The magnetism and electronic structure of Li-doped SnO 2 are investigated using first-principles LDA/LDA+U calculations. We find that Li induces magnetism in SnO 2 when doped at the Sn site but becomes nonmagnetic when doped at the O and interstitial sites. The calculated formation energies show that Li prefers the Sn site as compared with the O site, in agreement with previous experimental works. The interaction of Li with native defects (Sn V Sn and O V O vacancies) is also studied, and we find that Li not only behaves as a spin polarizer, but also a vacancy stabilizer, i.e., Li significantly reduces the defect formation energies of the native defects and helps the stabilization of magnetic oxygen vacancies. The electronic densities of states reveals that these systems, where the Fermi level touches the conduction (valence) band, are nonmagnetic (magnetic).

Abstract:In this work, we summarize the recent progress made in constructing time-dependent density-functional theory (TDDFT) exchange-correlation (XC) kernels capable to describe excitonic effects in semiconductors and apply these kernels in two important cases: a "classic" bulk semiconductor, GaAs, with weakly-bound excitons and a novel two-dimensional material, MoS 2 , with very strongly-bound excitonic states. Namely, after a brief review of the standard many-body semiconductor Bloch and Bethe-Salpether equation (SBE and BSE) and a combined TDDFT+BSE approaches, we proceed with details of the proposed pure TDDFT XC kernels for excitons. We analyze the reasons for successes and failures of these kernels in describing the excitons in bulk GaAs and monolayer MoS 2 , and conclude with a discussion of possible alternative kernels capable of accurately describing the bound electron-hole states in both bulk and two-dimensional materials.

Selective oxidation of V is observed when coordinated with a redox-active ligand, forming a single VO product.

Direct measurements of photoexcited carrier dynamics in nickel are made using few-femtosecond extreme ultraviolet (XUV) transient absorption spectroscopy at the nickel M 2,3 edge. It is observed that the core-level absorption line shape of photoexcited nickel can be described by a Gaussian broadening (σ ) and a red shift (ω s ) of the ground-state absorption spectrum. Theory predicts and the experimental results verify that after initial rapid carrier thermalization, the electron temperature increase ( T ) is linearly proportional to the Gaussian broadening factor σ , providing quantitative real-time tracking of the relaxation of the electron temperature. Measurements reveal an electron cooling time for 50 nm thick polycrystalline nickel films of 640 ± 80 fs. With hot thermalized carriers, the spectral red shift exhibits a power-law relationship with the change in electron temperature of ω s ∝ T 1.5 . Rapid electron thermalization via carrier-carrier scattering accompanies and follows the nominal 4-fs photoexcitation pulse until the carriers reach a quasithermal equilibrium. Entwined with a <6 fs instrument response function, carrier thermalization times ranging from 34 fs to 13 fs are estimated from experimental data acquired at different pump fluences and it is observed that the electron thermalization time decreases with increasing pump fluence. The study provides an initial example of measuring electron temperature and thermalization in metals in real time with XUV light, and it lays a foundation for further investigation of photoinduced phase transitions and carrier transport in metals with core-level absorption spectroscopy.

The energy demand for computing and data storage will continue to rise exponentially unless non-traditional computing architectures and innovative storage solutions are explored. Low-energy computing, including compute-in-memory architectures, has the potential to address these energy and environmental challenges and, in particular, tetrahedral (wurtzite-type) ferroelectrics are promising options for both performance and integration with existing semiconductor processes. The AlScN alloy is among the few tetrahedral materials that exhibit ferroelectric switching, but the electric field required to switch the polarization i.e., the coercive field, E_c, is on the order of MV/cm, which is about 1–2 orders of magnitude higher than more traditional oxide perovskite ferroelectrics (E_c < 100 kV/cm). Instead of further engineering AlScN and related alloys, we explore the alternative route of computationally identifying new materials with switching barriers lower than AlN while still possessing high enough intrinsic breakdown fields. Going beyond binary compounds, we explore the search space of multinary compounds with wurtzite-type structures. Through this large-scale search, we identify four promising ternary nitrides and oxides, including Mg2PN3, MgSiN2, Li2SiO3, and Li2GeO3, for future experimental realization and engineering. In > 90% of the considered multinary materials, we identify unique switching pathways and non-polar structures that are distinct from the commonly assumed switching mechanism in AlN-based materials. Our results disprove the existing design principle based on reduction of wurtzite c/a lattice parameter ratio while supporting two emerging design principles – ionicity and bond strength.

