We have developed a broadly tunable microwave‐frequency alternating current scanning tunneling microscope which employs a cavity with no resonances in the frequency range used. This results in a low, flat background for spectroscopic measurements with this instrument.

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These S21 (transmission) traces are sampled at 25 MHz intervals with an inherent bandwidth of less that 10 Hz, so that many high Q modes may not be observed.
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The term Sji is a scattering parameter (Ref. 14) which is the ratio of the signal measured at port j to that applied at port i̇Ssi is a complex quantity with both magnitude and phase components. We use the term, Sji, loosely in the text to mean the magnitude, |Sji|. Hence S11 represents that ratio of reflected to incident power (the power reflection coefficient), while S21 is the ratio of the transmitted to the incident power (the power transmission coefficient).
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This approach was first taken by M. G. Yoshikawa Youngquist, J. H. .Ferris, and P. S. Weiss. It has the additional advantage that the coarse approach centers the STM probe tip on small samples.
36.
We note that due to the well-defined ground plane (see Sec. II C) the ACSTM works nearly as well even with incomplete electromagnetic enclosure. We will use this configuration in ultrahigh vacuum where full enclosure by a liquid metal seal would not be possible.
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