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Distinct behaviors of suppression to superconductivity in LaRu3Si2 induced by Fe and

Co dopants

Sheng Li, Jian Tao, Xiangang Wan, Xiaxin Ding, Huan Yang and Hai-Hu Wen∗

Center for Superconducting Physics and Materials,

National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University, Nanjing 210093, China

In the superconductor LaRu3Si2 with the Kagome lattice of Ru, we have successfully doped theRu with Fe and Co atoms. Contrasting behaviors of suppression to superconductivity is discoveredbetween the Fe and the Co dopants: Fe-impurities can suppress the superconductivity completely ata doping level of only 3%, while the superconductivity is suppressed slowly with the Co dopants. Asystematic magnetization measurements indicate that the doped Fe impurities lead to spin-polarizedelectrons yielding magnetic moments with the magnitude of 1.6 µB per Fe, while the electrons givenby the Co dopants have the same density of states for spin-up and spin-down leading to much weakermagnetic moments. It is the strong local magnetic moments given by the Fe-dopants that suppressthe superconductivity. The band structure calculation further supports this conclusion.

PACS numbers: 74.20.Rp, 74.70.Dd, 74.62.Dh, 65.40.Ba

I. INTRODUCTION

Superconductivity in the systems RT3Si2 or RT3B2 (Rstands for the rare earth elements, like La, Ce, Y, etc.,T for the transition metals, like Ru, Co and Ni, etc.) isvery interesting because it concerns the conduction of thed-band electrons of the 3d- or 4d- transition metals. Byhaving different combinations of chemical compositions,one can tune the system from a superconducting (SC)ground state to a magnetic one, and sometimes have bothphases coexisting in one single sample.1,2 The LaRu3Si2has a SC transition temperature at about 7.8 K.3,4 Sincethe superconductivity is at the vicinity of the magneticorder, some unconventional pairing mechanisms, suchas the charge fluctuation5, or anti-ferromagnetic spinfluctuations6 mediated pairings are possible. Recently,we find that both the superconducting state and the nor-mal state exhibits some anomalous properties, suggest-ing that the electronic correlation plays important rolesin the occurrence of superconductivity.7 Another reasonfor doing research on this system is that it may havesome odd pairing symmetries, such as d − wave, s + d,px + ipy or dx2

−y2 + idxy,8,9 because the electric conduc-

tion is dominated by the 4d band of Ru atoms whichconstruct a Kagome lattice (a mixture of the honeycomband triangular lattice, as shown in Fig.1). Furthermore,the electric conduction in this system is strongly favoredby the Ru-chains along the z-axis, as evidenced by ourband structure calculations, this may induce quite strongsuperconducting fluctuations.7

In a superconductor, the impurity induced pair break-ing depends strongly on the structure of the pairinggap and the feature of the impurities, such as mag-netic or non-magnetic. Therefore it is very importantto measure the impurity induced scattering effect inthe superconducting state of LaRu3Si2. According tothe Anderson’s theorem,10,11 in a conventional s-wavesuperconductor, nonmagnetic impurities will not leadto apparent pair-breaking effect. This theoretical ex-

pectation has been well illustrated in the conventionalsuperconductors.12However, a magnetic impurity, due tothe effect of breaking the time reversal symmetry, canbreak Cooper pairs easily. In sharp contrast, in a d-wavesuperconductor, nonmagnetic impurities can significantlyalter the pairing interaction and induce a high density ofstates (DOS) due to the sign change of the gap on aFermi surface. This was indeed observed in cuprate su-perconductors where Zn-doping induces Tc-suppressionas strong as other magnetic disorders, such as Mn andNi.13,14 In LaRu3Si2, preliminary experiment indicatedthat the SC transition temperature drops only 1.4 K withthe substitution of 16 % La by Tm (supposed to possess amagnetic moment of about 8µB), suggesting that the su-perconductivity is robust against the local paramagneticmoment1. This kind of doping is induced at the sitesof the rare earth elements, which may give very weakpair breaking effect on the Cooper pairs (3d electrons ofRu). Therefore it is very interesting to investigate whatwill happen if we dope impurity atoms directly to theRu sites. In this paper, we report the doping effect onthe Ru sites by the Fe and Co dopants. We find a con-trasting suppression effect to the superconductivity withthese two kind of dopants. Possible reasons are given toexplain this effect.

