Van Hove singularities, chemical pressure and phonons: an angle-resolved photoemission study of KFeAs and CsFeAs
arXiv:1807.00193
Abstract
We report an angle-resolved photoemission spectroscopy (ARPES) study of KFeAs and CsFeAs, 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 KFeAs than in CsFeAs, notably for the 3 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 KFeAs 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 AFeAs (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 . 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.
arXiv comment11 pages, 7 figures. New title. Version significantly extended by the addition of a section about the high-temperature crossover
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