First-Principles Calculations of Electron-Defect Interactions and Defect-Limited Charge Transport

Crystallographic defects and impurities govern charge transport at low temperature, where the electron-defect (e-d) interactions limit the carrier mobility and manifest themselves in a wide range of phenomena of broad relevance in condensed matter physics. Theoretical treatments of e-d interactions have so far relied on heuristic approaches and analytic models. However, the band structure, electronic wave functions, and defect perturbation potential are far more complex in real materials than in these simplified models. First-principles calculations can provide atomistic details of the atomic and electronic structures of the material and make accurate predictions of their properties. Yet, ab initio calculations of e-d interactions are still in their infancy, mainly because they require large simulation cells and computationally expensive workflows. This thesis aims to overcome the open challenge of computing the e-d interactions and the associated e-d matrix elements, e-d relaxation times, and defect-limited mobility using first-principles methods. We develop an efficient first-principles method to compute the e-d matrix elements and apply it to neutral vacancy and interstitial defects in silicon. Using the new approach, we demonstrate systematic convergence of the e-d relaxation times with respect to supercell size, defect position, and Brillouin zone sampling. To speed up the e-d calculations, we formulate and implement an interpolation scheme to compute the e-d matrix elements using maximally-localized Wannier functions. We show for the first time fully ab initio calculations of the temperature dependent defect-limited carrier mobility and investigate its numerical convergence. To treat charged defects, we develop a different interpolation method and apply it to a charged point defect in silicon. We use this approach together with importance sampling integration to effectively compute the e-d relaxation times for charged defects. Finally, we provide technical details of the e-d routines and discuss their integration in the open source code PERTURBO developed in the Bernardi group. In summary, the methods developed in this thesis have laid a solid foundation for future ab initio e-d interaction calculations, which can be applied broadly to address materials design challenges in electronics, energy, and quantum technologies.</p

Efficient Ab Initio Calculations of Electron-Defect Scattering and Defect-Limited Carrier Mobility

Electron-defect ($e$-d) interactions govern charge carrier dynamics at low temperature, where they limit the carrier mobility and give rise to phenomena of broad relevance in condensed matter physics. Ab initio calculations of $e$-d interactions are still in their infancy, mainly because they require large supercells and computationally expensive workflows. Here we develop an efficient ab initio approach for computing elastic $e$-d interactions, their associated $e$-d relaxation times (RTs), and the low-temperature defect-limited carrier mobility. The method is applied to silicon with simple neutral defects, such as vacancies and interstitials. Contrary to conventional wisdom, the computed $e$-d RTs depend strongly on carrier energy and defect type, and the defect-limited mobility is temperature dependent. These results highlight the shortcomings of widely employed heuristic models of $e$-d interactions in materials. Our method opens new avenues for studying $e$-d scattering and low-temperature charge transport from first principles.Comment: 11 pages, 5 figures, submitte

Ab initio electron-defect interactions using Wannier functions

Computing electron–defect (e–d) interactions from first principles has remained impractical due to computational cost. Here we develop an interpolation scheme based on maximally localized Wannier functions (WFs) to efficiently compute e–d interaction matrix elements. The interpolated matrix elements can accurately reproduce those computed directly without interpolation and the approach can significantly speed up calculations of e–d relaxation times and defect-limited charge transport. We show example calculations of neutral vacancy defects in silicon and copper, for which we compute the e–d relaxation times on fine uniform and random Brillouin zone grids (and for copper, directly on the Fermi surface), as well as the defect-limited resistivity at low temperature. Our interpolation approach opens doors for atomistic calculations of charge carrier dynamics in the presence of defects

Using defects to store energy in materials – a computational study

Energy storage occurs in a variety of physical and chemical processes. In particular, defects in materials can be regarded as energy storage units since they are long-lived and require energy to be formed. Here, we investigate energy storage in non-equilibrium populations of materials defects, such as those generated by bombardment or irradiation. We first estimate upper limits and trends for energy storage using defects. First-principles calculations are then employed to compute the stored energy in the most promising elemental materials, including tungsten, silicon, graphite, diamond and graphene, for point defects such as vacancies, interstitials and Frenkel pairs. We find that defect concentrations achievable experimentally (~0.1–1 at.%) can store large energies per volume and weight, up to ~5 MJ/L and 1.5 MJ/kg for covalent materials. Engineering challenges and proof-of-concept devices for storing and releasing energy with defects are discussed. Our work demonstrates the potential of storing energy using defects in materials

