Temperature-dependent change of Cu—O bond length in Y${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$

X-ray photoelectron spectroscopy of the high-$T_c$ superconductor Y${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ is performed, varying the temperature of the measurement (350, 230, and 180 K). The intensity of the O $1s$ peak corresponding to the Cu—O bond decreased with decreasing temperature. The intensity of the peak due to the electrons in the Cu $3d-\mathrm{O} 2p$ antibonding orbitals with $x^2-y^2$ symmetry measured at 180 K is much smaller than that measured at 350 K. The top of the valence-band spectrum shifted -0.5 eV at 180 K relative to that at 350 K. A decrease of the O atoms in the crystal at lower temperature is observed in this experiment.