Insights into Structural, Electronic, and Transport Properties of Pentagonal PdSe2 Nanotubes Using First-Principles Calculations

One-dimensional (1D) novel pentagonal materials have gained significant attention as a new class of materials with unique properties that could influence future technologies. In this report, we studied the structural, electronic, and transport properties of 1D pentagonal PdSe₂ nanotubes (p-PdSe₂ NTs). The stability and electronic properties of p-PdSe₂ NTs with different tube sizes and under uniaxial strain were investigated using density functional theory (DFT). The studied structures showed an indirect-to-direct bandgap transition with slight variation in the bandgap as the tube diameter increased. Specifically, (5 × 5) p-PdSe₂ NT, (6 × 6) p-PdSe₂ NT, (7 × 7) p-PdSe₂ NT, and (8 × 8) p-PdSe₂ NT are indirect bandgap semiconductors, while (9 × 9) p-PdSe₂ NT exhibits a direct bandgap. In addition, under low uniaxial strain, the surveyed structures were stable and maintained the pentagonal ring structure. The structures were fragmented under tensile strain of 24%, and compression of -18% for sample (5 × 5) and -20% for sample (9 × 9). The electronic band structure and bandgap were strongly affected by uniaxial strain. The evolution of the bandgap vs. the strain was linear. The bandgap of p-PdSe₂ NT experienced an indirect-direct-indirect or a direct-indirect-direct transition when axial strain was applied. A deformability effect in the current modulation was observed when the bias voltage ranged from about 1.4 to 2.0 V or from -1.2 to -2.0 V. Calculation of the field effect I-V characteristic showed that the on/off ratio was large with bias potentials from 1.5 to 2.0 V. This ratio increased when the inside of the nanotube contained a dielectric. The results of this investigation provide a better understanding of p-PdSe₂ NTs, and open up potential applications in next-generation electronic devices and electromechanical sensors.

Compounds Isolated from Lawsonia inermis L. Collected in Vietnam and Evaluation of Their Potential Activity Against the Main Protease of SARS-CoV-2 Using In silico Molecular Docking and Molecular Dynamic Simulation

Since late 2019 to early 2020, an outbreak caused by severe acute respiratory syndrome-coronavirus 2 (SARS-CoV-2) has become a worldwide health emergency due to its rapid infection and mortality of millions of people around the world. As the main protease Mpro or 3CLpro produced by the virus plays an important role in coronavirus survival and proliferation, it becomes an excellent drug target to identify COVID-19 inhibitors. Lawsonia inermis L. (henna) is a medicinal plant that has been used for a long time for the treatment of many fungal and bacterial infections. In the search for new anti-COVID agents from medicinal plants, we report the results of our study into the potential inhibition of Mpro by the compounds isolated from the extracts of L. inermis roots and leaves using molecular docking and molecular dynamics simulation. The molecular modeling results showed that all isolated compounds bonded spontaneously into the catalytic pockets of Mpro with binding energies <0. The docking and calculated pharmacokinetic results of the compounds (1-3, 6-8) were similar to and even better than those of the commercial COVID-19 inhibitor remdesivir. In particular, the triterpenoid glycoside suavissimoside R1 (8) showed the best binding to SARS-CoV Mpro, with the lowest binding energy ΔG and IC50,calc. values of −8.19 kcal/mol and 0.98 μM, respectively. Furthermore, the calculations of ADMET (absorption, distribution, metabolism, excretion, and toxicity) showed that it had the lowest toxicity, with a predicted LD50 value of 3320 mg/kg. These triterpenoids are worthy of further study to evaluate their actual bioactivity against SARS-CoV-2 in vitro and in vivo in the hope of contributing valuable scientific data for natural resources for the development novel drug formulations for either the prevention or treatment of COVID-19.

Zoology of Multiple-Q Spin Textures in a Centrosymmetric Tetragonal Magnet with Itinerant Electrons

Magnetic skyrmion is a topologically stable particle-like swirling spin texture potentially suitable for high-density information bit, which was first observed in noncentrosymmetric magnets with Dzyaloshinskii-Moriya interaction. Recently, nanometric skyrmion has also been discovered in centrosymmetric rare-earth compounds, and the identification of their skyrmion formation mechanism and further search of nontrivial spin textures are highly demanded. Here, magnetic structures in a prototypical skyrmion-hosting centrosymmetric tetragonal magnet GdRu₂ Si₂ is exhaustively studied by performing the resonant X-ray scattering experiments. A rich variety of double-Q magnetic structures, including the antiferroic order of meron(half-skyrmion)/anti-meron-like textures with fractional local topological charges are identified. The observed intricate magnetic phase diagram is successfully reproduced by the theoretical framework considering the four-spin interaction mediated by itinerant electrons and magnetic anisotropy. The present results will contribute to the better understanding of the novel skyrmion formation mechanism in this centrosymmetric rare-earth compound, and suggest that itinerant electrons can ubiquitously host a variety of unique multiple-Q spin orders in a simple crystal lattice system.

Direct visualization of the three-dimensional shape of skyrmion strings in a noncentrosymmetric magnet

Magnetic skyrmions are topologically stable swirling spin textures that appear as particle-like objects in two-dimensional (2D) systems. Here, utilizing scalar magnetic X-ray tomography under applied magnetic fields, we report the direct visualization of the three-dimensional (3D) shape of individual skyrmion strings in the room-temperature skyrmion-hosting non-centrosymmetric compound Mn_1.4Pt_0.9Pd_0.1Sn. Through the tomographic reconstruction of the 3D distribution of the [001] magnetization component on the basis of transmission images taken at various angles, we identify a skyrmion string running through the entire thickness of the sample, as well as various defect structures, such as the interrupted and Y-shaped strings. The observed point defect may represent the Bloch point serving as an emergent magnetic monopole, as proposed theoretically. Our tomographic approach with a tunable magnetic field paves the way for direct visualization of the structural dynamics of individual skyrmion strings in 3D space, which will contribute to a better understanding of the creation, annihilation and transfer of these topological objects.

