Combined use of a lock-in detector and a multichannel analyser for 1/f noise application to tunneling spectroscopy
Résumé
Obtaining a good signal-to-noise ratio (S/N) is of great importance in tunneling spectroscopy, as it has often led to the discovery of new phenomena. The quantities of interest are the derivatives of the tunnel junction I-V curve ; the usual approach to obtain these characteristics is to apply a modulation voltage around a D. C. bias and to detect the proper harmonic with a lock-in detector. In order to improve the S/N ratio, one would think of increasing the detector time-constant (and total sweep time), but experimentalists know that, above a few seconds, such an increase is illusive and even misleading. This is because there are low frequency instabilities in the equipment, but also because there is noise present in the junction characteristic itself. The combined use of a lock-in detector and a multichannel analyser is proposed, in order to average a large number (n) of short sweeps instead of using one single long sweep. The S/N ratio is calculated for such an experiment, and it is found that it depends strongly upon the noise spectral density. For a 1/f spectrum a large improvement (n) is obtained with respect to the usual method, for the same total experimental duration. A system for adapting a commercial multichannel to the plotting of junction characteristics is described, and experiments are performed that demonstrate the advantages of this method. In conclusion, it is emphasized that the limit of the possible improvement of the S/N ratio in tunneling spectroscopy is removed and that the system can be applied to many other types of spectroscopy.
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