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NbAs is a Weyl semimetal and belongs to the group of topological phases that exhibit distinct quantum and topological attributes. Topological phases have a fundamentally different response to external perturbations, such as magnetic fields. To obtain insights into the response of such phases to pressure, we conducted a comprehensive study on the pressure-induced electronic and structural transitions in NbAs. We used micro-X-ray diffraction (XRD) and micro-X-ray spectroscopy (XAS) techniques to elucidate the changes at different atomic and electronic length scales (local, medium, and bulk) as combined with theoretical calculations. High-pressure XRD measurements revealed a rather common compression behavior up to ~12 GPa that could be fitted to an equation of state formalism with a bulk modulus of K 0 = 179.6 GPa. Complementary Nb K-edge XAS data unveiled anomalies at pressure intervals of ~12–15 and ~25–26 GPa in agreement with previous literature data from XRD studies. We attribute these anomalies to a previously reported topological Lifshitz transition and the tetragonal-to-hexagonal phase transition, respectively. Analysis of EXAFS results revealed slight changes in the mean next-nearest neighbor distance Nb–As(1) (~2.6 Å) at ~15 GPa, while the second nearest neighboring bond Nb–Nb(1) (~3.4 Å) shows a pronounced anomaly. This indicates that the electronic changes across the Lifshitz transition are accommodated first in the medium-range atomic structure and then at the local range and bulk. The variances of these bonds show anomalous but progressive evolutions close to the tetragonal-to-hexagonal transition at ~25 GPa, which allowed us to derive the evolution of vibration properties in this material. We suggest a prominent displacive character of the I 4 1 m d → P 6 ¯ m 2 transition facilitated by phonon modes. [ABSTRACT FROM AUTHOR]

Two-dimensional borophene nanosheets are sensitive to oxygen, which has hindered their development and use in scientific research and subsequent applications. Conventional methods of borophene synthesis are based on its epitaxial growth on single-crystal metal substrates by using molecular beam epitaxy (MBE). The electron deficiency of boron atoms in the borophene structure has made the use of metal substrates in the growth process inevitable. Hence, a method to stabilize the borophene sheets for further applications seems necessary. In this work, the oxidation arrangement of Al-activated chemical vapor deposition (CVD) of borophene sheets was investigated via scanning photoemission microscopy (SPEM). This method allows the investigation of oxidation ordering via its high spatial resolution and elemental sensitivity. The results indicate the persistence of the borophene χ3 (and some β12) phase after exposure to air. The formation of boron oxide is primarily limited to the Al aggregated islands and the edges while maintaining the borophene configuration intact. Our results show that the electronic exchange between the Al aggregates and the interface of boron atoms at the edges of the borophene sheet can prevent the oxidation process and preserve the borophene sheets for long periods. The lower oxidation tendency of the Al-saturated borophene was confirmed via density functional theory calculation, showing a lower oxygen binding energy on Al/borophene compared with that of the metal-free borophene. The oxidation-resistive borophene can be exploited in innovative high-efficiency catalytic, electronic, and sensing devices. [ABSTRACT FROM AUTHOR]

Pressure-induced transformations in an archetypal chalcogenide glass (GeSe2) have been investigated up to 157 GPa by X-ray absorption spectroscopy (XAS) and molecular dynamics (MD) simulations. Ge and Se K-edge XAS data allowed simultaneous tracking of the correlated local structural and electronic changes at both Ge and Se sites. Thanks to the simultaneous analysis of extended X-ray absorption fine structure (EXAFS) signals of both edges, reliable quantitative information about the evolution of the first neighbor Ge-Se distribution could be obtained. It also allowed to account for contributions of the Ge-Ge and Se-Se bond distributions (chemical disorder). The low-density to high-density amorphous-amorphous transformation was found to occur within 10 to 30 GPa pressure range, but the conversion from tetrahedral to octahedral coordination of the Ge sites is completed above ~ 80 GPa. No convincing evidence of another high-density amorphous state with coordination number larger than six was found within the investigated pressure range. The number of short Ge-Ge and Se-Se "wrong" bonds was found to increase upon pressurization. Experimental XAS results are confirmed by MD simulations, indicating the increase of chemical disorder under high pressure. [ABSTRACT FROM AUTHOR]

X-ray absorption spectroscopy (XAS) is a powerful technique to investigate the short-range order around selected atomic species in condensed matter. The theoretical framework and previous applications to undercooled elemental liquid metals are briefly reviewed. Specific results on undercooled liquid Ni obtained using a peak fitting approach validated on the spectra of solid Ni are presented. This method provides a clear evidence that a signature from close packed triangular configurations of nearest neighbors survives in the liquid state and is clearly detectable below k ≈ 5 Å−1, stimulating the improvement of data-analysis methods that account properly for the ensemble average, such as Reverse Monte Carlo.