Lattice Effects in High-Temperature Superconducting Cuprates Revealed by Thin Film Single Crystals

In contrast to conventional superconductivity where phonons lead to the formation of Cooper pairs, in high-temperature superconductivity (HTSC), the role of electron–phonon coupling has long been neglected. The in-plane Cu–O bonds in HTSC cuprates show unconventional broadening at low temperature as carriers are doped. Here, we focus on the high-quality polarized X-ray absorption spectroscopy (XAS) data for a model HTSC system, (La,Sr)2CuO4 (LSCO). Thin film single crystal samples were prepared by state-of-the-art MBE, precisely controlling compositions. High-quality data was obtained by use of a segmented X-ray detector. The in-plane Cu–O radial distribution function (RDF) in LSCO (x=0.15) shows broadening as temperature is lowered, which shows a sharp drop at the critical temperature, which is followed by a gradual increase (disorder). Comparing the data with resistivity, we find a remarkable coincidence between the sharpening and the onset of superconductivity. Since the sharpening of RDF is interpreted as correlated motion of oxygen atoms (phase coherence due to superconductivity), the results demonstrate that the superconducting state directly relates to the unconventional oxygen displacements in a bond stretching mode. The results will be discussed in relation to local models of distortion of the different nature (metallic vs. insulating), that is, strongly influenced by strain.