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Magnetic properties near the surface of cuprate superconductors studied using Beta-Detected NMR Saadaoui, Hassan

Abstract

Beta-detected Nuclear Magnetic Resonance (β-NMR) uses highly spin polarized β-emitting nuclei as a probe. Besides its use in nuclear physics, it has also become a powerful and sensitive tool in condensed matter physics and materials science. At TRIUMF, β-NMR of ⁸Li+ has been developed to study materials in a depth-resolved manner, where the implantation depth of ⁸Li+ is controlled via electrostatic deceleration. In this thesis, β-NMR of ⁸Li+ has been used to study high-Tc cuprate superconductors (HTSC). The objective of this work is to search for spontaneous magnetic fields generated by a possible time-reversal symmetry breaking (TRSB) superconducting state near the surface of hole-doped YBa₂Cu₃O₇−d (YBCO), and study the nature of the vortex lattice (VL) in YBCO and electron-doped Pr₂−xCexCuO₄−d (PCCO). For several advantages, our measurements were carried out by implanting ⁸Li+ in thin silver films evaporated on the superconductors. In our TRSB studies, the magnetic field distribution p(B) is measured 8 nm away from the Ag/YBCO interface in magnetic fields B₀ = 5 to 100 G, applied parallel to the interface. p(B) showed significant broadening below the Tc of ab- and c-axis oriented YBCO films. The broadening signals the existence of weak disordered magnetic fields near the surface of YBCO. From the broadening’s temperature and field dependence we draw an upper limit of 0.2 G on the magnitude of spontaneous magnetic fields associated with TRSB. To study the VL, p(B) is measured at average implantation depths ranging from 20 to 90 nm away from the Ag/YBCO or Ag/PCCO interface in B₀ = 0.1 to 33 kG, applied perpendicular to the surface. p(B) showed a dramatic broadening below Tc as expected from the emerging field lines of the VL in the superconductor. In YBCO, p(B) is symmetric and the dependence on B0 is much weaker than expected from an ideal VL, indicating that the vortex density varies across the face of the sample on a long length scale, likely due to vortex pinning at twin boundaries. In PCCO, a 2D VL is established due to the high anisotropy of the superconductor leading to a nearly symmetric p(B).

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