When low- and high-energy electronic responses meet in cuprate superconductors

Abstract

The existence of coherent quasiparticles near the Fermi energy in the low-temperature state of high-temperature superconductors has been well established by angle-resolved photoemission spectroscopy (ARPES). We present a study of ${\mathrm{La}}_{1.83}{\mathrm{Sr}}_{0.17}\mathrm{Cu}{\mathrm{O}}_{4}$ in the superconducting state and report an abrupt change in the quasiparticle spectral function, as we follow the dispersion of the ARPES signal from the Fermi energy to $0.6\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. The interruption in the quasiparticle dispersion separates coherent quasiparticle peaks at low energies from broad incoherent excitations at high energies. We find that the boundary between these low-energy and high-energy features exhibits a cosine-shaped momentum dependence, reminiscent of the superconducting $d$-wave gap. Further intriguing similarities between characteristics of the incoherent excitations and quasiparticle properties suggest a close relation between the electronic response at high and low energies in cuprate superconductors.