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Realization of the quantum anomalous Hall effect and axion electrodynamics in topological materials are among the paradigmatic phenomena in condensed matter physics. Recently, signatures of both phases are observed to exist in thin films of MnBi$_2$Te$_4$, a stoichiometric antiferromagnetic topological insulator. Direct evidence of the bulk topological magnetoelectric response in an axion insulator requires an energy gap at its topological surface state (TSS). However, independent spectroscopic experiments revealed that such a surface gap is absent, or much smaller than previously thought, in MnBi$_2$Te$_4$. Here, we utilize angle resolved photoemission spectroscopy and density functional theory calculations to demonstrate that a sizable TSS gap unexpectedly exists in Sb-doped MnBi$_2$Te$_4$. This gap is found to be topologically nontrivial, insensitive to the bulk antiferromagnetic-paramagnetic transition, while enlarges along with increasing Sb concentration. Our work shows that Mn(Bi$_{1-x}$Sb$_x$)$_2$Te$_4$ is a potential platform to observe the key features of the high-temperature axion insulator state, such as the topological magnetoelectric responses and half-integer quantum Hall effects.

Layered honeycomb lattice antiferromagnetic $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Ru}{\mathrm{Cl}}_{3}$ has been studied intensively recently as a Kitaev spin liquid candidate. Here we present strong evidence that this material host realistic Weyl fermions derived from Cl $2p$ electrons. Our theoretical analysis suggests the existence of quadratic Weyl fermions without spin-orbit coupling and linear Weyl fermion with spin-orbit coupling. Angle-resolved photoemission spectroscopy with systematic photon energy-dependent measurements demonstrates that this Weyl cone possesses linear dispersion at bulk Brillouin zone center along all the three reciprocal directions and hyperbolic dispersion off center. Our work reveals the rich topological physics lying in the band structure of Kitaev QSL candidates and will stimulate further investigation not only limited to $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Ru}{\mathrm{Cl}}_{3}$ but also honeycomb iridate family of materials.

Temperature dependent measurements of $^{57}\mathrm{Fe}$ M\"ossbauer spectra on ${\mathrm{CaFe}}_{2}{\mathrm{As}}_{2}$ single crystals in the tetragonal and collapsed tetragonal phases are reported. Clear features in the temperature dependencies of the isomer shift, relative spectra area, and quadrupole splitting are observed at the transition from the tetragonal to the collapsed tetragonal phase. From the temperature dependent isomer shift and spectral area data, an average stiffening of the phonon modes in the collapsed tetragonal phase is inferred. The quadrupole splitting increases by $\ensuremath{\sim}25%$ on cooling from room temperature to $\ensuremath{\sim}100$ K in the tetragonal phase and is only weakly temperature dependent at low temperatures in the collapsed tetragonal phase, in agreement with the anisotropic thermal expansion in this material. In order to gain microscopic insight about these measurements, we perform ab initio density functional theory calculations of the electric field gradient and the electron density of ${\mathrm{CaFe}}_{2}{\mathrm{As}}_{2}$ in both phases. By comparing the experimental data with the calculations we are able to fully characterize the crystal structure of the samples in the collapsed-tetragonal phase through determination of the As $z$ coordinate. Based on the obtained temperature dependent structural data we are able to propose charge saturation of the Fe-As bond region as the mechanism behind the stabilization of the collapsed-tetragonal phase at ambient pressure.

The relationship between spin, electron, and crystal structure has been one of the foremost issues in understanding the superconducting mechanism since the discovery of iron-based high temperature superconductors. Here, we report M\"ossbauer and first-principles calculations studies of the parent compound SrFeAsF with the largest temperature gap ($\sim$50\,K) between the structural and antiferromagnetic (AFM) transitions. Our results reveal that the structural transition has little effect on the electronic structure of the compound SrFeAsF while the development of the AFM order induces a redistribution of the charges near the Fermi level., Comment: 6 Pages, 7 Figures

Structural and magnetic properties of ${({\mathrm{Li}}_{x}{\mathrm{V}}_{1\ensuremath{-}x})}_{3}\mathrm{B}{\mathrm{O}}_{5}$ powders $(x=0.33)$ and single crystals $(x=0.40)$ were studied by x-ray diffraction, magnetization, and NMR measurements. Both powder and single crystal x-ray diffraction data are consistent with the previously reported structure of the system. Magnetization measurements show an overall antiferromagnetic interaction among vanadium spins and reveal a transition into a spin glass state at a sample and magnetic field dependent temperature below $\ensuremath{\sim}10\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The high temperature $(Tg20\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ susceptibility is analyzed using a linear spin trimer model suggested in the literature but such a model is found to be insufficient to explain the data. $^{7}\mathrm{Li}$ and $^{11}\mathrm{B}$ NMR studies indicate an inhomogeneous dynamics close to the zero-field spin-glass transition temperature. The distribution of electronic spin relaxation times is derived using a recently proposed method and the broad temperature-dependent distribution obtained gives a consistent description of the NMR results. The temperature dependence of the distribution indicates a strong slowing down of the local moment spin dynamics as the system cools toward the zero-field spin-glass transition temperature even in the presence of a strong applied magnetic field up to $4.7\phantom{\rule{0.3em}{0ex}}\mathrm{T}$.

The phase relations in the ${\mathrm{Li}}_{2}\mathrm{O}\text{\ensuremath{-}}{\mathrm{V}}_{2}{\mathrm{O}}_{3}\text{\ensuremath{-}}{\mathrm{V}}_{2}{\mathrm{O}}_{5}$ ternary system at $700\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ for compositions in equilibrium with $\mathrm{Li}{\mathrm{V}}_{2}{\mathrm{O}}_{4}$ are reported. This study clarified the synthesis conditions under which low and high magnetic defect concentrations can be obtained within the spinel structure of $\mathrm{Li}{\mathrm{V}}_{2}{\mathrm{O}}_{4}$. We confirmed that the $\mathrm{Li}{\mathrm{V}}_{2}{\mathrm{O}}_{4}$ phase can be obtained containing low $(0.006\phantom{\rule{0.3em}{0ex}}\mathrm{mol}\phantom{\rule{0.2em}{0ex}}%)$ to high $(0.83\phantom{\rule{0.3em}{0ex}}\mathrm{mol}\phantom{\rule{0.2em}{0ex}}%)$ magnetic defect concentrations ${n}_{\text{defect}}$ and with consistently high magnetic defect spin $S$ values between 3 and 6.5. The high ${n}_{\text{defect}}$ values were obtained in the $\mathrm{Li}{\mathrm{V}}_{2}{\mathrm{O}}_{4}$ phase in equilibrium with ${\mathrm{V}}_{2}{\mathrm{O}}_{3}$, ${\mathrm{Li}}_{3}\mathrm{V}{\mathrm{O}}_{4}$, or $\mathrm{Li}\mathrm{V}{\mathrm{O}}_{2}$ and the low values in the $\mathrm{Li}{\mathrm{V}}_{2}{\mathrm{O}}_{4}$ phase in equilibrium with ${\mathrm{V}}_{3}{\mathrm{O}}_{5}$. A model is suggested to explain this correlation.