© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-rapid.com pss Phys. Status Solidi RRL, 1–3 (2012) / DOI 10.1002/pssr.201206006 Early View publication on wileyonlinelibrary.com (issue and page numbers not yet assigned; citable using Digital Object Identifier – DOI) Pressure effects on the super- conducting transition of ytterbium doped Ce 0.6 Yb 0.4 FeAsO 0.9 F 0.1 S. Arumugam *, 1 , M. Kanagaraj 1 , N. R. Tamil Selvan 2 , S. Esakki Muthu 1 , J. Prakash 3 , G. S. Thakur 3 , A. K. Ganguli *, 3 , H. Yoshino 2 , K. Murata 2 , K. Matsubayashi 4 , and Y. Uwatoko 4 1 Centre for High Pressure Research, School of Physics, Bharathidasan University, Tiruchirapalli 620024, India 2 Department of Materials Science, Graduate School of Science, Osaka City University, Osaka, Japan 3 Department of Chemistry, Indian Institute of Technology, New Delhi 110016, India 4 ISSP, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8581, Japan Received 10 January 2012, revised 20 March 2012, accepted 30 March 2012 Published online 5 April 2012 Keywords superconductivity, iron pnictides, Ce compounds, high pressures, magnetisation, resistivity * Corresponding authors: e-mail sarumugam1963@yahoo.com, ashok@chemistry.iitd.ernet.in, * Phone: +91-431-2407118, 91-11-26591511, Cell: +91-95009 10310, Fax: +91-431-2407045, 2407032, +91-11-26854715 © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 Introduction The recent discovery of new super- conductors based on iron oxypnictides or ferropnictides has opened new possibilities in superconducting research and condensed matter physics. A similar superconducting family of F doped LaFeAsO 1–x F x (x = 0.1) with T c = 26 K was reported in 2008 [1]. These families of superconduc- tors have a layered structure at room temperature, which belongs to the tetragonal ZrCuSiAs type structure (P4/nmm). Electrical properties of these LnFeAsO com- pounds can be varied by doping of charge carriers (elec- trons or holes) by substitution of ions or by application of external pressure which results in modification of the elec- tronic structure. For example, electron doping by substitu- tion of fluoride ions at O sites or by oxygen deficiency in CeFeAsO induces charge carriers in FeAs plane which leads to the development of superconducting state with maximum T c of ~ 41 K in CeFeAsO 0.8 F 0.2 and ~ 46.5 K in CeFeAsO 1–x [2, 3]. The internal pressure produced by chemical doping, which leads to an increase of T c up to 55 K for the substitution of smaller lanthanide ions (Nd, Tb, Gd and Sm) in place of La ions [4–7]. The external pres- sure effect on these F doped LaFeAs(O/F) compounds shows interesting variation in T c . Takahashi et al. showed that T c could be enhanced from 26 K to 43 K by applica- tion of pressure (4 GPa) and it decreases gradually from 43 K to 9 K on increasing pressure beyond 4 GPa [8]. On the contrary, the application of external pressure on CeFeAsO 0.88 F 0.12 results in a monotonous decrease in T c [9]. Here we report the effect of external pressure on magnetization and resistivity of superconducting Ce 0.6 Yb 0.4 FeAsO 0.9 F 0.1 . 2 Experiment Polycrystalline sample of Ce 0.6 Yb 0.4 O 0.9 F 0.1 FeAs was synthesized [10] by a two-step solid state method. High purity Ce, CeO 2 , Yb 2 O 3 , CeF 3 , and FeAs were used as starting precursors. FeAs was ob- tained by reacting Fe chips and As powder at 800 °C for 24 h. A stoichiometric amount of raw materials was taken and then finely grinded, sealed in evacuated silica am- poules (10 −4 Torr) and heated at 900 °C for 30 h. The pow- der was then compacted (5 tons) and the disks were wrapped in tantalum foil, sealed in evacuated silica am- poules and annealed at 1150 °C for 30 h and then furnace- cooled to room temperature. All the chemical manipula- The Effect of pressure on the superconducting transition tem- perature of Yb doped Ce 0.6 Yb 0.4 FeAsO 0.9 F 0.1 has been inves- tigated for the first time using resistivity and magnetization studies. Increase in chemical pressure by substitution of smaller Yb 3+ ions in place of Ce 3+ ions results in a significant enhancement of T c from 38 K (Yb free) to 47 K (40% Yb). Enhancement in T c with external pressure has been observed for this compound up to a maximum value of T c = 48.7 K at 1 GPa, beyond which T c starts decreasing monotonously.