Pairing fluctuations in the pseudogap state of copper-oxide superconductors probed by the Josephson effect

The phase diagram of high-temperature superconductors is still to be understood(1). In the low-carrier-doping regime, a loss of spectral weight in the electronic excitation spectrum-the so-called pseudogap-is observed above the superconducting temperature T c, and below a characteristic temperature T* (ref.2). First observed in the spin channel by NMR measurements, the pseudogap has also been observed in the charge channel by scanning probe microscopy and photoemission experiments, for instance(2). An important issue to address is whether this phenomenon is related to superconductivity or to a competing 'hidden' order. In the superconductivity case, it has been suggested that superconducting pairing fluctuations may be responsible, but this view remains to be tested experimentally. Here, we have designed a Josephson-like experiment to probe directly the fluctuating pairs in the normal state. We show that fluctuations survive only in a restricted range of temperature above T c, well below T*, and therefore cannot explain the opening of the pseudogap at higher temperature.