Why scanning tunneling spectroscopy of Sr₂RuO₄ sometimes doesn't see the superconducting gap

Scanning tunneling spectroscopy (STS) and scanning tunneling microscopy (STM) are perhaps the most promising ways to detect the superconducting gap size and structure in the canonical unconventional superconductor Sr₂RuO₄ directly. However, in many cases [1-3], researchers have reported being unable to detect the gap at all in STM conductance measurements, while in others they were able to find the gap [4,5]. Recently, an investigation of this issue on various local topographic structures on a Sr-terminated surface found that superconducting spectra appeared only in the region of small nanoscale canyons [6,7], corresponding to the removal of one RuO surface layer. In this talk, we analyze the electronic structure of various possible surface structures using ab initio density functional theory (DFT), and argue that bulk conditions, favorable for superconductivity, can be achieved, when removal of the RuO layer suppresses the RuO₄ octahedral rotation locally. Our findings are supported by a paper recently published using numerical methods beyond DFT - random phase approximation (RPA) and functional renormalization group (FRG) [8]. We further propose alternative terminations to the most frequently reported Sr termination where superconductivity surfaces should be observed.