Direct evidence for Cooper pairing without a spectral gap in a disordered superconductor above <i>T</i>_c

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 present scanning tunneling noise spectroscopy experiments [1,2] that unambiguously show that preformed Cooper pairs exist up to temperatures much higher than the zero-resistance critical temperature T_c 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 [3]. Our data further reveals that the spectroscopic gap fills up rather than closes when increasing temperature. Our results thus demonstrate the existence of a novel state above T_c that, much like an ordinary metal, has no (pseudo)gap, but carries charge via paired electrons. Finally, I will discuss the relation of our work to the pseudogap state of the cuprates.

[1] Amplifier for scanning tunneling microscopy at MHz frequencies, KM Bastiaans, et al. Rev. Sci. Instrum. 89, 093709 (2018)
[2] Imaging doubled shot noise in a Josephson scanning tunneling microscope, KM Bastiaans, et al., Phys. Rev. B 100, 104506 (2019)
[3] KM Bastiaans, et al. (2020)