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Our examination of the interplay of ultrafast charge dynamics and electron–phonon interaction in the AA′ stacked bilayer MoS2 provides a microscopic basis for understanding the features (two peaks) in the emission spectrum. We demonstrate that while the initial accumulation of excited charge occurs at and near the Q point of the two-dimensional Brillioun zone, emission takes place predominantly through two pathways: direct charge recombination at the K point and indirect phonon-assisted recombination of electrons at the K valley and holes at the Γ hill of the Brillouin zone. Analysis of the wave vector dependencies of the electron–phonon interaction traces the higher energy peak to phonon-assisted relaxation of the excited electrons from the Q to the K valley in the conduction band. Our results thus reveal the importance of ultrafast charge dynamics in understanding photoemissive properties of a few-layer transition-metal dichalcogenide. These calculations are based on time dependent density functional theory in the density matrix formulation.

Using LDA+U , we investigate Li-doped rutile SnO2(001) surface. The surface defect formation energy shows that it is easier for Li to be doped at surface Sn site than bulk Sn site in SnO2. Li at surface and sub-surface Sn sites has a magnetic ground state, and the induced magnetic moments are not localized at Li site, but spread over Sn and O sites. The surface electronic structures show that Li at surface Sn site shows 100% spin-polarization (half metallic), whereas Li at sub-surface Sn site does not have half metallic state due to Li-Sn hybridized orbitals. The spin-polarized surface has a ferromagnetic ground state, therefore, ferromagnetism is expected in Li-doped SnO2(001) surface.In the past, decade density functional theory (DFT) has proven to be a predictive tool to discover new materials for certain applications, specially in the area of magnetism. With DFT, many new materials have been discovered and then synthesised.1-5 DFT has also predicted spin polarized materials [6][7][8] . One of the new materials is oxide-based diluted magnetic semiconductor, which has potential applications in spintronics. The main quest in this area is to discover magnetic materials having transition temperature (T c ), which is the temperature at which a system changes from a paramagnetic(disorder phase) to a magnetic phase(order phase), well above room temperature and large magnetization and spin-polarization. With this hope, transition-metals (TMs) were doped into nonmagnetic (NM) semiconductor hosts, 9,10 but later on these TM doped systems were found to have inherent issues, i.e., clustering, antisite defects. 11SnO 2 -based diluted system evoked particular attention when S. B. Ogale et al.12 found a giant magnetic moment (GMM) in Co-doped SnO 2 . Following this discovery, TM doped-SnO 2 has been extensively studied both experimentally and theoretically.13-19 Later on in 2008, our theoretical calculations showed that the Sn vacancies are responsible for magnetism in SnO 2 .20 This opened a new area of magnetism, where magnetism is made possible without doping of magnetic impurities, which are confirmed experimentally. [21][22][23] To go beyond vacancy-induced magnetism, we also proposed possible magnetism induced by light elements, e.g., C, and Li. 24,25Recent theoretical calculations further show that magnetism can be induced with NM impurities.2-4,26,27 A good example of NM impurity is carbon, which has been shown theoretically and experimentally that C-doped SnO 2 films can exhibit feromagnetic behaviour at room temperature, 24,28 where C does not induce magnetism in bulk SnO 2 , when located at the oxygen site.24,28 Now, it is a firm belief that magnetism in NM hosts can be tuned either by vacancies or light elements. In oxides, the magnetic vacancies can be created either at cation site or anion site. Most of the theoretical work show that the cation vacancies are magnetic, 29-32 but there has remained an open question that how to stabilize magnetic vacancies due to their higher formation energies? Very recently, th...