Microwave-frequency alternating current scanning tunneling microscopy by difference frequency detection: Atomic resolution imaging on graphite

We present a detailed description of an experimental setup for alternating current scanning tunneling microscopy, in which two slightly detuned high f requency signals are mixed at the tunneling junction and the resulting diff erence frequency signal is amplified using conventional scanning tunneling microscope electronics. This signal is used to control the distance between the microscope tip and the sample. With graphite as a model surface atomic resolution images have been obtained. It is demonstrated that the origin o f the generated signal on graphite is the nonlinearity of the static curren t-voltage characteristics. (C) 1999 American Institute of Physics. [S0034-6 748(99)03608-4].