UC Berkeley

The demands for new technologies with fast computation, efficient data storage, and low energy cost encourage the search for new matter of phases as well as novel quantum mechanisms. Exotic phenomena, such as high-temperature superconductivity, colossal mag- netoresistence, and topological insulator, naming three examples out of many, of quantum materials open the route to achieve these goals in which the conventional materials can- not easily accomplish. The underlying physics of these novel properties originate from the crystal symmetry, band topology, and the interplay between different degrees of freedom in the condensed phase of matter including electron charge, spin, orbital, and lattice, can- not be explained by the traditional quantum theory. Understand the multiple role of these determinants in quantum materials is not only pivotal from the viewpoint of fundamental physics research, it also encapsulates potential for exciting future applications, such as fast electronics and spintronics.

Angle-resolved photoemission spectroscopy (ARPES) has shown its power of revealing the electronic band structure in the reciprocal space by measuring the energy distribution of photo-ejected electrons with respect to kinetic energy and momentum. The development of spin polarimetry and efficient association with the energy analyzer allow direct measurements of the spin, energy, and momentum of the photoelectrons at the same time. As the electron spin has become one of the focus in condensed matter research, direct probe of such degrees of freedom is essential. By utilizing the unique power of spin-resolved ARPES (SARPES), we show the impacts of the crystal symmetry on the electron spin in the selected quantum materials, one with strong atomic spin-orbit coupling (SOC) but the electrons are weakly correlated and another with strong electron-electron correlation but weak atomic SOC, in this dissertation.

Content is organized into six chapters. Chapter 1 gives an introduction to the symme- try operations and the symmetry-driven spin asymmetry in the condensed phase. Chapter 2 reviews the working principle and the corresponding efficiency of various spin polarime- ters including the home-built SARPES station which collected the data presented in chap- ters 3–5. Chapter 3 contains a brief discussion of topology physics and the detailed spin-momentum locking pattern of topologically trivial and non-trivial surface states of a strong three-dimensional topological insulator. Chapter 4 details the hidden spin texture found a high Tc superconductor. The strong spin polarization observed in such bulk centrosym- metric crystal with weak atomic SOC suggests the importance of local crystal symmetry which depends on the lattice distortion and atomic valency, which is addressed in Chapter 5. Finally, conclusions and future research directions are outlined in Chapter 6.