Why the cuprate superconductors are unusual: an ionic approach

Abstract

A new mobile charge carrier in hole-doped high Tc cuprates is proposed. The configuration is based upon electrostatic considerations which show that a doped hole binds to an exciton and forms a linear, charge one singlet. In an ionic representation this exciton- hole (eh) particle is represented as O-1Cu+1O-1. The eh particle lives mostly in one dimension, being confined to either [10] rows and [01] columns of Cu-O ions in the CuO2 layer. This model provides a simple qualitative understanding of the transport behavior reported in the literature for (La,Sr)2CuO4 and a natural explanation for the two relaxation rates suggested by the Hall data. It further offers a model for why coherence in two dimensions requires d-wave symmetry. However additional pairing interactions in the intervening layers between the CuO2 layers are required to explain why the Tcs found in some cuprates are enhanced beyond what is expected by comparison with other cuprates. It is postulated that these interactions are based on eh partictles which occur in double chains of the 248 cuprates, and in negative U center ions in the charge reservoir layers of all the other cuprates with enhanced Tc > 100 K. We are lead to speculate that all cuprates with Tc > 100 K have pairing interactions off as well as on the CuO2 layers. The model suggests a much more enhanced Tc would be achieved with proper doping if the CuO2 layers could be modified by the addition of Cu to form Cu2O2 layers.