The probability I + 12 of finding a conduction electron a distance t outside a metal even at very low temperatures is not zero, but is governed by the metal's work function C#I through an exponential law I #I 1 2 N exp ( -Z K t ) , with K 2: (2m/h2)1/2 C#IW. This basic consequence of quantum mechanics makes possible measurable electrical currents between two conductors separated by a sufficiently thin uniform ( -20 A) insulator.The energy-spectroscopic information that can be, and has been, derived from careful measurement of the current-voltage (I-V) relation in such experiments is the subject of the present review.
I n probably the most important application of the technique Giaever in 1960demonstrated that when one member of the junction is superconducting, the differential conductance d1jdV as a function of bias energy eV directly measures the density of quasiparticle excitations of the superconductor. Such data are shown to permit the most complete characterisation of the strong coupling superconducting state that is presently available, and have contributed in a most important way to our present understanding of superconductivity and superconductors.