Possibility of a resonating-valence-bond state in the high-$T_c$ superconductor ${\mathrm{Bi}}_4$${\mathrm{Ca}}_3$${\mathrm{Sr}}_3$${\mathrm{Cu}}_4$${\mathrm{O}}_{16+\mathit{x}}$

We compared the photoemission edge line shape of ${\mathrm{Bi}}_4$ ${\mathrm{Ca}}_3$ ${\mathrm{Sr}}_3$ ${\mathrm{Cu}}_4$ ${\mathrm{O}}_{16+\mathit{x}}$ with those of the Au and Al Fermi edges. Specifically, we searched for the intensity decrease in the neighborhood of the Fermi level, predicted in the case of photoemission from a resonating-valence-bond (RVB) state. We found no positive evidence for this decrease and we estimated that, if the RVB state exists, then the upper limit for the maximum spinon energy is 65 meV. Our data analysis also indicates that the mere observation of an apparent spectral edge is not sufficient per se to rule out a RVB state.