Direct evidence for Cooper pairing without a spectral gap in a disordered superconductor above Tc
Koen M. Bastiaans (Universiteit Leiden)
Damianos Chatzopoulos (Universiteit Leiden)
Jian Feng Ge (Universiteit Leiden)
Willem O. Tromp (Universiteit Leiden)
Jan M. van Ruitenbeek (Universiteit Leiden)
Pieter J. de Visser (SRON–Netherlands Institute for Space Research)
David J. Thoen (Kavli institute of nanoscience Delft, TU Delft - Tera-Hertz Sensing)
Eduard F.C. Driessen (Institut de Radioastronomie Millimétrique (IRAM))
Teunis M. Klapwijk (TU Delft - QN/Afdelingsbureau, Institute of Topological Materials, Kavli institute of nanoscience Delft, Julius-Maximilians-Universität Würzburg)
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Abstract
The idea that preformed Cooper pairs could exist in a superconductor at temperatures higher than its zero-resistance critical temperature (Tc) has been explored for unconventional, interfacial, and disordered superconductors, but direct experimental evidence is lacking. We used scanning tunneling noise spectroscopy to show that preformed Cooper pairs exist up to temperatures much higher than Tc in the disordered superconductor titanium nitride by observing an enhancement in the shot noise that is equivalent to a change of the effective charge from one to two electron charges. We further show that the spectroscopic gap fills up rather than closes with increasing temperature. Our results demonstrate the existence of a state above Tc that, much like an ordinary metal, has no (pseudo)gap but carries charge through paired electrons.