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Neutron Scattering Study on BiS2-Based Superconductors

Lee, Jooseop
Thesis/Dissertation; Online
Lee, Jooseop
Lee, Seung-Hun
The recent discovery of a new type of superconductors based on BiS2 layers has excited the scientific community due to its structural similarities with other unconventional superconductors, such as cuprates and Fe-based superconductors. If the superconducting mechanism of BiS2-based superconductors is unconventional, it will yield a new route to better understand the long-unsolved puzzle of unconventional superconductivity. Hence, it is crucial to determine the superconducting mechanism and the nature of superconductivity in this system. There are contradicting scenarios regarding the superconducting mechanism in this system. The band structure calculation indicates the existence of Fermi surface nesting, like in iron pnictides, supporting a non-phonon mediated superconductivity. On the other hand, the electron-phonon coupling constant was calculated to be large enough to reproduce the reported T c , suggesting an conventional phonon mediated superconductivity. Experiments to test these theoretical predictions are limited, however. To tackle the controversy over the role of phonons in this system, we have performed elastic and inelastic neutron scattering measurements on polycrystalline samples of non-magnetic LaO1-xFxBiS2 and magnetic CeO0.3F0.7BiS2 . We have examined its crystal structure and lattice vibrational modes, and compared these results with density functional perturbation calculations. We could not find any meaningful change related to superconductivity in the phonon density-of-states either by F-doping or by cooling through the transition temperature. This suggests that the possible electron-phonon coupling in this material is much weaker than expected based on theoretical calculations, implying this new BiS2-based superconductors can be unconventional. For the ferromagnetic CeO0.3F0.7BiS2 , we have studied the interplay of magnetism with superconductivity. This system exhibits the rare and interesting case of the coexistence of ferromagnetism and superconductivity, which is not realized in in the conventional phonon mediated superconductors due to their antagonistic nature. The crystal and magnetic structures have been investigated together with the spin fluctuations, and we determined the spin Hamiltonian describing the spin dynamics in this system. In addition, we have examined the external magnetic field dependence of both magnetic structure and its excitation. Under field, the magnetic structure changes from ferromagnetic with the spins pointing along the c-axis to ferromagnetic in the ab-plane, and the spin fluctuation dispersion splits into two. While the splitting of spin excitations in a ferromagnet is untypical which needs further investigation, it seems that superconductivity is robust against a magnetic field and there is no direct relationship between magnetism and superconductivity in this system. Our neutron scattering study on the crystal structure, lattice vibrations, and magnetism in the BiS2-based superconductors will help us to understand the mechanism of superconductivity and its relationship with other degrees of freedom.
University of Virginia, Department of Physics, PHD (Doctor of Philosophy), 2013
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PHD (Doctor of Philosophy)
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