We report high magnetic field scanning tunneling microscopy and Landau level spectroscopy of twisted graphene layers grown by chemical vapor deposition. For twist angles exceeding ~3° the low energy carriers exhibit Landau level spectra characteristic of massless Dirac fermions. Above 20° the layers effectively decouple and the electronic properties are indistinguishable from those in single-layer graphene, while for smaller angles we observe a slowdown of the carrier velocity which is strongly angle dependent. At the smallest angles the spectra are dominated by twist-induced van Hove singularities and the Dirac fermions eventually become localized. An unexpected electron-hole asymmetry is observed which is substantially larger than the asymmetry in either single or untwisted bilayer graphene.

Single-Layer Behavior and Its Breakdown in Twisted Graphene Layers

Sustainability, as a key concept in the education field, has submitted a relevant change during the last years. Thus, there is a growing debate about its meaning. It has undergone a crucial merging of significances from many fields: Ecology, environmental awareness, but also from politics, ethics or even spiritual approaches. All these fields have been co-involved in the building of such subject concept. In this sense, this article addresses the different ways of understanding sustainability as a polyhedral concept and how sustainability can be understood under the umbrella of the Sustainable Development Goals (SDGs). Furthermore, it is proposed a conceptual framework to teach this UN Program at Higher Education, contributing to the training of undergraduate and postgraduate students from both a professional and a personal point of view. This framework is applied in a case study—in particular, in a course of Primary Teacher Degree called Didactics of Matter and Energy. This article finishes with practical consideration to build a change-maker University.

Teaching for a Better World. Sustainability and Sustainable Development Goals in the Construction of a Change-Maker University

We review some salient points in the derivation of density functional theory (DFT) and of the local density approximation (LDA) of it. We then articulate an understanding of DFT and LDA that seems to be ignored in the literature. We note the well-established failures of many DFT and LDA calculations to reproduce the measured energy gaps of finite systems and band gaps of semiconductors and insulators. We then illustrate significant differences between the results from self consistent calculations using single trial basis sets and those from computations following the Bagayoko, Zhao, and Williams (BZW) method, as enhanced by Ekuma and Franklin (BZW-EF). Unlike the former, the latter calculations verifiably attain the absolute minima of the occupied energies, as required by DFT. These minima are one of the reasons for the agreement between their results and corresponding, experimental ones for the band gap and a host of other properties. Further, we note predictions of DFT BZW-EF calculations that have been...

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Understanding Density Functional Theory (DFT) and Completing it in Practice

A superconducting quantum interference device (SQUID) with single-walled carbon nanotube (CNT) Josephson junctions is presented. Quantum confinement in each junction induces a discrete quantum dot (QD) energy level structure, which can be controlled with two lateral electrostatic gates. In addition, a backgate electrode can vary the transparency of the QD barriers, thus permitting change in the hybridization of the QD states with the superconducting contacts. The gates are also used to directly tune the quantum phase interference of the Cooper pairs circulating in the SQUID ring. Optimal modulation of the switching current with magnetic flux is achieved when both QD junctions are in the ‘on’ or ‘off’ state. In particular, the SQUID design establishes that these CNT Josephson junctions can be used as gate-controlled π-junctions; that is, the sign of the current–phase relation across the CNT junctions can be tuned with a gate voltage. The CNT-SQUIDs are sensitive local magnetometers, which are very promising for the study of magnetization reversal of an individual magnetic particle or molecule placed on one of the two CNT Josephson junctions.

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Carbon Nanotube Superconducting Quantum Interference Device.

Safety and immunogenicity of CpG 1018 and aluminium hydroxide-adjuvanted SARS-CoV-2 S-2P protein vaccine MVC-COV1901: interim results of a large-scale, double-blind, randomised, placebo-controlled phase 2 trial in Taiwan.

The three-dimensional ternary Li2GeO3 compound presents various unusual essential properties. The main features are thoroughly explored from the first-principles calculations. The concise pictures, the critical orbital hybridizations in Li–O and Ge–O bonds, are clearly examined through the optimal geometric structure, the atom-dominated electronic energy spectrum, the spatial charge densities, the atom and orbital-decomposed van Hove singularities, and the strong optical responses. The unusual optical transitions cover the red-shift optical gap, various frequency-dependent absorption structures and the most prominent plasmon mode in terms of the dielectric functions, energy loss functions, reflectance spectra, and absorption coefficients. Optical excitations, depending on the directions of electric polarization, are strongly affected by excitonic effects. The close combinations of electronic and optical properties can identify a significant orbital hybridization for each available excitation channel. The developed theoretical framework will be very useful in fully understanding the diverse phenomena of other emergent materials.

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Orbital-hybridization-created optical excitations in Li2GeO3

The theoretical frameworks

The 3D ternary Li2GeO3 compound, which could serve as the electrolyte material in Li+-based batteries, exhibits an unusual lattice symmetry (orthorhombic crystal), band structure, charge density distribution and density of states. The essential properties are fully explored through the first-principles method. In the delicate calculations and analyses, the main features of atom-dominated electronic energy spectrum, space-charge distribution, and atom-/orbital-projected density of states are sufficient to identify the critical multi-orbital hybridizations of the chemical bonds: 2s-(2px, 2py, 2pz) and (4s, 4px, 4py, 4pz)-(2s, 2px, 2py, 2pz), respectively, for Li-O and Ge-O. This system possesses a large indirect gap of Eg=3.77 eV. There exist a lot of significant covalent bonds, with an obvious non-uniformity and anisotropy. In addition, spin-dependent magnetic configurations are completely absent. The theoretical framework could be developed to investigate the important features of anode and cathode materials related to lithium oxide compounds.

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Geometric and Electronic Properties of Li2GeO3

The rich optical properties of AA-bottom-top (bt) bilayer silicene (BS) in a uniform perpendicular electric field (E) are investigated through the use of the tight-binding model. The distinctive multivalley band structure presents a semimetallic behavior but with a sizeable intraband gap. The main features of the energy dispersion appear in the optical absorption spectrum through transitions between band-edge states obeying specific selection rules. The E field clearly modifies the energy dispersion, especially opening a band gap, leading to a substantial influence on the optical properties. This field creates additional excitation channels and increases the frequency and intensity of the spectral structures. The inclusion of electron-hole interaction gives rise to a slight difference in the spectral intensity and strongly affects the fluctuation in the threshold intensity as the field strength is varied. The interplay between the special lattice structure, atomic interactions, electron-hole interaction, and a E field robustly diversifies the absorption spectra.

Electric-field-diversified optical properties of bilayer silicene.

3D ternary Li4Ti5O12, a Li+-based battery anode, presents an unusual lattice symmetry (triclinic crystal), band structure, charge density, and density of states under first-principles calculations. It is a large direct-gap semiconductor with Edg ∼ 2.98 eV. The atom-dominated valence and conduction bands, the spatial charge distribution and the atom- and orbital-decomposed van Hove singularities are available for delicate identifications of multi-orbital hybridizations in Li–O and Ti–O bonds. The extremely non-uniform chemical environment, which induces very complicated hopping integrals, directly arises from the large bonding fluctuations and the highly anisotropic configurations. Also, the developed theoretical framework is very useful for fully understanding cathodes and electrolytes of oxide compounds.

https://pubs.rsc.org/en/content/articlepdf/2020/ra/d0ra00818d

Hai Duong Pham

Papers

Orbital-hybridization-created optical excitations in Li2GeO3

The theoretical frameworks

Geometric and Electronic Properties of Li2GeO3

Electric-field-diversified optical properties of bilayer silicene.

Featured properties of Li+-based battery anode: Li4Ti5O12