Physical Origin of the One-Quarter Exact Exchange in Density Functional Theory

Exchange interactions are a manifestation of the quantum mechanical nature of the electrons and play a key role in predicting the properties of materials from first principles. In density functional theory (DFT), a widely used approximation to the exchange energy combines fractions of density-based and Hartree-Fock (exact) exchange. This so-called hybrid DFT scheme is accurate in many materials, for reasons that are not fully understood. Here we show that a 1/4 fraction of exact exchange plus a 3/4 fraction of density-based exchange is compatible with a correct quantum mechanical treatment of the exchange energy of an electron pair in the unpolarized electron gas. We also show that the 1/4 exact-exchange fraction mimics a correlation interaction between doubly-excited electronic configurations. The relation between our results and trends observed in hybrid DFT calculations is discussed, along with other implications

First-principles dynamics of electrons and phonons

First-principles calculations combining density functional theory and many-body perturbation theory can provide microscopic insight into the dynamics of electrons and phonons in materials. We review this theoretical and computational framework, focusing on perturbative treatments of scattering, dynamics and transport of coupled electrons and phonons. We discuss application of these first-principles calculations to electronics, lighting, spectroscopy and renewable energy.Comment: 14 pages, 1 figur

Ab Initio Electron-Phonon Interactions Using Atomic Orbital Wavefunctions

The interaction between electrons and lattice vibrations determines key physical properties of materials, including their electrical and heat transport, excited electron dynamics, phase transitions, and superconductivity. We present a new ab initio method that employs atomic orbital (AO) wavefunctions to compute the electron-phonon (e-ph) interactions in materials and interpolate the e-ph coupling matrix elements to fine Brillouin zone grids. We detail the numerical implementation of such AO-based e-ph calculations, and benchmark them against direct density functional theory calculations and Wannier function (WF) interpolation. The key advantages of AOs over WFs for e-ph calculations are outlined. Since AOs are fixed basis functions associated with the atoms, they circumvent the need to generate a material-specific localized basis set with a trial-and-error approach, as is needed in WFs. Therefore, AOs are ideal to compute e-ph interactions in chemically and structurally complex materials for which WFs are challenging to generate, and are also promising for high-throughput materials discovery. While our results focus on AOs, the formalism we present generalizes e-ph calculations to arbitrary localized basis sets, with WFs recovered as a special case

Optoelectronic Properties and Excitons in Hybridized Boron Nitride and Graphene Hexagonal Monolayers

We explain the nature of the electronic band gap and optical absorption spectrum of Carbon - Boron Nitride (CBN) hybridized monolayers using density functional theory (DFT), GW and Bethe-Salpeter equation calculations. The CBN optoelectronic properties result from the overall monolayer bandstructure, whose quasiparticle states are controlled by the C domain size and lie at separate energy for C and BN without significant mixing at the band edge, as confirmed by the presence of strongly bound bright exciton states localized within the C domains. The resulting absorption spectra show two marked peaks whose energy and relative intensity vary with composition in agreement with the experiment, with large compensating quasiparticle and excitonic corrections compared to DFT calculations. The band gap and the optical absorption are not regulated by the monolayer composition as customary for bulk semiconductor alloys and cannot be understood as a superposition of the properties of bulk-like C and BN domains as recent experiments suggested

Novel materials, computational spectroscopy, and multiscale simulation in nanoscale photovoltaics

