Studies of Charge Density Waves in Cuprate Superconductors

Abstract

The cuprate superconductors are high-Tc superconductors, with highly anisotropic properties, and which are defined by their crystallographic structure consisting of layers of CuO2 planes separated by charge reservoirs. Superconductivity seems to reside in these planes but the origin of the mechanism leading to Cooper pairs remains a mystery. The different phases that can be reached by variation of temperature, doping or application of a magnetic field produce intricate phase diagrams. Of particular interest is the charge density wave (CDW) order that seems to be ubiquitous in the different families of the cuprates and appears to be competing with superconductivity. This thesis presents studies on the electron-doped compound NCCO and the hole-doped compounds YBCO and LSCO. These involved the preparation, including the crystal growth for NCCO and YBCO, and characterisation of high quality single crystals. Then, a combination of x-ray diffraction (XRD), resonant inelastic x-ray scattering (RIXS) and inelastic neutron scattering (INS) measurements were used to investigate their properties. Three dopings of NCCO were successfully grown using the travelling-solvent floating-zone technique. A hard x-ray (14 keV) scattering experiment performed on a high-quality single crystal of NCCO x = 0.15 highlights the differences in the CDW between electron- and hole-doped cuprates, in particular in the amplitude of the modulations. Uniaxial strain was used as a tuning parameter to study the interplay between superconductivity and CDW order in YBCO. A mechanical device, whose design is presented, allowed uniaxial pressure to be applied on thin YBCO single crystals. Measurements of superconductivity as a function of uniaxial pressure on underdoped and optimally-doped YBCO show the expected suppression of superconductivity with pressure applied along the a-axis. The device is also compatible with hard x-ray diffraction measurements, and multiple CDW peaks were measured on a detwinned underdoped (p = 0.11) YBCO single crystal mounted in the device. The temperature and uniaxial pressure dependence of the CDW peak centred on (h, k, l) = (0, 0.685, 16.5), in particular the increased correlation length over a significant range of temperatures at 0.14 GPa, might hint that the strengthening of CDW with uniaxial pressure is not just due to the suppression of superconductivity. Finally, INS measurements in LSCO x = 0.12 combined with high energy resolution RIXS measurements at the Cu-L3 edge in LSCO x = 0.12 and 0.16 allow an in-depth analysis of the quasi-elastic and low-energy excitations in this compound. Phonon behaviours, and especially the anomaly in the CuO bond-stretching phonon due to coupling to charge order, are studied in detail. Thanks to the sensitivity of RIXS, these measurements also show that the CDW is stronger in this material than previously thought, with CDW modulations in underdoped LSCO x = 0.12 persisting at least up to 150 K, above the pseudogap onset temperature, and weaker modulations present in overdoped LSCO x = 0.16.vvvvv

Date of Award	28 Nov 2019
Original language	English
Awarding Institution	University of Bristol
Supervisor	Stephen M Hayden (Supervisor) & Antony Carrington (Supervisor)