Electron tunnelling spectroscopy

A basic consequence of quantum mechanics makes possible measurable electrical currents between two conductors separated by a sufficiently thin uniform ( approximately 20 AA) 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. Giaever in 1960 demonstrated that when one member of the junction is superconducting, the differential conductance dI/dV as a function of bias energy eV directly measures the density of quasiparticle excitations of the superconductor. A second type of energy spectroscopy, that of inelastic excitation thresholds observed in the second derivative d²I/dV ², was demonstrated initially by Esaki (1958) in phonon spectroscopy of semiconductor tunnel diodes. Finally, the present state of the field is assessed and some guesses are hazarded as to those areas most likely to lead to important future advances.