Most metal-to-insulator and metal-to-superconductor transitions are so dramatic that certain symmetries are also concurrently broken, and an energy gap opens during this process. But there are electronic systems that develop energy gaps without any broken symmetry, most notably the "pseudogap" in strongly correlated cuprate superconductors. I will show two examples of electronic pseudogap in unexpected places: the heavily hole-doped cuprates [1], and an excitonic insulator candidate Ta2NiSe5 [2]. The former is supposedly a good metal where mean-field BCS is thought to apply, and the latter is a structural symmetry-breaking system with strong electron-phonon coupling. Via angle-resolved photoemission spectroscopy and x-ray scattering, we show the electronic gap to persist well above the transition temperature in both systems. With insights from controlled numerical calculations, we show that fluctuation is an important factor when describing the properties of low dimensional material systems. Finally, I will discuss a few new directions in the study of fluctuations.
[1] Phys Rev X 11, 031068 (2021); Nat Mater 22, 671 (2023); [2] Phys Rev Research 5, 043089 (2023); Nature Communications 14, 7512 (2023)