Nuclear magnetic resonance studies of competing orders in high temperature superconductors

Orateur : Soutenance de Thèse de Igor VINOGRAD

Cuprates are materials that can be tuned from an antiferromagnetic insulator to a normal metal by increasing the carrier density through chemical doping. At intermediate doping, a rich variety of electronic phases emerges alongside, or intertwined, with the superconducting phase. The aim of this thesis was to characterise various aspects of the competition between superconductivity and charge or spin order, using nuclear magnetic resonance (NMR). A first part of the work consisted in improving the modelling of 17O NMR spectra in the two charge-density wave (CDW) phases present in YBa2Cu3Oy : the short-range order and the (magnetic-field induced) long-range order. Besides providing a much more accurate analysis framework for NMR data as a function of field, doping and pressure (see hereafter), the results indicate that the CDW in high-fields is uniaxial (i.e. single wave vector q) and commensurate with the lattice, with a period of three unit cells (q=1/3). The second aspect of phase competition addressed in this work is the (controversial) effect of hydrostatic pressure. Our measurements show that a pressure of 1.9 GPa weakens the short-range CDW in the normal state and the long-range CDW observed in high fields only slightly. The results support the proposal that the continuous rise in Tc upon increasing pressure up to ∼15 GPa arises almost entirely from a gradual decrease of the CDW strength. This establishes hydrostatic pressure as a tuning parameter of the competition between CDW order and superconductivity in the cuprates. In the third part of the thesis, 139La spin-lattice relaxation rate (1/T1) measurements were used to study the effect of a magnetic field on magnetic correlations in La2-xSrxCuO4. Using high fields up to 45 T, we reveal that the field is able to induce a frozen, or nearly so, state at doping levels much higher than previously thought, namely up to the putative endpoint of the pseudogap boundary, but not, or not far, beyond that point. This result has important implications for understanding the pseudogap phase.

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