By modulating the scanning tunneling microscope junction bias voltage at microwave frequencies, imaging and spectroscopy of insulating surfaces have become possible. In order to explore the spectroscopic capabilities of this instrument, we have developed a tunable microwave frequency alternating current scanning tunneling microscope. We combine the reliable beetle‐style sample approach with coaxial sample and tip contacts. This provides us with a stable microwave‐frequency‐compatible scanning tunneling microscope. This alternating current scanning tunneling microscope design is compatible with ultrahigh vacuum and low‐temperature operation.

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In place of a dc voltage source, we use a microwave signal source with 1 Hz resolution [Hewlett-Packard 83620A/008 (Hewlett-Packard Co., Palo Alto, CA)] attached via microwave coaxial cable to either the ACSTM tip or a conducting substrate behind the sample. The signal at the modulation frequency is detected using a microwave network analyzer [Hewlett-Packard 8510C/8514B (Hewlett-Packard Co., Palo Alto, CA)]. The background “pickup” signal which made measurements at the fundamental difficult in the experiments of Kochanski (Ref. 1) can be blanked using this scheme. In order to record the amplitudes at the harmonics of the modulation frequency or to use a particular (i.e., third) harmonic for feedback control of the tip height, we use a microwave spectrum analyzer [Hewlett-Packard 71210C (Hewlett-Packard, Palo Alto, CA)]. Both source and detection electronics have narrow bandwidths, making impedance matching less critical than in the scheme described in Ref. 3.
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