Abstract: The magnetic structure of the interfaces between organic semiconductors and ferromagnetic contacts plays a key role in the spin injection and extraction processes in organic spintronic devices. We present a combined computational (density functional theory) and experimental (X-ray magnetic circular dichroism) study on the magnetic properties of interfaces between bcc-Fe(001) and C-60 molecules. C-60 is an interesting candidate for application in organic spintronics due to the absence of hydrogen atoms and the associated hyperfine fields. Adsorption of C-60 on Fe(001) reduces the magnetic moments on the top Fe layers by similar to 6%, while inducing an antiparrallel magnetic moment of similar to-0.2 mu(B) on C-60. Adsorption of C-60 on a model ferromagnetic substrate consisting of three Fe monolayers on W(001) leads to a different structure but to very similar interface magnetic properties.

Abstract: Zirconium trisulphide (ZrS3), a member of the layered transition metal trichalcogenides (TMTCs) family, has been studied by angle-resolved photoluminescence spectroscopy (ARPLS). The synthesized ZrS3 layers possess a pseudo one-dimensional nature where each layer consists of ZrS3 chains extending along the b-lattice direction. Our results show that the optical properties of few-layered ZrS3 are highly anisotropic as evidenced by large PL intensity variation with the polarization direction. Light is efficiently absorbed when the E-field is polarized along the chain (b-axis), but the field is greatly attenuated and absorption is reduced when it is polarized vertical to the 1D-like chains as the wavelength of the exciting light is much longer than the width of each 1D chain. The observed PL variation with polarization is similar to that of conventional 1D materials, i.e., nanowires, and nanotubes, except for the fact that here the 1D chains interact with each other giving rise to a unique linear dichroism response that falls between the 2D (planar) and 1D (chain) limit. These results not only mark the very first demonstration of PL polarization anisotropy in 2D systems, but also provide novel insight into how the interaction between adjacent 1D-like chains and the 2D nature of each layer influences the overall optical anisotropy of pseudo-1D materials. Results are anticipated to have an impact on optical technologies such as polarized detectors, near-field imaging, communication systems, and bio-applications relying on the generation and detection of polarized light.

Abstract: The quantum anomalous Hall (QAH) effect is a topologically nontrivial phase, characterized by a non-zero Chern number defined in the bulk and chiral edge states in the boundary. Using first-principles calculations, we demonstrate the presence of the QAH effect in a 1T-YN2 monolayer, which was recently predicted to be a Dirac half metal without spin-orbit coupling (SOC). We show that the inclusion of SOC opens up a large nontrivial bandgap of nearly 0.1 eV in the electronic band structure. This results in the nontrivial bulk topology, which is confirmed by the calculation of Berry curvature, anomalous Hall conductance and the presence of chiral edge states. Remarkably, a QAH phase of high Chern number C = 3 is found, and there are three corresponding gapless chiral edge states emerging inside the bulk gap. Different substrates are also chosen to study the possible experimental realization of the 1T-YN2 monolayer, while retaining its nontrivial topological properties. Our results open a new avenue in searching for QAH insulators with high temperature and high Chern numbers, which can have nontrivial practical applications.

Abstract: The interface-dependent electronic, vibrational, piezoelectric, and optical properties of van der Waals heterobilayers, formed by buckled GeO (b-GeO) and Janus MoSO structures, are investigated by means of first-principles calculations. The electronic band dispersions show that O/Ge and S/O interface formations result in a type-II band alignment with direct and indirect band gaps, respectively. In contrast, O/O and S/Ge interfaces give rise to the formation of a type-I band alignment with an indirect band gap. By considering the Bethe-Salpeter equation (BSE) on top of G(0)W(0) approximation, it is shown that different interfaces can be distinguished from each other by means of the optical absorption spectra as a consequence of the band alignments. Additionally, the low- and high-frequency regimes of the Raman spectra are also different for each interface type. The alignment of the individual dipoles, which is interface-dependent, either weakens or strengthens the net dipole of the heterobilayers and results in tunable piezoelectric coefficients. The results indicate that the possible heterobilayers of b-GeO/MoSO asymmetric structures possess various electronic, optical, and piezoelectric properties arising from the different interface formations and can be distinguished by means of various spectroscopic techniques.

Abstract: Purpose: To compare the results obtained by using numerical flow simulations with the results of combined single photon emission computed tomography (SPECT) and computed tomography (CT) and to demonstrate the importance of correct boundary conditions for the numerical methods to account for the large amount of interpatient variability in airway geometry. Materials and Methods: This study was approved by all relevant institutional review boards. All patients gave their signed informed consent. In this study, six patients with mild asthma (three men; three women; overall mean age, 46 years ± 17 [standard deviation]) underwent CT at functional residual capacity and total lung capacity, as well as SPECT/CT. CT data were used for segmentation and computational fluid dynamics (CFD) simulations. A comparison was made between airflow distribution, as derived with (a) SPECT/CT through tracer concentration analysis, (b) CT through lobar expansion measurement, and (c) CFD through flow computer simulation. Also, the heterogeneity of the ventilation was examined. Results: Good agreement was found between SPECT/CT, CT, and CFD in terms of airflow distribution and hot spot detection. The average difference for the internal airflow distribution was less than 3% for CFD and CT versus SPECT/CT. Heterogeneity in ventilation patterns could be detected with SPECT/CT and CFD. Conclusion: This results of this study show that patient-specific computer simulations with appropriate boundary conditions yield information that is similar to that obtained with functional imaging tools, such as SPECT/CT.

