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Nematic response of charge order to uniaxial pressure in superconducting cuprates

Resonant X-ray scattering was used to monitor the uniaxial pressure response of two-dimensional charge density waves (2D CDW) in a cuprate. It was found that the pressure enhances the CDW perpendicular to the stress direction, providing a definitive confirmation of its uniaxial character.

Despite considerable research effort over several decades, the mechanism of high-temperature superconductivity in cuprates remains unsettled. A common approach to tackle this issue aims at understanding the normal state from which superconductivity arises and in which CDW (periodic modulations of the electronic charge density) have proven ubiquitous, making them the subject of intense scrutiny [1]. Resonant X-ray scattering (RXS) is one of the best experimental techniques to study CDW, since RXS data represent the Fourier transform of the charge density in space, and the resonant scattering process is sensitive enough to detect even small charge modulations. In RXS experiments, CDW appear as peaks in momentum space, whose position, width, and intensity inform on the periodicity, correlation length, and amplitude of the density modulation, respectively.

This work combined uniaxial stress with RXS to explore a segment of the phase space of correlated electrons in the cuprates that had so far been inaccessible. Previous

investigations using non-resonant inelastic hard X-ray scattering [2] revealed that a pressure-induced 3D CDW appears above Tc and is rapidly and unlike the 2D CDW completely suppressed in the superconducting state, testifying to a very strong competition to superconductivity. In that work, a parasitic signal arising from CuO chains (running parallel to the crystallographic b-axis) completely masks the CDW satellite in the (h,0,l) plane of the reciprocal space under non-resonant conditions, henceforth limiting investigations only to the (0,k,l) plane.

The use of RXS combined with the four-circle in-vacuum diffractometer of the ERIXS spectrometer at beamline ID32 made it possible to collect a complete dataset with the full information of 2D CDW in both (h,0,l) and (0,k,l) planes for uniaxial pressure applied along both a and b axes. The RXS

Fig. 91: Sketch of the strain device with the needle-shaped sample and measurement geometry.

Fig. 92: Uniaxial pressure dependence of the CDW. Evolution of width of 2D CDW peaks in reciprocal space under a) a-axis and b) b-axis compression. Semi-major (-minor) axes are half widths at half maximum of the peaks along H (K). c) Real-space cartoon of CDW domains with different modulation directions in the CuO2 plane without strain. ξa// and ξb// (ξa⊥ and ξb⊥) are longitudinal (transverse) correlation lengths of the a- and b-CDW, respectively. Real-space pictures of CDW domains in the CuO2 plane under d) a-axis and e) b-axis compression.