II. EXPERIMENTAL METHODS AND

CHARACTERIZATION

The samples of La(Ru1−xTx)3Si2 (T=Fe and Co) werefabricated by the arc melting method.1,3,4,7 The startingmaterials: La metal pieces (99.9%, Alfa Aesar), Fe pow-der (99.99%), Co powder (99.99%), Ru powder (99.99%)and Si powder (99.99%) were weighed, mixed well, andpressed into a pellet in a glove box filled with Ar (wa-ter and the oxygen compositions were below 0.1 PPM).In order to avoid the formation of the LaRu2Si2 phase,we intentionally add a small amount of extra Ru pow-der (about 15% more) in the starting materials. Three

2

LaLa

Ru

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FIG. 1: (color online) Top view of the atomic structure ofLaRu3Si2. The Ru atoms construct a Kagome lattice (bluemiddle size circles), while the Si (red small size circles) andLa atoms (yellow large size circles) form a honeycomb anda triangle structure, respectively. The three different atomsdon’t overlap each other from a top view. The prism at thetop corner illustrates one unit cell of the structure.

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FIG. 2: (color online) (a) X-ray diffraction patterns of thesample La(Ru1−xFex)3Si2. One can see that the main phase isthe 132 structure, with slight Ru impurity phase. (b) Dopingdependence of the a-axis and c-axis lattice constants. BecauseFe doping is only up to 3%, so there is no distinct change ofthe lattice constant.

rounds of welding with the alternative upper and bottomside of the pellet were taken, in order to achieve the ho-mogeneity. The X-ray diffraction (XRD) measurementwas performed on the Brook Advanced D8 diffractome-ter with Cu Kα radiation. The analysis of XRD data wasdone with the softwares Powder-X and Fullprof. The re-sistivity and magnetization measurements were done onthe Quantum Design physical property measurement sys-tem (PPMS-16T) and SQUID-VSM.

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FIG. 3: (color online) (a) X-ray diffraction patterns of thesample La(Ru1−xCox)3Si2, up to the doping level of 8% thesample is still quite clean. (b) Doping dependence of both aand c lattice constants with the increase of doped Co concen-tration.

The XRD patterns for Fe- and Co- doped samples areshown in Fig.2 and Fig.3, respectively. One can see thatthe samples are rather clean, except for a small amountof Ru impurity. For the Fe-doped samples, we don’t seeclear change of the lattice constant a and c. This could bedue to two reasons: (1) The maximal doping level here is3% which is already enough to kill the superconductivitycompletely; (2) The atoms of Fe and Ru are in the samecolumn and close to each other in the periodic table, wewould assume that the ions of Fe and Ru have the similarradii. For the Co-doping, however, there is an obviousdecrease of a and c lattice constant with doping, as shownin Fig.3. The variation of the lattice constants in theCo-doped samples are well associated with the resistivitydata shown below, clearly suggesting that the Co atomsare also successfully doped into the LaRu3Si2 system.

III. RESULTS AND DISCUSSION

A. Suppression to superconductivity

In Fig.4(a) and (b), we present the temperature depen-dence of the normalized resistivity of Fe- and Co- dopedsamples. It can be seen that the transition temperaturewas suppressed remarkably with Fe-doping, and shiftedto below 2 K at only a doping level of 3%. However,for the Co-doped ones, there is no significant change ofTc, up to 8% Co-doping. These behaviors are also re-vealed by the magnetization of the samples, as shown inFig.4(c). For the superconducting samples, the resistivityincreases monotonously with the increase of doping level,both for the Fe and Co doping. But it is clear that, theenhancement of the residual resistivity, although weaker

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FIG. 4: (color online) (a) Temperature dependence of nor-malized resistivity with Fe doping, there is no superconduct-ing transition with the doping level at only 3%. Slightlyenhancement of the residual resistivity is observed, indicat-ing an enhanced scattering. (b) Temperature dependenceof the normalized resistivity with Co doping, the suppres-sion to the superconducting transition by Co doping is ratherweak. (c) Temperature dependence of DC susceptibility of theLa(Ru1−xTx)3Si2 (T=Fe and Co) under H = 50 Oe, measuredin the zero-field-cooled (ZFC) and field-cooled (FC) processes.(d) Doping dependence of Tc in Fe- and Co-doped samples,the suppression to Tc in Fe doped samples is drastically fast,but that by Co doping is very slow.

