Los congresos internacionales de la lengua española

Desde 1997, con aparente periodicidad trienal, se celebra el Congreso Internacional de la Lengua Española (CILE) y en este documento se recogen las ideas más destacadas ofrecidas en los diferentes congresos a lo largo de los años. En todas sus ediciones expertos hispanistas y demás intelectuales discuten acerca de la lengua española, su situación actual, sus dilemas y sus desafíos de futuro. En su última edición, los debates han girado en torno al libro, en todos sus formatos, como herramienta de educación, de aprendizaje y de difusión del idioma español

Layer-by-Layer Dielectric Breakdown of Hexagonal Boron Nitride

Hexagonal boron nitride (BN) is widely used as a substrate and gate insulator for two-dimensional (2D) electronic devices. The studies on insulating properties and electrical reliability of BN itself, however, are quite limited. Here, we report a systematic investigation of the dielectric breakdown characteristics of BN using conductive atomic force microscopy. The electric field strength was found to be ∼12 MV/cm, which is comparable to that of conventional SiO₂ oxides because of the covalent bonding nature of BN. After the hard dielectric breakdown, the BN fractured like a flower into equilateral triangle fragments. However, when the applied voltage was terminated precisely in the middle of the dielectric breakdown, the formation of a hole that did not penetrate to the bottom metal electrode was clearly observed. Subsequent <i>I–V</i> measurements of the hole indicated that the BN layer remaining in the hole was still electrically inactive. On the basis of these observations, layer-by-layer breakdown was confirmed for BN with regard to both physical fracture and electrical breakdown. Moreover, statistical analysis of the breakdown voltages using a Weibull plot suggested the anisotropic formation of defects. These results are unique to layered materials and unlike the behavior observed for conventional 3D amorphous oxides

Anisotropic Wet Etching of WSe₂ and MoS₂ for Twist-Angle Extraction of Heterobilayers

We focus on the formation of hexagonal holes in WSe2 and MoS2 multilayers by anisotropic wet etching. It is found that anisotropic wet etching occurs for a high H2O2 concentration in an etchant and large thicknesses of WSe2 and MoS2. Furthermore, photoexcited carriers accelerate the anisotropic wet etching of the WSe2 multilayers. It is found that the oxidation process at the edges of WSe2 crystals significantly impacts anisotropic wet etching, and photoexcited carriers facilitate the WSe2 oxidation reaction with H2O2. We also demonstrate that anisotropic wet etching can be used to determine the twist angle in WSe2/MoS2 heterobilayers. The photoluminescence intensities of the infrared interlayer exciton are proportional to the twist angle in the heterobilayers. These results suggest the potential for exploiting the orientation-dependent phenomena of transition metal dichalcogenides

Defect Control and <i>n</i>‑Doping of Encapsulated Graphene by Helium-Ion-Beam Irradiation

We study with Raman spectroscopy the influences of He⁺ bombardment and the environment on beam-induced defects in graphene encapsulated in hexagonal boron nitride (h-BN). We show for the first time experimentally the autonomous behavior of the D′ defect Raman peak: in contrast to the D defect peak, the D′ defect peak is sensitive to the local environment. In particular, it saturates with ion dose in the encapsulated graphene. Electrical measurements reveal <i>n</i>-type conduction in the BN-encapsulated graphene. We conclude that unbound atoms (“interfacials”) between the sp²-layers of graphene and h-BN promote self-healing of the beam-induced lattice damage and that nitrogen–carbon exchange leads to <i>n</i>-doping of graphene

Nonlinear Landau Fan Diagram for Graphene Electrons Exposed to a Moiré Potential

Due to Landau quantization, the conductance of two-dimensional electrons exposed to a perpendicular magnetic field exhibits oscillations that generate a fan of linear trajectories when plotted in the parameter space spanned by density and field. This fan looks identical, irrespective of the dispersion and field dependence of the Landau level energy. This is no surprise because the position of conductance minima depends solely on the level degeneracy that is linear in flux. The fractal energy spectrum that emerges within each Landau band when electrons are also exposed to a two-dimensional superlattice potential produces numerous additional oscillations, but they also create just linear fans for identical reasons. Here, we report conductance oscillations of graphene electrons exposed to a moiré potential that defy this general rule and form nonlinear trajectories in the density-field plane. We attribute this anomalous behavior to the simultaneous occupation of multiple minibands and magnetic breakdown-induced open orbits

