A VERSATILE SYSTEM FOR INELASTIC ELECTRON TUNNELLING SPECTROSCOPY

A spectrometer system for sensitive measurements of inelastic tunneling spectra is described. Modulation and bridge techniques are used to determine the current I, and the derivatives dI/dv and d**2I/dv**2, for a four-terminal tunneling junction subject to an applied bias potential v. An operational amplifier circuit sweeps the dc bias voltage across the sample while holding the superimposed ac modulation voltage constant, independent of conductance changes. The current I and dI/dv are determined using dc and lock-in voltmeters connected across a current-sensing resistor, while a bridge configuration permits low-noise measurements of the second derivative.