Strain-induced change in electronic structure of Cu(100)

Abstract

We have studied the electronic structure change of the Cu(100) surface due to the lattice strain both experimentally and theoretically. In the experiments, the surface lattice is compressed by partial nitrogen adsorption. Detailed measurements were made using angle-resolved photoemission spectroscopy with synchrotron and $\mathrm{He}\phantom{\rule{0.2em}{0ex}}I$ radiations. We mainly focused on surface states at the $d$-band top and bottom, and also $sp$ states (Shockley state) at $\overline{\mathrm{X}}$. The $d$-band bottom shifts toward higher binding energy, while the $d$-band top shifts toward the Fermi level. This is the direct evidence experimentally indicating the $d$-band broadening due to the lattice-constant reduction. The observation of the shift of the Shockley state beyond the Fermi level indicates the large electronic redistribution in the $sp$ band. The changes in the electronic structures in the experiments are in good agreement with the results by first-principles calculations. The directions of the energy shifts due to the lattice contraction are well understood by considering the symmetry of the corresponding wave functions at each point of the surface Brillouin zone. An increase of the work function due to the lattice contraction is also discussed in terms of the first-principles calculations. This also implies the special redistribution of $\mathrm{Cu}\phantom{\rule{0.2em}{0ex}}4sp$ electrons, which can significantly influence the chemical reaction on noble metals. Finally, the lattice-constant reduction is quantitatively estimated from the folding point shift of a surface state at the Brillouin-zone boundary.