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Mark T. Lusk

Publications and source records attributed to Mark T. Lusk.

At least 37 records · Page 2Linked to original sources
arXiv2019

Identification and Resolution of Unphysical Multielectron Excitations in the Real-Time Time-Dependent Kohn-Sham Formulation

We resolve a fundamental issue associated with the conventional Kohn-Sham formulation of real-time time-dependent density functional theory. We show that unphysical multielectron excitations, generated during time propagation of the Kohn-Sham equations due to fixation of the total number of Kohn-Sham orbitals and their occupations, result in incorrect electron density and, therefore, wrong predictions of physical properties. A new formulation is proposed in that the number of Kohn-Sham orbitals and their occupations are updated on the fly, the unphysical multielectron excitations are removed, and the correct electron density is determined. The correctness of the new formulation is demonstrated by simulations of Rabi oscillation, as analytical results are available for comparison in the case of noninteracting electrons.

physics.comp-ph↗
arXiv2019

Entangled Excitons via Spontaneous Downconversion

A class of centrosymmetric molecules support excitons with a well-defined quasi-angular momentum. Cofacial arrangements of these molecules can be engineered so that quantum cutting produces a pair of excitons with angular momenta that are maximally entangled. The Bell state constituents can subsequently travel in opposite directions down molecular chains as ballistic wave packets. This is a direct excitonic analog to the entangled polarization states produced by the spontaneous parametric downconversion of light. As in optical settings, the ability to produce Bell states should enable foundational experiments and technologies based on non-local excitonic quantum correlation. The idea is elucidated with a combination of quantum electrodynamics theory and numerical simulation.

quant-ph↗
arXiv2019

Exciton Interferometry

An exciton beam splitter is designed and computationally implemented, offering the prospect of excitonic interferometry. Exciton interaction between propagation conduits is modeled using a coupling parameter that varies with position. In practice, this variation can be realized by a change in the distance separating conduits as would occur if they crossed at oblique angles. Two such excitonic beam splitters can be combined to comprise an excitonic analog to a Mach-Zehnder interferometer, allowing the relative phase shift between two signals to be used to tailor the output populations on each channel. In contrast to optical splitters, an excitonic signal can be coherently split among more than two channels. These ideas are computationally demonstrated within an idealized setting in which each site is idealized as a two-level system. Physical implementations include molecular and coupled cavity settings as well as combinations of these. This adds to the developing inventory of excitonic analogs to optical elements.

quant-ph↗
arXiv2017

Angular momentum transport with twisted exciton wave packets

A chain of cofacial molecules with CN or CNh symmetry supports excitonic states with a screw-like structure. These can be quantified with the combination of an axial wavenumber and an azimuthal winding number. Combinations of these states can be used to construct excitonic wave packets that spiral down the chain with well-determined linear and angular momenta. These twisted exciton wave packets can be created and annihilated using laser pulses, and their angular momentum can be optically modified during transit. This allows for the creation of opto-excitonic circuits in which information, encoded in the angular momentum of light, is converted into excitonic wave packets that can be manipulated, transported, and then re-emitted. A tight-binding paradigm is used to demonstrate the key ideas. The approach is then extended to quantify the evolution of twisted exciton wave packets in a many-body, multi-level time-domain density functional theory setting. In both settings, numerical methods are developed that allow the site-to-site transfer of angular momentum to be quantified.

cond-mat.str-el↗
arXiv2017

Twisted Molecular Excitons as Mediators for Changing the Angular Momentum of Light

Molecules with C_N or C_Nh symmetry can absorb quanta of optical angular momentum to generate twisted excitons with well-defined quasi-angular momenta of their own. Angular momentum is conserved in such interactions at the level of a paraxial approximation for the light beam. A sequence of absorption events can thus be used to create a range of excitonic angular momenta. Subsequent decay can produce radiation with a single angular momentum equal to that accumulated. Such molecules can thus be viewed as mediators for changing the angular momentum of light. This sidesteps the need to exploit nonlinear light-matter interactions based on higher-order susceptibilities. A tight-binding paradigm is used to verify angular momentum conservation and demonstrate how it can be exploited to change the angular momentum of light. The approach is then extended to a time-dependent density functional theory setting where the key results are shown to hold in a many-body, multi-level setting.

physics.atm-clus↗
arXiv2017

Engineering and Manipulating Exciton Wave Packets

When a semiconductor absorbs light, the resulting electron-hole superposition amounts to a uncontrolled quantum ripple that eventually degenerates into diffusion. If the conformation of these excitonic superpositions could be engineered, though, they would constitute a new means of transporting information and energy. We show that properly designed laser pulses can be used to create such excitonic wave packets. They can be formed with a prescribed speed, direction and spectral make-up that allows them to be selectively passed, rejected or even dissociated using superlattices. Their coherence also provides a handle for manipulation using active, external controls. Energy and information can be conveniently processed and subsequently removed at a distant site by reversing the original procedure to produce a stimulated emission. The ability to create, manage and remove structured excitons comprises the foundation for opto-excitonic circuits with application to a wide range of quantum information, energy and light-flow technologies. The paradigm is demonstrated using both Tight-Binding and Time-Domain Density Functional Theory simulations.

