Pairing fluctuation in the pseudogap phase of superconducting cuprates

The origin of the pseudogap phase and its relation with superconductivity in underoped cuprates is a hotly debated question in the community. For the first time we probed the pairing fluctuations with an experiment directly sensitive to the pairs. The existence of pairing fluctuations is revealed by measuring the current-voltage characteristics of a Josephson junction in which one side of the junction is the underdoped superconductor above its Tc, while the other side is an optimally doped superconductor well below its own Tc. To realize this experiment, we developed a new technique to make trilayer micro-junctions (underdoped YBaCuO/PrBaCuO/optimally doped YBaCuO). At low temperature, when both electrodes are in the superconducting state, a Josephson behavior is observed. In the intermediate regime, when one electrode is in the superconducting state and the other one in the pseudogap state, we observed a signature of pairing fluctuations in a temperature range of 15K above the transition temperature, which is much lower than the characteristic temperature of the pseudogap (250K). This result is in favor of a classical scenario where the pairing fluctuations are limited to the vicinity of the transition.

Figure 1 : a) Schematic view of a c axis YBa2Cu2.8Co0.2O7 (100 nm)/ PrBa2Cu2.8Ga0.2O7 (30 or 50 nm)/NdBa2Cu3O7 (200 nm) junction. b) Excess conductance Gex as a function of voltage (bottom axis) and corresponding frequency (top axis) at T-TcUD =6 K and 9 K. Solid lines correspond to experimental data and dashed lines to the theoretical calculation. Inset: Temperature dependence of the computed normalized excess conductance at V =0
(solid line) and experimental normalized excess conductance (circles)

References :

Probing pairing fluctuations in the pseudogap state of cuprates by Josephson effect
N. Bergeal, J. Lesueur, M. Aprili, G. Faini, B. Leridon and J-P. Contour , Nature Physics
4, 608 - 611 (2008)

Using ion irradiation de make High-Tc Josephson junctions, N. Bergeal, J. Lesueur, M. Sirena, M. Aprili, J-P. Contour and B. Leridon , J. Appl. Phys. 102, 083903 (2007).