Hole and Electron Contributions to the Transport Properties of Ba(Fe_(1-x)Ru_x)_2As_2 Single Crystals
F. Rullier-Albenque, D. Colson, A. Forget, P. Thuery, S. Poissonnet
aa r X i v : . [ c ond - m a t . s up r- c on ] J un Hole and Electron Contributions to the Transport Properties of Ba(Fe − x Ru x ) As Single Crystals
F. Rullier-Albenque, ∗ D. Colson, A. Forget, P. Thu´ery, and S. Poissonnet Service de Physique de l’Etat Condens´e, Orme des Merisiers, IRAMIS,CEA Saclay (CNRS URA 2464), 91191 Gif sur Yvette cedex, France SIS2M, CEA/CNRS UMR 3299, LCCEf, IRAMIS, 91191 Gif-sur-Yvette cedex, France Service de Recherches de M´etallurgie Physique, CEA Saclay, 91191 Gif sur Yvette cedex, France (Dated: May 7th 2010)We report a systematic study of structural and transport properties in single crystals ofBa(Fe − x Ru x ) As for x ranging from 0 to 0.5. The isovalent substitution of Fe by Ru leadsto an increase of the a parameter and a decrease of the c parameter, resulting in a strong increaseof the AsFeAs angle and a decrease of the As height above the Fe planes. Upon Ru substitution,the magnetic order is progressively suppressed and superconductivity emerges for x ≥ .
15, withan optimal T c ≃
20K at x = 0 .
35 and coexistence of magnetism and superconductivity betweenthese two Ru contents. Moreover, the Hall coefficient R H which is always negative and decreaseswith temperature in BaFe As , is found to increase here with decreasing T and even change signfor x ≥ .
20. For x Ru = 0 .
35, photo-emission studies have shown that the number of holes andelectrons are similar with n e = n h ≃ .
11 carriers/Fe, that is twice larger than found in BaFe As [1]. Using this estimate, we find that the transport properties of Ba(Fe . Ru . ) As can be ac-counted for by the conventional multiband description for a compensated semi-metal. In particular,our results show that the mobility of holes is strongly enhanced upon Ru addition and overcomesthat of electrons at low temperature when x Ru ≥ . PACS numbers: 74.70.Xa, 74.62.Bf, 74.25.Dw, 74.25.fc
INTRODUCTION
In iron pnictides the appearance of high- T c super-conductivity induced by carrier doping or pressure inclose proximity to the antiferromagnetic (AF) phase, ap-pears very similar to the behavior of cuprates and heavy-fermion superconductors and has been taken as the sig-nature of unconventional superconductivity in these com-pounds. It seems now well established that magnetismand superconductivity (SC) are intimately correlated anddirectly connected to the peculiar features of the elec-tronic structures of these compounds. More precisely thechanges in the Fermi surface and the modifications of thenesting conditions between the hole and electron pock-ets have been proposed to be the driving force for thesuppression of antiferromagnetism and the emergence ofsuperconductivity with a sign reversing s ± symmetry [2].So far a lot of studies have been devoted to the 122family as superconductivity can be induced not only bydoping with holes [3] or electrons [4, 5] but also throughchemical or physical pressure [6, 7]. Several investiga-tions have been done in order to find out a relevant pa-rameter allowing to explain the modifications of the elec-tronic structure and the emergence of superconductivityin these different systems. On one hand, it has been ar-gued that structural modifications could be more impor-tant than doping in achieving superconductivity, eitherthrough the height of As with respect to Fe planes [8]or the value of the As-Fe-As bonding tetrahedral angle[9]. On the other hand, in the case of electron dopedcompounds, the steric effect due to different atomic sub- stitutions in the Fe planes, has been shown to be of minorimportance compared to the effect of doping [10]. Let usnote that the very low level of substitution sufficient toinduce SC in this latter case is compatible with weakstructural distortion effects. However, in the case of holedoping for which large substitution level (around 35%) isnecessary to get the optimal T c , it seems more difficult todistinguish between doping and structural modifications.Studies of transport properties are a priori one of thesimplest way to investigate the modifications of the elec-tronic structure. However the situation in the 122 familyis far from being clear. In the undoped BaFe As par-ent, for which the electron and hole contents are identical( n = n e = n h ), it is found quite surprisingly that the Hallcoefficient R H is always negative, indicating that elec-trons dominate the transport properties both above andbelow the structural/magnetic transition at T ⋍ R H havebeen only reported for K and Cr doped BaFe As , asnaturally expected for hole doped compounds [15, 16].The isovalent substitution of Fe by Ru provides an-other alternative to study the modifications of the trans-port properties in the 122 family. It has been shownrecently on polycrystalline samples [17, 18] that the in-troduction of Ru suppresses the SDW magnetic order andinduces SC. Density functional calculations show that Rusubstitution does not induce any charge imbalance be-tween the bands and no additional bands related to Ruappear at the Fermi level [19]. However a negative Hallcoefficient with a weak T dependence has also been re-ported for polycrystalline BaFe . Ru . As [17].Here we report on structural, resistivity and Hall effectdata obtained on single crystals of Ba(Fe − x Ru x ) As for x ranging from 0 to 0.5. As reported previously, weconfirm the suppression of the magneto-structural tran-sition and the emergence of SC for x & .
