Unidirectional Rashba spin splitting in single layer WS2(1-x)Se2xalloy.

Atomically thin two-dimensional (2D) layered semiconductors such as transition metal dichalcogenides have attracted considerable attention due to their tunable band gap, intriguing spin-valley physics, piezoelectric effects and potential device applications. Here we study the electronic properties of a single layer WS1.4Se0.6alloys. The electronic structure of this alloy, explored using angle resolved photoemission spectroscopy, shows a clear valence band structure anisotropy characterized by two paraboloids shifted in one direction of thek-space by a constant in-plane vector. This band splitting is a signature of a unidirectional Rashba spin splitting with a related giant Rashba parameter of 2.8 ± 0.7 eV Å. The combination of angle resolved photoemission spectroscopy with piezo force microscopy highlights the link between this giant unidirectional Rashba spin splitting and an in-plane polarization present in the alloy. These peculiar anisotropic properties of the WS1.4Se0.6alloy can be related to local atomic orders induced during the growth process due the different size and electronegativity between S and Se atoms. This distorted crystal structure combined to the observed macroscopic tensile strain, as evidenced by photoluminescence, displays electric dipoles with a strong in-plane component, as shown by piezoelectric microscopy. The interplay between semiconducting properties, in-plane spontaneous polarization and giant out-of-plane Rashba spin-splitting in this 2D material has potential for a wide range of applications in next-generation electronics, piezotronics and spintronics devices.

Exploring the Magnetic and Electric Side of Light through Plasmonic Nanocavities.

Light-matter interactions are often considered to be mediated by the electric component of light only, neglecting the magnetic contribution. However, the electromagnetic energy density is equally distributed between both parts of the optical fields. Within this scope, we experimentally demonstrate here, in excellent agreement with numerical simulations, that plasmonic nanostructures can selectively manipulate and tune the magnetic versus electric emission of luminescent nanocrystals. In particular, we show selective enhancement or decay of magnetic and electric emission from trivalent europium-doped nanoparticles in the vicinity of plasmonic nanocavities, designed to efficiently couple to either the electric or magnetic emission of the quantum emitter. Specifically, by precisely controlling the spatial position of the emitter with respect to our plasmonic nanostructures, by means of a near-field optical microscope, we record local distributions of both magnetic and electric radiative local densities of states (LDOS) with nanoscale precision. The distribution of the radiative LDOS reveals the modification of both the magnetic and electric optical quantum environments induced by the presence of the metallic nanocavities. This manipulation and enhancement of magnetic light-matter interaction by means of plasmonic nanostructures opens up new possibilities for the research fields of optoelectronics, chiral optics, nonlinear and nano-optics, spintronics, and metamaterials, among others.

Enhancing Magnetic Light Emission with All-Dielectric Optical Nanoantennas.

Electric and magnetic optical fields carry the same amount of energy. Nevertheless, the efficiency with which matter interacts with electric optical fields is commonly accepted to be at least 4 orders of magnitude higher than with magnetic optical fields. Here, we experimentally demonstrate that properly designed photonic nanoantennas can selectively manipulate the magnetic versus electric emission of luminescent nanocrystals. In particular, we show selective enhancement of magnetic emission from trivalent europium-doped nanoparticles in the vicinity of a nanoantenna tailored to exhibit a magnetic resonance. Specifically, by controlling the spatial coupling between emitters and an individual nanoresonator located at the edge of a near-field optical scanning tip, we record with nanoscale precision local distributions of both magnetic and electric radiative local densities of states (LDOS). The map of the radiative LDOS reveals the modification of both the magnetic and electric quantum environments induced by the presence of the nanoantenna. This manipulation and enhancement of magnetic light-matter interaction by means of nanoantennas opens up new possibilities for the research fields of optoelectronics, chiral optics, nonlinear and nano-optics, spintronics, and metamaterials, among others.

High p doped and robust band structure in Mg-doped hexagonal boron nitride.

In two dimensional materials, substitutional doping during growth can be used to alter the electronic properties. Here, we report on the stable growth of p-type hexagonal boron nitride (h-BN) using Mg-atoms as substitutional impurities in the h-BN honeycomb lattice. We use micro-Raman spectroscopy, angle-resolved photoemission measurements (nano-ARPES) and Kelvin probe force microscopy (KPFM) to study the electronic properties of Mg-doped h-BN grown by solidification from a ternary Mg-B-N system. Besides the observation of a new Raman line at ∼1347 cm-1 in Mg-doped h-BN, nano-ARPES reveals p-type carrier concentration. Our nano-ARPES experiments demonstrate that the Mg dopants can significantly alter the electronic properties of h-BN by shifting the valence band maximum about 150 meV toward higher binding energies with respect to pristine h-BN. We further show that, Mg doped h-BN exhibits a robust, almost unaltered, band structure compared to pristine h-BN, with no significant deformation. Kelvin probe force microscopy (KPFM) confirms the p-type doping, with a reduced Fermi level difference between pristine and Mg-doped h-BN crystals. Our findings demonstrate that conventional semiconductor doping by Mg as substitutional impurities is a promising route to high-quality p-type doped h-BN films. Such stable p-type doping of large band h-BN is a key feature for 2D materials applications in deep ultra-violet light emitting diodes or wide bandgap optoelectronic devices.

Strain and Spin-Orbit Coupling Engineering in Twisted WS2/Graphene Heterobilayer.

The strain in hybrid van der Waals heterostructures, made of two distinct two-dimensional van der Waals materials, offers an interesting handle on their corresponding electronic band structure. Such strain can be engineered by changing the relative crystallographic orientation between the constitutive monolayers, notably, the angular misorientation, also known as the "twist angle". By combining angle-resolved photoemission spectroscopy with density functional theory calculations, we investigate here the band structure of the WS2/graphene heterobilayer for various twist angles. Despite the relatively weak coupling between WS2 and graphene, we demonstrate that the resulting strain quantitatively affects many electronic features of the WS2 monolayers, including the spin-orbit coupling strength. In particular, we show that the WS2 spin-orbit splitting of the valence band maximum at K can be tuned from 430 to 460 meV. Our findings open perspectives in controlling the band dispersion of van der Waals materials.