2021, Huang, H.Y., Singh, A., Mou, C.Y., Johnston, S., Kemper, A.F., van den Brink, J., Chen, P.J., Lee, T.K., Okamoto, J., Chu, Y.Y., Li, J.H., Komiya, S., Komarek, A.C., Fujimori, A., Chen, C.T., Huang, D.J.

Quantum phase transitions play an important role in shaping the phase diagram of high-temperature cuprate superconductors. These cuprates possess intertwined orders which interact strongly with superconductivity. However, the evidence for the quantum critical point associated with the charge order in the superconducting phase remains elusive. Here we show the short-range charge orders and the spectral signature of the quantum fluctuations in La$_{2-x}$Sr$_x$CuO$_4$ (LSCO) near the optimal doping using high-resolution resonant inelastic X-ray scattering. On performing calculations through a diagrammatic framework, we discovered that the charge correlations significantly soften several branches of phonons. These results elucidate the role of charge order in the LSCO compound, providing evidence for quantum critical scaling and discommensurations associated with charge order.

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, 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.