15 with anoptimal T c ∼ K at x ≃ .
35. We find that the latticeparameters a ( c ) respectively increases (decreases) uponRu addition, so that a/c increases markedly. As for thetransport properties, our Hall effect measurements ev-idence the contribution of both holes and electrons inthe transport properties, as the Hall coefficient changessign, from negative to positive with decreasing tempera-ture. On the other hand, Angle Resolved Photo-EmissionSpectroscopy (ARPES) measurements performed on sim-ilar single crystals with x = 0 .
35 [1] confirm that Ru-substituted BaFe As behave as a compensated metalwith essentially the same number of holes and electrons,i.e. n = n e = n h ≃ .
11 carriers/Fe. Assuming a twoband model to describe the transport properties, we showthat it is possible here to disentangle the respective con-tributions of electrons and holes. Quite surprisingly thededuced electron and hole resistivity curves display sim-ilar T dependences as those found respectively for Co-doped and K-doped BaFe As at optimal doping. Theirevolutions with Ru content show that the mobility ofholes is more affected than that of electrons. The strongmodification of the electronic structure of BaFe As withRu substitution revealed by ARPES [1] might be the keyfactor for governing these properties. SAMPLES AND STRUCTURALMEASUREMENTS.
Single crystals of Ba(Fe − x Ru x ) As with Ru contents x ranging from 0 to 0.50 were grown using a FeAs/Ru+Asself-flux method. Small Ba chunks, FeAs powder andRuAs (or Ru+As) powders were mixed in the ratioBa:(FeAs+RuAs)=1:4. Starting products were put inan alumina crucible and sealed in an evacuated quartztube which was put into a tubular furnace. The sam-ples were heated to 1180 ◦ C, held at this temperaturefor 4h, cooled slowly first to 1000 ◦ C (3-6 ◦ C/h) and thenmore rapidly to room temperature. Clean crystals oftypical dimensions 0.5x0.5x0.05 mm were mechanicallyextracted from the flux. It is worth pointing out herethat it is very difficult to get homogeneous single crys-tals for x Ru ≥ .
2, probably due to the very high meltingtemperatures of Ru and RuAs with respect to that ofFeAs. Consequently, the Ru composition of each stud-
FIG. 1: (color on line) (a)Variation of the lattice parameters a and c , their ratio a/c and the unit cell volume V, normalizedto their values for the undoped compound, as a function ofthe Ru content x (determined by X-rays diffraction). (b)The same for different crystallographic parameters: the twoAsFeAs tetrahedral angles, α and α , the c-axis coordinateof As, z As , the Fe-As interatomic distance r ( F e − As ) and theheight of As above Fe layers, d F e − As . ied crystal has been determined with a Camebax SX50electron microprobe in several spots of the surfaces. Thestructural properties were characterized by single crys-tal X-ray diffraction on very thin platelets ( ∼ .
10 x0.05 x 0.01 mm ). The data were collected at room tem-perature on a Nonius Kappa-CCD area detector diffrac-tometer [20] using graphite-monochromated Mo K α ra-diation ( λ = 0 . ϕ -and ω -scans giving complete data sets up to θ = 27 . ∼
100 indepen-dent reflections. Fe and Ru were constrained to retainthe same displacement parameters, which enabled to re-fine the Ru content x . The final R indices are in therange 0 . − .
042 and the wR values in the range0 . − .
113 [23].The relative lattice parameters are plotted in Fig.1 ver-sus the refined value of x Ru , while numerical data arereported in table 1 for x = 0 (undoped BaFe As ) and x = 0 .
