Quantum dynamics plays an extraordinarily important role at the nanoscale level. Dynamics is an unavoidable ingredient at the nanoscale, whether the movement is electronic, ionic, or a near-equilibrium fluctuation. Dynamical processes occur when one wishes to control the nanoscale, e.g., to avoid local failures of gate dielectrics, to manipulate structures by electronic excitation, or to use the spin degrees of freedom in quantum information processing.
To describe and understand any process at the nanoscale level the essential complement to equilibrium structural information is dynamics. The ultimate timescale for atomic rearrangements in chemical and material systems is known to be that of a single vibrational period (~100 fs). Many important phenomena have their origins in processes that occur on this timescale. To get an insight of the dynamics at the nanoscale one needs a well-developed general method for the atomic-level structural description of short-lived transient states. The object of this proposal is to study electron dynamics at the nanoscale using a time-dependent first-principles framework by coupling the Schrodinger and the Maxwell equations. This proposal addresses the theoretical and quantum simulation challenges of nanoscale dynamics. The research will focus on simulating nonlinear dynamics in nanoscale systems, such as response to femtosecond laser pulses, dynamics of Fermi degenerate electron beams, and laser excitation of coherent acoustic phonon modes. The theoretical description and computational simulation of these systems is challenging because for proper description of electron dynamics one should use time-dependent quantum mechanical approach with time-varying electromagnetic fields. In certain cases, the quantum nuclear dynamics also plays important role and one has to go beyond the Born-Oppenheimer approach. The goal of this proposal is to provide a rigorous atomistic quantum mechanical framework to simulate the time-dependent processes at the nanoscale.
We have studied the of electron and nuclear dynamics induced by strong laser pulses in the framework of the time-dependent density functional theory in real-time and real-space. Several prototypical examples was used to highlight the correlated electron and nuclear dynamics in strong fields, including Coulomb explosion of clusters, laser-enhanced field emission from nanostructures, and laser-assisted desorption of hydrogen from surfaces of silicon clusters and graphene flakes. Our simulations of the Coulomb explosion of hydrocarbon molecules have shown that the dissociation is a sudden, all-at-once, "concerted" fragmentation where the ionization step is followed by an explosive ejection of the charged fragments. The study of the dynamics of hydrogen desorption from H-terminated silicon surface clusters have demonstrated that by choosing an appropriate frequency and intensity of the laser it is possible to remove the hydrogen layer from the surface without destroying the structure of underlying silicon . The possibility of creation of short electron pulses by laser illumination of nanostructures was also explored . The results of the NSF grant have been published in 34 peer reviewed publications, 19 of these publications is the direct result of the project, while 15 papers is describing collaborative research on quantum mechanical calculations related to the project. The results have also been presented in 15 conference contributions. The grant supported a postdoctoral researcher, Dr S. Bubin, three graduate students, J. Driscoll (he received his Ph D in 2011 and currently a tenure track assistant professor at Bradley University), and V. Goncharov and A. Russakoff, who currently working on their PhD.
