A universal high energy anomaly in angle resolved photoemission spectra   of high temperature superconductors - possible evidence of spinon and holon   branches
  A universal high energy anomaly in the single particle spectral function is reported in three different families of high temperature superconductors by using angle-resolved photoemission spectroscopy. As we follow the dispersing peak of the spectral function from the Fermi energy to the valence band complex, we find dispersion anomalies marked by two distinctive high energy scales, E_1=~ 0.38 eV and E_2=~0.8 eV. E_1 marks the energy above which the dispersion splits into two branches. One is a continuation of the near parabolic dispersion, albeit with reduced spectral weight, and reaches the bottom of the band at the gamma point at ~0.5 eV. The other is given by a peak in the momentum space, nearly independent of energy between E_1 and E_2. Above E_2, a band-like dispersion re-emerges. We conjecture that these two energies mark the disintegration of the low energy quasiparticles into a spinon and holon branch in the high T_c cuprates. 
J. Graf, G.-H. Gweon, K. McElroy, S. Y. Zhou, C. Jozwiak, E.   Rotenberg, A. Bill, T. Sasagawa, H. Eisaki, S. Uchida, H. Tak...