DIFFERENTIAL CONDUCTANCE OF A SADDLE-POINT CONSTRICTION WITH A TIME-MODULATED GATE-VOLTAGE

This work investigates how a time-modulated gate-voltage influences th e differential conductance G of a saddle-point constriction. The const riction is modeled by a symmetric saddle-point potential and the time- modulated gate-voltage is represented by a potential of the form V(0)T heta(a/2 - \x - x(c)\) cos(omega t). For h omega less than half of the transverse subband energy level spacing, gate-voltage-assisted (suppr essed) feature occurs when the chemical potential mu is less (greater) than but close to the threshold energy of a subband. Our results indi cate that as mu increases, G exhibits, alternatively, the assisted and the suppressed feature. For a larger h omega, these two features may overlap. In addition, dip structures are found in the suppressed regim e, and mini-steps are found in the assisted regime only when the gate- voltage covers a region sufficiently distant from the center of the co nstriction. (C) 1998 Elsevier Science B.V. All rights reserved.