in the Fe-doped samples than that in the Co-doped ones,but the suppression to the superconductivity is the op-posite. In Fig. 4(d) we illustrate the suppression of Tc

with doping concentrations of Fe and Co. This is easyto be understood in the way that, the suppression to thesuperconductivity in the Fe-doped samples is induced bythe local magnetic moments. These magnetic scatter-ing centers are detrimental to the Cooper pairs, and thussuppress the superconducting transition temperature sig-nificantly. However, in the normal state these impurities,although possessing strong magnetic moments, act as theusual scattering centers. In the Co-doped case, the in-crease of the residual resistivity is quite strong. For ex-ample, the residual resistivity increases more than 100 %with the Co doping level of about 8%. However, the su-perconducting transition temperature drops only about2 K. This sharp contrast between the behaviors of theFe and Co-doped samples is unexpected from a straightforward picture, since both Fe and Co would behave simi-larly, i.e., both would contribute local magnetic momentsand influence the electric conduction, as well as the su-perconductivity.

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FIG. 5: (color online) (a) Temperature dependence of DCmagnetic susceptibility for Co and Fe doped samples under 3T. A low-T diverging is observed for the Fe-doped samples,indicating an doping induced local magnetic moments. (b)-(e) The fit to the low temperature data yielding the magneticmoments (see Table I).

B. Doping induced magnetic moments

In order to unravel the puzzle concerning the sharpcontrast between the Fe and Co-doped samples, we havedone the magnetization measurements under high mag-netic fields. The raw data of magnetization measuredat 3 T up to room temperature are shown in Fig.5(a).The temperature dependence of the magnetic suscepti-bility look similar, however, it is only for the Fe-dopedsamples, that there is a diverging of the magnetic suscep-tibility at low temperatures. This diverging of χ at lowtemperatures can be understood as the formation of somestrong local magnetic moments. The magnetization forCo-doped samples reveals an itinerant moment. To illus-trate this point more clearly, we fit the low temperaturemagnetization with the Curie-Weiss law,

χ = χ0 + C/(T + T0), (1)

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eff(

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FIG. 6: (color online) Magnetic moment with Fe and Co dop-ing calculated by the constant C in the Curie-Weiss law (eq.1).(a) The doped Fe impurities lead to an enhanced magneticmoment. (b) The Co doping gives a gradually weakened mag-netic moment.

TABLE I: Fitting parameters with Curie-Weiss law for theCo- and Fe-doped samples.

doping C(K · emu/mol ·Oe) T0(K) µeff (µB)

Co-0.02 0.00883 5.596 1.1

Co-0.05 0.01046 12.608 0.747

Co-0.08 0.00984 9.307 0.572

Fe-0.01 0.00613 4.553 1.27

Fe-0.02 0.0188 5.097 1.58

Fe-0.025 0.02304 4.89 1.57

Fe-0.0275 0.02723 4.628 1.624

where C = µ0µ2eff/3kB, χ0 and T0 are the fitting pa-

rameters. The first term χ0 arises mainly from the Pauliparamagnetism of the conduction electrons, the secondterm is induced by the local magnetic moments, given bythe doped ions. In order to derive the correct values forC and χ0, we adjust χ0 value to make the 1/(χ−χ0) vs.T as a linear relation in the low temperature limit, theslope gives 1/C, and the intercept delivers the value of

