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- ItemA neutron diffraction study of crystal and low-temperature magnetic structures within the (Na,Li)FeGe2O6 pyroxene-type solid solution series(Berlin ; Heidelberg : Springer, 2017-5-12) Redhammer, Günther J.; Senyshyn, Anatoliy; Lebernegg, Stefan; Tippelt, Gerold; Dachs, Edgar; Roth, GeorgSolid solution compounds along the Li1–x Na x FeGe2O6 clinopyroxene series have been prepared by solid state ceramic sintering and investigated by bulk magnetic and calorimetric methods; the Na-rich samples with x(Na) > 0.7 were also investigated by low-temperature neutron diffraction experiments in a temperature range of 4–20 K. For samples with x(Na) > 0.76 the crystal structure adopts the C2/c symmetry at all measuring temperatures, while the samples display P21/c symmetry for smaller Na contents. Magnetic ordering is observed for all samples below 20 K with a slight decrease of T N with increasing Na content. The magnetic spin structures change distinctly as a function of chemical composition: up to x(Na) = 0.72 the magnetic structure can be described by a commensurate arrangement of magnetic spins with propagation vector k = (½, 0 0), an antiferromagnetic (AFM) coupling within the Fe3+O6 octahedra zig-zag chains and an alternating AFM and ferromagnetic (FM) interaction between the chains, depending on the nature of the tetrahedral GeO4 chains. The magnetic structure can be described in magnetic space group P a21/c. Close to the structural phase transition for sample with x(Na) = 0.75, magnetic ordering is observed below 15 K; however, it becomes incommensurately modulated with k = (0.344, 0, 0.063). At 4 K, the magnetic spin structure best can be described by a cycloidal arrangement within the M1 chains, the spins are within the a–c plane. Around 12 K the cycloidal structure transforms to a spin density wave (SDW) structure. For the C2/c structures, a coexistence of a simple collinear and an incommensurately modulated structure is observed down to lowest temperatures. For 0.78 ≤ x(Na) ≤ 0.82, a collinear magnetic structure with k = (0 1 0), space group P C21/c and an AFM spin structure within the M1 chains and an FM one between the spins is dominating, while the incommensurately modulated structure becomes dominating the collinear one in the samples with x(Na) = 0.88. Here the magnetic propagation vector is k = (0.28, 1, 0.07) and the spin structure corresponds again to a cycloidal structure within the M1 chains. As for the other samples, a transition from the cycloidal to a SDW structure is observed. Based on the neutron diffraction data, the appearance of two peaks in the heat capacity of Na-rich samples can now be interpreted as a transition from a cycloidal magnetic structure to a spin density wave structure of the magnetically ordered phase for the Na-rich part of the solid solution series.
- ItemUnraveling the Orbital Physics in a Canonical Orbital System KCuF3(College Park, Md. : APS, 2021) Li, Jiemin; Xu, Lei; Garcia-Fernandez, Mirian; Nag, Abhishek; Robarts, H.C.; Walters, A.C.; Liu, X.; Zhou, Jianshi; Wohlfeld, Krzysztof; van den Brink, Jeroen; Ding, Hong; Zhou, Ke-JinWe explore the existence of the collective orbital excitations, orbitons, in the canonical orbital system KCuF3 using the Cu L3-edge resonant inelastic x-ray scattering. We show that the nondispersive high-energy peaks result from the Cu2+ dd orbital excitations. These high-energy modes display good agreement with the ab initio quantum chemistry calculation, indicating that the dd excitations are highly localized. At the same time, the low-energy excitations present clear dispersion. They match extremely well with the two-spinon continuum following the comparison with Müller ansatz calculations. The localized dd excitations and the observation of the strongly dispersive magnetic excitations suggest that the orbiton dispersion is below the resolution detection limit. Our results can reconcile with the strong local Jahn-Teller effect in KCuF3, which predominantly drives orbital ordering.
- ItemMeasurement of Spin Dynamics in a Layered Nickelate Using X-Ray Photon Correlation Spectroscopy: Evidence for Intrinsic Destabilization of Incommensurate Stripes at Low Temperatures(College Park, Md. : APS, 2021) Ricci, Alessandro; Poccia, Nicola; Campi, Gaetano; Mishra, Shrawan; Müller, Leonard; Joseph, Boby; Shi, Bo; Zozulya, Alexey; Buchholz, Marcel; Trabant, Christoph; Lee, James C. T.; Viefhaus, Jens; Goedkoop, Jeroen B.; Nugroho, Agustinus Agung; Braden, Markus; Roy, Sujoy; Sprung, Michael; Schüßler-Langeheine, ChristianWe study the temporal stability of stripe-type spin order in a layered nickelate with x-ray photon correlation spectroscopy and observe fluctuations on timescales of tens of minutes over a wide temperature range. These fluctuations show an anomalous temperature dependence: they slow down at intermediate temperatures and speed up on both heating and cooling. This behavior appears to be directly connected with spatial correlations: stripes fluctuate slowly when stripe correlation lengths are large and become faster when spatial correlations decrease. A low-temperature decay of nickelate stripe correlations, reminiscent of what occurs in cuprates as a result of a competition between stripes and superconductivity, hence occurs via loss of both spatial and temporal correlations.
- ItemHidden Charge Order in an Iron Oxide Square-Lattice Compound(College Park, Md. : APS, 2021) Kim, Jung-Hwa; Peets, Darren C.; Reehuis, Manfred; Adler, Peter; Maljuk, Andrey; Ritschel, Tobias; Allison, Morgan C.; Geck, Jochen; Mardegan, Jose R. L.; Bereciartua Perez, Pablo J.; Francoual, Sonia; Walters, Andrew C.; Keller, Thomas; Abdala, Paula M.; Pattison, Philip; Dosanjh, Pinder; Keimer, BernhardSince the discovery of charge disproportionation in the FeO2 square-lattice compound Sr3Fe2O7 by Mössbauer 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 FeO2 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 of detecting them by conventional probes. We discuss the implications of these findings for research on “hidden order” in other materials.