Scanning Tunneling Microscopy studies of single layer high-Tc cuprate Bi2Sr2-xLaxCuO6+delta

Abstract

High temperature superconductivity has been one of the most challenging problems in condensed matter physics since its discovery. This dissertation presents systematic studies on the single layer high temperature superconductor Bi2Sr2-xLaxCuO6+delta by scanning tunneling microscopy. The STM results have been compared to Angle-resolved photoemission spectroscopy (ARPES) data. Using STM spectroscopy and ARPES we observed two distinct gaps that coexist both in real space and in the antinodal region of momentum space, below the superconducting transition temperature. By looking at the energy scale of these two gaps along with the temperature dependence data, we fnd that the small gap is associated with superconductivity. The large gap persists above Tc, and seems linked to observed charge ordering. We also find a strong correlation between the large and small gaps suggesting that they are affected by similar physical processes. This is the first time that two coexisting and competing energy scales have been directly observed in STM spectroscopy. Combining this with ARPES data, we show that the pseudogap may be a diffferent order parameter from the superconducting phase. This provides support to the recently proposed "two gaps scenario" and should lead to more experimental discovery and theoretical discussions. In this dissertation we also discuss the spatial properties of the scanning tunneling microscopy conductance maps, as well as the charge ordering pattern at high energies. We observe interesting periodic patterns at low energies which can not be explained by a simple charge density wave picture. We also fnd the surprising bias dependence in terms of the contrast reversal. We propose a model of STM measuring effect to explain these phenomena.