Field Dependent Superfluid Density in the Optimally Doped SmFeAsO1−xFy Superconductor

The magnetic-field dependence of the in-plane magnetic penetration depth �ab for optimally doped SmFeAsO1−xFy was investigated by combining torque magnetometry, SQUID magnetometry, and muon-spin rotation. The results obtained from these techniques show all a pronounced decrease of the superfluid density �s��ab− 2 as the field is increased to 1.4 T. This behaviour of �s is analysed within a two-band model with self-consistently derived coupled gaps and �s=�s1+�s2, where �s1 related to the larger gap is field independent, and �s2 related to the smaller gap is strongly suppressed with increasing field. DOI: https://doi.org/10.1209/0295-5075/91/47005 Posted at the Zurich Open Repository and Archive, University of Zurich ZORA URL: https://doi.org/10.5167/uzh-45363 Accepted Version Originally published at: Weyeneth, S; Bendele, M; Puzniak, R; Murányi, F; Bussmann-Holder, A; Zhigadlo, N D; Katrych, S; Bukowski, Z; Karpinski, J; Shengelaya, A; Khasanov, R; Keller, H (2010). Field-dependent superfluid density in the optimally doped SmFeAsO1-xFysuperconductor. EPL (Europhysics Letters), 91(4):47005. DOI: https://doi.org/10.1209/0295-5075/91/47005 ar X iv :0 91 1. 54 20 v2 [ co nd -m at .s up rco n] 9 S ep 2 01 0 epl draft Field Dependent Superfluid Density in the Optimally Doped SmFeAsO1−xFy Superconductor S. Weyeneth , M. Bendele, R. Puzniak, F. Murányi, A. Bussmann-Holder, N. D. Zhigadlo, S. Katrych, Z. Bukowski, J. Karpinski, A. Shengelaya, R. Khasanov and H. Keller 1 Physik-Institut der Universität Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland 2 Laboratory for Muon Spin Spectroscopy, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland 3 Institute of Physics, Polish Academy of Sciences, Aleja Lotników 32/46, PL-02-668 Warsaw, Poland 4 Max-Planck-Institut für Festkörperforschung, Heisenbergstrasse 1, D-70569 Stuttgart, Germany 5 Laboratory for Solid State Physics, ETH Zurich, Schafmattstrasse 16, CH-8093 Zurich, Switzerland 6 Department of Physics, Tbilisi State University, Chavchavadze 3, GE-0128 Tbilisi, Georgia PACS 74.70.Xa – Pnictides and chalcogenides PACS 74.25.Ha – Magnetic properties including vortex structures and related phenomena PACS 76.75.+i – Muon spin rotation and relaxation PACS 74.25.Bt – Thermodynamic properties Abstract. The magnetic field dependence of the in-plane magnetic penetration depth λab for optimally doped SmFeAsO1−xFy was investigated by combining torque magnetometry, SQUID magnetometry, and muon-spin rotation. The results obtained from these techniques show all a pronounced decrease of the superfluid density ρs ∝ λ −2 ab as the field is increased to 1.4 T. This behaviour of ρs is analysed within a two-band model with self-consistently derived coupled gaps and ρs = ρs1 + ρs2, where ρs1 related to the larger gap is field independent, and ρs2 related to the smaller gap is strongly suppressed with increasing field. The magnetic field dependence of the in-plane magnetic penetration depth λab for optimally doped SmFeAsO1−xFy was investigated by combining torque magnetometry, SQUID magnetometry, and muon-spin rotation. The results obtained from these techniques show all a pronounced decrease of the superfluid density ρs ∝ λ −2 ab as the field is increased to 1.4 T. This behaviour of ρs is analysed within a two-band model with self-consistently derived coupled gaps and ρs = ρs1 + ρs2, where ρs1 related to the larger gap is field independent, and ρs2 related to the smaller gap is strongly suppressed with increasing field. After the discovery of superconductivity in LaFeAsO1−xFx [1] with a transition temperature Tc ≃ 26 K, a whole new family of iron-based superconductors was found with a maximum Tc ≃ 55 K for SmFeAsO1−xFy [2], where y 6 x due to a possible oxygen deficiency in the chemical composition [3]. Various experiments indicate multi-gap superconductivity within the family REFeAsO1−xFy (RE = rare-earth element) [3–8]. Furthermore, the magnetic penetration depth anisotropy, γλ = λc/λab, increases with decreasing temperature, in contrast to the upper critical field anisotropy, γH = H ||ab c2 /H ||c c2 , which decreases with decreasing temperature [9–11], similar to those of the two-gap superconductor MgB2 [12, 13], although with reversed slopes [14–16]. Here λi and H ||i c2 denote the magnetic penetration depth and the upper critical field components along the crystallographic direction i (ab-plane or c-axis). Besides of the influence of the magnetic field H and temperature T on the anisotropy, the direct influence of H and T on λ is essential in probing multi-gap supercon(a)E-mail: [email protected] ductivity [17]. However, it is difficult to obtain reliable experimental evidence for multi-gap superconductivity from the temperature dependence of λ in samples containing magnetic ions such as SmFeAsO1−xFy . Importantly, the superfluid density ρs = ns/m , with ns being the superfluid carrier density and m the effective carrier mass, can be probed directly by measuring λ ∝ ρs. In the two-gap superconductor MgB2 [17] ρs was found to be field dependent due to a suppression of the superfluid density in the band with the smaller gap [18, 19]. A similar dependence was observed for NbSe2 [20, 21], V3Si [22], YNi2B2C [23], and La1.83Sr0.17CuO4 [24] and again suggested to stem from either multi-gap superconductivity or to be related to changes of the internal field distribution of the vortex lattice. Previous studies of λab in LaFeAsO1−xFx [25] and in SmFeAsO1−x and NdFeAsO1−x [26] by muon-spin rotation (μSR) experiments indicated a field dependent ρs up to 0.6 T in the REFeAsO1−xFy family. Also in the related Ba(Fe0.926Co0.074)2As2 compound a field dependent ρs was reported for fields up to 0.2 T [27], and being interpreted in terms of multi-gap superconductivity. However,