Transient dynamics of charge transport in a nanoswitch and a nanojunction with a time-varying gate potential
Eduardo C. Cuansing1*
1Institute of Mathematical Sciences and Physics, University of the Philippines, Los Banos, Laguna, Philippines
* presenting author:Eduardo Cuansing, email:eccuansing@up.edu.ph
We study the transport of charges in quantum devices we call the nanoswitch and the nanojunction. In the nanoswitch, two linear chains are coupled by a connection that varies in time. A potential source is attached to the linear chains, driving a time-dependent current through the chains whenever the connection between the left and right chains is established. To make the connection vary in time, we place the right chain on a rotatable disk so that the maximum value of the coupling, i.e., a switch-on, occurs whenever the left and right chains are aligned. The value of the coupling can be modulated as the right chain is rotated away from the horizontal. We determine the current through the nanoswitch using the fully time-dependent nonequilibrium Green’s functions techniques. We first construct the Hamiltonian of the linear chains and their time-varying coupling using the tight-binding method. Using the Hamiltonian, the current is then calculated from the expectation value of the rate of change of the number of charges within each chain. This expectation value is expressed in terms of nonequilibrium Green’s functions, whose values are determined via the Keldysh formalism wherein the time axis is extended into the complex plane, a Dyson equation is derived via a perturbation calculation, and then the time variable is returned back to the real-time axis using Langreth’s theorem and analytic continuation. For an abrupt step-like switch-on and a slowly increasing switch-on, we find that the time evolution of the current, including during the transient regime, can be fitted to a power-law. The long-time behavior of the dynamical current approaches a steady-state value. We also study the dynamics of charge transport in a nanojunction consisting of a central channel in contact with source and drain leads which are, in turn, attached to a bias potential. A gate, which is not in direct contact with the channel, exerts a time-varying gate potential on the channel. The value of the current through the channel changes as the gate potential varies in time. To determine the current through the nanojunction, we again make use of the techniques from fully time-dependent nonequilibrium Green’s functions. In addition, we calculate the current not just through the source or the drain but also across sites within the channel. It is therefore possible to determine how the influence of a change in the gate potential to the current propagates across the channel. As a special case, we also study how the device behaves if there is no bias potential but a time-varying gate potential is impinging on the channel. We find that a momentary transient current appears whenever the gate potential changes. It is therefore possible to propagate a current through the device by continually changing the gate potential. We also find that the reaction of the device to a change in the gate potential is not instantaneous.


Keywords: Nanodevices, Quantum Transport, Nonequilibrium Green's Functions, Many-Body Physics, Nonequilibrium Systems