Renormalization of f-levels away from the Fermi energy in electron excitation spectroscopies: Density functional results of Nd$_{2-x}$Ce$_x$CuO$_4$

TL;DR

This study uses density functional theory to calculate relaxation energies in Nd$_{2-x}$Ce$_x$CuO$_4$, revealing that f-electron bands are shifted away from the Fermi energy in spectroscopic measurements.

Contribution

It demonstrates that f-electron levels are significantly renormalized away from the Fermi energy due to relaxation effects, explaining experimental observations.

Findings

f-electron relaxation energies are several eV

f-bands appear away from $E_F$ in spectra

Observation of f electrons at $E_F$ is challenging

Abstract

Relaxation energies for photoemission, when an occupied electronic state is excited, and for inverse photoemission, when an empty state is filled, are calculated within the density functional theory with application to Nd$_{2-x}$Ce$_x$CuO$_4$. The associated relaxation energies are obtained by computing differences in total energies between the ground state and an excited state in which one hole or electron is added into the system. The relaxation energies of f-electrons are found to be of the order of several eV's, indicating that f-bands will appear substantially away from the Fermi energy ($E_F$) in their spectroscopic images, even if these bands lie near $E_F$. Our analysis explains why it would be difficult to observe f electrons at the $E_F$ even in the absence of strong electronic correlations.