What Does the Josephson Effect Tell Us about the Superconducting State of the Cuprates?

The defining property of a superconductor is the stiffness of its phase. In conventional lowTc superconductors this stiffness is so large that in order to directly observe its finite value one has to create a region where it is locally suppressed, the so-called superconducting weak link. The resulting weak link region gives rise to a variety of Josephson effects (for a review, see [1]), in which the superconducting phase becomes a measurable quantity. In the cuprate superconductors the sensitivity of the Josephson effect to the phase has played a decisive role in establishing a phase-sensitive test of the (now well established) unconventional d-wave symmetry of their pairing state (for a review, see [2]). The purpose of this paper which is based on the results of our measurements of the current-phase relation in the cuprates [3, 4, 5, 6] is to argue that besides providing a phase-sensitive test of the pairing symmetry, a quantitative analysis of the Josephson effect in the cuprates provides new insights into the microscopic physics of the cuprates, in particular into the reduced phase stiffness of the cuprates. Our experiments were carried out on two types of Josephson junctions: [001] tilt grain boundary junctions [3, 4, 6] and c-axis junctions between a cuprate and a low-Tc superconductor [5]. Let us start with a brief discussion of these two types of junctions. 1. Grain boundary junctions have been studied since the very beginning of the cuprate