The Role of Nanoscale Atomic Fluctuations in the Superconducting Mechanism of the BiS2 Superconductors

The interplay of disorder and superconductivity has been a topic of intense interest for decades. Several exemplary systems can be found in the literature that demonstrate how disorder, manifested through doping, vacancies and strain in both two and three dimensions, plays an important role in the pairing mechanism. In the cuprates for example, it is well understood by now that disorder induced by doping leads to charge puddles creating an inhomogeneous lattice, and superconductivity is the result of percolation through filamentary regions surrounded by the puddles.

The newly discovered Bismuth Sulfide superconductors provide a testbed for investigating the effects of disorder on superconductivity in the absence of magnetism. The superconducting BiS2 planes consist of the same building blocks as in the cuprate and Fe-based superconductors. Charge carriers are introduced either via doping in the blocking layers or by introducing vacancies. The pairing mechanism is electron-phonon mediated, possibly of the conventional type but with strong electronic correlations indicated by electron phonon coupling constant of λ~0.8. Moreover, they have been theoretically predicted to have unstable phonon modes, a charge density wave (CDW) instability, and strong spin orbit coupling.

In this dissertation, the characteristic nature of the lattice distortions, the effects of atomic inhomogeneities and how they couple to the mechanism of superconductivity are investigated in the BiS2 class. By combining neutron and high resolution X-ray diffraction, the structural investigation is carried out on polycrystalline and single crystal samples of LnO1-xFxBiS2 (Ln = La, Nd and Pr).

While on average, the crystal symmetry is presumed to be tetragonal, our results provide evidence for charge disproportionation of the Bi ions linked to a locally distorted lattice. In-plane ferro lattice distortions are evident from the local displacements of sulfur and bismuth in the basic building blocks giving rise to superlattice reflections. The in-plane short-range distortions break the 2-fold and 4-fold symmetry operations and create distinct environments around Bi. The ferro-distortive sulfur displacements occur in both non-superconducting and superconducting phases with diminished magnitude of displacements in the latter. In the superconducting phase, the out-of-plane motion of apical sulfur becomes significant that can act as charge transfer between the donor layers and superconducting layers. The lattice distortions that arise from these local modes may trap charge carriers decreasing the number of pairing electrons. Thus, the work presented in this thesis provides insight into the structural distortions coupled to the charge fluctuations and carrier transfer in BiS2 superconductors.