Quantum transport in the presence of a finite-range time-modulated potential

Quantum transport in a narrow constriction, and in the presence of a finite-range time-modulated potential, is studied. The potential is taken the form $V(x,t)=V_0\theta (x)\theta (a-x)\cos (\omega t)$, with $a$ the range of the potential and $x$ the transmission direction. As the chemical potential $\mu$ is increasing, the dc conductance $G$ is found to exhibit dip, or peak, structures when $\mu$ is at $n\mathrm{\hslash }\omega$ above the threshold energy of a subband. These structures in $G$ are found in both the small $a$ ($a \ll \lambda_{F}$)andthelarge$a$($a \gg \lambda_{F}$) regime. The dips, which are associated with the formation of quasi-bound states, are narrower for smaller $a$, and for smaller $V_0$. The locations of these dips are essentially fixed, with small shifts only in the case of large $V_0$. Our results can be reduced to the limiting case of a delta-profile oscillating potential when both $a$ and $V_{0}a$ are small. The assumed form of the time-modulated potential is expected to be realized in a gate-induced potential configuration.