Trajectory sampling and finite-size effects in first-principles stopping power calculations

Real-time time-dependent density functional theory (TDDFT) is presently the most accurate available method for computing electronic stopping powers from first principles. However, obtaining application-relevant results often involves either costly averages over multiple calculations or ad hoc selection of a representative ion trajectory. We consider a broadly applicable, quantitative metric for evaluating and optimizing trajectories in this context. This methodology enables rigorous analysis of the failure modes of various common trajectory choices in crystalline materials. Although randomly selecting trajectories is common practice in stopping power calculations in solids, we show that nearly 30% of random trajectories in an FCC aluminium crystal will not representatively sample the material over the time and length scales feasibly simulated with TDDFT, and unrepresentative choices incur errors of up to 60%. We also show that finite-size effects depend on ion trajectory via "ouroboros" effects beyond the prevailing plasmon-based interpretation, and we propose a cost-reducing scheme to obtain converged results even when expensive core-electron contributions preclude large supercells. This work helps to mitigate poorly controlled approximations in first-principles stopping power calculations, allowing 1-2 order of magnitude cost reductions for obtaining representatively averaged and converged results

Electron cascades and secondary electron emission in graphene under energetic ion irradiation

Highly energetic ions traversing a two-dimensional material such as graphene produce strong electronic excitations. Electrons excited to energy states above the work function can give rise to secondary electron emission, reducing the amount of energy that remains in graphene after the ion impact. Electrons can be either emitted (kinetic energy transfer) or captured by the passing ion (potential energy transfer). To elucidate this behavior that is absent in three-dimensional materials, we simulate the electron dynamics in graphene during the first femtoseconds after ion impact. We employ two conceptually different computational methods: a Monte Carlo (MC)-based one, where electrons are treated as classical particles, and time-dependent density functional theory (TDDFT), where electrons are described quantum mechanically. We observe that the linear dependence of electron emission on deposited energy, emerging from MC simulations, becomes sublinear and closer to the TDDFT data when the electrostatic interactions of emitted electrons with graphene are taken into account via complementary particle-in-cell simulations. Our TDDFT simulations show that the probability for electron capture decreases rapidly with increasing ion velocity, whereas secondary electron emission dominates in the high-velocity regime. We estimate that these processes reduce the amount of energy deposited in the graphene layer by 15%-65%, depending on the ion and its velocity. This finding clearly shows that electron emission must be taken into consideration when modeling damage production in two-dimensional materials under ion irradiation.Peer reviewe

Real-time exciton dynamics with time-dependent density-functional theory

Linear-response time-dependent density-functional theory (TDDFT) can describe excitonic features in the optical spectra of insulators and semiconductors, using exchange-correlation (xc) kernels behaving as $-1/k^{2}$ to leading order. We show how excitons can be modeled in real-time TDDFT, using an xc vector potential constructed from approximate, long-range corrected xc kernels. We demonstrate for various materials that this real-time approach is consistent with frequency-dependent linear response, gives access to femtosecond exciton dynamics following short-pulse excitations, and can be extended with some caution into the nonlinear regime.Comment: 7 pages, 4 figure

Non-equilibrium dynamics of electron emission from cold and hot graphene under proton irradiation

Characteristic properties of secondary electrons emitted from irradiated two-dimensional materials arise from multi-length and time-scale relaxation processes that connect the initial non-equilibrium excited electron distribution with their eventual emission. To understand these processes, which are critical for using secondary electrons as high-resolution thermalization probes, we combine first-principles real-time electron dynamics with modern experiments. Our data for cold and hot proton-irradiated graphene shows signatures of kinetic and potential emission and generally good agreement for electron yields between experiment and theory. The duration of the emission pulse is about 1.5 femtoseconds, indicating high time resolution when used as a probe. Our newly developed method to predict kinetic energy spectra shows good agreement with electron and ion irradiation experiments and prior models. We find that lattice temperature significantly increases secondary electron emission, whereas electron temperature has a negligible effect

