Measurement of bias voltage dependence of local barrier height at constant tip-sample separation

We measured the bias voltage dependence of the apparent local barrier height (LBH) by STM on an Au (1 1 1) surface with an Au tip. The results under two different conditions, a constant tip-sample separation and a constant current, were compared in the low bias voltage range (−30 to −5 mV). Under constant separation, the LBH does not change with the bias voltage, indicating that the LBH is independent of the bias voltage. Under constant current, the LBH increases with increasing absolute value of the bias voltage due to the increase of the tip-sample separation with increasing absolute bias voltage. We found that the LBH depends only on the separation in the low bias voltage range.