Bulk Photovoltaic Effect Along the Nonpolar Axis in Organic-Inorganic Hybrid Perovskites.

The origin of the bulk photovoltaic effect (BPVE) was considered as a built-in electric field formed by the macroscopic polarization of materials. Alternatively, the "shift current mechanism" has been gradually accepted as the more appropriate description of the BPVE. This mechanism implies that the photocurrent generated by the BPVE is a topological current featuring an ultrafast response and dissipation-less nature, which is very attractive for photodetector applications. Meanwhile, the origin of the BPVE in organic-inorganic hybrid perovskites (OIHPs) has not been discussed and is still widely accepted as the classical mechanism without any experimental evidence. Herein, we observed the BPVE along the nonpolar axis in OIHPs, which is inconsistent with the classical explanation. Furthermore, based on the nonlinear optical tensor correlation, we substantiated that the BPVE in OIHPs is originated in the shift current mechanism.

Exciton-Phonon Interaction and Relaxation Times from First Principles.

Electron-phonon interactions are key to understanding the dynamics of electrons in materials and can be modeled accurately from first principles. However, when electrons and holes form Coulomb-bound states (excitons), quantifying their interactions and scattering processes with phonons remains an open challenge. Here we show a rigorous approach for computing exciton-phonon (ex-ph) interactions and the associated exciton dynamical processes from first principles. Starting from the ab initio Bethe-Salpeter equation, we derive expressions for the ex-ph matrix elements and relaxation times. We apply our method to bulk hexagonal boron nitride, for which we map the ex-ph relaxation times as a function of exciton momentum and energy, analyze the temperature and phonon-mode dependence of the ex-ph scattering processes, and accurately predict the phonon-assisted photoluminescence. The approach introduced in this work is general and provides a framework for investigating exciton dynamics in a wide range of materials.

Theory and Ab Initio Computation of the Anisotropic Light Emission in Monolayer Transition Metal Dichalcogenides.

Monolayer transition metal dichalcogenides (TMDCs) are direct gap semiconductors with a unique potential for use in ultrathin light emitters. However, their photoluminescence (PL) is not completely understood. We develop an approach to compute the radiative recombination rate in monolayer TMDCs as a function of photon emission direction and polarization. Using exciton wavefunctions and energies obtained with the ab initio Bethe-Salpeter equation, we obtain polar plots of the PL for different scenarios. Our results can explain the PL anisotropy and polarization dependence measured in recent experiments and predict that light is emitted with a peak intensity normal to the exciton dipole in monolayer TMDCs. We show that excitons emit light anisotropically upon recombination when they are in any quantum superposition state of the K and K' inequivalent valleys. When averaged over the emission angle and exciton momentum, our new treatment recovers the temperature-dependent radiative lifetimes that we previously derived. Our work demonstrates a generally applicable first-principles approach to studying anisotropic light emission in two-dimensional materials.

Radiographic characteristics of odontomas in patients in the National Taiwan University Children's Hospital.

Background/purpose: Odontoma is the most common odontogenic tumor. A literature review revealed limited data for odontomas in Taiwan. This study evaluated the radiographic characteristics of odontomas in 1280 patients in the National Taiwan University Children's Hospital (NTUCH). Materials and methods: This retrospective study analyzed the odontomas based on examination of mainly panoramic radiographs and related radiographs of 1280 patients in the NTUCH. Chi-square test was used for trend analysis. Results: Among 1280 patients (710 boys and 570 girls), a total of 16 patients with odontomas (10 boys and 6 girls) were identified. There were 14 compound and 2 complex odontomas. The mean age of the patients was 10.38 years with the majority of odontomas occurring in the first (37.5%) and second decade (62.5%) of life. Odontomas had a marked predilection for the mandible (56.25%) and for the anterior region of the jaws (75%), particularly for the anterior maxilla (43.75%). Ten odontomas (62.5%) were associated with the impacted corresponding permanent teeth. The sagittal position of odontomas and vertical relation of odontomas to the corresponding permanent teeth were significantly correlated with the eruption rate of corresponding permanent teeth (P < 0.05). Conclusion: The odontomas occur most commonly in male patients with a male to female ratio of 1.67:1. The incidence of odontomas in our 1280 patients is 1.25%. The age range of patients with odontomas is 6-17 years. The most frequent location of odontomas is the anterior maxillary region. Odontoma is frequently associated with an impacted corresponding permanent tooth.

Precise radiative lifetimes in bulk crystals from first principles: the case of wurtzite gallium nitride.

Gallium nitride (GaN) is a key semiconductor for solid-state lighting, but its radiative processes are not fully understood. Here we show a first-principles approach to accurately compute the radiative lifetimes in bulk uniaxial crystals, focusing on wurtzite GaN. Our computed radiative lifetimes are in very good agreement with experiment up to 100 K. We show that taking into account excitons (through the Bethe-Salpeter equation) and spin-orbit coupling is essential for computing accurate radiative lifetimes. A model for exciton dissociation into free carriers allows us to compute the radiative lifetimes up to room temperature. Our work enables precise radiative lifetime calculations in III-nitrides and other anisotropic solid-state emitters.

Ab initio electron-two-phonon scattering in GaAs from next-to-leading order perturbation theory.

Electron-phonon (e-ph) interactions are usually treated in the lowest order of perturbation theory. Here we derive next-to-leading order e-ph interactions, and compute from first principles the associated electron-two-phonon (2ph) scattering rates. The derivations involve Matsubara sums of two-loop Feynman diagrams, and the numerical calculations are challenging as they involve Brillouin zone integrals over two crystal momenta and depend critically on the intermediate state lifetimes. Using Monte Carlo integration together with a self-consistent update of the intermediate state lifetimes, we compute and converge the 2ph scattering rates, and analyze their energy and temperature dependence. We apply our method to GaAs, a weakly polar semiconductor with dominant optical-mode long-range e-ph interactions. We find that the 2ph scattering rates are as large as nearly half the value of the one-phonon rates, and that including the 2ph processes is necessary to accurately predict the electron mobility in GaAs from first principles.