ScholarBank@NUShttps://scholarbank.nus.edu.sgThe DSpace digital repository system captures, stores, indexes, preserves, and distributes digital research material.Wed, 28 Sep 2022 01:21:52 GMT2022-09-28T01:21:52Z5051- Time-dependent quantum transport and power-law decay of the transient current in a nano-relay and nano-oscillatorhttps://scholarbank.nus.edu.sg/handle/10635/57671Title: Time-dependent quantum transport and power-law decay of the transient current in a nano-relay and nano-oscillator
Authors: Cuansing, E.C.; Liang, G.
Abstract: Time-dependent nonequilibrium Green's functions are used to study electron transport properties in a device consisting of two linear chain leads and a time-dependent interlead coupling that is switched on non-adiabatically. We derive a numerically exact expression for the particle current and examine its characteristics as it evolves in time from the transient regime to the long-time steady-state regime. We find that just after switch-on, the current initially overshoots the expected long-time steady-state value, oscillates and decays as a power law, and eventually settles to a steady-state value consistent with the value calculated using the Landauer formula. The power-law parameters depend on the values of the applied bias voltage, the strength of the couplings, and the speed of the switch-on. In particular, the oscillating transient current decays away longer for lower bias voltages. Furthermore, the power-law decay nature of the current suggests an equivalent series resistor-inductor-capacitor circuit wherein all of the components have time-dependent properties. Such dynamical resistive, inductive, and capacitive influences are generic in nano-circuits where dynamical switches are incorporated. We also examine the characteristics of the dynamical current in a nano-oscillator modeled by introducing a sinusoidally modulated interlead coupling between the two leads. We find that the current does not strictly follow the sinusoidal form of the coupling. In particular, the maximum current does not occur during times when the leads are exactly aligned. Instead, the times when the maximum current occurs depend on the values of the bias potential, nearest-neighbor coupling, and the interlead coupling. © 2011 American Institute of Physics.
Sat, 15 Oct 2011 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/576712011-10-15T00:00:00Z
- Role of the on-site pinning potential in establishing quasi-steady-state conditions of heat transport in finite quantum systemshttps://scholarbank.nus.edu.sg/handle/10635/51030Title: Role of the on-site pinning potential in establishing quasi-steady-state conditions of heat transport in finite quantum systems
Authors: Cuansing, E.C.; Li, H.; Wang, J.-S.
Abstract: We study the transport of energy in a finite linear harmonic chain by solving the Heisenberg equation of motion, as well as by using nonequilibrium Green's functions to verify our results. The initial state of the system consists of two separate and finite linear chains that are in their respective equilibriums at different temperatures. The chains are then abruptly attached to form a composite chain. The time evolution of the current from just after switch-on to the transient regime and then to later times is determined numerically. We expect the current to approach a steady-state value at later times. Surprisingly, this is possible only if a nonzero quadratic on-site pinning potential is applied to each particle in the chain. If there is no on-site potential a recurrent phenomenon appears when the time scale is longer than the traveling time of sound to make a round trip from the midpoint to a chain edge and then back. Analytic expressions for the transient and steady-state currents are derived to further elucidate the role of the on-site potential. © 2012 American Physical Society.
Mon, 24 Sep 2012 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/510302012-09-24T00:00:00Z
- Quantum transport in honeycomb lattice ribbons with armchair and zigzag edges coupled to semi-infinite linear chain leadshttps://scholarbank.nus.edu.sg/handle/10635/97712Title: Quantum transport in honeycomb lattice ribbons with armchair and zigzag edges coupled to semi-infinite linear chain leads
Authors: Cuansing, E.; Wang, J.-S.
Abstract: We study quantum transport in honeycomb lattice ribbons with either armchair or zigzag edges. The ribbons are coupled to semi-infinite linear chains serving as the input and output leads and we use a tight-binding Hamiltonian with nearest-neighbor hops. The input and output leads are coupled to the ribbons through bar contacts. In narrow ribbons we find transmission gaps for both types of edges. The appearance of this gap is due to the enhanced quantum interference coming from the multiple channels in bar contacts. The center of the gap is at the middle of the band in ribbons with armchair edges. This particle-hole symmetry is because bar contacts do not mix the two sublattices of the underlying bipartite honeycomb lattice when the ribbon has armchair edges. In ribbons with zigzag edges the gap center is displaced to the right of the band center. This breakdown of particle-hole symmetry is the result of bar contacts now mixing the two sublattices. We also find transmission oscillations and resonances within the transmitting region of the band for both types of edges. Extending the length of a ribbon does not affect the width of the transmission gap, as long as the ribbon's length is longer than a critical value when the gap can form. Increasing the width of the ribbon, however, changes the width of the gap. In ribbons with zigzag edges the gap width systematically shrinks as the width of the ribbon is increased. In ribbons with armchair edges the gap is not well-defined because of the appearance of transmission resonances. We also find only evanescent waves within the gap and both evanescent and propagating waves in the transmitting regions. © 2009 EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg.
Mon, 01 Jun 2009 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/977122009-06-01T00:00:00Z
- Transient behavior of heat transport in a thermal switchhttps://scholarbank.nus.edu.sg/handle/10635/98432Title: Transient behavior of heat transport in a thermal switch
Authors: Cuansing, E.C.; Wang, J.-S.
Abstract: We study the time-dependent transport of heat in a nanoscale thermal switch. The switch consists of left and right leads that are initially uncoupled. During switch on the coupling between the leads is abruptly turned on. We use the nonequilibrium Green's function formalism and numerically solve the constructed Dyson equation to determine the nonperturbative heat current. At the transient regime we find that the current initially flows simultaneously into both of the leads and then afterwards oscillates between flowing into and out of the leads. At later times the oscillations decay away and the current settles into flowing from the hotter to the colder lead. We find the transient behavior to be influenced by the extra energy added during switch on. Such a transient behavior also exists even when there is no temperature difference between the leads. The current at the long-time limit approaches the steady-state value independently calculated from the Landauer formula. © 2010 The American Physical Society.
Fri, 05 Feb 2010 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/984322010-02-05T00:00:00Z
- Tunable heat pump by modulating the coupling to the leadshttps://scholarbank.nus.edu.sg/handle/10635/98459Title: Tunable heat pump by modulating the coupling to the leads
Authors: Cuansing, E.C.; Wang, J.-S.
Abstract: We follow the nonequilibrium Green's function formalism to study time-dependent thermal transport in a linear chain system consisting of two semi-infinite leads connected together by a coupling that is harmonically modulated in time. The modulation is driven by an external agent that can absorb and emit energy. We determine the energy current flowing out of the leads exactly by solving numerically the Dyson equation for the contour-ordered Green's function. The amplitude of the modulated coupling is of the same order as the interparticle coupling within each lead. When the leads have the same temperature, our numerical results show that modulating the coupling between the leads may direct energy to either flow into the leads simultaneously or flow out of the leads simultaneously, depending on the values of the driving frequency and temperature. A special combination of values of the driving frequency and temperature exists wherein no net energy flows into or out of the leads, even for long times. When one of the leads is warmer than the other, net energy flows out of the warmer lead. For the cooler lead, however, the direction of the energy current flow depends on the values of the driving frequency and temperature. In addition, we find transient effects to become more pronounced for higher values of the driving frequency. © 2010 The American Physical Society.
Mon, 16 Aug 2010 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/984592010-08-16T00:00:00Z