Nanoscale superconducting-gap variations and lack of phase separation in optimally dopedBaFe1.86Co0.14As2

F. Massee,* Y. Huang, R. Huisman, S. de Jong, J. B. Goedkoop, and M. S. GoldenVan der Waals–Zeeman Institute, University of Amsterdam, 1018XE Amsterdam, The Netherlands

Received 8 April 2009; revised manuscript received 20 May 2009; published 30 June 2009

We present tunneling data from superconducting BaFe1.86Co0.14As2 and its parent compound, BaFe2As2. Inthe superconductor, clear coherencelike peaks are seen across the whole field of view, and their analysis revealsnanoscale variations in the superconducting gap value, . The average peak-to-peak separation gives a 2 of7.4kBTc, which exceeds the BCS weak coupling value for either s- or d-wave superconductivity. The char-acteristic length scales of the deviations from the average gap value and of anticorrelations observed betweenthe gap magnitude and both the zero-bias conductance and coherence peak strength match well with theaverage separation between the Co dopant ions in the superconducting FeAs planes.

DOI: 10.1103/PhysRevB.79.220517 PACS numbers: 74.70.b, 68.37.Ef, 74.25.Jb

The pnictide high-Tc superconductors,1 with maximal Tc’scurrently exceeding 55 K,2 are the subject of global scrutinyon a level at par with that seen for the cuprates. One of themost debated issues are their similarities and differences withrespect to the cuprates.3 The pnictides display many featuresthat we recognize from the cuprate repertoire, yet there arearguments that they are essentially different.4 In the last fewyears, scanning tunneling microscopy and spectroscopySTM/STS have played a major role in investigating theelectronic structure of the cuprates on length scales down tothose of the atom.5–9 This effort has brought the role of in-trinsic disorder introduced by dopant atoms into sharp focus.Consequently, BaFe2−xCoxAs2 is of great interest not only asan electron-doped pnictide but also because the electroni-cally active dopants in this system are situated in the super-conducting layers themselves.

Single crystals of superconducting BaFe1.86Co0.14As2 andthe nonsuperconducting parent compound BaFe2As2 Ba122were grown using a self-flux method. Typical crystal sizesare 110.1 mm3 see Fig. 1b. The high quality of ourcrystals is illustrated in Fig. 1a, with the parent compounddisplaying the well-known resistivity kink at 130 K, whichhas been associated with a spin-density wave and accompa-nying orthorhombic phase transition.10–13 Co doping erasesany sign of these transitions in the resistivity but brings withit a very sharp transition into the superconducting state,which takes place at 220.25 K in this case.

For the STM investigation, crystals were cleaved at apressure of 510−10 mbar at room temperature in a prepa-ration chamber and were immediately transferred into theSTM chamber base pressure 1.510−11 mbar, wherethey were cooled to 4.2 K. The experiments were carried outusing electrochemically etched W and cut Pt/Ir tips, whichwere conditioned before each measurement on a Au778single crystal and yielded identical results. In all cases, thespectral shapes obtained were independent of the tip tosample distance.14

Low-energy electron diffraction LEED was performedin situ, directly after the STM/STS measurements. For allmeasured samples, only the tetragonal unit cell spots wereseen in LEED see Fig. 2a, with no sign of extra spots orextinctions as would occur as a result of a significant and

structurally coherent reconstruction of the atomic positions atthe surface.

We begin our discussion with the Co-doped crystals. Aconstant current image with atomic resolution is shown inFig. 2a. In general, over areas of up to 150150 nm2, wesaw no sign of steps on the surface, with the maximal appar-ent corrugation being less than 1.5 Å on all the data shownhere.

From the zooms shown in Figs. 2b and 2c, one canimmediately see that the surface atoms lie arranged along thedirection of the clear 11 tetragonal unit cell we measureusing LEED but that the inter-atomic spacing is quite irregu-lar. The most frequent separation seen is 8 Å, twice thetetragonal unit cell. Occasionally a row of atoms with a sepa-ration of 3.9 Å occurs, as seen in panel c, and sometimes,the bright atoms sit on a black background. The irregularityin interatomic distances is reflected in the Fourier transformshown in Fig. 2d in which smeared-out features predomi-nate, corresponding to a real-space separation of 8 Åmarked with an ’X’ on the FFT. The tetragonal lattice isbarely visible in the form of weak spots, highlighted in Fig.2d with a yellow arrow.