T0. The data treated in this way is shown in Fig.5(b)-(e).Here Fig.5(b) and Fig.5(c) are representing results for theFe-doped samples with x = 0.01 and x = 0.0275; Fig.5(d)and Fig.5(e) are for the Co-doped ones for x = 0.02 andx = 0.08. One can see that the low temperature partis indeed linear. Once C is determined, we can get themagnetic moment given by the Fe and Co ions µeff/Coor µeff/Fe. It turns out that µeff/Co = 0.572µB in theCo-doped (x=0.08) sample, 1.62µB/(Fe) in the Fe-dopedone (x = 0.0275). Fig.6(a) and Fig.6(b) show the de-rived µeff for Co- and Fe-doped samples, respectively.The decrease of the µeff in Co-doped samples indicatesthe weakening of the magnetic moments compared to theparent sample, which suggests that the density of statesof spin up and spin down contributed by the Co atomsare equal. This is also consistent with the theoreticalresults: Co-dopant introduces negligible magnetic mo-ments. While in Fe-doped samples, an increase of µeff isobserved showing the enhancement of magnetic momentsby the Fe impurities. This strongly suggest that the elec-trons given by the Fe ions are more polarized, yielding amagnetic moment of about 1.6µB/Fe, comparable to thetheoretical calculation: 2.05µB/Fe.It is interesting to mention that, although the Ru and

Fe are in the same column in the periodic table, the dopedFe atoms apparently play a very different role as the Rudoes. This is consistent with the common sense that the3d electrons (here contributed by Fe ions) are more lo-calized leading to the magnetic moments. This is verydifferent from that in the iron pnictide superconductorsin which many different kind of 3d or 4d transition metalscan be doped to the Fe sites for inducing superconduc-tivity, showing a wide flexibility.15–19 Doping many tran-sition metals, like Co, Ni, Pd, Ir, Pt and Ru does notinduce very strong magnetic moments, instead the anti-ferromagnetic order is suppressed. On the other hand, inLaRu3Si2, doping Co does not suppress the superconduc-tivity quickly, although the impurity scattering is strong.This effect manifests that the pairing gap is probably s-wave type, although gap anisotropy exists for the presentsystem.7 It remains to be explored that whether the Co-doping in LaRu3Si2 can result in a ”dome” like dopingdependence of superconducting transition temperature,or in other words, can we find an antiferromagnetic (AF)order as the parent phase and superconductivity can beinduced by suppressing this AF order.

C. Density-functional theory calculations

Using WIEN2k package20, we studied the elec-tronic structure based on the generalized gradientapproximation.21 To consider the low doping concentra-tion, we perform calculation for a 2 × 2 × 2 supercell,and replace one of the 48 Ru atoms in the supercell byFe/Co. In Fig.7, we show the Fe/Co 3d partial DOS. Itis interesting to find that the main part of Co 3d is lo-cated below EF . Therefore Co 3d band is close to fully

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FIG. 7: (color online) Calculated 3d partial DOS, (a) for Co3d orbitals; (b) for Fe 3d orbitals. The positive and negativevalue signal the spin up and spin down portion of the DOS.

occupied, although due to the hybridization with Si andRu, Co 3d has also distribution above Fermi level (EF ).Thus, it is natural to expect that the spin splitting is

very small, and Co becomes nonmagnetic as shown inFig.7(a). For Fe, while the spin-up channel is almostfully occupied like Co, the spin-down is clearly partiallyoccupied as shown in Fig.7(b). Therefore, there is a bigexchange splitting and the magnetic moment at Fe-siteis found to be 2.05 µB, close to our experimental value1.6 µB. Because of the strong hybridization with Fe 3delectrons, the neighboring Ru-site has also about 0.1 µB

magnetic moment.

IV. SUMMARY

In summary, contrasting behaviors of the suppressionto superconductivity has been observed in Fe and Codoped LaRu3Si2. In the case of doping Fe, the super-conductivity can be easily suppressed, while it is muchslower in the Co-doped samples. Measurements and anal-ysis on the DC magnetization suggest that the Fe-dopinginduce some strong local magnetic moments, while Co-doping does not. This is well consistent with our DFTcalculations. In the Fe-doped samples, the impuritiesact as strong pair breakers, which is caused by the lo-cal magnetic moment. While the doping of Co atomsbrings about equally spin-up and spin-down electronswhich contributes negligible magnetic moment. There-fore the pair breaking is much weaker in the Co-dopedsamples.

Acknowledgments

We appreciate the useful discussions with Jan Zaa-nen, Zidan Wang, and Jianxin Li. This work is sup-ported by the NSF of China (11034011/A0402), the Min-istry of Science and Technology of China (973 projects:2011CBA00102 and 2012CB821403) and PAPD.

⋆ hhwen@nju.edu.cn

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