Direct measurement of superconducting fluctuations in the underdoped cuprate phase

Numerous experiments have revealed a "pseudogap" in the spectrum of electronic excitations in superconducting cuprates, in the underdoped regime and below a critical temperature T* above Tc. The scientific community is still debating whether this is related to superconductivity or to a competing phase. In the former case, the emergence of uncondensed pairs could be responsible for the suppression of electronic excitations. Until now, the vast majority of results concerning the pseudogap have been obtained using experimental probes that are essentially sensitive to single-particle excitations. We have therefore set up a Josephson-type experiment to directly measure the presence of Cooper pairs above Tc. We show that superconducting fluctuations survive in a region limited to about 15 Kelvin above Tc, well below T*. This observation is hardly compatible with a scenario in which superconductivity is the origin of the pseudogap. More generally, we will discuss the role of fluctuations in the superconducting transition of cuprates. N. Bergeal et al, Nature Physics 4, 608 - 611 (2008).