Imaging the coupling between itinerant electrons and localised moments in the centrosymmetric skyrmion magnet GdRu2Si2

Magnetic skyrmions were thought to be stabilised only in inversion-symmetry breaking structures, but skyrmion lattices were recently discovered in inversion symmetric Gd-based compounds, spurring questions of the stabilisation mechanism. A natural consequence of a recent theoretical proposal, a coupling between itinerant electrons and localised magnetic moments, is that the skyrmions are amenable to detection using even non-magnetic probes such as spectroscopic-imaging scanning tunnelling microscopy (SI-STM). Here SI-STM observations of GdRu₂Si₂ reveal patterns in the local density of states that indeed vary with the underlying magnetic structures. These patterns are qualitatively reproduced by model calculations which assume exchange coupling between itinerant electrons and localised moments. These findings provide a clue to understand the skyrmion formation mechanism in GdRu₂Si₂.

GdRu₂Si₂ can host magnetic skyrmions, however, it does not have inversion symmetry breaking, a feature usually assumed necessary for skyrmion formation. Using scanning tunnelling microscopy, the authors visualise the double-Q structure in the itinerant electrons that mediate the skyrmion formation.

Nanometric square skyrmion lattice in a centrosymmetric tetragonal magnet

Magnetic skyrmions are topologically stable spin swirls with a particle-like character and are potentially suitable for the design of high-density information bits. Although most known skyrmion systems arise in non-centrosymmetric systems with a Dzyaloshinskii-Moriya interaction, centrosymmetric magnets with a triangular lattice can also give rise to skyrmion formation, with a geometrically frustrated lattice being considered essential in this case. Until now, it remains an open question if skyrmions can also exist in the absence of both geometrically frustrated lattice and inversion symmetry breaking. Here we discover a square skyrmion lattice state with 1.9 nm diameter skyrmions in the centrosymmetric tetragonal magnet GdRu₂Si₂ without a geometrically frustrated lattice by means of resonant X-ray scattering and Lorentz transmission electron microscopy experiments. A plausible origin of the observed skyrmion formation is four-spin interactions mediated by itinerant electrons in the presence of easy-axis anisotropy. Our results suggest that rare-earth intermetallics with highly symmetric crystal lattices may ubiquitously host nanometric skyrmions of exotic origins.

Propagation dynamics of spin excitations along skyrmion strings

Magnetic skyrmions, topological solitons characterized by a two-dimensional swirling spin texture, have recently attracted attention as stable particle-like objects. In a three-dimensional system, a skyrmion can extend in the third dimension forming a robust and flexible string structure, whose unique topology and symmetry are anticipated to host nontrivial functional responses. Here we experimentally demonstrate the coherent propagation of spin excitations along skyrmion strings for the chiral-lattice magnet Cu₂OSeO₃. We find that this propagation is directionally non-reciprocal and the degree of non-reciprocity, as well as group velocity and decay length, are strongly dependent on the character of the excitation modes. These spin excitations can propagate over a distance exceeding 50 μm, demonstrating the excellent long-range ordered nature of the skyrmion-string structure. Our combined experimental and theoretical analyses offer a comprehensive account of the propagation dynamics of skyrmion-string excitations and suggest the possibility of unidirectional information transfer along such topologically protected strings.

Quassinoids and Alkaloids From the Roots of Eurycoma longifolia

Five quassinoids and 2 alkaloids were isolated from the roots of Eurycoma longifolia. The structures of 3 new quassinoids, eurycomalide F, G, and H (1-3), were elucidated using a variety of spectroscopic methods. The known compounds were identified as eurycomalide A (4), laurycolactone B (5), 5-methoxycanthin-6-one (6), and canthin-6-one (7). Among the isolated compounds, canthin-6-one (7) exhibited the strongest inhibitory effect on nitric oxide production (IC50 = 16.9 μM). 5-Methoxycanthin-6-one (6) and eurycomalide F (1) exhibited weak inhibition with IC50 values of 23.4 and 32.7 μM, respectively.

The possibility of previous epidemiological data to serve as baseline for future national oral health surveys--a study in Vietnam

AIM

The purpose of this paper is to review the most recent epidemiological data (1985-2000) on dental caries and periodontal diseases in Vietnam in an attempt to obtain a 'baseline' for future national oral health surveys.

METHODS

Studies on periodontal diseases and caries were included when CPITN and WHO caries criteria had been applied and when the sample size was at least n = 200 for each age group.

RESULTS

Almost all subjects had calculus. The median number of sextants with calculus in the 15-19-year- and in the 35-44-year-olds was 4.2 and 5.0, respectively. Only 2-7% of the 35-44-year-olds had one or more deep periodontal pockets. The estimated DMFT of the 12-, 15- and 35-44-year-olds was 1.8, 2.0 and 5.4, respectively, in the years around 1990.

CONCLUSION

This review shows that (often neglected) epidemiological studies from the past on caries and periodontal diseases can contain comprehensive data sets that provide an estimate of the past oral health status which may serve as a baseline for future national surveys.