Multiple antiferromagnetic phases and magnetic anisotropy in exfoliated CrBr3_3 multilayers

In twisted two-dimensional (2D) magnets, the stacking dependence of the magnetic exchange interaction can lead to regions of ferromagnetic and antiferromagnetic interlayer order, separated by non-collinear, skyrmion-like spin textures. Recent experimental searches for these textures have focused on CrI$_3$, known to exhibit either ferromagnetic or antiferromagnetic interlayer order, depending on layer stacking. However, the very strong uniaxial anisotropy of CrI$_3$ disfavors smooth non-collinear phases in twisted bilayers. Here, we report the experimental observation of three distinct magnetic phases -- one ferromagnetic and two antiferromagnetic -- in exfoliated CrBr$_3$ multilayers, and reveal that the uniaxial anisotropy is significantly smaller than in CrI$_3$. These results are obtained by magnetoconductance measurements on CrBr$_3$ tunnel barriers and Raman spectroscopy, in conjunction with density functional theory calculations, which enable us to identify the stackings responsible for the different interlayer magnetic couplings. The detection of all locally stable magnetic states predicted to exist in CrBr$_3$ and the excellent agreement found between theory and experiments, provide complete information on the stacking-dependent interlayer exchange energy and establish twisted bilayer CrBr$_3$ as an ideal system to deterministically create non-collinear magnetic phases

Anomalous T-dependence of phonon lifetimes in metallic VO2

We investigate phonon lifetimes in VO2 single crystals. We do so in the metallic state above the metal-insulator transition (MIT), where strong structural fluctuations are known to take place. By combining inelastic X-ray scattering and Raman spectroscopy, we track the temperature dependence of several acoustic and optical phonon modes up to 1000 K. Contrary to what is commonly observed, we find that phonon lifetimes decrease with decreasing temperature. Our results show that pre-transitional fluctuations in the metallic state give rise to strong electron-phonon scattering that onsets hundreds of degrees above the transition and increases as the MIT is approached. Notably, this effect is not limited to specific points of reciprocal space that could be associated with the structural transition

Phonon promoted charge density wave in topological kagome metal ScV6_{6}Sn6_{6}

Charge density wave (CDW) orders in vanadium-based kagome metals have recently received tremendous attention due to their unique properties and intricate interplay with exotic correlated phenomena, topological and symmetry-breaking states. However, the origin of the CDW order remains a topic of debate. The discovery of ScV$_{6}$Sn$_{6}$, a vanadium-based bilayer kagome metal exhibiting an in-plane $\sqrt{3}$ x $\sqrt{3}$ $\textit{R}$30${\deg}$ CDW order with time-reversal symmetry breaking, provides a novel platform to explore the underlying mechanism behind the unconventional CDW. Here, we combine high-resolution angle-resolved photoemission spectroscopy, Raman scattering measurements and density functional theory to investigate the electronic structures and phonon modes of ScV$_{6}$Sn$_{6}$ and their evolution with temperature. We identify topologically nontrivial Dirac surface states and multiple van Hove singularities (VHSs) in the vicinity of the Fermi level, with one VHS near the K point exhibiting nesting wave vectors in proximity to the $\sqrt{3}$ x $\sqrt{3}$ $\textit{R}$30${\deg}$ CDW wave vector. Additionally, Raman measurements indicate a strong intrinsic electron-phonon coupling in ScV$_{6}$Sn$_{6}$, as evidenced by the presence of a two-phonon mode and a large frequency amplitude mode. Our findings highlight the fundamental role of lattice degrees of freedom in promoting the CDW in ScV$_{6}$Sn$_{6}$ and provide important insights into the fascinating correlation phenomena observed in kagome metals

Propriétés optiques non linéaires du deuxième et troisième ordre de nanoparticules de ZnO et de cristaux massifs de KDP dopés par des nanoparticules d’oxydes métalliques

This work focuses on the experimental characterization of the linear and nonlin-ear optical properties of harmonic nanoparticles - a new class of biological markers - and of composites based on KDP single crystals with incorporated metal oxide nanoparticles. We have developed several experimental setups, such as light scattering indicatrices technique, laser beam self-action analysis, the third harmonic generation efficiency (THG) measurements, and multiphoton microscopy with femtosecond laser excitation in the 710 - 1300 nm spectral range.The third order susceptibility measurements were performed by analyzing the THG at the interface of a cuvette filled with ZnO colloidal suspensions, showing a susceptibility value close to the reference value of the bulk crystal. In addition, the optical properties of these nanoparticles were studied with multiphoton mi-croscopy. The emission spectra of the nanoparticles show the second harmonic generation (SHG), the THG and photoluminescence signals. The spectral re-sponse has been correlated with the structure of defects in bulk ZnO crystals. In addition, we studied the influence of the incorporation of metal oxide nanopar-ticles into KDP crystals. It has been shown that these nanoparticles can increase the SHG efficiency compared to nominally pure KDP crystal. A set of optical measurements was carried out on these composite crystals, demonstrating the relationship between the third order nonlinear optical effects and the observed SHG efficiency enhancement.Ce travail porte sur la détermination expérimentale des propriétés optiques li-néaires et non linéaires de nanoparticules harmoniques - une nouvelle classe de marqueurs biologiques - et de cristaux massifs de KDP avec incorporation de nanoparticules d'oxydes métalliques. Nous avons mis au point une série de bancs expérimentaux dédiés, comme la mesure des indicatrices de diffusion, la technique I-scan pour la mesure de l’indice de réfraction non linéaire, la mesure de l’efficacité de génération de troisième harmonique (THG), ou le développe-ment d’un microscope multiphotonique sous excitation laser femtoseconde dans le domaine spectral 710 - 1300 nm.Des mesures de susceptibilité non linéaire d’ordre 3 ont été effectuées sur des suspensions colloïdales de nanoparticules de ZnO par la technique de la THG aux interfaces, montrant une valeur de susceptibilité proche de la valeur de réfé-rence des cristaux massifs. Par ailleurs, les propriétés optiques de ces nanoparti-cules ont été étudiées par microscopie multiphotonique. Les spectres d’émission des nanoparticules montrent des signaux de génération de second harmonique (SHG), de THG et de photoluminescence, dont la réponse spectrale a été corrélée avec une étude portant sur la spectroscopie de défauts de cristaux massifs de ZnO. Par ailleurs, nous avons étudié l’influence sur les propriétés optiques de l’incorporation de nanoparticules d’oxyde métalliques dans des cristaux KDP. Il a été démontré que ces nanoparticules peuvent être à l’origine d’une augmenta-tion de l'efficacité de SHG par comparaison avec un cristal de KDP pur. Un en-semble de mesures optiques a été réalisé sur ces cristaux composites, démontrant en particulier la corrélation entre les effets non linéaires d’ordre 3 et l’augmentation observée

Second harmonic spectroscopy of ZnO, BiFeO₃ and LiNbO₃ nanocrystals

We developed a wavelength-tunable second harmonic scattering experimental setup to investigate dispersion of the nonlinear optical response of three different nanocrystal suspensions (LiNbO3, ZnO and BiFeO3). Special attention was paid to reproducibility issues with the implementation of a detailed protocol that allows correcting for the setup spectral response. The absolute, orientation-averaged second order susceptibilities of the three nanomaterials were then assessed in the 700-1300 nm spectral range evidencing very specific optical signatures. The well-defined resonances observed for ZnO and BiFeO3 near their electronic transitions were found to be poorly described by the Miller’s rule