The effect of zinc as a nonmagnetic impurity onhigh-temperature superconducting cuprates

Abstract

In this work, the influence of nonmagnetic impurities in the form of zinc ions on the superconducting state of lanthanum strontium copper oxide (LSCO) will be studied. The superconducting system is described by the spin-fermion Hubbard model and its superconductivity characterised by the critical temperature. The replacement of copper by zinc atoms changes the superconducting coupling and the kinetic energy of the itinerant holes. This dependence on the zinc concentrations is found from a fit to experimental results and explained in the picture of an excluded area around each zinc impurity. Lanthanum copper oxide consists of layers of copper-oxygen planes and charge reservoirs. Superconductivity arises when sufficiently many lanthanum ions are replaced by strontium ions in the charge reservoirs, adding itinerant holes to the copper-oxygen planes. These itinerant holes on the oxygen ions and the original localized holes on the copper sites are described by the spin-fermion Hubbard model, which consists of a Hubbard Hamiltonian for the itinerant holes, an antiferromagnetic Heisenberg Hamiltonian for the localized holes and a Kondo interaction Hamiltonian between the two types of holes. As a result of the replacement of copper with zinc, the critical temperature of the superconducting phase is reduced until the superconducting state vanishes completely at the critical zinc concentration. The zinc dependence enters the critical temperature via the superconducting coupling, the critical coupling and the system’s typical energy scale, all of which depend in the same manner on the zinc doping and the critical zinc concentration.