Inspection of the crystal structure of Ba122 leads to thesupposition that cleavage occurs at the As-Ba interface. Inorder to avoid creation of a polar surface, the charge of the

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FIG. 1. Color online a Co-doped Ba122 shows a sharp su-perconducting transition Tc0.5 K at 22 K red curves. Theinset displays the full temperature range, with data from the parentcompound top trace. b Optical micrograph of the very flat cleav-age surface typically obtained.

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termination layer of a system such as the pnictides should be−1 /2 of that of the layer beneath,4 and this condition can bemet in Ba122 if exactly half of the Ba atoms remains on eachof the surfaces created by cleavage. For a room-temperaturecleave, the Ba ions may reorder to minimize their mutualCoulomb repulsion, resulting in interatomic distances largerthat the in-plane tetragonal lattice constant, as seen in Fig. 2.

Although the LEED patterns from all studied surfacesafter room-temperature cleavage show a nonreconstructed11 tetragonal pattern, on an STM length scale, the de-tails of the topography vary from cleave to cleave and occa-sionally also for different locations on the same cleave. Acommonly encountered variation is shown in Fig. 3a froma different Co-doped crystal of the same doping level inwhich the atomic contrast is absent, and a two-dimensional2D, mazelike network is seen, oriented along the tetragonalaxes with typical period of 12 Å. This imageresembles previously reported one-dimensional stripelikestructures,15,16 whereby our “stripes” appear cut into shorterand more disordered segments, probably as a result of thehigher cleavage temperature. This is in keeping with a recentreport of a temperature dependence of the surface topologyin a related system.17 In Fig. 3b, we show an image frompristine Ba122, which displays a very similar surface topol-ogy as in panel a, as can also be seen from the line scansthrough both images shown in panel c.

We now move on to the tunneling spectra. Differentialconductance spectra dI /dV of both the Co-doped and pris-tine Ba122 systems were recorded modulation amplitude of2mV at a lock-in frequency of 427.3 Hz on a square6464 pixel grid at 4.2 K. The spectra for the Co-dopedsystem vary significantly between different locations withina single field of view FOV.

In Fig. 4a we plot representative STS spectra for differ-ing gap values each spectrum is an average of four adjacentpixels. The spectra show not only a clear variation in peak-to-peak separation, hereafter taken as a measure of the super-conducting gap 2, but also in the value of the conduc-tance at zero bias ZBC and in the strength of the coherencepeaks. They also vary in their asymmetry and in the form ofthe higher-energy structures. While these latter variations aremore pronounced than those reported in Ref. 16, they areless dramatic than observed in a study of the relatedSr1−xKxFe2As2 compound.15 Naturally, we are interested inthe real-space 2 distribution and thus plot the peak-to-peakseparation from all 4096 spectra as a gap map in Fig. 4b.The map indicates that the major part of the FOV supports agap, , of 7 meV, with smaller patches of dimension between5 and 10 Å possessing significantly smaller darker andlarger brighter gaps. From this FOV, only a handful ofspectra did not exhibit coherencelike peaks, and thus if thesepeaked structures give the superconducting gap, we can ex-clude phase separation between superconducting and nonsu-perconducting magnetic regions from these data.

From Figs. 4a and 4b it is clear that BaFe1.86Co0.14As2supports 2 /kBTc values between 5.3 for the smaller gaps,through 7.4 for the modal gap value 7 meV to 10.6 for thelargest gaps. Our modal gap is close to that seen recently inangle resolved photoemission spectroscopy ARPES fromoptimally Co-doped Ba122 for the outermost centered 2or Fermi surface18 and is in keeping with data from an-

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FIG. 2. Color online a Constant current image Vsample=−35 mV, Isetup=40 pA of Co-doped Ba122. The inset showsLEED from the same surface with E0=114 eV. b Zoom of panela: the surface atoms can be seen as bright dots. c Further zoomof b showing a row of four surface atoms separated by the tetrag-onal cell dimension of 3.9 Å. d Fourier transform of panel a,whereby the yellow X and arrow indicate real-space distances of 8and 3.9 Å, respectively. All data with exception of the LEEDwere recorded at 4.2 K.

