External control of electronic phases in correlated-electron materials is a long-standing challenge of condensed-matter research. Layered cuprates exhibit antiferromagnetic, charge-density-wave (CDW), and high-temperature superconducting ground states which can be tuned by doping and external magnetic fields. However, disorder generated by lattice defects and randomly pinned magnetic vortices greatly complicates the interpretation of these experiments. Here, we report a high-resolution inelastic x-ray scattering (IXS) study of the high-temperature superconductor YBa2Cu3O6.67 under hydrostatic  and uniaxial  stress. We show that the former rapidly suppresses the CDW, while the second can induce a three-dimensional long-range-ordered CDW state by compressing the sample along the a-axis, in the absence of magnetic fields. The amplitude of the CDW is strongly suppressed below the superconducting transition temperature, indicating strong thermodynamic competition with superconductivity. We also show that the transition is driven by the complete softening of an optical phonon mode.
Non-resonant scattering can however not be used to monitor the evolution of the CDW with stress along the b-axis, due to a strong background arising from CuO chains superstructures. ... mehrThis can be overcome using resonant (inelastic) x-ray scattering and will be further discussed.
The results provide new insights into the anomalous normal-state properties of high-temperature superconductors an illustrate the potential of uniaxial-pressure control of competing orders in quantum materials.
 S. M. Souliou et al., Phys. Rev. B 97, R020503 (2018).
 H.-H. Kim, S. M. Souliou et al., Science 362, 1040 (2018).