Lattice instability in high temperature superconducting cuprates probed by x-ray absorption spectroscopy

In contrast to conventionl superconductivity where phonons lead to the formation of Cooper pairs, in high-tempratures superconductivity (HTSC), the role of electron-phonon coupling has long been neglected. The in-plane Cu-O bonds in HTSC cuprates show unconventionalbroadening at low temperature as carier are doped. Here we focus on high-quality polarized x-ray absorption spectroscopy (XAS)data for a model HTSC system, (La,Sr)2CuO4 (LSCO). Thin film signal crystal samples were prepared by state-of-the-arts MBE, precisely controlling compositions. High quality data was obtained by use of a segmented x-ray detector. the inplane CuO radial distribution function (RDF)in LSCO (x = 0.15)shows broadening as temprature is lowered, which shows a sharp drop at the critical temprature which is followed by a gradual increase (disorder). Comparing the data with resistivity, we find a remarkeble coinsidence between the sharpening and the onset supercondectivity. Since the sharpening of RDF is interpreated as correleted motion of oxyzen atoms (phase coherence due to supercondectivity), the result demonstrate that the super conducting state directly relates to the unconventionl oxyzen displacements in a bond stretching mode. The result will be discussed in relation to the local models of distortion of the diffrent nature (metallic vs. insulating) that is strongly influnced by strain.