Scale free distribution of oxygen interstitials wires in optimum doped HgBa$_2$CuO$_{4+y}$

Abstract

Novel nanoscale probes are opening new venues for understanding unconventional electronic and magnetic functionalities driven by multiscale lattice complexity in doped high temperature superconducting perovskites. In this work we focus on the multiscale texture at supramolecular level of atomic oxygen interstitials (O-i) stripes in HgBa$_2$CuO$_{4+y}$ at optimal doping for the highest superconducting critical temperature $T_C$=94K. We report compelling evidence for the nematic phase of oxygen-interstitial O-i atomic wires with fractal-like spatial distribution over multiple scales by using scanning micro and nano X-ray-diffraction. The scale free distribution of O-i atomic wires at optimum doping extending from micron scale down to nanoscale has been associated with the intricate filamentary network of hole rich metallic wires in the CuO$_2$ plane. The observed critical opalescence provides evidence for the proximity to a critical point controlling the emergence of high temperature superconductivity at optimum doping