Differential conductance of a saddle-point constriction with a time-modulated gate-voltage

This work investigates how a time-modulated gate-voltage influences the differential conductance G of a saddle-point constriction. The constriction is modeled by a symmetric saddle-point potential and the time-modulated gate-voltage is represented by a potential of the form » 0 #(a/2!Dx!x # D) cos(ut). For +u less than half of the transverse subband energy level spacing, gate-voltage-assisted (suppressed) feature occurs when the chemical potential k is less (greater) than but close to the threshold energy of a subband. Our results indicate that as k increases, G exhibits, alternatively, the assisted and the suppressed feature. For a larger +u, these two features may overlap. In addition, dip structures are found in the suppressed regime, and mini-steps are found in the assisted regime only when the gate-voltage covers a region sufficiently distant from the center of the constriction. ( 1998 Elsevier Science B.V. All rights reserved. PACS: 72.10.!d; 72.40.#w