Question1
The overall gain of an STM controller is 10000x (equivalent to40dB, where dB=10log10) at low frequencies. If piezo resonance occurs at 10 kHz, what is utmost 3dB bandwidth of the amplifier (the frequency at which the gain of the amplifier has fallen by 3 dB from its DC gain)? (The gain falls 2x or -3bB per octave for a plain RC circuit like the integrator in an STM). To do this figure out how several octaves below 10 kHz one should go to get to the full gain, then double frequency to find the -3dB point (this is how the bandwidth of amplifiers is generally specified.)
Question2
Operated in constant height mode, the STM forms a current image by recording current as the probe is scanned over a surface. A probe, 2A above a gold surface, consists of a double tip, with a second atom 0.25A higher than lowest atom (which is the one 2A above the surface) gives a ‘ghost’ image displaced from key image. What is relative intensity of this ghost image?