Abstract: Defect-free graphene is impermeable to all atoms(1-5) and ions(6,7) under ambient conditions. Experiments that can resolve gas flows of a few atoms per hour through micrometre-sized membranes found that monocrystalline graphene is completely impermeable to helium, the smallest atom(2,5). Such membranes were also shown to be impermeable to all ions, including the smallest one, lithium(6,7). By contrast, graphene was reported to be highly permeable to protons, nuclei of hydrogen atoms(8,9). There is no consensus, however, either on the mechanism behind the unexpectedly high proton permeability(10-14) or even on whether it requires defects in graphene's crystal lattice(6,8,15-17). Here, using high-resolution scanning electrochemical cell microscopy, we show that, although proton permeation through mechanically exfoliated monolayers of graphene and hexagonal boron nitride cannot be attributed to any structural defects, nanoscale non-flatness of two-dimensional membranes greatly facilitates proton transport. The spatial distribution of proton currents visualized by scanning electrochemical cell microscopy reveals marked inhomogeneities that are strongly correlated with nanoscale wrinkles and other features where strain is accumulated. Our results highlight nanoscale morphology as an important parameter enabling proton transport through two-dimensional crystals, mostly considered and modelled as flat, and indicate that strain and curvature can be used as additional degrees of freedom to control the proton permeability of two-dimensional materials. A study using high-resolution scanning electrochemical cell microscopy attributes proton permeation through defect-free graphene and hexagonal boron nitride to transport across areas of the structure that are under strain.

Abstract: Two-dimensional materials have been attracting increasing interests because of their outstanding properties for Lithium-ion battery applications. In particular, a material family called MXenes (Mn+1Cn, where n = 1, 2, 3) have been recently attracted immense interest in this respect due to their incomparable fast-charging properties and high capacity promises. In this article, we review the state-of-the-art computational progress on Li-ion battery applications of MXene materials in accordance with our systematical DFT calculations. Structural, mechanical, dynamical, and electrical properties of 20 distinct MXene (M: Sc, Ti, V, Cr, Nb, Mo, Hf, Ta, W, and Zr) have been discussed. The battery performances of these MXene monolayers are further investigated by Li-ion binding energies, open circuit voltage values, and Li migration energy barriers. The experimental and theoretical progress up to date demonstrates particularly the potential of non-terminated or pristine MXene materials in Li ion-storage applications. Stability analyses show most of the pristine MXenes should be achievable, however susceptible to the development progress on the experimental growth procedures. Among pristine MXenes, Ti2C, V2C, Sc2C, and Zr2C compounds excel with their high charge/discharge rate prospect due to their extremely low Li diffusion energy barriers. Considering also their higher predicted gravimetric capacities, Sc, Ti, V, and Zr containing MXenes are more promising for their utilization in energy storage applications.

Abstract: We present extensive calculations of the optical and plasmonic properties of a graphene sheet carrying a dc current. By calculating analytically the density-density response function of current-carrying states at finite temperature, we demonstrate that an applied dc current modifies the Pauli blocking mechanism and that absorption acquires a birefringent character with respect to the angle between the in-plane light polarization and current flow. Employing the random phase approximation at finite temperature, we show that graphene plasmons display a degree of non-reciprocity and collimation that can be tuned with the applied current. We discuss the possibility to measure these effects.

Abstract: We study the electronic properties of commensurate faulted bilayer graphene by diagonalizing the one-particle Hamiltonian of the bilayer system in a complete basis of Bloch states of the individual graphene layers. Our novel approach is very general and can be easily extended to any commensurate graphene-based heterostructure. Here, we consider three cases: (i) twisted bilayer graphene, (ii) bilayer graphene where triaxial stress is applied to one layer and (iii) bilayer graphene where uniaxial stress is applied to one layer. We show that the resulting superstructures can be divided into distinct classes, depending on the twist angle or the magnitude of the induced strain. The different classes are distinguished from each other by the interlayer coupling mechanism, resulting in fundamentally different low-energy physics. For the cases of triaxial and uniaxial stress, the individual graphene layers tend to decouple and we find significant charge transfer between the layers. In addition, this piezoelectric effect can be tuned by applying a perpendicular electric field. Finally, we show how our approach can be generalized to multilayer systems.

