Cuprate superconductors are known to harbor charge order in part of their phase diagram. The order has a two-dimensional (2D) character at zero magnetic field, whereas a 3D order appears at high fields. In x-ray diffraction measurements under high uniaxial pressure, we have discovered Bragg reflections characteristic of 3D charge order even at zero magnetic field. We also demonstrated a soft phonon mode that is associated with the formation of 3D order. The results provide insights into the normal-state properties of cuprates and illustrate the potential of uniaxial-pressure control of competing orders in quantum materials.

H.H. Kim, S.M. Souliou et al., Science 362, 1040 (2018)

moreTwo new papers report important progress towards the goal of designing new magnetic states in oxide heterostructures. First, we used high-resolution inelastic x-ray scattering to directly determine the magnon dispersion relations and exchange interactions in an epitaxial film of NdNiO_{3} [1]. We then established an elementary model system – NdNiO_{3} slabs embedded in a non-magnetic NdGaO_{3} matrix – to show that both collinear and non-collinear magnetic structures can be realized, depending on the slab thickness. [2] The crossover between both spin structures can be qualitatively understood in a low-energy spin model with the competing exchange interactions determined by RIXS. Magnetic slabs thus provide new perspectives for research on complex magnetism, in analogy to two-dimensional materials created by exfoliation. [1] Y. Lu et al., Phys. Rev. X 8, 031014 (2018)

[2] M. Hepting et al., Nature Physics (2018)

Over the past several years, charge density waves have turned out to be the major competitor of high-temperature superconductivity in copper oxides. In resonant x-ray scattering experiments on YBCO thin films grown epitaxially on SrTiO_{3}, we made the surprising discovery that three-dimensional charge order is present in the absence of magnetic fields, and persists to much higher temperatures than the two-dimensional order investigated in prior work. The results offer new insight into the unusual stability of charge density waves in YBCO, as well as fresh perspectives for experiments elucidating the influence of charge order on the electronic properties of cuprates without the need to apply high magnetic fields.

See also the time lapse video of him, Simon Mayer, Hasan Yavas and others assembling the spectrometer.

_{2}RuO

_{4}revealed a well-defined, dispersive "Higgs" mode that modulates the amplitude of the ordered moment. The mode quickly decays into transverse spin waves at the antiferromagnetic ordering wavevector, and through a complete mapping of the transverse modes in the reciprocal space we were able to obtain a quantitative understanding of the decay process. The results establish a novel condensed matter platform for research on the dynamics of the Higgs mode.