Thesis (Ph. D.)--University of Texas at Austin, 2001. / Vita. Includes bibliographical references. Available also from UMI/Dissertation Abstracts International.

Thesis--M. Phil., University of Hong Kong, 1978. / Typewritten.

Thesis (Ph. D.)--University of Texas at Austin, 2001. / Vita. Includes bibliographical references (leaves 190-216). Available also in a digital version from Dissertation Abstracts.

Within the exact framework established recently, which is a successful marriage between the time dependent density functional theory for open electronic system and quantum dissipation theory formulated in the hierarchical equations of motion, an entirely new scheme is proposed in this thesis to simulate the time-dependent quantum transport in nano-devices at both zero and finite temperature equally without relying on the pole structure of the Fermi distribution function. Neither does it depend on any non-unique parametrization of the line-width matrix, hence, this new practical approach can be integrated with the first principles simulations seamlessly. Beyond the exact framework, a reliable method which works under the Wide- Band-Limit approximation at zero temperature is also developed. At the price of loss of some non-Markovian memory effects on the dynamics, a set of equations of motion which terminates at the first tier instead of the second tier is obtained. Benefiting from the latest advancement of numerical analysis, a hybrid fourth-order Runge-Kutta algorithm is proposed to solve this set of equations of motion which comprises stiff ones. Based on this result, an alternative scheme is considered to deal with the same approximation at finite temperature. As an illustration of these new approaches, the transient current of the one dimensional tight-binding periodical chain with and without a single impurity, driven by some time alternating and/or static bias voltages, are investigated. The influence of temperature and switch-on rate of bias voltage is exemplified. Particularly, in the one dimensional tight-binding chain with a single impurity which breaks its perfect periodicity, an asymmetry between the left and right transient current is found. Comparison between the results under the Wide-Band-Limit approximation and those with the exact description is carried out. / published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

The time-dependent quantum transport theory has attracted a great deal of interest in the past decade. However, some concepts in the frequency-dependent transport theory remain confused. Based on the Keldysh non-equilibrium green’s function formalism for time-dependent quantum transport, new expressions for dynamic current and admittance are derived in this thesis, which satisfy gauge invariance and current continuity. The key concepts in this field are clarified. The derivation is under wideband limit (WBL) and first order approximations. This new formalism is validated by first-principle time-dependent calculations of three carbon-based nano-devices. Later a study on asymmetric systems is carried out by this new theory, and discussion on current conversation problem is presented. This new ac quantum transport theory can cooperate with electromagnetic method to solve a mesoscopic problem. The active core part which is usually under atomistic scale is simulated by frequency-domain quantum transport theory, while the broad environment is tackled with electromagnetic solver. By a careful treatment at the interface between two solvers, this quantum mechanic / electromagnetic (QM/EM) method is implemented self-consistently. This QM/EM method is also validated by calculations of the transient current through a carbon-nanotube based device surrounded by silicon environment under a small ac bias voltage. The small signal and WBL approximations are also adopted in the development of this method. As a supplementary to the family of QM/EM methods (static and time-dependent QM/EM method are already established), this method shows very good efficient and high accuracy. Beyond linear response in frequency domain, we have also studied some nonlinear effects. As one application of nonlinear effect, memristor has attracted great attention in the past few years. Through Fourier analysis method, we have now understood the physical and mathematical mechanisms of ideal memristor, memcapacitor and meminductor. We have proposed methods to verify ideal memristor, memcapacitor and memdicutor, and to find out their intrinsic parameters which can be employed to predict their behavior under various input signals. This study may also provide an instruction on experimental research. / published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

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