Quantum computation of stopping power for inertial fusion target design

Stopping power is the rate at which a material absorbs the kinetic energy of a charged particle passing through it -- one of many properties needed over a wide range of thermodynamic conditions in modeling inertial fusion implosions. First-principles stopping calculations are classically challenging because they involve the dynamics of large electronic systems far from equilibrium, with accuracies that are particularly difficult to constrain and assess in the warm-dense conditions preceding ignition. Here, we describe a protocol for using a fault-tolerant quantum computer to calculate stopping power from a first-quantized representation of the electrons and projectile. Our approach builds upon the electronic structure block encodings of Su et al. [PRX Quantum 2, 040332 2021], adapting and optimizing those algorithms to estimate observables of interest from the non-Born-Oppenheimer dynamics of multiple particle species at finite temperature. Ultimately, we report logical qubit requirements and leading-order Toffoli costs for computing the stopping power of various projectile/target combinations relevant to interpreting and designing inertial fusion experiments. We estimate that scientifically interesting and classically intractable stopping power calculations can be quantum simulated with roughly the same number of logical qubits and about one hundred times more Toffoli gates than is required for state-of-the-art quantum simulations of industrially relevant molecules such as FeMoCo or P450

Electronic ripples in ion-irradiated graphene

Delicate ripples flow through the electrons in graphene after impact by an ion, reminiscent of the patterns formed when a raindrop lands in a still puddle. Graphene is composed of a single layer of carbon atoms; it's an ultrathin flake of graphite, the material comprising pencil "lead". Because their electrons are so confined, two-dimensional materials like graphene have unique properties which hold promise for innovations in solar cells, flexible electronics, water desalination, and even quantum computing. However, these applications rely on precise control of defects in atomic structure: either avoiding them to maintain a pristine material or intentionally introducing them to alter properties. Both cases depend on high-resolution imaging and patterning techniques, which typically involve ion beams. With the ultimate goal of improving ion beam techniques and enabling technological advancements based on atomically thin materials, my research uses cutting-edge computational tools to perform extremely accurate simulations of these materials under ion-irradiation. In the process, we uncover beautiful parallels between the quantum world of electrons and the macroscopic world of our daily lives.Ope

Quantum capacitance measurements of single-layer molybdenum disulfide

Thesis: S.B., Massachusetts Institute of Technology, Department of Physics, 2014.Cataloged from PDF version of thesis.Includes bibliographical references (pages 45-46).Through this thesis, heterostructures composed of a thin layer of hexagonal boron nitride atop a monolayer of molybdenum disulfide were fabricated with the goal of measuring quantum capacitance and probing the transition metal dichalcogenide's density of states. In the final devices, no modulation of the quantum-capacitance was observed due to large Schottky barriers between the metal contacts and the molybdenum disulfide. Lessons learned from this investigation inform improved fabrication and measurement techniques for future iterations of these fascinating devices.by Alina Kononov.S.B

Simulation of aluminum sheet under proton radiation

This is a visualization of the electron density in a simulation of a proton traversing an aluminum sheet. The electrons within the aluminum sheet and the electron cloud around the exiting proton (direction of motion indicated by the arrow) are represented by the white and yellow regions of high electron density on the left side of the image. Orange and red shades show electrons ejected from the aluminum in response to energy deposited in the material by the proton. All lengths are in units of the Bohr radius (0.0529 nm). I am investigating how the electronic response depicted here (specifically, the number of electrons ejected from the material and the number of electrons transferred to the projectile) depends on the type of projectile, the projectile's velocity and trajectory, and the type of target material and its thickness. Understanding the behavior of materials under radiation is crucial for improving techniques for materials imaging and processing and developing materials for nuclear and space applications. The simulations are performed using time-dependent density functional theory and Ehrenfest dynamics, methods that approximate non-equilibrium quantum dynamics, on supercomputers such as Blue Waters at the National Center for Supercomputing Applications.Ope