Anomaly of phonon state of superconducting ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ studied by inelastic neutron scattering

Inelastic neutron scattering measurements over a wide Q-E range have been performed on superconducting ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$. Fine structure in its phonon density of states as well as the Q dependence of excitations was revealed by means of a high-energy-resolution chopper spectrometer at a pulsed neutron source. The temperature dependence showed an anomalous behavior at ${\mathit{T}}_{\mathit{c}}$ similar to critical phenomena, which could be due to a local distortion and an expansion of the dynamical correlation length. The distortion is associated with a 〈110〉 buckling structure, which is commonly observed in the orthorhombic phase of ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$ ${\mathrm{Ba}}_{\mathit{x}}$ ${\mathrm{CuO}}_4$ and in ${\mathrm{Tl}}_2$ ${\mathrm{Ba}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_8$ in the superconducting state.