March 1st, 10:00 CET: Milan Allan (Leiden University, Netherlands)
Cooper pairing without superconductivity
The idea that preformed Cooper pairs could exist in a superconductor above its zero-resistance state has been explored for unconventional, interface, and disordered superconductors, yet direct experimental evidence is lacking. In this talk, I will introduce the use of scanning tunneling noise spectroscopy to unambiguously detect and quantify the number of Cooper pairs in a sample [1-3]. We show that preformed Cooper pairs exist up to temperatures much higher than the zero-resistance critical temperature Tc in the disordered superconductor titanium nitride, by observing a clear enhancement in the shot noise that is equivalent to a change of the effective charge from 1 to 2 electron charges [4]. We further show that spectroscopic gap fills up rather than closes when increasing temperature. Our results thus demonstrate the existence of a novel state above Tc that, much like an ordinary metal, has no (pseudo)gap, but carries charge via paired electrons.
[1] KM Bastiaans et al., RSI 89, 093709 (2018)
[2] KM Bastiaans, D. Cho et al., Nature Physics 14, 1183 (2018)
[3] KM Bastiaans et al., Phys. Rev. B 100, 104506 (2019)
[4] KM Bastiaans, et al. arXiv:2101.08535 (2021)