Low-energy quasiparticles in high- T c cuprates

The aim of this review is to summarize existing experimental investigations on the nature of the low-energy quasiparticle excitations in high- T c cuprates, and to examine critically recent claims of consistency between the experimentally determined low-temperature thermodynamic and transport properties in certain cuprates and theoretical predictions based on standard perturbation theory for a BCS d-wave superconductor. Measurements of the low-temperature specific heat, thermal conductivity, microwave conductivity, penetration depth and scanning tunnelling microscopy are described, both in the Meissner state and in the mixed state. These results are then compared with the predictions of quasi-classical theory of a d-wave BCS superconductor and the self-consistent T-matrix approximation for both a single impurity and a finite impurity concentration. Detailed inspection reveals that significant discrepancies still exist between experiment and theory, with important implications for the development of a coherent model, perhaps beyond standard perturbation theory. Finally, I discuss how considerations of the possible effects of band structure, anisotropic scattering and low carrier concentration in the underdoped region of the cuprate phase diagram might reconcile some of the discrepancies that have emerged.