Terahertz scanning tunneling microscopy of metal and superconductor surfaces

Abstract

The advent of the terahertz scanning tunneling microscope (THz-STM) brings with it a new method of observing and characterizing the ultrafast dynamics of materials. The sub-nanometer spatial resolution of an ultra-high vacuum scanning tunneling microscope (UHV-STM) coupled with the sub-picosecond time resolution from a terahertz (THz) pulse allows for ultrafast measurements of surface dynamics across varying surface features down to the atomic scale. This thesis explores STM and THz-STM of metal and superconductor surfaces. High critical temperature superconductivity has been an area Bi2Sr2CaCu2O8 is an excellent candidate to study the superconductive process due to its high critical temperature of 95 K and ease of cleaving. STM and scanning tunneling spectroscopy measurements on the high-temperature superconductor Bi2Sr2CaCu2O8 are performed at 100 K and 55 K. STM and STS measurements were also performed on the surface of flat Au(111). Given gold's versatility as a substrate for many STM measurements, it is important to ensure that the STM system used in this study can replicate results that coincide with many other studies done on the surface of gold. The characteristics of the substrate such as flatness and cleanliness prior to deposition of other materials were investigated. The herringbone surface reconstruction of Au(111) was imaged and the topography was analysed with good agreement compared to other studies. Finally, optical-pump/THz-STM-probe measurements are performed on a silver-coated gold surface. These measurements, with tip-sample distances outside of the typical tunneling regime of the THz-STM, use photo-excited electrons from an optical pump beam to tunnel between the sample and tip. A THz pulse is then focused onto the junction between the sample and tip. The results show an interesting behaviour in the regime where the pump beam is not focused on the junction but elsewhere on the surface of the sample. These preliminary results may help gain a better understanding of the photoemission-based THz waveform measurements, as well as demonstrate a new use of the THz-STM for propagation dynamics of surface excitations.