Delayed Airway Obstruction after Internal Jugular Venous Catheterization in a Patient with Anticoagulant Therapy

Delayed onset of neck hematoma following central venous catheterization without arterial puncture is uncommon. Herein, we present a patient who developed a delayed neck hematoma after repeated attempts at right internal jugular venous puncture and subsequent enoxaparin administration. Progressive airway obstruction occurred on the third day after surgery. Ultrasound examination revealed diffuse hematoma of the right neck, and fibreoptic examination of the airway revealed pharyngeal edema. After emergent surgical removal of the hematoma, the patient was extubated uneventfully

Perturbo: a software package for ab initio electron-phonon interactions, charge transport and ultrafast dynamics

Perturbo is a software package for first-principles calculations of charge transport and ultrafast carrier dynamics in materials. The current version focuses on electron-phonon interactions and can compute phonon-limited transport properties such as the conductivity, carrier mobility and Seebeck coefficient. It can also simulate the ultrafast nonequilibrium electron dynamics in the presence of electron-phonon scattering. Perturbo uses results from density functional theory and density functional perturbation theory calculations as input, and employs Wannier interpolation to reduce the computational cost. It supports norm-conserving and ultrasoft pseudopotentials, spin-orbit coupling, and polar electron-phonon corrections for bulk and 2D materials. Hybrid MPI plus OpenMP parallelization is implemented to enable efficient calculations on large systems (up to at least 50 atoms) using high-performance computing. Taken together, Perturbo provides efficient and broadly applicable ab initio tools to investigate electron-phonon interactions and carrier dynamics quantitatively in metals, semiconductors, insulators, and 2D materials

Electron-Photon Exchange-Correlation Approximation for QEDFT

Quantum-electrodynamical density-functional theory (QEDFT) provides a promising avenue for exploring complex light-matter interactions in optical cavities for real materials. Similar to conventional density-functional theory, the Kohn-Sham formulation of QEDFT needs approximations for the generally unknown exchange-correlation functional. In addition to the usual electron-electron exchange-correlation potential, an approximation for the electron-photon exchange-correlation potential is needed. A recent electron-photon exchange functional [C. Sch\"afer et al., Proc. Natl. Acad. Sci. USA, 118, e2110464118 (2021), https://www.pnas.org/doi/abs/10.1073/pnas.2110464118], derived from the equation of motion of the non-relativistic Pauli-Fierz Hamiltonian, shows robust performance in one-dimensional systems across weak- and strong-coupling regimes. Yet, its performance in reproducing electron densities in higher dimensions remains unexplored. Here we consider this QEDFT functional approximation from one to three-dimensional finite systems and across weak to strong light-matter couplings. The electron-photon exchange approximation provides excellent results in the ultra-strong-coupling regime. However, to ensure accuracy also in the weak-coupling regime across higher dimensions, we introduce a computationally efficient renormalization factor for the electron-photon exchange functional, which accounts for part of the electron-photon correlation contribution. These findings extend the applicability of photon-exchange-based functionals to realistic cavity-matter systems, fostering the field of cavity QED (quantum electrodynamics) materials engineering.Comment: 15 pages, 4 figure

Efficient ab initio calculations of electron-defect scattering and defect-limited carrier mobility

Electron-defect (e−d) interactions govern charge carrier dynamics at low temperature, where they limit the carrier mobility and give rise to phenomena of broad relevance in condensed matter physics. Ab initio calculations of e−d interactions are still in their infancy, mainly because they require large supercells and computationally expensive workflows. Here we develop an efficient ab initio approach for computing elastic e−d interactions, their associated e−d relaxation times (RTs), and the low-temperature defect-limited carrier mobility. The method is applied to silicon with simple neutral defects, such as vacancies and interstitials. Contrary to conventional wisdom, the computed e−d RTs depend strongly on carrier energy and defect type, and the defect-limited mobility is temperature dependent. These results highlight the shortcomings of widely employed heuristic models of e−d interactions in materials. Our method opens avenues for studying e−d scattering and low-temperature charge transport from first principles