Andreev Reflection and Related Studies in Low-Dimensional Superconducting Systems

Point- contact Andreev- reflection spectroscopy (PCARS) is one of the most direct techniques to study the number, amplitude, and symmetry (in the k space) of the energy gap(s) in superconductors. In the past 15 years, it has turned out to be a powerful and versatile alternative to more demanding and expensive techniques, for example scanning tunnel spectroscopy, for the study of unconventional multiband superconductors. Indeed, when high- quality single crystals are available, and if suitable models are used to analyze the data, PCARS can provide information on the detailed structure of the gap and on the shape of the Fermi surface. In short, PCARS consists in measuring the differential conductance of a contact between a normal metal and a superconductor, that must be ballistic to ensure a good energy resolution. In practice, point contacts are usually mesoscopic, as is the region of the superconductor probed by PCARS, because of the constraints posed by the electronic mean free path and the coherence length. However, PCARS has been seldom used to investigate “small” or more generally low- dimensional systems. In this chapter we will give a brief description of PCARS and analyze the limits of applicability of this technique to the study of low- dimensional superconductors. Then, we will review and discuss some examples of PCARS on quasi- 2D, quasi- 1D, and quasi- 0D systems reported in literature.