Hexagonal boron nitride is a large band-gap insulating material which complements the electronic and optical properties of graphene and the transition metal dichalcogenides. However, the intrinsic optical properties of monolayer boron nitride remain largely unex- plored. In particular, the theoretically expected crossover to a direct-gap in the limit of the single monolayer is presently not confirmed experimentally. Here, in contrast to the tech- nique of exfoliating few-layer 2D hexagonal boron nitride, we exploit the scalable approach of high-temperature molecular beam epitaxy to grow high-quality monolayer boron nitride on graphite substrates. We combine deep-ultraviolet photoluminescence and reflectance spectroscopy with atomic force microscopy to reveal the presence of a direct gap of energy 6.1eV in the single atomic layers, thus confirming a crossover to direct gap in the monolayer limit.

The light emission properties of color centers emitting in 3.3–4 eV region are investigated for hydrostatic pressures ranging up to 5 GPa at liquid helium temperature. The light emission energy decreases with pressure less sensitively than the bandgap. This behavior at variance from the shift of the bandgap is typical of deep traps. Interestingly, hydrostatic pressure reveals the existence of levels that vary differently under pressure (smaller increase of the emission wavelength compared to the rest of the levels in this energy region or even decrease of it) with pressure. This discovery enriches the physics of the color centers operating in the UV in hBN.

We demonstrate room-temperature stabilization of dipolar magnetic skyrmions with diameters in the range of 100 nm in a single ultrathin layer of the Heusler alloy Co2FeAl (CFA) under moderate magnetic fields. Current-induced skyrmion dynamics in microwires is studied with a scanning nitrogen-vacancy magnetometer operating in the photoluminescence quenching mode. We first demonstrate skyrmion nucleation by spin-orbit torque and show that its efficiency can be significantly improved using tilted magnetic fields, an effect that is not specific to Heusler alloys and could be advantageous for future skyrmion-based devices. We then show that current-induced skyrmion motion remains limited by strong pinning effects, even though CFA is a magnetic material with a low magnetic damping parameter.

We present a detailed analysis of the temperature dependence of the interlayer shear mode that reveals a variation of the four-phonon scattering coupling with isotopic mass. Phonon anharmonic decay and isotopic disorder effects are very weak for the low-energy interlayer shear mode, and the overall temperature dependence is mainly governed by the thermal lattice expansion. This allows us to observe systematic differences in the temperature dependence of the low-energy mode with the isotopic mass that are related to the four-phonon scattering contribution in quartic anharmonicity. The change in the quartic anharmonicity coefficient between the isotopically pure samples is larger than would be purely expected from the mass variation.

Zn1-xMgxO microcrystals are produced in the 0.15 & x & 1 composition range by calcination of ZnO + Mg(OH)2 after hydro micro mechanical activation according to the patent WO2018065735A1. The structural properties of the samples have revealed the cohabitation of wurtzite and rock-salt phases for x values ranging from 0.15 up to 0.6 with a clear increase of the proportion of cubic phase with x. A single cubic phase is observed in the range 0.66