High-temperature superconductivity in copper-based materials, with critical temperature well above what was anticipated by the BCS theory, remains a major unsolved physics problem more than 30 years after its discovery. The problem is fascinating because it is simultaneously simple - being a single band and ½ spin system, yet extremely rich - d-wave superconductivity, pseudogap, spin and charge orders, and strange metal. The resulting electronic phase diagram consists of intertwined states with unusual properties. As such, the cuprates remain as the most important model system for correlated electrons.

Angle-resolved photoemission spectroscopy (ARPES) has emerged as the leading experimental tool to understand the electronic structure of these states and their relationships [1]. In this talk, I will describe our recent progress on the subjects. With the instrumentation improvements, ARPES measurements in the normal state with unprecedented precision became possible in hole-doped cuprates Bi_2 Sr_2 Ca Cu_2 O_{8+δ}. Our study revealed that the incoherent strange metal abruptly reconstructs into a more conventional metal with quasiparticles across the putative critical doping around 19%, defining a temperature-independent vertical phase boundary [2], providing insights into the nature of the pseudogap, strange metal, and putative quantum critical point. In the overdoped regime above such critical doping, an unconventional signature of Tc in the spectral weight was revealed, making direct semi quantitative connection between ARPES spectra and thermodynamic property [3]. More recently, our ARPES studies were expanded to the electron-doped cuprates Nd_{2–x} Ce_x Cu O_4, leading to the observation of Bogoliubov quasiparticle peaks in this system, revealing how the superconductivity emerges under the influence of antiferromagnetic correlations [4]. Time permits, I will also show our results from oxide MBE grown thin films. I will discuss the benchmarking of 1D Hubbard model and the necessity of an additional attractive term to describe the doped 1D cuprate chains Ba_{2-x} Sr_x Cu O_{3+δ [5]. In thin-film La_{2-x} Sr_x Cu O_4, the Lifshitz transition around 19% doping was revealed, and its impact on specific heat and the phase diagram will be discussed [6].

[1] M. Hashimoto, I. M. Vishik, R.-H. He, T. P. Devereaux, Z.-X. Shen, Nat Phys 10, 483 (2014)
[2] S. Chen et al. Science 366, 6469 (2019)
[3] S. Chen et al., Nature 601, 562 (2022)
[4] K. Xu et al., Nature Phys, in press
[5] Z. Chen et al., Science 373, 6560 (2021)
[6] Y. Zhong et al., PNAS 119, e2204630119 (2022)