38 (optimal doping). Upon substitution, the a pa- TABLE I: Crystallographic data taken at room tem-perature for the parent and Ru-substituted (x= 0.38)Ba(Fe x Ru x ) As . The space group of both compoundsis I4/mmm and the atomic coordinates are: Ba(0,0,0),Fe/Ru(0.5,0,0.25) and As (0,0,z). α and α are the bond-ing tetrahedral angles as sketched in Fig.1. x=0 x=0.38 a (˚A) 3.9633(4) 4.0342(5)c (˚A) 13.022(2) 12.749(2)V (˚A ) 204.55(4) 207.49(5) z As α x 2 (deg) 111.18(3) 113.73(4) α x 4 (deg) 108.624(15) 107.39(2)As height (˚A) 1.3575 1.3165Fe-As interatomic distance (˚A) 2.402(2) 2.409(2)FeAs layer spacing (˚A) 3.796 3.741 rameter and the cell volume increase while the c parame-ter decreases in about the same proportion (by 2-3% for x = 0 .
5) in agreement with data on polycrystalline sam-ples [17]. The main effect of Ru substitution is thus tostrongly increase the ratio ( a/c ). This results in a strongincrease of the As-Fe-As tetrahedral bonding angle α displayed in fig.1(b) which varies by 4% for x = 0 . z parameter of As and the Fe-Asinteratomic distance are nearly unaffected by Ru sub-stitution, the vertical distance of As from the Fe layersdecreases, due to the strong c decrease. These tenden-cies can be explained quite naturally by the larger sizeof Ru compared to Fe which expands the distanceswithin Fe-Ru planes. Also the larger delocalization ofthe Ru 4d orbitals with respect to Fe 3d reinforces thehybridization with As and then reduces the c axis pa-rameter [19]. Let us note that the variation of the lat-tice parameters upon Ru substitution displays the sametrend as that observed in electron doped BaFe As com-pounds, although the incidence on the structure appearsmuch weaker in this latter case [10]. However it is atodds to the tendency found for hole doping or pressure[9, 24] for which both a and the As-Fe-As α angle arefound to decrease. RESISTIVITY MEASUREMENTS AND PHASEDIAGRAM
Transport measurements were performed on crystalscleaved to thicknesses lower than 20 µ m and cut to getsquare samples with ∼ . − . ρ ( T ) curves as a function of Ru con-tent. In BaFe As , the combined structural and mag-netic (S-M) transition at T =137 K is signalled by a FIG. 2: (color on line) (a) Temperature dependence of the in-plane resistivity ρ ( T ) of Ba(Fe − x Ru x ) As single crystals.The arrows point the strong anomalies in the ρ ( T ) curvesthat signal the occurrence of the magneto-structural transi-tion. They are determined more precisely in (b) which shows dρ/dT versus T . For the x = 0 .
26 sample, the arrow corre-sponds to the maximum of the Hall coefficient as explained inthe text. (c) Picture of a typical sample mounted in the Vander Pauw configuration. decrease of the resistivity [25]. For the lowest Ru con-tent x = 0 .
05 studied here, we still observe a resistivitydecrease at the S-M transition but with a much widertransition as seen in fig.2(b) which shows the resistivityderivative dρ/dT . Then for larger Ru contents, the resis-tive signature of the S-M transition, determined by thedeviations in dρ/dT , changes shape towards a step-likeincrease of the resistivity as observed in electron dopedcompounds. In contrast, let us point out that the S-Mtransition is always signalled by a decrease of the resis-tivity in polycrystalline samples [17, 18], which might berelated to inhomogeneities in the samples or directionalaveraging due to different in-plane and out of plane re-sistivity variations at the S-M transition. For x = 0 . dρ/dT as proposed for Co-doped samples(arrows in fig.2(b)) [5, 27]. This would give 95 and 88K FIG. 3: (color on line) Phase diagram of theBa(Fe − x Ru x ) As system showing the evolution of thestructural-magnetic transition T S − M and critical tempera-tures T c versus Ru content x Ru . Values of T c are taken at themid-point of superconducting transitions. The sample with x = 0 . respectively for T S and T SDW . For the other samples,this distinction is not possible and microscopic investiga-tions are needed to determine the respective values of T S and T SDW .Superconductivity appears for x ≥ .
15 and a maxi-mum T c of 19 . ± .
5K is found for x = 0 .
