Scanning tunneling microscopy (STM) methods for detecting ESR of a single spin center

The STM (Scanning Tunneling Microscope) is a new revolutionary technique which was invented approximately 15 years ago in IBM Zürich Laboratories by Binnig et al. (1982, 1983). Since this invention the STM has been found to be an extremely useful technique for many applications. The fundamental characteristic of quantum mechanical tunneling of electrons which is used by the STM is the extreme dependence of the tunneling probability and current on the tunneling barrier width. This phenomenon has been measured many times in bulk tunneling junctions. Such junctions are constructed from two conducting electrodes (c — in Fig. la) separated by a thin, uniform insulating layer (i — in Fig. la). When the thickness (d — in Fig. la) of the insulating layer is reduced below a certain value (≈ 100 Å) tunneling of electrons becomes possible, and as a result the conductivity of the junction increases from zero to a certain value. The conductivity of the junction was found to be exponentially dependent on the thickness of the insulating layer. Decreasing this thickness by 1 Å increases the conductivity by an order of magnitude.