  We performed temperature- and doping-dependent high-resolution Raman
spectroscopy experiments on YBa$_2$Cu$_3$O$_{7-\delta}$ to study $B$$_{\rm 1g}$
phonons. The temperature dependence of the real part of the phonon self-energy
shows a distinct kink at $T=T_{\rm B1g}$ above $T$$_{\rm c}$ due to softening,
in addition to the one due to the onset of the superconductivity. $T$$_{\rm
B1g}$ is clearly different from the pseudogap temperature with a maximum in the
underdoped region. The region between $T$$_{\rm B1g}$ and $T$$_{\rm c}$
resembles that of superconducting fluctuation or charge density wave order.
While the true origin of the $B$$_{\rm 1g}$ phonon softening is not known, we
can attribute it to a gap on the Fermi surface due to an electronic order. Our
results may reveal the role of the $B$$_{\rm 1g}$ phonon not only in the
superconducting state but also in the intertwined orders in multilayer copper
oxide high-$T$$_{\rm c}$ superconductors.
