= Constant-current scanning tunneling microscopy
{title2=$I(x,y,z)=I_0$}
Choose a fixed tip-sample <electric potential difference> and a positive target <electric current> magnitude $I_0$. At each lateral position $(x,y)$, <feedback control> adjusts $z$ until the measured <electric current> approaches $I_0$. Since increasing the gap reduces <quantum tunnelling>, excessive <electric current> calls for retraction; insufficient <electric current> calls for approach. The image is the calibrated vertical displacement $z(x,y)$, while the residual <electric current> error tests how closely the loop followed its target.
This is a contour of equal <electric current>, not necessarily a contour of equal separation. The <electronic contrast in scanning tunneling microscopy> model below shows why. It also explains the value of <vibration isolation>: an unintended displacement changes the same sensitive gap as the feature being measured. A scan must be slow enough for the finite <bandwidth> of <feedback control>; otherwise $z(x,y)$ lags the desired contour.
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