THEORETICAL-STUDY OF CU-O CHAIN FRAGMENTS IN THE OXYGEN-DEFICIENT PLANES OF YBA2CU3O6+X .2. INTRACHAIN INTERACTIONS AND THE CHAIN FRAGMENTSLENGTH DISTRIBUTION

We study some models of the strongly correlated Cu-O chain fragments t hat form on the oxygen-deficient planes of YBa2Cu3O6+x. We use analyti cal and numerical methods to investigate the stability of the chain fr agments as a function of their size. We discuss also the size and x de pendence of the O-O intrachain interactions that enter in the phenomen ological descriptions of the CuOx planes. Only the fermionic degrees o f freedom of the chain fragments are considered.