YTTRIUM SUBSTITUTED IRON-BASED SUPERCONDUCTORS S.V. Chong, T. Goya, T. Mochiji, and K. Kadowaki Institute of Materials Science and Graduate School of Pure & Applied Sciences, University of Tsukuba, Japan Thank you madam chairperson On behalf of my coworkers in the Kadowaki group at the University of Tsukuba, I would like to present to you our work on Yttrium substituted iron-based or iron-arsenide superconductors. Special Romp Session on Oxypnictides

Fe-based Superconductors 37 K 38 K 26 K 41K 52 K 55 K 34.7 46 K 45 K Rare-earth oxypnictides, ReFeAsO 122 iron-arsenide, MFe2As2 Rare-earth oxypnictides from La to Dy (excluding Pm and Eu) show superconductivity when they are electron or hole doped; Hole (K, Cs, Na) and electron (Co into Fe) doped Sr, Ba, and Eu-Fe2As2 are superconductors; Yttrium oxypnictide (doped) has been predicted to show superconductivity… To begin with, let us look at the cut-out of the periodic table As you can see, half of the elements surrounding Yttrium have been found to be either the rare-earth oxypnictides or the 122 iron-arsenide superconductors And it has been suggested earlier on when the oxypnictides were booming that fluoride doped Yttrium oxypnictide should also be superconducting

Fluoride-doped YFeAsO with x = 0.1 So this was what we are trying to find out here, i.e. can fluoride doped yttrium iron oxy-arsenide be superconducting? The reaction actually took a long time before you could see a change in the powder XRD pattern, which we believe the Bragg reflection here at 2.907 Angstrom belongs to the (102) reflection in the oxypnictide family So this number fits in between Neodynium and Samarium; it correlates better by comparing with the atomic radii of the rare-earths, but not that well unfortunately by comparing the 3+ ionic radii… Yttrium iron oxy-arsenide requires prolong reaction time: 1150 °C for 50 hrs and 1200 °C for 72 hrs; still there were many impurity phases – YAs, Y2O3, FeAs, etc. XRD shows the development of a Bragg reflection at 2.907Å; c.f. ReFeAsO1-xFx

YFeAsO0.9F0.1 So what we observed from magnetisation measurements was that after annealing at 1200 deg C, this magnetic ordering was almost all being suppressed and we observed both Meissner effect and Magnet Screening starting at 10.2 K With R-T two transition temperatures were observed, one at 9.3 K and the other at 10.2 K; most likely due to mixed phase in the presence of large amount of impurities. Magnetic ordering at 62 K was suppressed after 1200 °C annealing, following that Meissner effect and magnetic screening transition was observed at onset Tc ~10.2 K; R-T shows two transition temperatures perhaps due to the presence of large amount of impurities and mixed phases.

Gd1-yYyFeAsO0.8F0.2 GdFeAsO1-xFx: Ambient pressure preparation Tc ~36.6 K  High pressure preparation Tc ~53.5 K  Electron doping via Th4+ Tc went up to ~55 K Substitution with small amount of Yttrium did not alter the Bragg reflections position related to GdFeAsO0.8F0.2, but Tc was lowered from 40.8 K to 30.7 K  Perhaps due to some electronic effect(s) by the presence of Yttrium? We have also tried to see by substituting a small amount of the Gadolinium with Yttrium whether there will be an enhancement in Tc, But the opposite was observed. Tc was lowered from 40.8 to 30.7 K, with no alteration in the XRD pattern, perhaps due to some electronic effects in the presence of yttrium And we are currently making samples with different Yttrium doping level to try to understand this result.

Electron doping of SrFe2As2 Superconductivity in electron-doped SrFe2As2 was achieved by substituting a small amount of Fe with Co Tc ~20 K;1 Ba1-xLaxFe2As2 was previously attempted by Wu et al. but was not superconducting, but with RH < 0.2 (1-x)SrAs + xYAs + FeAs + Fe  Sr1-xYxFe2As2 e- SC The last system that I will talk about is the electron doping of Strontium-122 As you know by now, you can do an intralayer electron doping of this by substituting a small amount of Fe with Co and the reported Tc was around 20 K; Incidentally, an earlier report by Wu and co-workers by doping Lanthanum into Ba-122 was not superconducting, but electron was the major carriers. Here we are trying to use our favourite Yttrium and hopefully it will dope electron into the FeAs layers and induced superconductivity 1 A. Leithe-Jasper et al. Cond-mat/0807.2223v1. 2 G. Wu et al. Cond-mat/0806.1459v1.

Sr0.6Y0.4Fe2As2 Indeed, superconductivity was observed with the suppression of magnetic orderings found in the undoped sample at 210 and 60 K; However, R-T still shows the anomaly above 200 K, with a onset Tc of 26.4 K; the mid point Tc is around 24.2 K as compare to an onset Tc of 23.6 K with M-T. That is all I want to present, thank you for your attention. Interlayer electron-doping with yttriumIII, Sr0.6Y0.4Fe2As2, caused the suppression of magnetic orderings at 210 and 60 K; R-T still shows an anomaly at ~207 K; Tc at midpoint ~24.2 K, c.f. onset Tc ~23.6 K from M-T.