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Effects of In-Plane Impurity Substitution in Sr2ruo4

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Effects of In-Plane Impurity Substitution in Sr2ruo4 typeset using JPSJ.sty <ver.1.0b> Effects of In-Plane Impurity Substitution in Sr2RuO4 Naoki Kikugawa1,2, Andrew Peter Mackenzie3 and Yoshiteru Maeno2,4 1Venture Business Laboratory, Kyoto University, Kyoto 606-8501 2Department of Physics, Kyoto University, Kyoto 606-8502 3School of Physics and Astronomy, University of St. Andrews, St. Andrews, Fife KY16 9SS, Scotland 4International Innovation Center, Kyoto University, Kyoto 606-8501 (Received November 18, 2018) We report comparative substitution effects of nonmagnetic Ti4+ and magnetic Ir4+ impurities 4+ for Ru in the spin-triplet superconductor Sr2RuO4. We found that both impurities suppress the superconductivity completely at a concentration of approximately 0.15%, reflecting the high sensitivity to translational symmetry breaking in Sr2RuO4. In addition, a rapid enhancement of residual resistivity is in quantitative agreement with unitarity-limit scattering. Our result suggests that both nonmagnetic and magnetic impurities in Sr2RuO4 act as strong potential 2+ scatterers, similar to the nonmagnetic Zn impurity in the high-Tc cuprates. KEYWORDS: Sr2RuO4, impurity effects, unitarity scattering Unconventional superconductors such as heavy fermion cidentally during crystal growth. After considerable ef- 14) compounds and high-Tc cuprates have attracted much fort to optimize the growth conditions, we can now attention in the last two decades, since highly correlated constantly obtain high quality crystals with minimal ac- f- and d-electrons in these materials play essential roles cidental contamination and Tc > 1.4 K. This had allowed in the emergence of the unconventional superconduc- us to embark on a systematic study of the effects of con- tivity.1) In contrast to conventional superconductivity trolled substitution of Ru4+ with nonmagnetic Ti4+ and with s-wave symmetry, nonmagnetic impurities as well magnetic Ir4+ ions. as magnetic impurities act as strong pair breakers and The effect of impurity substitution into correlated elec- severely suppress the transition temperature Tc of un- tron systems can be subtle, with even nonmagnetic ions conventional superconductivity. The suppression of Tc introducing magnetic effects. Before describing the scat- reflects the sensitivity to translational symmetry break- tering effects of very low concentrations of Ti4+ and Ir4+ ing2) and is a characteristic of anisotropic pairing. Sys- on the superconductivity, it is therefore useful to review tematic studies of impurity substitution have also been the effects of higher level doping on the magnetic prop- used to obtain information on the underlying strongly erties. Recently, we reported that the substitution of 3, 4) 4+ 0 correlated electronic states in both f- and d-electron nonmagnetic impurity Ti (3d ) in Sr2RuO4 induces 5) systems. a local moment with the effective moment peff ∼ 0.5 15) Here, we study the effects of such substitution in the µB/Ti. The induced moment has Ising anisotropy layered perovskite ruthenate Sr2RuO4, whose supercon- with an easy axis along the c direction. Furthermore, ductivity6) is unconventional, most probably involving magnetic ordering with glassy behavior appears for x(Ti) 7) spin-triplet pairing. The normal state properties of ≥ 2.5% in Sr2Ru1−xTixO4 while keeping metallic con- Sr2RuO4 are described quantitatively within the frame- duction along the in-plane direction. When x(Ti) is fur- work of a quasi-two-dimensional Fermi liquid8) with the ther increased to 9%, elastic neutron scattering measure- Fermi surface consisting of three nearly cylindrical sheets ments detect an incommensurate Bragg peak16) whose 9) arXiv:cond-mat/0210190v1 [cond-mat.str-el] 9 Oct 2002 (α, β and γ). Comparison with the band-structure cal- wave vector Qic ∼ (2π/3, 2π/3, 0) is close to the po- culation10) indicates that strong correlations among the sition of the inelastic neutron scattering peak seen in 4+ 4 17) electrons originating from the Ru ions (4d in the low pure Sr2RuO4. In the vicinity of the magnetic or- spin configuration) hybridized with p-electrons of sur- dering with x ≥ 2.5%, deviation from the Fermi-liquid rounding oxygen play an essential role in the physical behavior seen in the pure Sr2RuO4 is observed with properties of Sr2RuO4. the resistivity and the specific heat data showing linear- Early studies of the impurity effects in Sr2RuO4 re- temperature dependence and logarithmic temperature vealed several features reflecting the unconventional su- dependence, respectively.18) These results indicate that perconductivity: rapid suppression of Tc by native im- the two-dimensional antiferromagnetic spin fluctuations 11) purities and defects and a large enhancement of the at Qic arising from the nesting mainly in the β band be- residual density of states in the superconducting state comes a static spin density wave (SDW) by nonmagnetic seen in specific heat measurements12) and NMR mea- Ti substitution. 