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Energy and symmetry of dd excitations in undoped layered cuprates measured By Cu L3 resonant inelastic x-ray scattering
2011, Moretti Sala, M., Bisogni, V., Aruta, C., Balestrino, G., Berger, H., Brookes, N.B., De Luca, G.M., Di Castro, D., Grioni, M., Guarise, M., Medaglia, P.G., Miletto, Granozio, F., Minola, M., Perna, P., Radovic, M., Salluzzo, M., Schmitt, T., Zhou, K.J., Braicovich, L., Ghiringhelli, G.
We measured the high-resolution Cu L3 edge resonant inelastic x-ray scattering (RIXS) of undoped cuprates La2CuO4, Sr2CuO2Cl2, CaCuO2 and NdBa 2Cu3O6. The dominant spectral features were assigned to dd excitations and we extensively studied their polarization and scattering geometry dependence. In a pure ionic picture, we calculated the theoretical cross sections for those excitations and used these to fit the experimental data with excellent agreement. By doing so, we were able to determine the energy and symmetry of Cu-3d states for the four systems with unprecedented accuracy and confidence. The values of the effective parameters could be obtained for the singleion crystal field model but not for a simple two-dimensional cluster model. The firm experimental assessment of dd excitation energies carries important consequences for the physics of high-Tc superconductors. On the one hand, we found that the minimum energy of orbital excitation is always ≥ 1.4 eV, i.e. well above the mid-infrared spectral range, which leaves to magnetic excitations (up to 300 meV) a major role in Cooper pairing in cuprates. On the other hand, it has become possible to study quantitatively the effective influence of dd excitations on the superconducting gap in cuprates.
Measurement of Spin Dynamics in a Layered Nickelate Using X-Ray Photon Correlation Spectroscopy: Evidence for Intrinsic Destabilization of Incommensurate Stripes at Low Temperatures
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.