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We report on the structural and spectroscopic characterization of the multiferroic Fe$_2$Mo$_3$O$_8$. Synchrotron x-ray and neutron diffraction, as well as thermal expansion measurements reveal a lattice anomaly at $T_{mathrm{N}}simeq 60,$K but do not show any symmetry lowering in the magnetically ordered state. The lattice parameter $c$ exhibits a non-monotonic behavior with a pronounced minimum around $200,$K, which is also reflected in an anomalous behavior of some of the observed infrared-active optical excitations and parallels the onset of short-range magnetic order. The infrared reflectivity spectra measured between 5 and 300$,$K in the frequency range of $100-8000,$cm$^{-1}$ reveal most of the expected phonon modes in comparison with the eigenfrequencies obtained by density-functional calculations. The $A_1$ phonons show an overall hardening upon cooling, whereas a non-monotonic behavior is observed for some of the $E_1$ modes. These modes also show a strongly increased phonon lifetime below $T_mathrm{N}$, which we associate with the quenched direction of the orbital moment in the magnetically ordered state. A similar increase is observed in the lifetime of the higher-lying $d$-$d$ excitations of the tetrahedral Fe$^{2+}$ site, which become clearly visible below $T_mathrm{N}$ only.
We report on optical excitations in the magnetically ordered phases of multiferroic Fe$_{1.86}$Zn$_{0.14}$Mo$_3$O$_8$ in the frequency range from 10-130 cm$^{-1}$ (0.3-3.9 THz). In the collinear easy-axis antiferromagnetic phase below $T_N=50$~K elev
We present both static and time-resolved second harmonic generation (SHG) measurements on polar antiferromagnet Fe$_2$Mo$_3$O$_8$ to monitor the evolution of the electric polarization change and its coupling to magnetic order. We find that only one o
LiZn$_2$Mo$_3$O$_8$ has been proposed to contain $S~=~1/2$ Mo$_3$O$_{13}$ magnetic clusters arranged on a triangular lattice with antiferromagnetic nearest-neighbor interactions. Here, microwave and terahertz electron spin resonance (ESR), $^7$Li nuc
Theoretical studies have predicted the existence of topological magnons in honeycomb compounds with zig-zag antiferromagnetic (AFM) order. Here we report the discovery of zig-zag AFM order in the layered and non-centrosymmetric honeycomb nickelate Ni
La$_2$O$_3$Fe$_2$Se$_2$ can be explained in terms of Mott localization in sharp contrast with the metallic behavior of FeSe and other parent parent compounds of iron superconductors. We demonstrate that the key ingredient that makes La$_2$O$_3$Fe$_2$