cond-mat.mes-hall↗
arXiv2017

Improved Energy Pooling Efficiency Through Inhibited Spontaneous Emission

The radiative lifetime of molecules or atoms can be increased by placing them within a tuned conductive cavity that inhibits spontaneous emission. This was examined as a possible means of enhancing three-body, singlet-based upconversion, known as energy pooling. Achieving efficient upconversion of light has potential applications in the fields of photovoltaics, biofuels, and medicine. The affect of the photonically constrained environment on pooling efficiency was quantified using a kinetic model populated with data from molecular quantum electrodynamics, perturbation theory, and ab initio calculations. This model was applied to a system with fluorescein donors and a hexabenzocoronene acceptor. Placing the molecules within a conducting cavity was found to increase the efficiency of energy pooling by increasing both the donor lifetime and the acceptor emission rate--i.e. a combination of inhibited spontaneous emission and the Purcell effect. A model system with a free-space pooling efficiency of 23% was found to have an efficiency of 47% in a rectangular cavity.

physics.atm-clus↗
arXiv2016

The Centroid Shifts of Light Beams Reflected from Multi-Layers and the Effects of Angular Momentum Manifestations

Laguerre-Gaussian (LG) beams reflected from a multi-layered dielectric experience a shift in their centroid that is different than that from a single interface. This has been previously investigated for linearly polarized beams and, to a much lesser extent, beams with spin angular momentum. Here a combination of perturbation and computational analyses is used to provide a unified quantification of these shifts in layered dielectrics with two parallel interfaces. The approach is then extended to consider the qualitatively new behavior that results when the light is endowed with an intrinsic orbital angular momentum--i.e. vortex beams. Destructive interference causes singular lateral shifts in the centroid of the reflected vortex beam for which spin alone produces only a mild modulation. In the case of total internal reflection, both spin and intrinsic orbital angular momentum contribute to an enhancement of these lateral shifts as the interlayer thickness is decreased. This is just the opposite of the trend associated with longitudinal shifts. Two geometries are considered: air/glass/air and glass/air/glass multi-layers. A commonly available glass is used to show that vortex beams can result in centroid shifts on the order of microns for beams with a significant reflection coefficient.

physics.optics↗
arXiv2016

Electric Dipole Coupling in Optical Cavities and Its Implications for Energy Transfer, Upconversion and Pooling

Resonant energy transfer, energy transfer upconversion, and energy pooling are considered within optical cavities to elucidate the relationship between exciton dynamics and donor/acceptor separation distance. This is accomplished using perturbation theory to derive analytic expressions for the electric dipole coupling tensors of perfect planar and rectangular channel reflectors---directly related to a number of important energy transfer processes. In the near field, the separation dependence along the cavity axis is not influenced by the cavity and is essentially the same as for three-dimensional, free space. This is in sharp contrast to the reduced sensitivity to separation found in idealized low-dimensional settings. The cavity dynamics only correspond to their reduced dimensional counterparts in the far field where such excitonic processes are not typically of interest. There is an intermediate regime, though, where sufficiently small cavities cause a substantial decrease in separation sensitivity that results in one component of the dipole-dipole coupling tensor being much larger than those of free space.

quant-ph↗
arXiv2016

Confinement of Vibrational Modes in Nanocrystalline Silicon

It is possible to confine vibrational modes to silicon nanocrystals by encapsulating them within hydrogenated amorphous silicon. This is not because of the small impedance mismatch between materials but, rather, is due to higher order moments in the distribution of density and stiffness in the amorphous phase--i.e. it is a result of material substructure. The concept is elucidated using an idealized one-dimensional setting and then demonstrated for a realistic nanocrystalline geometry. Beyond the immediate focus on amorphous encapsulation, this offers the prospect of specifically engineering higher order property distributions as an alternate means of managing phonons. The approach could be applied, for instance, to design deterministically ordered materials which exploit this means of control.

cond-mat.mes-hall↗
arXiv2015

Control of Exciton Transport using Quantum Interference

It is shown that quantum interference can be employed to create an exciton transistor. An applied potential gates the quasi-particle motion and also discriminates between quasi-particles of differing binding energy. When implemented within nanoscale assemblies, such control elements could mediate the flow of energy and information. Quantum interference can also be used to dissociate excitons as an alternative to using heterojunctions. A finite molecular setting is employed to exhibit the underlying discrete, two-particle, mesoscopic analog to Fano anti-resonance. Selected entanglement measures are shown to distinguish regimes of behavior which cannot be resolved from population dynamics alone.

