxation rate, in a quantitatively consistent manner. These results are discussed in the light of nematic fluctuations involving domain wall motion. The analogies with the behaviour observed in the cuprates are also outlined

Evidence for a vortex-glass transition in superconducting Ba(Fe0.9_{0.9}Co0.1_{0.1})2_{2}As2_{2}

Measurements of magneto-resistivity and magnetic susceptibility were performed on single crystals of superconducting Ba(Fe$_{0.9}$Co$_{0.1}$)$_{2}$As$_{2}$ close to the conditions of optimal doping. The high quality of the investigated samples allows us to reveal a dynamic scaling behaviour associated with a vortex-glass phase transition in the limit of weak degree of quenched disorder. Accordingly, the dissipative component of the ac susceptibility is well reproduced within the framework of Havriliak-Negami relaxation, assuming a critical power-law divergence for the characteristic correlation time $\tau$ of the vortex dynamics. Remarkably, the random disorder introduced by the Fe$_{1-x}$Co$_{x}$ chemical substitution is found to act on the vortices as a much weaker quenched disorder than previously reported for cuprate superconductors such as, e.g., Y$_{1-x}$Pr$_{x}$Ba$_{2}$Cu$_{3}$O$_{7-\delta}$.Comment: 10 pages, 8 figure

Glassy Transition in the Vortex Lattice of Ba(Fe0.93Rh0.07)2As2 superconductor, probed by NMR and ac-susceptibility

By using Nuclear Magnetic Resonance and ac-susceptibility, the characteristic correlation times for the vortex dynamics, in an iron-based superconductor, have been derived. Upon cooling, the vortex dynamics displays a crossover consistent with a vortex glass transition. The correlation times, in the fast motions regime, merge onto a universal curve which is fit by the Vogel-Fulcher law, rather than by an Arrhenius law. Moreover, the pinning barrier shows a weak dependence on the magnetic field which can be heuristically justified within a fragile glass scenario. In addition, the glass freezing temperatures obtained by the two techniques merge onto the de Almeida-Thouless line. Finally the phase diagram for the mixed phase has been derived.Comment: 6 pages, 9 figure

Enhancement of low-frequency fluctuations and superconductivity breakdown in Mn-dopedLa1−yYyFeAsO0.89F0.11superconductors

19F NMR measurements in optimally electron-doped La1-yYyFe1-xMnxAsO0.89F0.11 superconductors are presented. The effect of Mn doping on the superconducting phase is studied for two series of compounds (y = 0 and y = 0.2) where the chemical pressure is varied by substituting La with Y. In the y = 0.2 series superconductivity is suppressed for Mn contents an order of magnitude larger than for y = 0. For both series a peak in the 19FNMRnuclear spin-lattice relaxation rate 1/T1 emerges upon Mn doping and becomes significantly enhanced on approaching the quantum phase transition between the superconducting and magnetic phases. 19F NMR linewidth measurements show that for similar Mn contents magnetic correlations are more pronounced in the y = 0 series, at variance with what one would expect for Q = (π/a,0) spin correlations. These observations suggest that Mn doping tends to reduce fluctuations at Q = (π/a,0) and to enhance other low-frequency modes. The effect of this transfer of spectral weight on the superconducting pairing is discussed along with the charge localization induced by Mn

Enhancement of low-frequency fluctuations and superconductivity breakdown in Mn-doped La

19F NMR measurements in optimally electron-doped La1-yYyFe1-xMnxAsO0.89F0.11 superconductors are presented. The effect of Mn doping on the superconducting phase is studied for two series of compounds (y = 0 and y = 0.2) where the chemical pressure is varied by substituting La with Y. In the y = 0.2 series superconductivity is suppressed for Mn contents an order of magnitude larger than for y = 0. For both series a peak in the 19FNMRnuclear spin-lattice relaxation rate 1/T1 emerges upon Mn doping and becomes significantly enhanced on approaching the quantum phase transition between the superconducting and magnetic phases. 19F NMR linewidth measurements show that for similar Mn contents magnetic correlations are more pronounced in the y = 0 series, at variance with what one would expect for Q = (π/a,0) spin correlations. These observations suggest that Mn doping tends to reduce fluctuations at Q = (π/a,0) and to enhance other low-frequency modes. The effect of this transfer of spectral weight on the superconducting pairing is discussed along with the charge localization induced by Mn

Anisotropy dependence of the fluctuation spectroscopy in the critical and gaussian regimes in superconducting NaFe1-xCoxAs single crystals

We investigate thermal fluctuations in terms of diamagnetism and magnetotransport in superconducting NaFe1-xCoxAs single crystals with different doping levels. Results show that in the case of optimal doped and lightly overdoped (x= 0.03, 0.05) crystals the analysis in the critical as well as in the Gaussian fluctuation regions is consistent with the Ginzburg-Landau 3D fluctuation theory. However, in the case of strongly overdoped samples (x &gt;= 0.07) the Ullah-Dorsey scaling of the fluctuation induced magnetoconductivity in the critical region confirms that thermal fluctuations exhibit a 3D anisotropic nature only in a narrow temperature region around T-c(H). This is consistent with the fact that in these samples the fluctuation effects in the Gaussian region above T-c may be described by the Lawrence-Doniach approach. Our results indicate that the anisotropy of these materials increases significantly with the doping level