Three-Dimensional Location of a Single Dopant with Atomic Precision by Aberration-Corrected Scanning Transmission Electron Microscopy

Materials properties, such as optical and electronic response, can be greatly enhanced by isolated single dopants. Determining the full three-dimensional single-dopant defect structure and spatial distribution is therefore critical to understanding and adequately tuning functional properties. Combining quantitative Z-contrast scanning transmission electron microscopy images with image simulations, we show the direct determination of the atomic-scale depth location of an optically active, single atom Ce dopant embedded within wurtzite-type AlN. The method represents a powerful new tool for reconstructing three-dimensional information from a single, two-dimensional image

Tunneling Spin Valves Based on Fe₃GeTe₂/hBN/Fe₃GeTe₂ van der Waals Heterostructures

Thin van der Waals (vdW) layered magnetic materials hold the possibility of realizing vdW heterostructures with new functionalities. Here, we report on the realization and investigation of tunneling spin valves based on van der Waals heterostructures consisting of an atomically thin hBN layer acting as tunnel barrier and two exfoliated Fe₃GeTe₂ crystals acting as ferromagnetic electrodes. Low-temperature anomalous Hall effect measurements show that thin Fe₃GeTe₂ crystals are metallic ferromagnets with an easy axis perpendicular to the layers and a very sharp magnetization switching at magnetic field values that depends slightly on their geometry. In Fe₃GeTe₂/hBN/Fe₃GeTe₂ heterostructures, we observe textbook behavior of the tunneling resistance, which is minimum (maximum) when the magnetization in the two electrodes is parallel (antiparallel) to each other. The magnetoresistance is 160% at low temperature, from which we determine the spin polarization of Fe₃GeTe₂ to be 0.66, corresponding to 83% and 17% of the majority and minority carriers, respectively. The measurements also show that, with increasing temperature, the evolution of the spin polarization extracted from the tunneling magnetoresistance is proportional to the temperature dependence of the magnetization extracted from the analysis of the anomalous Hall conductivity. This suggests that the magnetic properties of the surface are representative of those of the bulk, as may be expected for vdW materials

Long Rayleigh length confocal microscope: A fast evaluation tool for obtaining quantum propensities of color centers

Color centers in wide band-gap semiconductors, which have superior quantum properties even at room temperature and atmospheric pressure, have been actively applied to quantum sensing devices. Characterization of the quantum properties of the color centers in the semiconductor materials and ensuring that these properties are uniform over a wide area are key issues for developing quantum sensing devices based on color center. In this article, we will describe the principle and performance of a newly developed confocal microscope system with a long Rayleigh length (LRCFM). This system can characterize a wider area faster than the confocal microscope systems commonly used for color center evaluation

Optical Imaging and Spectroscopic Characterization of Self-Assembled Environmental Adsorbates on Graphene

Topographic studies using scanning probes have found that graphene surfaces are often covered by micron-scale domains of periodic stripes with a 4 nm pitch. These stripes have been variously interpreted as structural ripples or as self-assembled adsorbates. We show that the stripe domains are optically anisotropic by imaging them using a polarization-contrast technique. Optical spectra between 1.1 and 2.8 eV reveal that the anisotropy in the in-plane dielectric function is predominantly real, reaching 0.6 for an assumed layer thickness of 0.3 nm. The spectra are incompatible with a rippled graphene sheet but would be quantitatively explained by the self-assembly of chainlike organic molecules into nanoscale stripes

Exciton-Sensitized Second-Harmonic Generation in 2D Heterostructures

The efficient optical second-harmonic generation (SHG) of two-dimensional (2D) crystals, coupled with their atomic thickness, which circumvents the phase-match problem, has garnered considerable attention. While various 2D heterostructures have shown promising applications in photodetectors, switching electronics, and photovoltaics, the modulation of nonlinear optical properties in such heterosystems remains unexplored. In this study, we investigate exciton-sensitized SHG in heterobilayers of transition metal dichalcogenides (TMDs), where photoexcitation of one donor layer enhances the SHG response of the other as an acceptor. We utilize polarization-resolved interferometry to detect the SHG intensity and phase of each individual layer, revealing the energetic match between the excitonic resonances of donors and the SHG enhancement of acceptors for four TMD combinations. Our results also uncover the dynamic nature of interlayer coupling, as made evident by the dependence of sensitization on interlayer gap spacing and the average power of the fundamental beam. This work provides insights into how the interlayer coupling of two different layers can modify nonlinear optical phenomena in 2D heterostructures