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Quantum Material, as exemplified by unconventional superconductors and topological insulators, is a fascinating and rapidly developing field of modern physics. High-temperature superconductivity in cupper and iron based materials, with critical temperature well above what was anticipated by the BCS, remains a major unsolved physics problem today. The challenge of this problem is symbolized by a complex phase diagram consists of intertwined states with extreme properties in addition to unconventional superconductivity. None of them can be described by conventional theory, thus compounding the difficulty to understand high-temperature superconductivity itself as these states are different manifestations of the same underlying physical system, making an integrated understanding a necessity.

Angle-resolved photoemission spectroscopy (ARPES), derived from Einstein’s formulation of photoelectric effect, has emerged as a leading experimental tool to push the frontier of this important field of modern physics. Over the last three decades, the improved resolution and carefully matched experiments have been the keys to turn this technique into a sophisticated many-body physics tool. As a result, ARPES played a critical role in setting the intellectual agenda by testing new ideas and discovering surprises. ARPES has impacted both the field of unconventional superconductors and topological phases of matter.




In this talk, we discuss ARPES evidence for a general theme of high temperature superconductivity - cooperative enhancement and positive feedback loop of different interactions exemplified by electron-electron and electron-phonon interactions. The accumulated evidence comes from an expanded version of angle-resolved photoemission spectroscopy and its match to in-situ material synthesis. In such experiments, the precision measurements of electron’s energy, momentum and time dynamics provide evidence for cooperative interactions as a pathway to increase the superconducting transition temperature. An outlook for ARPES development and application for other quantum materials will also be discussed.

2-D electron gases residing on the crystallographic surfaces of atomically flat and ultra-clean metal surfaces have been studied in detail by measurements of the energy bands in momentum as well as in real space. For instance, on various Cu and Au surfaces, the energy vs. wavevector relationship, including the effects of (Rashba) spin-orbit interaction, have been measured by ARPES. On the other hand, the local density of states on unperturbed facets and on surface positions close to atomic defects has been studied in detail by scanning tunnelling microscopy and spectroscopy.
More recently, the surface state electrons have been moulded into artificial molecules and 2-D lattices created by manipulation of ad-atoms and CO molecules on metallic surfaces in an STM. Here, manipulation in an STM is used to position ad-atoms or CO molecules on the metal surface in order to form arrays or repulsive scatters that force the surface electrons into certain patterns. In this way, arrays of artificial atomic sites can be engineered, as well as the quantum coupling between these sites in a given lattice. In such a way, artificial lattices form a nearly ideal platform to study the relationship between the lattice geometry and electronic band structure of 2-D systems. This includes the study of Dirac bands and topological electronic phases emerging from the lattice geometry and the nature of the scatters1-4. I will discuss the progress obtained in this field in recent years by us and other groups. I will also show the relevance of this work for real materials with a Dirac or topological band structure5,6.