Van Hove singularities, chemical pressure and phonons: an angle-resolved photoemission study of KFe${}_2$As${}_2$ and CsFe${}_2$As${}_2$

Abstract

We report an angle-resolved photoemission spectroscopy (ARPES) study of KFe${}_2$As${}_2$ and CsFe${}_2$As${}_2$, revealing the existence of a van Hove singularity affecting the electronic properties. As a result of chemical pressure, we find a stronger three-dimensionality in KFe${}_2$As${}_2$ than in CsFe${}_2$As${}_2$, notably for the 3$d_{z^2}$ states responsible for the small three-dimensional hole-like Fermi surface pocket reported by quantum oscillations. Supported by first-principles calculations, our ARPES study shows that the van Hove singularity previously reported in KFe${}_2$As${}_2$ moves closer to the Fermi level under negative chemical pressure. This observation, which suggests that the large density-of-states accompanying the van Hove singularity contributes to the large Sommerfeld coefficient reported for the AFe${}_2$As${}_2$ (A = K, Rb, Cs) series, is also consistent with the evolution of the inelastic scattering revealed by transport under external pressure, thus offering a possible interpretation for the origin of the apparent change in the superconducting order parameter under pressure. We find that the coherent spectral weight decreases exponentially upon increasing temperature with a characteristic temperature $T^{\ast }$. We speculate how the low-energy location of the van Hove singularity and the presence of a low-energy peak in the phonon density-of-states can relate to the high-temperature crossover observed in various electronic and thermodynamic quantities.

11 pages, 7 figures. New title. Version significantly extended by the addition of a section about the high-temperature crossover