Tunneling spectroscopy of the underdoped high-Tcsuperconductors

We study the tunneling conductance of underdoped cuprate superconductors to investigate how the two energy scales, superconducting and pseudogaps, manifest themselves in tunneling below the superconducting transition temperature $T_c.$ We consider two phenomenological models of the pseudogap state: (a) the case where the pseudogap is a simple suppression of the density of states from an unspecified origin and (b) the case where it is due to the short-range antiferromagnetic correlation. Case (b) seems unsatisfactory in comparison with the experimental observations, but (a) gives a good description of the recent c-axis tunneling experiment by Krasnov et al. [Phys. Rev. Lett. 84, 5860 (2000)].