Antiferromagnetism in High-Temperature Superconductors

Abstract

One of the universal features of copper-oxide compounds is the antiferromagnetic ordering of the copper spins in the CuO₂ planes. In stoichiometric compounds, the copper ions are in the state Cu²⁺ and have one hole with spin S = 1/2 in the 3d shell. A strong superexchange interaction (via oxygen ions) between hole spins at copper sites gives rise to a three-dimensional long-range antiferromagnetic order with relatively high Neel temperatures T _N = 300−500 K. Although the long-range order disappears in the metallic and superconducting phases, strong dynamical spin fluctuations with a wide spectrum of excitations are observed even at temperatures above 100 K. This fact has led to a number of hypotheses on possible electron pairing in copper-oxide compounds via magnetic degrees of freedom (Sect. 7.3). The study of the antiferromagnetic properties of high-temperature superconductors is thus important for checking hypotheses on magnetic mechanisms of superconductivity. The interaction of copper spins in a plane is of two-dimensional nature. Their small value, S = 1/2, is the reason behind the important quantum fluctuations. In this respect, besides exploring the interplay of antiferromagnetism and superconductivity, the study of quantum two-dimensional antiferromagnets is of great interest in its own right.