35, in excel-lent agreement with the value found for polycrystallinesamples of Ru substituted Ba(Sr)Fe As [17, 18]. The T − x phase diagram obtained for Ba(Fe − x Ru x ) As isdisplayed in fig.3. One can notice that the superconduct-ing region is rather small, particularly in the ”overdoped”region where T c is found to drop rapidly beyond x = 0 . T c occurring when long range magnetic orderis fully suppressed. This points here again to the impor-tance of spin fluctuations for superconductivity in thesesystems. From these resistivity data, it is not possible toknow whether the coexistence between magnetism andsuperconductivity occurs at the atomic scale like in Co-doped samples [29] or comes from phase segregation asobserved in K-doped samples [30, 31] and further studiesat the miscoscopic level are needed to assess this point.We find here that superconductivity is induced uponRu addition while the FeAs tetrahedra become stronglydistorted as both the α and α angles deviate from 109.5deg, the ideal tetrahedral value. This observation con-flicts with the claim that the regularisation of tetrahedrais the optimal condition for achieving superconductivityin pnictides [9, 34, 35]. Let us also note that Ru substi-tution in the 1111 PrFeAsO compound induces similar crystallographic modifications as those observed here inBaFe As [36] with suppression of the magnetic order butno apparition of superconductivity.On another hand, band structure calculations havepointed out the important impact of the vertical dis-tance d F e − As on the Fermi surface topology of iron pnic-tices [37]. Mizugushi et al. [8] have recently shownthat a striking correlation between T c and the Fe-Asdistance is followed by a lot of different FeAs super-conductors. This plot is symmetric with a peak around d F e − As =1.38˚A. We find that the point corresponding toBa(Fe . Ru . ) As ( d F e − As =1.3165˚A) and T c ≃ T c and d F e − As may provide a help-ful hint to understand the modifications of the electronicproperties. HALL EFFECT AND ANALYSIS OFTRANSPORT PROPERTIES
The temperature dependences of the Hall coefficient R H are displayed in Fig.4 for different Ru concentra-tions. In the paramagnetic state of the samples, we havechecked that the Hall resistivity is always linear in fieldup to 14T, which allows to define R H unambiguously[28]. This linearity is illustrated in the inset of Fig.4 forthe x = 0 .
35 sample. The strong reduction in the Hallcoefficient at the S-M transition is well correlated to theanomalies seen in dρ/dT and represented by arrows in thefigure. It can be associated, as in the undoped parent, tothe reduction of carrier density due to the reconstructionand/or partial gaping of the Fermi surfaces. The factthat R H remains negative indicates that electrons stilldominate the transport properties in the magnetic phaseof Ru-substituted samples.Fig.5(a) shows an enlarged view of the evolution of theHall coefficient in the paramagnetic phase. For compar-ison, we have also plotted in Fig.5(b) similar data ob-tained for Ba(Fe − x Co x ) As at various dopings [11]. Inthis latter case, R H is always found negative, indicatingthat the contribution of electrons dominate the transportproperties. However an opposite trend appears as soonas Ru is added to BaFe As . For x = 0 . R H nearlyreaches zero before dropping at the S-M transition andfor higher Ru contents, a change of sign of R H occursat low temperature. In particular for the x ∼ .
25 sam-ple, we observe that R H increases and becomes positiveupon cooling and then appears to slightly decrease againfor T ≤ K . This can be related to the flattening of the ρ ( T ) curves observed in the same temperature range andthis is for us the sign that the S-M transition takes placeat T ≃
50K in this sample.
FIG. 4: (color on line) T dependence of the Hall coefficient R H ( T ) for various compositions. The temperatures at which R H starts decreasing due to the apparition of the M-S transi-tion correspond exactly to those where anomalies are seen inthe resistivity curves(arrows). The inset shows that the Hallresistivity ρ xy of the x = 0 .
35 sample is linear in magneticfield up to 14T whatever T . A sign change of the slope occursbetween 93 and 120K.FIG. 5: (color on line) T dependence of the Hall coefficient R H ( T ) in the vicinity of R H = 0 for (a) Ba(Fe − x Ru x ) As and (b) Ba(Fe − x Co x ) As , from ref.[11]. In multiband systems, it is well known that a temper-ature variation of the Hall coefficient can be assigned todifferent variations of hole and electron mobilities withtemperature. The observation of a sign change of R H in Ba(Fe − x Ru x ) As indicates that holes and electronscontribute similarly to the transport in a large tempera-ture range. More precisely ARPES data on crystals with35% Ru [1] have shown that the number of holes and elec-trons are similar, i.e. n = n e = n h ≃ .
