The nuclear quadrupole interaction in high temperature superconductors

Abstract Since their discovery in 1986 [11], the high temperature superconducting (HTS) copper oxides have presented a continuing challenge to both experiment and theory. The identification of the underlying mechanism (or mechanisms) responsible for their superconductivity remains an unanswered question. Numerous theories have been proposed ranging from phonon-mediated pairing of the charge carriers, similar to the Bardeen–Cooper–Schrieffer (BCS) [2] theory developed for conventional low-temperature superconductors, to novel concepts independent of phonons [3–-l0]. For conventional superconductors the variation of the transition temperature Tc , with isotopic mass M (from BCS theory Tc ∼ M−a ) was an important verification of the contribution of electron-phonon interactions to electron pairing. Measurements of this effect of HTS cuprates resulted in isotope shifts much smaller than predicted by theory [ll-14], raising doubts about the role of phonons. However, Barbee [15] argued that the size of the isoto...