Disorder, critical currents, and vortex pinning energies in isovalently substituted BaFe2_{2}(As1−x_{1-x}Px_{x})2_{2}

We present a comprehensive overview of vortex pinning in single crystals of the isovalently substituted iron-based superconductor BaFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$, a material that qualifies as an archetypical clean superconductor, containing only sparse strong point-like pins [in the sense of C.J. van der Beek {\em et al.}, Phys. Rev. B {\bf 66}, 024523 (2002)]. Widely varying critical current values for nominally similar compositions show that flux pinning is of extrinsic origin. Vortex configurations, imaged using the Bitter decoration method, show less density fluctuations than those previously observed in charge-doped Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ single crystals. Analysis reveals that the pinning force and -energy distributions depend on the P-content $x$. However, they are always much narrower than in Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$, a result that is attributed to the weaker temperature dependence of the superfluid density on approaching $T_{c}$ in BaFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$. Critical current density measurements and pinning force distributions independently yield a mean distance between effective pinning centers $\bar{\mathcal L} \sim 90$ nm, increasing with increasing P-content $x$. This evolution can be understood as being the consequence of the P-dependence of the London penetration depth. Further salient features are a wide vortex free "Meissner belt", observed at the edge of overdoped crystals, and characteristic chain-like vortex arrangements, observed at all levels of P-substitution.Comment: 11 page

Strong pinning and vortex energy distributions in single-crystalline Ba(Fe1−xCox)2As2

International audienceThe interrelation between heterogeneity and flux pinning is studied in Ba(Fe1−xCox)2As2 single crystals with widely varying Co content x. Magnetic Bitter decoration of the superconducting vortex ensemble in crystals with x=0.075 and x=0.1 reveals highly disordered vortex structures. The width of the Meissner belt observed at the edges of the crystals, and above the surface steps formed by cleaving, as well as the width of the intervortex distance distribution, indicate that the observed vortex ensemble is established at a temperature just below the critical temperature Tc. The vortex interaction energy and pinning force distributions extracted from the images strongly suggest that the vortex lattice disorder is attributable to strong pinning due to spatial fluctuations of Tc and of the superfluid density. Correlating the results with the critical current density yields a typical length scale of the relevant disorder of 40-60 nm

Vortex pinning : a probe for nanoscale disorder in iron-based superconductors

4 pagesInternational audienceThe pinning of quantized flux lines, or vortices, in the mixed state is used to quantify the effect of impurities in iron-based superconductors (IBS). Disorder at two length scales is relevant in these materials. Strong flux pinning resulting from nm-scale heterogeneity of the superconducting properties leads to the very disordered vortex ensembles observed in the IBS, and to the pronounced maximum in the critical current density jc at low magnetic fields. Disorder at the atomic scale, most likely induced by the dopant atoms, leads to "weak collective pinning" and a magnetic field-independent contribution jcoll. The latter allows one to estimate quasi-particle scattering rates

Electron irradiation of Co, Ni, and P-doped BaFe2As2 - type iron-based superconductors

17 pages, soumis pour publications dans les actes de M2S-2012International audienceHigh energy electron irradiation is used to controllably introduce atomic-scale point defects into single crystalline Ba(Fe_1-xCo_x)_2As_2, Ba(Fe_1-xNi_x)_2As_2, and BaFe_2(As_1-xP_x)_2. The appearance of the collective pinning contribution to the critical current density in BaFe_2(As_1-xP_x)_2, and the magnitude of its enhancement in Ba(Fe_1-xCo_x)_2As_2, conform with the hypothesis of quasi-particle scattering by Fe vacancies created by the irradiation. Whereas the insignificant modification of the temperature dependence of the superfluid density in Ba(Fe_1-xCo_x)_2As_2 and Ba(Fe_1-xNi_x)_2As_2 points to important native disorder present before the irradiation, the critical temperatures of these materials undergo a suppression equivalent to that observed in the much cleaner BaFe_2(As_1-xP_x)_2. This lends credence to the hypothesis of line nodes of the order parameter (at finite k_{z}) in the former two materials

