Comment on "Distinction of Electron Dispersion in Time-Resolved Photoemission Spectroscopy"

TL;DR

This paper critically examines a recent theory on photoemission delay, arguing that its core assumptions about photoelectron phase and effective mass are invalid for real solids, thus questioning the theory's applicability.

Contribution

It provides an ab initio analysis showing the inapplicability of the phase and effective mass assumptions in real crystalline materials, challenging the validity of the proposed interferometric method.

Findings

The assumptions of well-defined phase velocity are invalid in real solids.

Effective mass concept does not hold for photoelectrons in crystals.

The proposed method is not applicable to real solid-state systems.

Abstract

In a recent paper [Phys. Rev. Lett. 125, 043201 (2020)] (Ref.1) Liao et al. propose a theory of the interferometric photoemission delay based on the concepts of the photoelectron phase and photoelectron effective mass. The present comment discusses the applicability and limitations of the proposed approach based on an ab initio analysis supported by vast literature. Two central assumptions of the paper are questioned, namely that the photoelectron can be characterized by a phase (have a well-defined phase velocity), and that it can always be ascribed an effective mass Theories based on these concepts are concluded to be inapplicable to real solids, which is illustrated by the example of the system addressed in Ref. 1. That the basic assumptions of the theory are never fulfilled in nature discredits the underlying idea of the "time-domain interferometric solid-state energy-momentum-dispersion imaging method" suggested in Ref. 1. Apart from providing a necessary caution to experimentalists, the present comment also gives an insight into the photoelectron wave function and points out problems and pitfalls inherent in modeling real crystals.