Anisotropic photoelectron emission delay in two-dimensional atomic arrangements

In two-dimensional semiconductors, the density fluctuation potential created by a screening of the photohole is intrinsically anisotropic because of the infinitely periodic planar arrangements of atoms. Modeling the anisotropic-localized screening with the cylindrical geometry, we investigate the emission delay of the photoelectron from 2s and 2pz states of graphene in an angle-resolved mode of the attosecond streaking by solving the time-dependent Schrödinger equation. Strong angle dependencies in absolute emission delays of 2s and 2pz states are obtained, from which the effects of the infrared-induced continuum transition are ruled out and those of the anisotropic screening could be solely extracted. Consequently, the anisotropic scattering induces photoelectrons to be emitted with substantial negative delays within small angles (i.e., θ≲θc) from the normal direction to the atomic arrangement, that is, a conical electron emission in a very early stage of the photoemission.