11 carriers/Fe.It is worth pointing out that this value is significantlylarger than that determined by ARPES in the paramag-netic phase of BaFe As : n = 0 . n e and n h is consistent with theobservation that the Hall resistivity ρ xy is always linearwith magnetic field. Indeed in a two band model, theHall resistivity ρ xy can be written out as: ρ xy = 1 e n h µ h − n e µ e + ( µ h µ e ) ( n h − n e ) H ( n h µ h + n e µ e ) + ( µ h µ e ) ( n h − n e ) H H (1)where µ h = | e | τ h /m h ( µ e = | e | τ e /m e ) are the mobili-ties of holes (electrons) and τ h ( τ e ) and m h ( m e ) theirrelaxation rates and effective masses. For n e = n h = n ,the H term in the numerator and denominator of Eq.(1)vanishes resulting in linear variation of ρ xy with H , what-ever H and T .Knowing n , it is then straightforward to deduce therespective contributions of electrons of holes to the trans-port for the x = 0 .
35 sample from the resistivity and Hallcoefficient data, using:1 /ρ = σ = σ e + σ h (2)and R H = 1 ne µ e − µ h µ e + µ h = 1 ne σ e − σ h σ e + σ h (3)The resulting resistivity curves obtained for electrons andholes are displayed in Fig.6(a). It is striking to see thatthe shapes of the curves resemble those obtained respec-tively for electron and hole doped BaFe As at optimaldoping. These similarities give strong support to the va-lidity of the decomposition using the two band model.One can also notice that ρ h ( T ) displays a nearly T de-pendence as in Ba . K . Fe As [15] while ρ e ( T ) ex-hibits a nearly linear T-dependence up to 150K as inBa(Fe . Co . ) As [11]. These temperature variationsappear then tightly connected to the type of carriers -hole or electrons - and indicate an intrinsic disparity be-tween their respective properties.It has been suggested that the linear T dependence ofresistivity found in Ba(Fe − x Co x ) As or BaFe As − x P x near optimal doping might be a general property of un-conventional superconductors near a SDW instability [33] FIG. 6: (color on line) (a) Respective resistivities of electronsand holes for Ba(Fe . Ru . ) As obtained from the dataof resistivity and Hall coefficient using Equations (2) and (3)with n e = n h = 0 .
11, as given by ARPES measurements onsimilar samples [1]. The raw data for resistivity and Hallcoefficient are recalled in the inset. (b) The resistivity curvesfor Ba(Fe . Co . ) As [11] and Ba . K . Fe As [15] aregiven for comparison. or a signature of non Fermi liquid behavior [14]. The ob-servation of a linear behavior for the electrons and notfor the holes in the same sample clearly addresses thequestion of the real physical origin of this linearity. InBa(Fe − x Co x ) As , the analysis of combined data of re-sistivity and Hall effect for n e > n h led us to suggestthat this linearity comes from an artefact due to a smallvariation of n e with temperature and that the scatteringrates obey the T behavior expected for Fermi liquids[11]. CONTRIBUTION OF ELECTRONS AND HOLESTO THE ELECTRONIC TRANSPORT
The most striking result of this study is that elec-trons and holes contribute similarly to the transport inBa(Fe − x Ru x ) As . This is in contrast to the observa-tions of negative Hall coefficient in the undoped parent[11, 12] or upon isovalent exchange of As by P [14]. Animportant question is thus to understand why the holesare more scattered than the electrons in these latter cases. FIG. 7: (color on line) Respective resistivities of electrons andholes for BaFe As in the paramagnetic phase ( T > T SM ) ob-tained from resistivity and Hall coefficient data using Equa-tions (2) and (3) using different estimates for the carrier num-ber. The full symbols are for n = n e = n h = 0 .
06 carriers/Feas determined by ARPES measurements [32] while the fulland dotted lines correspond to the extremal values of n dueto the ± .
02 error bar. The empty symbols are for the LDAestimate n = n e = n h = 0 .
15 carriers/Fe.
In BaFe As , ARPES data have shown that the num-ber of carriers is n e = n h = 0 . As in the paramagnetic phase, i.e. for T > n = 0 . n = 0 .
