Investigating the relationship between the superconducting and pseudogap states of the high-temperature superconductor Bi-2201 using scanning tunneling microscopy

Abstract

There is considerable controversy regarding the nature of the relationship between the superconducting and pseudogap states of high-temperature superconductors. Although there exist a large number of theories regarding their relationship, the theories fall into one of two broad classes. The first class views the superconducting state as intrinsically tied to the pseudogap state. The second class views the pseudogap state as unrelated to or possibly competing with superconductivity. To address this controversy, we utilize a custom, home-built scanning tunneling microscope (STM) with the ability to follow the same atomically resolved location as a function of temperature to study the high-temperature superconductor Bi2Sr2CuO6s+. In our studies we are able to track the spatial evolution of the local density of states as the temperature is raised through the superconducting transition temperature, Tc, and in doing so, understand how the density of states evolves from the superconducting to pseudogap states. This thesis contains three complementary studies, all of which give some insight into the two states. The main study of this thesis focuses on the spatial evolution of the gap in the density of states, from below to above Tc in overdoped Bi2Sr2CuO6+8 (Tc = 15 K). Initially we find a spatially inhomogeneous gap which smoothly evolves with temperature through Tc. However, from the temperature and spatial dependence of the spectra, we are able to employ a normalization scheme which leads us to uncover a small, spatially homogeneous gap which coexists with the larger gap throughout the sample and disappears at Tc. From a study of the doping dependence of the two gaps, we determine that the large gap is the pseudogap, that the small gap is the superconducting gap, and that they appear to be two independent gaps indicating that they are associated with two apparently separate phases.

(cont.) The second study focuses on the evolution of native impurity states in overdoped Bi2Sr2CuO6+8 (Tc = 15 K) through Tc. We find that the impurity states persevere virtually unchanged with temperature, save for the effects of thermal broadening, indicating an interaction of impurities with the pseudogap state both above and below Tc. The existence of a satellite peak in the impurity spectrum below TC shows an interaction of impurities with the superconducting state. This study provides confirmation that the pseudogap state exists below Tc and coexists with superconductivity. Our final study is of the doping and temperature dependence of the electronic checkerboard pattern observed in underdoped and optimally doped Bi2Sr2CuO6+8. We find the pattern exists both above and below Tc, and, with the support of other experimental evidence, believe this pattern represents the ordering of the pseudogap state. Our doping-dependence measurements show a pattern with a wavelength periodicity which increases with doping, which is consistent with a charge-density-wave origin for the checkerboard ordering and hence the pseudogap state. Taken together our studies show that the superconducting and pseudogap states appear to be separate states that coexist at the same spatial locations below Tc. And, we find evidence for charge density waves as the likely origin for the pseudogap state.