Workshop "NMR, µSR, Mössbauer spectroscopies in the study of Fe-based and other unconventional high-Tc superconductors"
s-wave superconductivity in high-pressure annealed LaO0.5F0.5BiS2 polycrystals
Gianrico Lamura, CNR-SPIN and Università di Genova, Italy
Layered superconductors have always attracted the interest of condensed matter scientists since low dimensionality is generally associated to high transition temperatures and exotic pairing mechanisms. Very recently bulk superconductivity (SC) with Tc in the range 4-10 K was discovered in bismuth-based compounds such as Bi4O4S3 [1, 2] and LaO0.5F0.5BiS2  that belong to the class of layered materials. In these systems BiS2 planes are expected to play the same role as CuO2 planes in cuprates or the FeAs and Fe(Se,Te) planes in iron-based superconductors (IBS). Differently from the latter, the conduction should be dominated by bismuth 6p orbitals, spatially extended and hybridized with sulfur 3p orbitals. This should give a more delocalized and anisotropic (2D) character to the conduction bands . In particular, in LaO0.5F0.5BiS2 four bands are expected to cross the Fermi level giving rise to a 2D Fermi surface strongly nested at (π, π, 0) wave vector as in IBS, whose consequences on SC are still under debate.
Here we present a µSR study of the superconducting properties of high-pressure annealed polycrystalline LaO0.5F0.5BiS2 . ZF-µSR spectroscopy shows that, contrary to IBS, no hidden magnetic order static on SR time scale is present. In the SC phase TF-µSR was used to study the evolution of the muon-spin depolarization with the applied field (see Fig. 1-a) and that of the superfluid density with temperature (see Fig. 1-b). We will show that both these dependencies hint at a fully-gapped conventional BCS picture of superconductivity with a marked 2D character.
FIG. 1: (a) Field dependence of the square of superconducting depolarization rate σsc. The red line represents a linear fit in the low-field region. The grey area delimits the region of validity of σsc = γµ (0.00371)1/2 Φ0/λ2, where Φ0 is the quantum of magnetic flux and λ is the effective magnetic penetration depth. Inset: reduced field dependence of λ(h = H/Hc2)/λ(0) as extracted from the data in the grey region by means of the up-cited equation. (b) Temperature dependence of the normalized superfluid density measured at μ0H = 0.3 T (full circles) and at 0.07 T (open triangles). Low-temperature (T < 1.7 K) LTF data at 0.07 T are shown by solid triangles. The full and the dotted lines represent best fits to an s-wave gap in the clean limit and a numerical calculation with an s-wave anisotropic gap respectively.
 Y. Mizuguchi et al., Phys. Rev. B 86, 220510(R) (2012).
 S. K. Singh et al., J. Am. Chem. Soc. 134, 16504 (2012).
 Y. Mizuguchi et al., J. Phys. Soc. Jpn. 81, 114725 (2012).
 X. Wan et al., Phys. Rev. B 87, 115124 (2013).
 G. Lamura et al., Phys. Rev. B 88, 180509(R) (2013).