¹⁹F-NMR and magnetic susceptibility in the normal state of LaO_0.5F_0.5BiS₂ superconductor

We report ¹⁹F-NMR spectral parameters together with complementary magnetic susceptibility measurements in an ambient-pressure-annealed powder of LaO_0.5F_0.5BiS₂; T_c ~3K. NMR measurements and bulk magnetization experiments were performed as functions of temperature T and applied magnetic field H. The ¹⁹F-NMR linewidth grows slowly with decreasing temperature from room temperature to about T*~10 K. Below T*, the linewidth increase is anomalous and consistent with the appearance of superconducting regions in the sample. The lineshift, measured with respect to CaF₂, is T-independent above T* and decreases noticeably with decreasing T below it. Although the linewidth above T* can be attributed to random paramagnetic impurities, the lineshift cannot be understood in terms of impurity magnetization effects. Instead, the line position is seen to be dominated by the chemical shift and by the direct-contact interaction of ¹⁹F with a relatively low density of conduction electrons. The decrease in lineshift below T* together with the lineshape symmetry suggest that the effect is happening throughout the sample and not just in some regions. T~10 K is the maximum T_c this material displays under pressure, or when samples of it are pressure annealed. We conjecture that we are seeing either a lack of superconducting coherence or the effects of pair-breaking fluctuations; either or both being caused by the presence of strong paramagnetic impurities in the starting materials.