This choice prevents the feedback control from compensating for the modulation by changing the tip-sample spacing and minimizes the displacement current 90?out-of-phase with the applied bias modulation.
2.
A small, high frequency sinusoidal modulation voltage is superimposed on the A . C . component of the tunneling current is recorded using a lock-in amplifier, and the component in-phase with the tip-sample bias modulation gives dI / dV directly.
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