Direct evidence for Cooper pairing without a spectral gap in a disordered superconductor above Tc

• Published in: Science (American Association for the Advancement of Science) Vol. 374; no. 6567; pp. 608 - 611
• Main Authors: Bastiaans, Koen M, Chatzopoulos, Damianos, Jian-Feng, Ge, Cho, Doohee, Tromp, Willem O, van Ruitenbeek, Jan M, Fischer, Mark H, de Visser, Pieter J, Thoen, David J, Driessen, Eduard F C, Klapwijk, Teunis M, Allan, Milan P
• Format: Journal Article
• Language: English
• Published: Washington The American Association for the Advancement of Science 29.10.2021
• Subjects: Climate; Cooper pairs; Current carriers; Electrons; Noise; Noise measurement; Shot noise; Spectroscopy; Spectrum analysis; Superconductivity; Superconductors; Temperature; Titanium; Titanium nitride; Transition temperature
• ISSN: 0036-8075; 1095-9203
• DOI: 10.1126/science.abe3987

Measuring the effective chargeAt low enough temperatures, superconductors are capable of conducting electricity without any resistance because of the formation of so-called Cooper pairs of electrons. Cooper pairs typically form at the same critical temperature at which superconductivity sets in. In certain materials, they are thought to form above that temperature, but showing this property directly in an experiment is tricky. Bastiaans et al. used tunneling noise spectroscopy to measure the effective charge of current carriers in the disordered superconductor titanium nitride. As expected, below the critical temperature, the effective charge was equal to two electron charges. However, this behavior persisted above the critical temperature, indicating that electron pairs exist in that regime. —JSThe 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.