Pseudogap in a crystalline insulator doped by disordered metals*

The electronic structure of a crystalline solid can be described based on the periodic order of constituent atoms, but little is known about how it changes in the presence of disorder. Back in the 1960s, theoretical models for the band structure of disordered systems, such as a liquid or a glassy solid, was formulated. [1] However, the key feature of theoretical models, the back-bending dispersion and pseudogap, has remained unobserved experimentally. In this talk, we will report the discovery of such features in black phosphorus whose surface is decorated by alkali-metal dopants (Na, K, Rb, Cs) with a liquid-like spatial distribution. Then, the doped electrons in black phosphorus are subject to multiple scattering by the effect of ionized dopants, modifying the band structure to that of liquid metals. Using angle-resolved photoemission spectroscopy, we observed the back-bending dispersion and pseudogap as a consequence of wavenumber renormalizations in the presence of short-range order. [1]