cond-mat.mes-hall↗
arXiv2015

Designing Small Silicon Quantum Dots with Low Reorganization Energy

A first principles, excited state analysis is carried out to identify ways of producing silicon quantum dots with low excitonic reorganization energy. These focus on the general strategy of either reducing or constraining exciton-phonon coupling, and four approaches are explored. The results can be implemented in quantum dot solids to mitigate polaronic effects and increase the lifetime of coherent excitonic superpositions. It is demonstrated that such designs can also be used to alter the shape of the spectral density for reorganization so as to reduce the rates of both decoherence and dissipation. The results suggest that it may be possible to design quantum dot solids that support partially coherent exciton transport.

cond-mat.mes-hall↗
arXiv2011

Efficient Exciton Transport in Strongly Quantum-Confined Silicon Quantum Dots

First-order perturbation theory and many-body Green function analysis are used to quantify the influence of size, surface reconstruction and surface treatment on exciton transport between small silicon quantum dots. Competing radiative processes are also considered in order to determine how exciton transport efficiency is influenced. The analysis shows that quantum confinement causes small (~1 nm) Si quantum dots to exhibit exciton transport efficiencies far exceeding that of their larger counterparts. We also find that surface reconstruction significantly influences the absorption cross section and leads to a large reduction in both transport rate and efficiency. Exciton transport efficiency is higher for hydrogen passivated dots as compared with those terminated with more electronegative ligands. This is because such ligands delocalize electron wave functions towards the surface and result in a lower dipole moment. Such behavior is not predicted by Förster theory, built on a dipole-dipole approximation, because higher order multi-poles play a significant role in the exciton dynamics.

cond-mat.mes-hall↗
arXiv2010

A Two-Dimensional Carbon Semiconductor

We show that patterned defects can be used to disrupt the sub-lattice symmetry of graphene so as to open up a band gap. This way of modifying graphene's electronic structure does not rely on external agencies, the addition of new elements or special boundaries. The method is used to predict a planar, low energy, graphene allotrope with a band gap of 1.2 eV. This defect engineering also allows semiconducting ribbons of carbon to be fabricated within graphene. Linear arrangements of defects lead to naturally embedded ribbons of the semiconducting material in graphene, offering the prospect of two-dimensional circuit logic composed entirely of carbon.

cond-mat.mes-hall↗
arXiv2010

Parallelized Hybrid Monte Carlo Simulation of Stress-Induced Texture Evolution

A parallelized hybrid Monte Carlo (HMC) methodology is devised to quantify the microstructural evolution of polycrystalline material under elastic loading. The approach combines a time explicit material point method (MPM) for the mechanical stresses with a calibrated Monte Carlo (cMC) model for grain boundary kinetics. The computed elastic stress generates an additional driving force for grain boundary migration. The paradigm is developed, tested, and subsequently used to quantify the effect of elastic stress on the evolution of texture in nickel polycrystals. As expected, elastic loading favors grains which appear softer with respect to the loading direction. The rate of texture evolution is also quantified, and an internal variable rate equation is constructed which predicts the time evolution of the distribution of orientations.

cond-mat.mtrl-sci↗
arXiv2010

Size Dependence of the Multiple Exciton Generation Rate in CdSe Quantum Dots

The multiplication rates of hot carriers in CdSe quantum dots are quantified using an atomistic pseudopotential approach and first order perturbation theory. Both excited holes and electrons are considered, and electron-hole Coulomb interactions are accounted for. We find that holes have much higher multiplication rates than electrons with the same excess energy due to the larger density of final states (positive trions). When electron-hole pairs are generated by photon absorption, however, the net carrier multiplication rate is dominated by photogenerated electrons, because they have on average much higher excess energy. We also find, contrary to earlier studies, that the effective Coulomb coupling governing carrier multiplication is energy dependent. We show that smaller dots result in a decrease in the carrier multiplication rate for a given absolute photon energy. However, if the photon energy is scaled by the volume dependent optical gap, then smaller dots exhibit an enhancement in carrier multiplication for a given relative energy.

cond-mat.mes-hall↗
arXiv2010

Toroidal Carbon Nanotubes with Encapsulated Atomic Metal Loops

Toroidal carbon nanotubes can serve as hosts for encapsulated loops of atomic metal wires. Such composite structures have been analyzed using density functional theory for a semiconducting C$_{120}$ torus encapsulating chains of Fe, Au and Cu atoms. The sheathed metal necklaces form a zigzag structure and drops the HOMO/LUMO bandgap to less than 0.1 eV. The iron composite is ferromagnetic with a magnetic moment essentially the same as that of bcc iron. The azimuthal symmetry of these toroidal composites suggests that they may offer novel elecromagnetic properties not associated with straight, metal-encapsulated carbon nanotubes.

cond-mat.mtrl-sci↗