Abstract: <script type='text/javascript'>document.write(unpmarked('Quantum electrodynamics predicts that heavy atoms (Z \u003E Z(c) approximate to 170) will undergo the process of atomic collapse where electrons sink into the positron continuum and a new family of so-called collapsing states emerges. The relativistic electrons in graphene exhibit the same physics but at a much lower critical charge (Z(c) approximate to 1) which has made it possible to confirm this phenomenon experimentally. However, there exist conflicting predictions on the effect of a magnetic field on atomic collapse. These theoretical predictions are based on the continuum Dirac-Weyl equation, which does not have an exact analytical solution for the interplay of a supercritical Coulomb potential and the magnetic field. Approximative solutions have been proposed, but because the two effects compete on similar energy scales, the theoretical treatment varies depending on the regime which is being considered. These limitations are overcome here by starting from a tight-binding approach and computing exact numerical results. By avoiding special limit cases, we found a smooth evolution between the different regimes. We predict that the atomic collapse effect persists even after the magnetic field is activated and that the critical charge remains unchanged. We show that the atomic collapse regime is characterized: (1) by a series of Landau level anticrossings and (2) by the absence of root B scaling of the Landau levels with regard to magnetic field strength.'));

Abstract: Using the non-equilibrium Green's function formalism, we study carrier transport through imperfect two-dimensional (2D) topological insulator (TI) ribbons. In particular, we investigate the effect of vacancy defects on the carrier transport in 2D TI ribbons with hexagonal lattice structure. To account for the random distribution of the vacancy defects, we present a statistical study of varying defect densities by stochastically sampling different defect configurations. We demonstrate that the topological edge states of TI ribbons are fairly robust against a high concentration (up to 2%) of defects. At very high defect densities, we observe an increased inter-edge interaction, mediated by the localisation of the edge states within the bulk region. This effect causes significant back-scattering of the, otherwise protected, edge-states at very high defect concentrations (>2%), resulting in a loss of conduction through the TI ribbon. We discuss how this coherent vacancy scattering can be used to our advantage for the development of TI-based transistors. We find that there is an optimal concentration of vacancies yielding an ON-OFF current ratio of up to two orders of magnitude. Finally, we investigate the importance of spin-orbit coupling on the robustness of the edge states in the TI ribbon and show that increased spin-orbit coupling could further increase the ON-OFF ratio.

Abstract: Atomic collapse is a phenomenon inherent to relativistic quantum mechanics where electron states dive in the positron continuum for highly charged nuclei. This phenomenon was recently observed in graphene. Here we investigate a novel collapse phenomenon when multiple sub- and supercritical charges of equal strength are put close together as in a molecule. We construct a phase diagram which consists of three distinct regions: (1) subcritical, (2) frustrated atomic collapse, and (3) molecular collapse. We show that the single impurity atomic collapse resonances rearrange themselves to form molecular collapse resonances which exhibit a distinct bonding, anti-bonding and non-bonding character. Here we limit ourselves to systems consisting of two and three charges. We show that by tuning the distance between the charges and their strength a high degree of control over the molecular collapse resonances can be achieved.

Abstract: Dirac plasmons in graphene are very sensitive to the dielectric properties of the environment. We show that this can be used to probe the structure and composition of van der Waals heterostructures (vdWh) put underneath a single graphene layer. In order to do so, we assess vdWh composed of hexagonal boron nitride and different types of transition metal dichalcogenides (TMDs). By performing realistic simulations that account for the contribution of each layer of the vdWh separately and including the importance of the substrate phonons, we show that one can achieve single-layer resolution by investigating the nonlocal nature of the Dirac plasmon-polaritons. The composition of the vdWh stack can be inferred from the plasmon-phonon coupling once it is composed by more than two TMD layers. Furthermore, we show that the bulk character of TMD stacks for plasmonic screening properties in the terahertz regime is reached only beyond 100 layers.

Abstract: Dirac plasmons in graphene hybridize with phonons of transition metal dichalcogenides (TMDs) when the materials are combined in so-called van der Waals heterostructures (vdWh), thus forming surface plasmon-phonon polaritons (SPPPs). The extend to which these modes are coupled depends on the TMD composition and structure, but also on the plasmons' properties. By performing realistic simulations that account for the contribution of each layer of the vdWh separately, we calculate how the strength of plasmon-phonon coupling depends on the number and composition of TMD layers, on the graphene Fermi energy and the specific phonon mode. From this, we present a semiclassical theory that is capable of capturing all relevant characteristics of the SPPPs. We find that it is possible to realize both strong and ultra-strong coupling regimes by tuning graphene's Fermi energy and changing TMD layer number.

Abstract: When using hexagonal boron-nitride (hBN) as a substrate for graphene, the resulting moire pattern creates secondary Dirac points. By encapsulating a multilayer graphene within aligned hBN sheets the controlled moire stacking may offer even richer benefits. Using advanced tight-binding simulations on atomistically-relaxed heterostructures, here we show that the gap at the secondary Dirac point can be opened in selected moire-stacking configurations, and is independent of any additional vertical gating of the heterostructure. On the other hand, gating can broadly tune the gap at the principal Dirac point, and may thereby strongly compress the first moire mini-band in width against the moire-induced gap at the secondary Dirac point. We reveal that in hBN-encapsulated bilayer graphene this novel mechanism can lead to isolated bands flatter than 10 meV under moderate gating, hence presenting a convenient pathway towards electronically-controlled strongly-correlated states on demand.