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FIG. 3. Color online a Constant current image from a differ-ent sample of BaFe1.86Co0.14As2 Vsample=−88 mV, Isetup=40 pA.b Analogous topography from BaFe2As2 Vsample=−100 mV, Isetup=33 pA. c Line scans from the superconductingtop and parent bottom compounds taken along the arrows. Typi-cal distances between the bright rows in the images is 12 Å indi-cated by gray ticks in the line scans. d Overlay of 400 dI /dVconductance spectra from pristine Ba122, together with their aver-age bold line. All data were recorded at 4.2 K.

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other STS experiment on Co-doped Ba122.16 We also notethat our normalized gap values are in line with ARPES datafrom the outer hole pocket Fermi surface in hole-dopedBa,K-122 system.19 At present it is unclear whether theSTS tunneling matrix elements favor the hole or electronpocket Fermi surfaces, but in any case, all of the normalizedgap values we observe are greater than those expected for aweakly coupled BCS s- or d-wave superconductor.

Closer inspection of Fig. 4a reveals a relation among theZBC, 2, and the coherence peak strength: when 2 is smalllarge, the ZBC is large small, and the coherence peaks arestrong weak. To see whether these relations hold for thewhole data set, we conduct three qualitative tests. First, weplot in Fig. 4c the results of binning all the STS spectra intofive groups, depending on their peak-to-peak separation.20

Second, we show in Fig. 4d a map of the ZBC value, withan inverted color scale to ease comparison with panel a.Third, we map the spatial distribution of the coherence peakstrength for this sample Fig. 4f and note that this gives anearly identical image to the ZBC map in panel d. The factthat the inverse relation between 2 and ZBC and between2 and the coherence peak strength is also clearly visible inthe binned i.e., highly averaged STS spectra and the factthat the gap, ZBC and peak strength maps resemble eachother closely providing the color scale in the latter two mapsis inverted indeed indicate that the relation among 2, ZBC,and peak strength holds for the data set as a whole and notjust for the STS spectra shown in Fig. 4a.

In a next step, we formalize the relations between the2⇔ZBC and peak-strength maps by comparing theirazimuthal-integrated cross correlation traces. Figure 4eshows the results, indicating a clear anticorrelation that diesoff over 8–10 Å. The question arises as to the origin of theobserved spatial disorder in the 2 values. As a first suspect,we consider the nontrivial surface topography illustrated inFigs. 2 and 3. Here, pristine Ba122 offers the best test of thispoint as it possesses the same complex Ba termination topog-raphy and yet is void of substitutional disorder within theFeAs structural unit. Figure 3d shows a compilation of one-tenth of a complete STS data set 400 of 4096 STS spectraand their average—taken from pristine Ba122 across thesame FOV as the topograph shown in Fig. 3b. The near-EFelectronic states in the parent compound are fairly spatiallyhomogeneous in terms of both the shape of the spectra andthe absolute dI /dV value.

To compare the variation in absolute conductance withthat seen in the superconductor, we show a ZBC map frompristine Ba122 in Fig. 4g, using the same color scale as Fig.4d, after correction for the difference in junction resistance.Although the two ZBC maps are from different cleaves ofdiffering materials, it is still obvious that the pristine materialhas much less variation and does not possess structure on thelength scales of the superconductor. Thus, the gap disorderseen in Fig. 4 is a property of the superconducting system,and the finger is quickly pointed in the direction of the Codopants as the culprit. For our doping level, 1 in 14 Fe atomsis replaced by a Co dopant, and at a zeroth level this gives aCo-Co separation of a little over 10 Å. The first length scaleover which Co disorder would be visible is therefore 10 Å,as observed here. In Fig. 4e we show autocorrelation tracesfor the gap map in Fig. 4b and the analogous trace for a gapmap recorded from an identical FOV as Fig. 3a, i.e., asuperconductor cleave with a clearly different topographyyet similar STS spectra. Both these positive correlationtraces show that 8 Å is the characteristic length scale of thesignificant gap variations, which is very close to the Co-Colength scale. The fact that the atomically resolved cleavesFig. 2a and those with the 2D mazelike structure Fig.3a both give the same length scales for the deviations from2 is a further indication that in these cases the topographicdetails—which most likely track the particulars of the sur-face Ba disorder—do not have much direct effect on thesuperconducting system. We note that—albeit on a some-what coarser energy scale—similar conclusions have beendrawn from a recent photoemission study of the Ba122

