Alexandra V. Koroleva

arXiv2025

Atomic and electronic structure of poly-[Ni(Salen)]: combined study by XPS, UV PES, NEXAFS and DFT methods

A detailed study of poly-[Ni(Salen)] polymer in its oxidized (Ox) and reduced (Red) states was conducted using X-ray photoelectron (XPS) and ultraviolet photoemission (UV PES) spectroscopy, near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, and quantum-chemical calculations. XPS analysis revealed significant energy shifts (-1.5 to -1.8 eV) and broadening of the PE lines for all atoms upon polymerization, indicating a major redistribution of valence electron density between the monomer fragments. In the oxidized polymer, new features in the Ni 2p and O 1s PE spectra were associated with the formation of polarons with weakened Ni-O bonds; this effect diminished upon reduction as the number of polarons decreased. Quantum-chemical calculations attributed the valence band broadening to enhanced C 2p contributions from $π$-conjugation between monomers. NEXAFS spectroscopy confirmed the stability of the ethylenediamine fragment and the direct involvement of the phenolic rings of the salen ligand in polymerization, also revealing a partial weakening and incomplete restoration of the $π$ bonding between O and Ni atoms upon reduction. Furthermore, it was shown that it is the $BF_{4}^-$ anions that weaken the Ni-O bonds during oxidation, which are partially preserved in the reduced state.

physics.chem-ph↗

arXiv2025

Prediction and observation of the first antiferromagnetic topological insulator

Magnetic topological insulators (MTIs) are narrow gap semiconductor materials that combine non-trivial band topology and magnetic order. Unlike their nonmagnetic counterparts, MTIs may have some of the surfaces gapped due to breaking the time-reversal symmetry, which enables a number of exotic phenomena having potential applications in spintronics. So far, MTIs have only been created by means of doping nonmagnetic TIs with 3d transition metal elements, however, such an approach leads to strongly inhomogeneous magnetic and electronic properties of these materials, restricting the observation of important effects to very low temperatures. Finding intrinsic MTI, i.e. a stoichiometric well-ordered magnetic compound, could be an ideal solution to these problems, but no such material was observed to date. Here, using density functional theory we predict and further confirm by means of structural, transport, magnetic, angle- and spin-resolved photoemission spectroscopy measurements the realization of the antiferromagnetic (AFM) TI phase, that is hosted by the van der Waals layered compound MnBi$_2$Te$_4$. An interlayer AFM ordering makes MnBi$_2$Te$_4$ invariant with respect to the combination of the time-reversal ($Θ$) and primitive-lattice translation ($T_{1/2}$) symmetries, $S = ΘT_{1/2}$, giving rise to the $Z_2$ topological classification of AFM insulators. We find $Z_2 = 1$ for MnBi$_2$Te$_4$, which confirms its topologically nontrivial nature. The $S$-breaking (0001) surface of MnBi$_2$Te$_4$ exhibits a giant bandgap in the topological surface state as evidenced by ab initio calculations and photoemission measurements. These results culminate almost a decade-long search of an AFMTI, predicted in 2010. Furthermore, MnBi$_2$Te$_4$ is the first intrinsic magnetic TI realized experimentally.

cond-mat.mtrl-sci↗