Electronic Pseudogap from Fluctuations in Low Dimensional Excitonic and Superconducting Materials

Many metal-to-insulator and metal-to-superconductor transitions happen concurrently with an electronic energy gap opening and a broken symmetry. But there are electronic systems that develop energy gaps without any broken symmetry, most notably the "pseudogap" in cuprate superconductors. In this talk, 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 discuss fluctuation as an important factor when describing the properties of low dimensional material systems. Finally, I will discuss potential new directions in sensing and metrology enabled by low dimensional fluctuations.<br/><br/>[1] Phys Rev X 11, 031068 (2021); Nat Mater 22, 671 (2023); [2] Phys Rev Research 5, 043089 (2023); Nat Commun 14, 7512 (2023)