15 carriers/Fe (Fig.7). It is clear thatthe results are much more sensitive to the actual valueof n for the holes than for the electrons: the electron re-sistivity always displays a metallic behavior with similarvalues while the hole one tends towards a semiconductingbehavior when approaching the magneto-structural tran-sition. The disparity between the two types of carriersappears more or less pronounced depending on the valuetaken for the carrier number.It has been suggested that spin fluctuations due tointerband electron-hole scattering might play a crucialrole to explain the asymmetric behaviors of holes andelectrons in undoped and electron doped BaFe As [12].On the contrary, our Hall coefficient data displayed inFig.5(a) for different Ru contents seem to indicate thatthe proximity of magnetism does not play here an impor-tant role on the respective mobilities of the carriers. Thisis more visible in Fig.8 where the decompositions in ρ e and ρ h are reported for different Ru contents, assuminga linear variation of n with x Ru .Even though this analysis is tentative, it gives sometrends on the evolution of the transport properties of FIG. 8: (color on line) Same decompositions as that per-formed in fig.6(a) for different Ru contents, assuming a linearvariation of the number of carriers with x : n = 0 .
06 + 0 . x carriers/Fe. Full and dotted lines are for ρ e and ρ h respec-tively. For the sake of clarity, the data corresponding to x = 0 .
35 (fig.6(a)) are not reported in this plot
BaFe As upon Ru addition. In particular, one can no-tice that the decrease of the hole and electron resistivitiescannot be entirely explained by the increase of n . Thistherefore points to a concomitant increase of their re-spective mobilities. As shown by ARPES measurementson the x = 0 .
35 sample, Ru substitution strongly mod-ifies the electronic structure with respect to that of un-doped BaFe As : not only the number of carriers hasdoubled but also the Fermi velocities have increased bya factor 2 or 3 [1]. In fact the electronic structure ofBa(Fe . Ru . ) As can be reasonably accounted forby LDA calculations with negligible electron correlationeffects. This tendency is not observed for electron orhole doped compounds with similar T c [32, 39, 40] andseems then to be a specific feature of this Ru substitutedsystem. One might reasonably think that these modi-fications would be at the origin of the evolution of thetransport properties observed here.Nevertheless the way how these band structure modi-fications can also affect the strength of spin fluctuationsor the carrier mobilities is not clear at present. More-over, the resemblance displayed in fig.6 between the re-sistivity curves found for electrons and holes and thosemeasured in electron and hole doped compounds appearsvery puzzling, as it suggests that the transport propertiesof electrons and holes are defined by their own and arenot tightly dependent on the system of interest. CONCLUSION
The results presented here and in ref.[1] clearly showthat Ru is isovalent of Fe. We have confirmed that Rusubstitution suppresses the magnetic state and inducessuperconductivity, which coexist in a given concentrationrange. Therefore it appears qualitatively very similar tothe other types of substitutions.From the structural point of view, we have shownthat superconductivity can be induced although theFeAs tetrahedra are strongly distorted upon Ru addi-tion. These structural modifications are in total con-trast to those induced under pressure or by hole dop-ing. This thus demonstrates that the regularisation oftetrahedra cannot be the key structural factor for theoccurrence of superconductivity as proposed recently [9].However, we find that the relationship between the opti-mal T c and the anion height above the Fe planes obeysthe same plot as found for a lot of different iron basedcompounds [8]. Even though this cannot be consideredas the only parameter to drive superconductivity, thisindicates that subtle details of crystal structure mighttune specific properties of the Fermi surface necessary tooptimize it.We have demonstrated that a two band model ap-proach is a prerequisite to get insight into the respectivecontribution of electrons and holes to the transport prop-erties of these multi-band materials. Using combinedstudies of transport and ARPES measurements on thesame samples, we have been able to disentangle the re-spective contributions of electrons and holes to transportproperties. We have evidenced that their mobilities be-come comparable upon Ru addition, even in the closeproximity to magnetism. In addition, we find that themobility of holes overcomes that of electrons at low T insuperconducting samples. The observation that the ρ ( T )curves deduced for electrons and holes are very similar tothose measured in electron or hole doped compounds sug-gests that the occurrence of optimal T c in all these com-pounds is linked with well defined features of the electronand hole bands. Further work, specifically studying thestrength and the evolution of antiferromagnetic spin fluc-tuations with Ru contents, will hopefully allow to clarifythe incidence of spin fluctuations, electronic correlationsand filling of the electronic bands on the transport prop-erties of these compounds.We would like to acknowledge H. Alloul and V.Brouet for fruitful discussions and critical reading of themanuscript. ∗ Electronic address: fl[email protected][1] V. Brouet, F. Rullier-Albenque, M. Marsi, B. Mansart,J. Faure, L. Perfetti, A. Taleb-Ibrahimi, P. Le F`evre, F.
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