Mise en page 1 - Laboratoire National des Champs Magnétiques ...

METALS, SUPERCONDUCTORS ... 2009Coexistence of magnetic order and superconductivity in iron pnictidesHow long range magnetic order can coexist with bulk superconductivityis a central question in a number of unconventionalsuperconductors. Both in the copper oxideand in the ”new” iron pnictide family of high temperaturesuperconductors the superconducting phase is obtained byadding charge carriers into a phase with antiferromagneticorder. Observation of phase coexistence in such a situationimmediately raises two important questions. The firstone is about the intrinsic character of the coexistence. Thisquestion is obviously linked to a rather subtle issue in theseoff-stoichiometric materials: to which extent is chemicalinhomogeneity of the samples intrinsic or not, i.e. unavoidableor not? The next important question is: Do the phasesoverlap in space, or do they occupy mutually exclusive volumes?In short, on which length scale do the two phasescoexist? This is an equally delicate problem because unambiguousdirect experimental proofs are in most cases difficultto obtain, even for true local probes such as nuclearmagnetic resonance (NMR), muon spin rotation (µSR) orscanning tunneling microscopy (STM). As a matter of fact,the debate is still going on in the cuprates. In the ironpnictides, magnetic order (a spin density wave state) hasrecently been found to coexist with superconductivity inseveral (but not all) families. Yet, a global picture of theconditions for this coexistence in the pnictides is still lacking.We have measured the NMR properties of 75 As nucleiin single crystals of Ba(Fe 1.95 Co 0.05 ) 2 As 2 andBa 0.6 K 0.4 Fe 2 As 2 , both of which show 100% Meissner volumefraction [Julien et al., EPL 87, 37001 (2009)]. Ourresults demonstrate that both samples show the coexistenceof bulk superconductivity with magnetic (SDW) order. Yet,the details appear different:- In Ba(Fe 1.95 Co 0.05 ) 2 As 2 , the fact that the NMR signalat the paramagnetic resonance frequency vanishes abruptlyand completely below TSDW onset = 56 K indicates that the twophases coexist at the microscopic scale probed by NMR, asituation which is often referred to as ”homogeneous mixing”in the literature. This implies either that both types oforders are simultaneously defined at each Fe site (owing tothe multiple bands present at the Fermi level), or that theyare mixed on the scale not greater than one or two latticespacing. A nanoscale coexistence involving superconductingislands (without magnetic order) of typical size definedby the coherence length ξ ≃ 2.8 nm appears to be unlikely.In this case, some paramagnetic NMR signal from regionsas large as ten times the Fe-Fe distance should have beenobserved.- In Ba 0.6 K 0.4 Fe 2 As 2 , on the other hand, the NMR signalhas decreased but remains finite in the whole temperaturerange, including the superconducting state. Thus, the magneticregions do not occupy the full sample volume. Inthis sense, the coexistence might be qualified as ”inhomogeneous”.Figure 80: Top panel: In-plane resistivities. Bottom panel: NMRsignal intensities (renormalized by temperature). Excessive signalloss is due to the spin-density-wave transition in all (Co-dopedmaterial) or part (K-doped material) of the samplePerhaps unexpectedly, the inhomogeneity is considerablystronger in the potassium-doped sample. Actually, the factthat substitutions at the Fe site, unlike substitutions at theCu site in the cuprates, improve conductivity and even inducesuperconductivity is one of the most remarkable surprisesof these new superconductors. It is difficult to fullyunderstand the Ba 1−x K x Fe 2 As 2 system from the presentNMR data only. The important inhomogeneity/disordercould be due to inhomogeneity of K + concentration and/orto a particularly strong impact of these ions on the localelectronic structure in FeAs layers. No evidence for phaseseparation could be detected in the K-doped sample, andthe single crystal exhibits 100% Meissner fraction. Wepoint out that T SDW vs. x is extremely steep near x = 0.4.Phase separation or inhomogeneous coexistence could thusoriginate from K-doping inhomogeneity around this particularconcentration. Therefore, they might not reflect theproperties at somewhat lower x values. In other words,Ba 1−x K x Fe 2 As 2 should perhaps be simply viewed as a disorderedversion of Ba(Fe 1−x Co x ) 2 As 2 . Anyhow, the coexistenceof magnetic and superconducting phases clearlyemerges as a cornerstone of the iron-pnictide physics.M. Horvatić, C. BerthierM.-H. Julien, H. Mayaffre (Laboratoire de Spectrométrie Physique, Université J. Fourier, Grenoble, France), X. D.Zhang, W. Wu, G.F. Chen, N.L. Wang, J.L. Luo (Beijing National Laboratory for Condensed Matter Physics andInstitute of Physics, Chinese Academy of Science, Beijing, China)60