Ryan M. Hatcher

Microscopic current dynamics in nanoscale junctions

So far, transport properties of nanoscale contacts have been studied mostly within the static scattering approach. The electron dynamics and the transient behavior of current flow, however, remain poorly understood. We present a numerical study of microscopic current flow dynamics in nanoscale quantum point contacts. We employ an approach that combines a microcanonical picture of transport with time-dependent density-functional theory. We carry out atomic and jellium model calculations to show that the time evolution of the current flow exhibits several noteworthy features, such as nonlaminarity and edge flow. We attribute these features to the interaction of the electron fluid with the ionic lattice, to the existence of pressure gradients in the fluid, and to the transient dynamical formation of surface charges at the nanocontact-electrode interfaces. Our results suggest that quantum transport systems exhibit hydrodynamical characteristics, which resemble those of a classical liquid.

Quantum Mechanical Description of Displacement Damage Formation

Atomic-scale processes during displacement damage formation have been previously studied using molecular dynamics (MD) calculations and empirical potentials. Low-energy displacements (1 keV) are characterized by a high cross-section for producing secondary knock-on atoms and damage clusters, and determine the threshold displacement energy (an important parameter in NIEL calculations). Here we report first-principles, parameter-free quantum mechanical calculations of the dynamics of low-energy displacement damage events. We find that isolated defects formed by direct displacements result from damage events of les100 eV. For higher energy events, the initial defect profile, which subsequently undergoes thermal annealing to give rise to a final stable defect profile, is the result of the relaxation and recrystallization of an appreciable volume of significantly disordered and locally heated crystal surrounding the primary knock-on atom displacement trajectory.

Representing the thermal state in time-dependent density functional theory

Classical molecular dynamics (MD) provides a powerful and widely used approach to determining thermodynamic properties by integrating the classical equations of motion of a system of atoms. Time-Dependent Density Functional Theory (TDDFT) provides a powerful and increasingly useful approach to integrating the quantum equations of motion for a system of electrons. TDDFT efficiently captures the unitary evolution of a many-electron state by mapping the system into a fictitious non-interacting system. In analogy to MD, one could imagine obtaining the thermodynamic properties of an electronic system from a TDDFT simulation in which the electrons are excited from their ground state by a time-dependent potential and then allowed to evolve freely in time while statistical data are captured from periodic snapshots of the system. For a variety of systems (e.g., many metals), the electrons reach an effective state of internal equilibrium due to electron-electron interactions on a time scale that is short compared to electron-phonon equilibration. During the initial time-evolution of such systems following electronic excitation, electron-phonon interactions should be negligible, and therefore, TDDFT should successfully capture the internal thermalization of the electrons. However, it is unclear how TDDFT represents the resulting thermal state. In particular, the thermal state is usually represented in quantum statistical mechanics as a mixed state, while the occupations of the TDDFT wavefunctions are fixed by the initial state in TDDFT. We work to address this puzzle by (A) reformulating quantum statistical mechanics so that thermodynamic expectations can be obtained as an unweighted average over a set of many-body pure states and (B) constructing a family of non-interacting (single determinant) TDDFT states that approximate the required many-body states for the canonical ensemble.