Point-contact spectroscopy on tunable constrictions in GaAs

In quasi-one-dimensional channels formed in the two-dimensional electron gas of an ${\mathrm{Al}}_{\mathit{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As/GaAs heterostructure, the second derivative ${\mathit{d}}^2$V/${\mathit{dI}}^2$ of the voltage-current characteristic is measured at low temperatures as a function of the voltage drop V over the channel length. In increasing the lateral confinement, strong resonant structures appear at voltages V that correspond to energies of GaAs phonon density-of-states maxima for acoustical as well as optical phonons. Compared with metal point-contact spectroscopy, this technique offers reproducible tunability of buried point contacts.