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-Jin

We 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.

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, Christian

We 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.

2021, Sauter, E., Abrosimov, N.V., Hübner, J., Oestreich, M.

We measure the spin-lattice relaxation of donor bound electrons in ultrapure, isotopically enriched, phosphorus-doped 28Si:P. The optical pump-probe experiments reveal at low temperatures extremely long spin relaxation times which exceed 20 h. The 28Si:P spin relaxation rate increases linearly with temperature in the regime below 1 K and shows a distinct transition to a T9 dependence which dominates the spin relaxation between 2 and 4 K at low magnetic fields. The T7 dependence reported for natural silicon is absent. At high magnetic fields, the spin relaxation is dominated by the magnetic field dependent single phonon spin relaxation process. This process is well documented for natural silicon at finite temperatures but the 28Si:P measurements validate additionally that the bosonic phonon distribution leads at very low temperatures to a deviation from the linear temperature dependence of Γ as predicted by theory.

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, Bernhard

Since 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.

2021, Golias, E., Kumberg, I., Gelen, I., Thakur, S., Gördes, J., Hosseinifar, R., Guillet, Q., Dewhurst, J.K., Sharma, S., Schüßler-Langeheine, C., Pontius, N., Kuch, W.

We present evidence for an ultrafast optically induced ferromagnetic alignment of antiferromagnetic Mn in Co/Mn multilayers. We observe the transient ferromagnetic signal at the arrival of the pump pulse at the Mn L3 resonance using x-ray magnetic circular dichroism in reflectivity. The timescale of the effect is comparable to the duration of the excitation and occurs before the magnetization in Co is quenched. Theoretical calculations point to the imbalanced population of Mn unoccupied states caused by the Co interface for the emergence of this transient ferromagnetic state.

2021, Windsor, Y.W., Zahn, D., Kamrla, R., Feldl, J., Seiler, H., Chiang, C.-T., Ramsteiner, M., Widdra, W., Ernstorfer, R., Rettig, L.

We use femtosecond electron diffraction to study ultrafast lattice dynamics in the highly correlated antiferromagnetic (AFM) semiconductor NiO. Using the scattering vector (Q) dependence of Bragg diffraction, we introduce Q-resolved effective temperatures describing the transient lattice. We identify a nonthermal lattice state with preferential displacement of O compared to Ni ions, which occurs within ∼0.3  ps and persists for 25 ps. We associate this with transient changes to the AFM exchange striction-induced lattice distortion, supported by the observation of a transient Q asymmetry of Friedel pairs. Our observation highlights the role of spin-lattice coupling in routes towards ultrafast control of spin order.