UC Davis

This work will present experimental results on two projects measuring the electronic properties of unconventional superconductors. The first project focuses on the iron-based superconductor system of FeSe1-xSx. We used scanning tunneling microscopy/spectroscopy (STM/S) to make detailed measurements of the electronic states of tetragonal FeSe0.89S 0.19. We also performed theoretical band structure calculations that were calibrated by angle-resolved photoemission spectroscopy measurements for comparison. We utilize the high spatial resolution of the STM/S measurements to separately analyze modulations in the local density of states (LDOS) in regions near and away from iron-vacancies. This analysis revealed two types of features: (i) energy dispersive quasiparticle interference patterns which can explained by our band structure calculations and (ii) a much stronger modulation just above the Fermi level centered at q=0.12 Å-1 which does not disperse with energy and cannot be explained by our band structure model. Local rotational symmetry analysis shows that while the modulations are four-fold symmetric on average, they are actually comprised of small domains with two-fold symmetry. Statistical analysis demonstrates that the boundaries of these domains are spatially correlated with the locations of iron-vacancies.

The second project studies charge order in the La-based cuprate La1.475Nd0.4Sr0.125CuO4. We performed temperature and uniaxial strain dependent resonant x-ray scattering studies on the charge order and the low-temperature orthorhombic to low-temperature tetragonal (LTO-LTT) structural transition. Before applying any strain, we found a precursor charge order peak existing up to 200 K, well above the static charge order onset temperature. Upon applying uniaxial tensile strain of about 0.1%, we observed a reduction in the onset of charge order by 50 K and a 20 K reduction in the LTO-LTT transition temperature. We also saw a preference for the charge order to form in the direction of applied strain due to a 6 K difference in onset temperatures for the two directions.