Tunneling spectroscopy: A probe for high-Tc superconductivity

After the pioneering work of Giaever [Phys. Rev. Lett. 5 (1960) 147. [1]] and Shapiro in the 1960s, tunneling spectroscopy has become an important method for studying different properties of superconductors, such as the superconducting gap and the density of states. At the interface with a superconductor there is a variety of tunneling processes depending on barrier properties and the type of materials in both sides of the junction. Among other phenomena, we have single electron (Giaever) tunneling, cooper pairs (Josephson) tunneling [B.D. Josephson, Phys. Rev. Lett. 1 (1962) 251. [17]] and mixed (Andreev-Saint-James reflections [A.F. Andreev, Zh. Eksp. Teor. Fiz. 46 (1964) 1823, Saint-James D., J. Phys 25 (1964) 899]) tunneling. In high-Tc superconductors these phenomena are enriched by the anisotropy of the energy gap, which adds new features, like the so-called 'Andreev-Saint-James bound states', which originate the zero bias conductance peak. These peculiarities add to this technique the capability of studying the symmetry of the superconducting gap and the normal state pseudogap. In this report we present a brief review of tunneling spectroscopy applied to high-Tc superconductors and some results about the symmetry of the superconducting gap, supporting the hypothesis of a quantum critical point, which may be at the origin of the superconductive transition.