The Nuclear Quadrupole Interaction in High Temperature Superconductors

摘要

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#x2013;Cooper#x2013;Schrieffer (BCS) 2 theory developed for conventional low-temperature superconductors, to novel concepts independent of phonons 3#x2013;-l0. For conventional superconductors the variation of the transition temperatureTc, with isotopic massM(from BCS theoryTc#x223C;M#x2212;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 isotope shift is not a unique indicator of phonon-mediated pairing. Since the HTS materials contain Cu ions with partially filled 3dshells, many of the alternative theories of HTS have focused on magnetic interactions and associated spin fluctuations 3#x2013;10. The reader is referred to Ref. 16 for the details of other theories that have been proposed and to the article by Schrieffer and Anderson 17) for an overview discussion of the theory of high temperature superconductivity.