Direct test of pairing fluctuations in the pseudogap phase of an underdoped cuprate

In underdoped cuprates, many experiments have provided evidence for the presence of a gap-like structure in the electronic excitations spectrum, in a region above the critical temperature and below a characteristic temperature T*. The origin of this so-called pseudogap is still hardly debated and the answer to this question turns out to be essential for the understanding of high-Tc superconductivity. One doesn't know if the pseudogap is related to superconductivity or to an order in competition. In the former case, it has been suggested that superconducting pairing fluctuations may be responsible for the partial suppression of electronic excitations. This remains to be tested experimentally, but most of the probes used to investigate the pseudogap are not sensitive to pairs and therefore cannot provide such a test. Here, we report for the first time on a direct test of pairing fluctuations in the pseudogap regime using a Josephson-like experiment. Our results shows that fluctuations survive only in a restricted range of temperature close to Tc (T-Tc$<$15K), and therefore cannot be responsible for the opening of the pseudogap at high temperature. Nature Physics 4, 608 - 611 (2008)