Effets du désordre dans les supraconducteurs à base de fer

The pinning of vortices is used as a probe for the identification of disorder and its effect on superconductivity in 122-type iron-based superconductors. Using a new analysis method taking into account the interaction of individual vortices with their neighbors, pinning energies and pinning forces in Ba(Fe1-xCox)2As2 are extracted from the vortex distributions in the regime of small fields. The correlation of measurements of the critical current density Jc with the spatial distribution of vortices shows that pinning in this particular regime is due to the heterogeneity of superconducting properties, on the scale of 20-100 nm. Application of the same analysis procedure on the vortex structure in BaFe2(As1-xPx)2 with less density fluctuations, shows that the pinning forces and energies depend on the doping level x. Both Jc measurements and pinning force distributions independently yield a mean distance between pinning centers of about 90 nm, increasing with increasing P-content x. Combination of the above results and critical current density measurements lead to the conclusion that the low field plateau observed in Jc curves, followed by a power-law decrease, emerges from strong pinning due to nm scale heterogeneity of superconducting properties. Attention is also paid to the weak collective pinning contribution that manifests itself at higher fields > 1 T. Notably, this contribution is consistently analyzed in terms of quasiparticle scattering and mean free path fluctuations. In order to test this premise irradiation of Co, Ni and P-doped 122-type iron-based compounds with high-energy 2.5 MeV electrons is performed for several doping levels of the materials and to different doses. Such irradiation introduces atomic sized point-like defects. Following irradiation it appears that the critical temperature Tc shows a similar depression for all studied materials. The weak collective contribution to Jc in Co-doped is found to clearly increase. Moreover this contribution appears after irradiation of the P-doped compound in which it was previously absent. This allows one to confirm the role of atomic point-like pins as scatterers in Ni and Co-doped compounds, as well as the hypothesis that these defects are at the origin of the weak collective pinning contribution to Jc at larger fields.L'ancrage des vortex est utilisé comme une sonde pour l'identification du type de désordre et son effet sur la supraconductivité dans la famille 122 des supraconducteurs à base de fer. Une nouvelle technique d'analyse obtenue d'images de décoration de Bitter prenant en compte l'interaction de chaque vortex avec ses voisins, a permis d'obtenir l'énergie et la force de piégeage dans Ba(Fe1-xCox)2As2, dans le régime de bas champ magnétique. La corrélation avec des mesures de courant critique Jc a montré que le piégeage des vortex dans ce composé est due à l'hétérogénéité des propriétés supraconductrices sur une échelle de 20-100 nm. Application de la même méthode d'analyse pour les vortex dans le BaFe2(As1-xPx)2 a montré que l'énergie et la force d'ancrage dépendent du dopage x. Les mesures de Jc et de la distribution des forces de piégeage ont montré que la distance moyenne entre différents centres de piégeage est de l'ordre de 90 nm et que cette distance augmente quand on augmente le conteneur en P. La combinaison de ces résultats avec les mesures de Jc mène à la conclusion que l'ancrage fort des lignes de flux due à l'hétérogénéité des propriétés supraconductrice à l'échelle de nm est à l'origine de la constante observé à des champ faibles dans les courbes de Jc ainsi que la diminution en loi de puissance qui suit. On traite également la contribution d'ancrage faible collectif à Jc, qui se manifeste à des champs magnétiques plus importants, de l'ordre de 1 T. Cette contribution a été analysée en terme de la diffusion des quasiparticules et de la fluctuation spatiale du libre parcours moyen. Afin de tester l'hypothèse avancé ci-dessus, l'irradiation aux électrons d'énergie 2.5 MeV, sur les composés dopé au Co, Ni et P de la famille 122 a été réalisé à des différentes doses pour plusieurs dopage de ces matériaux. Ce type d'irradiation introduit des défauts ponctuels de taille atomique dans le matériau. La température critique Tc de tous les matériaux étudiés diminue après irradiation de façon similaire. Une claire augmentation de la contribution d'ancrage faible collectif à Jc dans le composé dopé au Co a été observée. De plus, cette contribution qui, avant irradiation, était absente dans tous les dopages du composé au P, apparait après irradiation. Les défauts ponctuels de taille atomique, diffuseur des quasiparticules, dans les supraconducteurs à base de fer sont donc à l'origine de la contribution d'ancrage faible collectif à Jc

Absence of magnetism in the superconductor B a 2 T i 2 F e 2 A s 4 O : Insights from inelastic neutron scattering measurements and ab initio calculations of phonon spectra

Ba2Ti2Fe2As4O is a self-doped superconductor exhibiting a Tc∼21.5K and containing, distinctively with respect to other Fe-based superconductors, not only [Fe2As2] layers but also conducting [Ti2O] sheets. This compound exhibits a transition at T∗∼125K, which has tentatively been assigned in the literature to a possible density-wave order. However, the nature of this density wave (whether it is charge- or spin-induced) is still under debate. Magnetism in Ba2Ti2Fe2As4O has never been experimentally confirmed, which raises the question whether this superconductor might be nonmagnetic or exhibiting a very weak magnetism. Here, we report evidence from inelastic neutron scattering (INS) measurements and ab initio calculations of phonon spectra pointing towards a possible absence of magnetism in Ba2Ti2Fe2As4O. The INS measurements did not reveal any noticeable magnetic effects in Ba2Ti2Fe2As4O, within the accessible (Q, E) space. The effect of magnetism on describing phonon spectra was further investigated by performing ab initio calculations. In this context, nonmagnetic calculations reproduced well the measured phonon spectra. Therefore, our results indicate a nonmagnetic character of the superconductor Ba2Ti2Fe2As4O

Magnetic polarization of Ir in underdoped nonsuperconducting Eu(Fe0.94Ir0.06)2As2

Abstract: Using polarized neutron diffraction and x-ray resonant magnetic scattering (XRMS) techniques, multiple phase transitions were revealed in an underdoped, nonsuperconducting Eu(Fe1−x Irx)2As2 (x = 0.06) single crystal. Compared with the parent compound EuFe2As2, the tetragonal-to-orthorhombic structural phase transition and the antiferromagnetic order of the Fe2+ moments are significantly suppressed to TS = 111(2) K and TN, Fe = 85(2) K by 6% Ir doping, respectively. In addition, the Eu2+ spins order within the ab plane in the A-type antiferromagnetic structure similar to the parent compound. However, the order temperature is evidently suppressed to TN, Eu = 16.0(5) K by Ir doping. Most strikingly, the XRMS measurements at the Ir L3 edge demonstrates that the Ir 5d states are also magnetically polarized, with the same propagation vector as the magnetic order of Fe. With TN, Ir = 12.0(5) K, they feature a much lower onset temperature compared with TN, Fe. Our observation suggests that the magnetism of the Eu sublattice has a considerable effect on the magnetic nature of the 5d Ir dopant atoms and there exists a possible interplay between the localized Eu2+ moments and the conduction d electrons on the FeAs layers