Angle resolved photoemission spectroscopy study of high temperature superconductors

High Temperature Superconductors (HTSCs) were discovered in 1986, but despite immense research for last two decades, these materials are not yet completely understood. HTSCs exhibit very complicated three dimensional phase diagram parameterized by temperature, magnetic filed and carrier concentration. Up to now we do not know how to properly characterize all of their different phases, in particular the so-called pseudogap (PG) phase where the system is not macroscopically a superconductor but shows properties similar to the superconducting (SC) state. Angle Resolved Photoemission Spectroscopy (ARPES) that probes the momentum space structure of a system has greatly contributed to our understanding of the electronic structure of HTSCs. In this thesis I will present various ARPES studies on HTSCs. To properly understand the PG phase we need to characterize its ground state, i.e. the zero temperature state. I will present our scaling analysis of the finite temperature ARPES data in the PG phase over a wide region in the phase diagram. Using this scaling analysis I will show that in the zero temperature limit the PG phase is a pure d-wave state just like the SC state. I will also present our detailed temperature dependent ARPES measurements on underdoped HTSCs to study the temperature evolution of the superconducting gap. Our measurements show for the first time that unlike the conventional superconductors SC energy gap in underdoped HTSCs do not show mean-filed temperature dependence. I will show, using our new formalism how we can evaluate various two-particle correlation functions from ARPES data and can understand different collective excitations in HTSCs in terms of common underlying physics of the single particle excitations. Finally, I will present our new autocorrelation analysis of ARPES data to look at very minute details of it that might not be possible in the framework of conventional analysis. In addition this analysis gives a new paradigm to correlate ARPES results with the Scanning Tunneling Spectroscopy measurements, probing the real space structure of the system. For the first time, a single experiment can give information about both the real space and the momentum space structure of a system.