Nuclear magnetic resonance: its role as a microscopic probe of the electronic and magnetic properties of high-T[subscript c] superconductors and related materials

摘要

A series of NMR experiments are reported for several representative high-T[subscript]c superconductors (HTSC) and related materials: Sr[subscript]2CuO[subscript]2Cl[subscript]2, HgBa[subscript]2CuO[subscript]4+[delta], YNi[subscript]2B[subscript]2C and YBa[subscript]2Cu[subscript]3O[subscript]7. The NMR studies reported here typify three different aspects of the microscopic properties of HTSC. In the non-superconducting antiferromagnetic (AF) prototype Sr[subscript]2CuO[subscript]2Cl[subscript]2 we used NMR to investigate the Cu[superscript]2+ correlated spin dynamics and the AF phase transition in the CuO[subscript]2 layers. In the remaining systems, which are superconductors, we used NMR both to investigate the electronic properties of the Fermi-liquid in the normal and superconducting state and to investigate the flux lattice and the flux-line dynamics in the superconducting state in the presence of an applied magnetic field. A summary of each of the studies is given in the following. [superscript]35Cl NMR measurements were performed in Sr[subscript]2CuO[subscript]2Cl[subscript]2 single crystals with T[subscript]N = 257 K. By analyzing the [superscript]35Cl NMR relaxation rates, we obtained evidence of a crossover of the Cu[superscript]2+ spin dynamics from Heisenberg to XY-like correlation at T≈290 K well above T[subscript]N. A field-dependent T[subscript]N for H ⊥ c was observed and explained by a field-induced Ising-like anisotropy in the ab plane. [superscript]199Hg NMR measurements performed on the HTSC HgBa[subscript]2CuO[subscript]4+[delta] are reported. The properties of the Fermi-liquid are found to be characterized by a single-spin fluid picture and the opening of a spin pseudo-gap at q = 0 above T[subscript]c. Below T[subscript]c, the spin component of the Knight shift decreases rapidly in agreement with predicted behavior for the d-wave pairing scheme. [superscript]11B and [superscript]89Y NMR and magnetization measurements were performed in YNi[subscript]2B[subscript]2C single crystals. From the temperature dependence of the [superscript]11B Knight shift and of the NSLR, we obtained a behavior in the normal state which agrees with the Korringa relation, indicating that the AF fluctuations on the Ni sublattice is negligible. The opening of the superconducting gap obeys the predictions of the BCS theory. A novel NMR approach to investigate thermal motion of vortices in HTSC is presented. This is based on a contribution of thermal flux-lines motion to both T[subscript]2[superscript]-1 and T[subscript]1[superscript]-1. The effects are demonstrated in YBa[subscript]2Cu[subscript]3O[subscript]7 and HgBa[subscript]2CuO[subscript]4+[delta].