Slope discontinuity and fluctuation of lattice expansion near ${\mathit{T}}_{\mathit{c}}$ in untwinned ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ single crystals

The thermal lattice expansion of untwinned ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ single crystals is measured from 10 to 300 K with a high-resolution x-ray-diffraction technique on beam-line X22B at the National Synchrotron Light Source. All the unit-cell parameters (a, b, c, and volume) show unexpectedly large slope discontinuities near ${\mathit{T}}_{\mathit{c}}$ and possible strong fluctuations over a wide temperature region (80<T<120 K). Possible implications for the nature of the superconducting mechanism are discussed.