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FIG. 4. Color online a STS spectra from BaFe1.86Co0.14As2

averages of four adjacent pixels displaying different values of ,taken from the region marked in panel b. b Gap map with iden-tical FOV as Fig. 2b, taken with the same setup conditions. cBinned spectra for five ranges of Ref. 20, plotted in colorsmatching those in the gap map. d Map of the zero-bias conduc-tance on an inverted color scale from the same data set as panel b.e Correlation functions. From top to bottom: autocorrelation of thegap map shown in panel b red squares and of an analogous dataset recorded from the FOV shown in Fig. 3a black triangles.Yellow dots: cross correlation of the gap map of panel b with thecorresponding topograph Fig. 2b. Blue green diamonds: crosscorrelations between the gap and ZBC peak-strength maps. fMap of the coherence peak strength for half of the FOV of panelsb and d, and on an inverted color scale, extracted from a com-parison of the coherence peak weight to the high energy back-ground. g ZBC map of pristine Ba122 recorded from half the FOVshown in Fig. 3b. The conductance scale shown on the right isthe same as that used in panel d.

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system.21 Finally, we remark that the cross correlation tracesbetween the gap map and the topography are zero for bothtypes of topography see Fig. 4e.

How do the spatial gap variations found here in an elec-tron doped pnictide compare with those from STS studies ofthe cuprate high Tc’s at analogous doping levels? Optimallydoped Bi2212 yields a similar total spread of a factor of 2 innormalized gap values but upshifted with respect to thosehere to lie between 6 and 13kBTc.

7 Recently, the emphasishas come to lie on the role played by the pseudogap in theobserved large apparent superconducting gap disorder seenin the cuprates.22 In the pnictide STS data presented here, itwould be natural to take the modal gap value as that repre-senting areas with Co doping occurring in the FeAs plane atthe nominal level. Consequently, the small gaps could eitherbe under- or overdoped regions formed due to clustering ofCo since both would—in principle—lead to a lower Tc andpresumably . This would leave only the larger peak-to-peak separations unaccounted for. To decide whether, as inthe cuprates, these large-gap regions are related to the pres-ence of a pseudogap or not will require detailed temperature-dependent measurements.

In summary, we present detailed STM and STS investiga-tions of pristine Ba122 and samples of the electron dopedsuperconductor BaFe1.86Co0.14As2. In the first part of theRapid Communication we describe the complex topography

of the surfaces of these single crystals, which is probably aresult of partial liftoff of the Ba ions upon cleavage. We goon to demonstrate that the termination-plane topographic dis-order encountered here has little effect on the low-lying elec-tronic states of these systems.

The STS data from the superconducting samples displayclear coherence-peak-like features defining an energy gap ofon average 7.4kBTc. There exist, however, significant spatialdeviations from this modal gap value, with the gap distribu-tion ranging from 5–10kBTc. If these gaps are indeed super-conducting gaps, we can clearly rule out nanoscopic phaseseparation in these samples. There is a robust anticorrelationbetween the peak-to-peak separation and the zero-bias con-ductance, and coherence peak strength, operating over lengthscales of 8 Å. The spatial correlation of the low and highgap deviations from the modal gap value also displays thesame length scale, one which is very close to the averageseparation of the Co atoms in the FeAs superconductingblocks, highlighting their importance as local dopants.

We thank H. Luigjes, H. Schlatter, and J. S. Agema forvaluable technical support, and J. van den Brink and A. deVisser for useful discussions. This work is part of the re-search program of FOM, which is financially supported bythe NWO.

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