The phase-sensitive c-axis twist experiments on cuprate superconductors

The phase-sensitive c-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 temperature Tc superconductors Bi₂Sr₂CaCu₂O8+δ (Bi2212) and Bi₂Sr₂CuO6+δ (Bi2201) are reviewed. The primary OP candidates are the conventional s-wave OP and the unconventional dx²-y²-wave OP. Although major portions of the oxygen-doping phase diagrams for these compounds are complicated by the so-called pseudogap (PG) that usually arises below Tₚ > 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 their TCDW >> 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). However, some workers assumed that the PG in the cuprates is due to a SC OP, and c-axis twist experiments in the PG region were claimed to provide support for a dx²-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 superconducting OP. In the small SC-only region in the overdoped region of the Bi2212 phase diagram, the c-axis twist experiments provide strong support for the conclusion that the orbital symmetry of the superconducting OP in both Bi2212 and Bi2201 contains at least a substantial s-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 Bi2212 surface also provides evidence of an s-wave OP in that material.