Photovoltaic (PV) solar cells convert solar energy to electricity using combinations of semiconducting sunlight absorbers and metallic materials as electrical contacts. Novel nanoscale materials introduce new paradigms for ultrathin, lightweight, solution processable PV as an alternative to conventional Si technology. For example, the ability to use deposition methods not viable in conventional inorganic PV is particularly exciting as products like paper, textiles, automobiles, and building materials could be coated with PV devices, thus making solar cells ubiquitous. In addition, the optical absorption, band gap, and charge carrier mobility of nanoscale materials can be tuned by tailoring their chemistry or using quantum confinement effects, thus creating novel opportunities for efficient and inexpensive solar cells. From the viewpoint of the fundamental processes involved in PV operation, nanoscale PV poses additional challenges due to the formation of strongly bound electron-hole pairs (excitons) upon photoabsorption requiring the presence of semiconductor heterointerfaces within the active layer to dissociate excitons and generate charge carriers. Such interfaces are known as donor-acceptor (D-A) interfaces, and their presence leads to correlated exciton and charge dynamics in nanoscale PV. Material combinations suitable for nanoscale PV can be predicted using atomistic quantum mechanical calculations, which further enable the computation of a small number of spectroscopic quantities necessary to estimate the power conversion efficiency. Our work shows the computational design of two novel classes of materials for nanoscale PV displaying optical absorption, stability, tunability, and carrier mobility superior to materials employed so far in nanoscale PV. To this end, we employed simulation techniques generally falling under the umbrella of ab initio atomistic electronic structure methods, including density functional theory (DFT) and the GW-Bethe-Salpeter approach. Proof-of-concept PV devices were fabricated and tested within our group and in collaboration with other experimental research groups. The two material families studied in this thesis include carbon based materials (both in nanoscale and bulk form) and two-dimensional monolayers such as graphene, reduced graphene oxide, boron nitride, and transition metal dichalcogenides. Our work demonstrates the feasibility of novel PV devices with a range of benefits employing such materials. It further develops a framework to accurately predict exciton dissociation at D-A interfaces and estimate efficiencies in nanoscale PV. Beyond our work on nanoscale materials, we introduce a combination of methods to enable simulation of nanoscale PV across time and length scales. We discuss modeling of subpicosecond dynamics at D-A interfaces, device-scale transport of excitons, charge carriers, and photons, and macroscopic sunlight management by arranging solar panels to best couple with the Sun's trajectory. We elaborate on the latter point and discuss our work on simulation and fabrication of macroscopic three-dimensional PV structures with promise to deliver a range of benefits for solar energy conversion, including reduced seasonal and latitude sensitivity and a doubling of peak power generation hours. Taken together, this thesis advances the computational design of nanoscale PV systems and introduces novel families of materials and PV structures with technological promise for next-generation PV. This thesis document is organized as follows: Chapter 1 and Chapter 2 introduce, respectively, nanoscale PV and ab initio atomistic simulation methods employed in this work. Chapter 3 is the core of our work on novel families of materials for nanoscale PV, and Chapter 4 illustrates multi-scale simulation methods in nanoscale PV as well as our work on three-dimensional PV. The key results are briefly summarized in Chapter 5

Respiratory muscle training with normocapnic hyperpnea improves ventilatory pattern and thoracoabdominal coordination, and reduces oxygen desaturation during endurance exercise testing in COPD patients

Background: Few data are available about the effects of respiratory muscle training with normocapnic hyperpnea (NH) in COPD. The aim is to evaluate the effects of 4 weeks of NH (Spirotiger®) on ventilatory pattern, exercise capacity, and quality of life (QoL) in COPD patients. Methods: Twenty-six COPD patients (three females), ages 49-82 years, were included in this study. Spirometry and maximal inspiratory pressure, St George Respiratory Questionnaire, 6-minute walk test, and symptom-limited endurance exercise test (endurance test to the limit of tolerance [tLim]) at 75%-80% of peak work rate up to a Borg Score of 8-9/10 were performed before and after NH. Patients were equipped with ambulatory inductive plethysmography (LifeShirt®) to evaluate ventilatory pattern and thoracoabdominal coordination (phase angle [PhA]) during tLim. After four supervised sessions, subjects trained at home for 4 weeks 10 minutes twice a day at 50% of maximal voluntary ventilation. The workload was adjusted during the training period to maintain a Borg Score of 5-6/10. Results: Twenty subjects completed the study. After NH, maximal inspiratory pressure significantly increased (81.5±31.6 vs 91.8±30.6 cmH2O, P<0.01); exercise endurance time (+150 seconds, P=0.04), 6-minute walk test (+30 meters, P=0.03), and QoL (-8, P<0.01) all increased. During tLim, the ventilatory pattern changed significantly (lower ventilation, lower respiratory rate, higher tidal volume); oxygen desaturation, PhA, and dyspnea Borg Score were lower for the same work intensity (P<0.01, P=0.02, and P<0.01, respectively; one-way ANOVA). The improvement in tidal volume and oxygen saturation after NH were significantly related (R2=0.65, P<0.01). Conclusion: As expected, NH improves inspiratory muscle performance, exercise capacity, and QoL. New results are significant change in ventilatory pattern, which improves oxygen saturation, and an improvement in thoracoabdominal coordination (lower PhA). These two facts could explain the reduced dyspnea during the endurance test. All these results together may play a role in improving exercise capacity after NH training