The phase-sensitivec-axis twist experiments on cuprate superconductors

The phase-sensitivec-axis twist experiments and the corresponding theoretical interpretations designed to test the orbital symmetry of the superconducting (SC) order parameter (OP) on the hole-doped high transition temperatureTcsuperconductors Bi2Sr2CaCu2O8+δ(Bi2212) and Bi2Sr2CuO6+δ(Bi2201) are reviewed. The primary OP candidates are the conventionals-wave OP of the standard Bardeen-Cooper-Schrieffer theory based upon the generally long-range electron pairing mediated by the electron-phonon interaction, and the unconventionaldx2-y2-wave OP that has been proposed to arise from some short-range attractive electronic pairing mechanism stronger than the Coulomb repulsion. Although major portions of the oxygen-doping phase diagrams for these compounds are complicated by the so-called pseudogap (PG) that usually arises belowTp>Tc, a large variety of hole-doped layered superconductors, such as the transition metal dichalcogenides, are well known to exhibit charge-density waves (CDWs) forming below theirTCDW>>Tc. These CDWs can be either incommensurate or commensurate with the underlying crystalline lattice, and have rather recently been observed to be present in underdoped Bi2212 and YBa2Cu3O7-δ(YBCO). Experimental evidence for the presence of an incoherent CDW in the PG region of the phase diagram of Bi2212 is provided. Some workers assumed that the PG in the cuprates is due to a SC OP, andc-axis twist experiments in the PG region claimed to provide support for adx2-y2-wave OP. However in Bi2212, there is a small SC-only region in the overdoped portion of the phase diagram where no PG has been observed, so that twist experiments in that overdoped region are only sensitive to the SC OP. In the small SC-only region in the overdoped region of the Bi2212 phase diagram, thec-axis twist experiments provide strong support for the conclusion that the orbital symmetry of the SC OP in both Bi2212 and Bi2201 contains at least a substantials-wave component, in agreement with scanning tunneling experiments on the electron-doped cuprate Sr1-xNdxCuO2, which does not exhibit any PG or CDW. A scanning tunneling experiment over a substantial portion of the freshly vacuum-cleaved surface of Bi2212 and the narrow linewidths of the THz emissions from three circular disk mesas of Bi2212 also provide evidence for ans-wave SC OP in that material.