13) surements. Throughout the above series of studies, On the other hand, the system Sr2Ru1−xIrxO4 in control of the impurity concentration within the crystals which the substitution is magnetic Ir4+ (5d5 in the low was not easy, because the impurities were introduced ac- spin configuration) shows weak ferromagnetism at x(Ir) 1 2 Naoki Kikugawa, Andrew Peter Mackenzie and Yoshiteru Maeno ≥ 30% occurring concomitantly with metal-insulator amount of x. The Tc is rapidly and systematically sup- 19) transition and the end-member material Sr2IrO4 is pressed in both cases. The result reflects the high sensi- a Mott insulator with canted antiferromagnetic order- tivity to translational symmetry breaking, characteristic ing.20) Thus, substitution of high levels of Ti4+ and Ir4+ of unconventional superconductivity. The inset shows impurities in Sr2RuO4 leads to different magnetic ground the dependence of Tc on the impurity concentration x. states, presumably reflecting the different magnetic char- We can see an almost universal suppression of Tc, irre- acter of the isolated ions. spective of the kind of impurity. The broken line shows In this letter, we show that both nonmagnetic Ti4+ the universal Abrikosov-Gor’kov pair-breaking function, and magnetic Ir4+ impurities suppress the superconduc- where the formulation is generalized to the case of non- tivity of Sr2RuO4 completely by a concentration of ∼ magnetic and magnetic impurities in an unconventional 21) 0.15%. Also, both impurities act as strong potential scat- superconductor. Based on this model, Tc(x) satisfies terers with the maximum phase shift δ ∼ π/2 (unitar- 0 T 1 1 ¯hΓ ity limit), as seen in high-T cuprates with nonmagnetic ln c = Ψ − Ψ + . c T 2 2 2πk T Zn2+ (3d10) impurity. c0 B c A series of single crystals of Sr2Ru1−xTixO4 and Here Ψ is the digamma function, ¯h the Dirac constant − 1 2x 2 Sr2Ru1 xIrxO4 with x up to 3% were grown by a and the scattering rate Γ = = sin δ0 +AS(S + floating-zone method with an infrared image furnace 2τ π¯hN0 1); the first and second terms in Γ represent the potential (NEC Machinery, model SC-E15HD). The detailed pro- and magnetic spin-flip scattering contributions, respec- cedure of the crystal growth is described elsewhere.14, 15) tively, where N is the density of states in the normal We only note here that an excess of 0.15 molar Ru was 0 state. From our best fitting by fixing T as 1.5 K, the added for each 2 molar Sr for the crystal growth. The c0 initial rate dT /dx ∼ −7.5 K/x(%) is obtained for both Ti and Ir concentrations in grown crystals were ana- c Ti and Ir substitutions. The critical concentration x for lyzed by electron-probe microanalysis (EPMA). The Ti c disappearance of the superconductivity is estimated as is well substituted for Ru as reported by Minakata and x ∼ 0.15%. Maeno.15) On the other hand, we found that the Ir as c By measuring the residual resistivity ρ (Fig. 1), well as Ru was heavily evaporated during crystal growth ab0 ◦ we can see a universal trend that the superconductiv- at the high temperature of ∼2200 C, so that excess ity of Sr RuO is completely suppressed at the critical amounts had to be added for the crystal growth: the 2 4 resistivity of ρ ∼ 1.1 µΩcm for both impurities, as re- analyzed Ir concentration x (Ir) is roughly connected ab0 a ported from previous studies with native impurities and with the nominal concentration xn(Ir) by xa ∼ 0.25xn for 11) defects. The critical value ρab0 is again in good agree- xn ≤ 9%. We note that the tetragonal crystal symme- ment with the mean free path lab falling below the su- try15, 19) for both Sr Ru − Ti O and Sr Ru − Ir O 2 1 x x 4 2 1 x x 4 perconducting coherence length ξ ∼ 900Awhen˚ super- with x up to 3% was confirmed at room temperature. ab conductivity is destroyed. The induced moment by magnetic impurities is isotropic The fact that Ti4+ and Ir4+ suppress T in the same in sharp contrast to the result of Ti substituted system c way in spite of their very different magnetic characters and estimated as 0.7 µ /Ir from susceptibility measure- B suggests that the magnetic contribution to pair break- ments. This value is much smaller than the previous ing is negligible, and potential scattering dominates. Al- report using polycrystals, ∼2 µ /Ir at x(Ir) ∼ 5%.19) B though it could also be explained in other ways, this For the resistivity measurements, the crystals were cut observation is qualitatively consistent with the existence into rectangles with a typical size of 3.5 × 0.4 × 0.05 of a spin-triplet state. Magnetic impurities break singlet mm3. The shortest dimension was along the c axis. Sil- pairs essentially because of exchange splitting of the sin- ver paste (Dupont, 6838) was used for attaching elec- ◦ gle particle state; equal spin paired triplet states would trodes and cured at 500 C for 5 minutes; the contact not be subject to such an effect.
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