Abstract
The large variations in Tc across the cuprate families is one of the
major unsolved puzzles in condensed matter physics and is poorly
understood. Although there appears to be a great deal of universality
in the cuprates, several orders of magnitude changes in Tc can be
achieved through changes in the chemical composition and structure
of the unit cell. In this paper we formulate a systematic examination
of the variations in electron-phonon coupling to oxygen phonons in
the cuprates, incorporating a number of effects arising from several
aspects of chemical composition and doping across cuprate families.
It is argued that the electron-phonon coupling is a very sensitive
probe of the material-dependent variations in chemical structure,
affecting the orbital character of the band crossing the Fermi level,
the strength of local electric fields arising from structural-induced
symmetry breaking, doping-dependent changes in the underlying band
structure, and ionicity of the crystal governing the ability of the
material to screen c-axis perturbations. Using electrostatic Ewald
calculations and known experimental structural data, we establish
a connection between the material’s maximal Tc at optimal doping
and the strength of coupling to c-axis modes. We demonstrate that
materials with the largest coupling to the out-of-phase bond-buckling
(B1g) oxygen phonon branch also have the largest Tc’s. In light of
this observation we present model Tc calculations using a two-well
model where phonons work in conjunction with a dominant pairing interaction,
presumably due to spin fluctuations, indicating how phonons can generate
sizeable enhancements to Tc despite the relatively small coupling
strengths. Combined, these results can provide a natural framework
for understanding the doping and material dependence of Tc across
the cuprates.
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