Finite-temperature properties of doped antiferromagnets

We review recent results for the properties of doped antiferromagnets, obta ined by the numerical analysis of the planar t-J model using the novel fini te-temperature Lanczos method for small correlated systems. First we briefl y summarize our present understanding of anomalous normal-state properties of cuprates, and present the electronic phase diagram, phenomenological sce narios and models proposed in this connection. The numerical method is then described in more detail. The following sections are devoted to various st atic and dynamical properties of the t-J model. Among the thermodynamic pro perties the chemical potential, entropy and the specific heat are evaluated . Discussing electrical properties the emphasis is on the planar optical co nductivity and the d.c. resistivity. Magnetic properties involve the static and dynamical spin structure factor, as measured via the susceptibility me asurements, the NMR relaxation and the neutron scattering, as well as the o rbital current contribution. The analysis of electron spectral functions, s tudied by photoemission experiments, and their relation to the c-axis condu ctivity, follows. Finally we discuss density fluctuations, the electronic R aman scattering and the thermoelectric power. Whenever feasible a compariso n with experimental data is performed. A general conclusion is that the r-J model captures well the main features of anomalous normal-state properties of cuprates, for a number of quantities the agreement is even a quantitati ve one. It is shown that several dynamical quantities exhibit at intermedia te doping a novel universal behaviour consistent with a marginal Fermi-liqu id concept, which seems to emerge from a large degeneracy of states and a f rustration induced by doping the antiferromagnet.