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Register a new userHidden Charge Order in an Iron Oxide Square-Lattice Compound
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Since the discovery of charge disproportionation in the FeO$_2$ square-lattice compound Sr$_3$Fe$_2$O$_7$ by Mossbauer spectroscopy more than fifty years ago, the spatial ordering pattern of the disproportionated charges has remained hidden to conventional diffraction probes, despite numerous x-ray and neutron scattering studies. We have used neutron Larmor diffraction and Fe K-edge resonant x-ray scattering to demonstrate checkerboard charge order in the FeO$_2$ planes that vanishes at a sharp second-order phase transition upon heating above 332 K. Stacking disorder of the checkerboard pattern due to frustrated interlayer interactions broadens the corresponding superstructure reflections and greatly reduces their amplitude, thus explaining the difficulty to detect them by conventional probes. We discuss implications of these findings for research on hidden order in other materials.
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Neutron diffraction has been used to determine the magnetic structure of Na$_8$Cu$_5$O$_{10}$, a stoichiometric compound containing chains based on edge-sharing CuO$_4$ plaquettes. The chains are doped with 2/5 hole per Cu site and exhibit long-range commensurate charge order with an onset well above room temperature. Below $T_N = 23$ K, the neutron data indicate long-range collinear magnetic order with a spin density modulation whose propagation vector is commensurate along and incommensurate perpendicular to the chains. Competing interchain exchange interactions are discussed as a possible origin of the incommensurate magnetic order.
We report susceptibility, specific heat, and neutron diffraction measurements on NaCu$_2$O$_2$, a spin-1/2 chain compound isostructural to LiCu$_2$O$_2$, which has been extensively investigated. Below 13 K, we find a long-range ordered, incommensurate magnetic helix state with a propagation vector similar to that of LiCu$_2$O$_2$. In contrast to the Li analogue, substitutional disorder is negligible in NaCu$_2$O$_2$. We can thus rule out that the helix is induced by impurities, as was claimed on the basis of prior work on LiCu$_2$O$_2$. A spin Hamiltonian with frustrated longer-range exchange interactions provides a good description of both the ordered state and the paramagnetic susceptibility.
Organic salts represent an ideal experimental playground for studying the interplay between magnetic and charge degrees of freedom, which has culminated in the discovery of several spin-liquid candidates, such as $kappa$-(ET)$_2$Cu$_2$(CN)$_3$ ($kappa$-Cu). Recent theoretical studies indicate the possibility of chiral spin liquids stabilized by ring-exchange, but the parent states with chiral magnetic order have not been observed in this material family. In this work, we discuss the properties of the recently synthesized $kappa$-(BETS)$_2$Mn[N(CN)$_2$]$_3$ ($kappa$-Mn). Based on analysis of specific heat, magnetic torque, and NMR measurements combined with ab initio calculations, we identify a spin-vortex crystal order. These observations definitively confirm the importance of ring-exchange in these materials, and support the proposed chiral spin-liquid scenario for triangular lattice organics.
Solution-grown single crystals of Fe2OBO3 were characterized by specific heat, Mossbauer spectroscopy, and x-ray diffraction. A peak in the specific heat at 340 K indicates the onset of charge order. Evidence for a doubling of the unit cell at low temperature is presented. Combining structural refinement of diffraction data and Mossbauer spectra, domains with diagonal charge order are established. Bond-valence-sum analysis indicates integer valence states of the Fe ions in the charge ordered phase, suggesting Fe2OBO3 is the clearest example of ionic charge order so far.
We report a novel quasi-two dimensional compound of EuTe4 hosting charge density waves (CDW) instability. The compound has a crystallographic structure in a orthorhombic space group Pmmn (No.59) with cell parameters a = 4.6347(2){AA}, b = 4.5119(2){AA}, c = 15.6747(10){AA} at room temperature. The pristine structure contains consecutive near-square Te sheets separated by corrugated Eu-Te slabs. Upon cooling, the compound experiences a phase transition near 255 K. X-ray crystallographic analysis and transmission electron microscopy (TEM) measurements reveal strong structural distortions in the low temperature phase, showing a superstructure with a periodic formation of Te-trimers in the monolayer Te sheets, yielding evidence for the formation of CDW order. The charge transport properties show a semiconducting behavior in the CDW state. Density functional theory calculations reveals a Fermi surface nesting driven instability with a nesting vector in good agreement with the one observed experimentally. Our finding provides a promising system for the study of CDW driven 2D semiconducting mechanisms, which would shed a new light on exploring novel 2D semiconductors with collective electronic states.
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